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About Google Book Search Google's mission is to organize the world's information and to make it universally accessible and useful. Google Book Search helps readers discover the world's books while helping authors and publishers reach new audiences. You can search through the full text of this book on the web at |http: //books .google .com/I GAGE DESIGN AND GAGE-MAKING i GAGE DESIGN AND GAGE-MAKING A TREATISE ON THE DEVELOPMENT OF GAGING SYSTEMS FOR INTERCHANGE- ABLE MANUFACTURE, THE DESIGN OF DIFFERENT TYPES OF GAGES AND THEIR PRODUCTION, INCLUDING PRECISION MACHINING OPERATIONS, LAPPING, AND VARIOUS MODERN APPLIANCES FOR GAGE MEASUREMENT BY - : .'.'..*.-.,». ERIK OBERG Editoh or MACHINERY Editos op MACHINERY'S HANDBOOK and MACHINERY'S ENCYCLOPEDIA, Auihos of "Handbook of Shall Tools," Etc. AND FRANKLIN D. JONES Associate Editos of MACHINERY avibok of "tukning and boming," "planing and mlllmq," "Mbchanisms and Mechanical Movements," "Tbebad-cuttimo Methods,** etc FIRST EDITION FIRST PRINTING NEW YORK THE INDUSTRIAL PRESS LONDW: THE MACHINERY PUBLISHING CO.. Ltd. 1920 \^\' J u'*«i It. ^ r- i\* 03 U * • • • ■ . - * • « » ' » • ' • ' • . • .■.,•.♦ •" . . ' • - - • • • • %»»•»•• COFYSIGBT, 1920, BY THE INDUSTRIAL PRESS NEW YORK COMPOSITION AND ELECTROTYPING BY F. H. GILSON COUPANY, BOSTON, U.8^ SC PREFACE The cost of manufacturing any mechanism may be greatly aflfected, first, by the tolerances specified, and, second, by the gages used to insure that these tolerances are adhered to. There has been, and still is, a great deal of misunderstanding with regard to tolerances and their application in shop work. Briefly stated, tolerances should be based upon the working require- ments of the mechanism to be made, and should be as large as possible, consistent with the purpose of the part or combination of parts to be made. Unnecessary accuracy which does not improve the product merely increases the manufacturing cost, and, hence, is sheer waste. After the tolerances have been established, the means for insuring that the desired tolerances are adhered to in the machining operations must be considered. The gages to be used must be so designed and made that they are not only accurate, but practicable, and adapted to shop use. The relation between the gaging system and the design of jigs and fixtures is also essential. This book deals with such fundamental and vital points as have been outlined above, and is, therefore, regarded as a valu- able addition to the technical literature of the day. Much has been published on manufacturing practice, but comparatively little on the design and making of gages; yet, gages control the different manufacturing processes and insure that the required degree of interchangeability is obtained. As the design of any one gage or group of gages is dependent upon such factors as the location of the finished surfaces, the shape of the part, its size, the degree of accuracy required, etc., it is, of course, necessary to adapt each gage to its particular purpose; but while these factors may be combined in an endless variety of ways, and gages, therefore, differ greatly in form, size, and arrangement of detail, they all conform to certain principles V 822783 vi PREFACE which experience and practice have proved to be correct. This book deals in part with these basic or fundamental principles, and includes information on the making of gages and on methods of measuring and testing them to insure conformity to given dimensions. That part of the book dealing with the development of gaging systems and gage design is based largely upon the experience and practice of the Pratt & Whitney Co., in making gaging equip- ment for small arms and heavy ordnance. The manufacture of munitions of war has emphasized the great importance of ade- quate gaging systems in a more forceful manner than the produc- tion of tools and machines used in peaceful pursuits, and the Pratt & Whitney Co., has probably done a greater pioneer work along these lines during the past few years than any other concern. In the preparation of this part of the material, the authors have been substantially aided by A. E. Larsson, engineer of the Equipment Division of the Pratt & Whitney Go. Valu- able material has also been obtained from articles in Machinery contributed by a number of other men experienced in gage-making. The Authors. Nkw Yosk, April, 1930. CONTENTS Chapter I DEVELOPING A GAGING SYSTEM Pages Interchangeability and Its Relation to Manufacturing — Definitions of Terms used in Gaging — Inspection, Working, and Reference Gages — Gage Tolerances — Marking Gages — Relation of Gaging System to Kind of Work — Procedure in Developing a Gaging System — Tolerances for Different Machining Operations 1-48 Chapter n SNAP AND PLUG GAGES Solid and Built-up Snap Gages — Reference Gages for Snap Gages — Modifications in Making Snap Gages — ^.Plug Gages — A Practical Ring, Plug, and Snap Gage System — Plug and Ring Gage Allowances — Windmill or Wing Gages 49~78 Chapter III CONTOUR OR PROFILE GAGES Guided Contour Gages — Matching Type of Contour Gages — Receiving Gages — Dial Indicator Contour Gages and Their Advantages — Indicating Tolerances for a Contour -r— Different Designs of Dial Indicator Contour Gages 79-101 Chapter IV FLUSH-PIN, SLIDING-BAR, AND HOLE GAGES Work for which Flush-pin Gages are Adapted — Different Types of Flush-pin Gages — Gage for Testing Direction of a Deep Drilled Hole — Advantages of Flush-pin Gages — Sliding- bar Gages and Their Application — Gages for Testing the Location of Holes 102-127 • • vu VIU CONTENTS Chapter V THREAD GAGES AND THEIR PRODUCTION Paces Complete Gage Sets for Screws and Tapped Holes — Special Forms of Thread Gages — Tools for Cutting Gage Threads — Gaging the Ends of Precision Thread-cutting Tools — Methods of Grinding Tools to the Required Angle — Tools for Cutting Whitworth Thread Gages — Methods of Compensating for Errors in a Screw-cutting Lathe 128-151 Chapter VI MAKING WHITWORTH THREAD GAGES Machining of Gage Blanks — Rough-machining the Thread — Heat-treatment of the Gages — Preliminary Grinding Operation on Thread — Finish-grinding Operations 152-170 Chapter VH GRINDING AND LAPPING THREAD GAGES Advantages of Grinding Thread before Lapping — General Types of Machines used for Thread Grinding — Grinder having Traversing Master Screw — Grinder having Traversing Wheel Slide — Forming Wheel for Grinding Crest of Whitworth Thread — Wheels used for Thread Gage Grinding — Allowances for Lapping — Timing the Lapping Periods — Special Lapping Machines 171-200 Chapter VHI LEAD AND DIAMETER MEASUREMENTS ON THREAD GAGES Floating Micrometer for Diameter Measurements — Deter- mining Error in Thread Angle by Three-wire Method — Testing Thread Gages for Lead Error — Micrometer Type of Lead- testing Machine — Lead Tester eqiupped with Micrometer and Dial Gage — Lead-testing Machine having Multiplying Indi- cator and Size-block Adjustment — Fluid-gage Type of Lead Tester — Lead-measuring Machines having Sliding Contact Point and Provision for Magnif3dng Errors — Improved Design of Optical Lead-measuring Apparatus , 201-232 • CONTENTS IZ Chaptek DC MICROSCOPIC MEASURING MACfflNES FOR TESTING ACCURACY OF CONTOUR AND THREAD GAGES Pages Microscopic Machine having Gage Adjustment — Contour- and Radius-measuring Instrument — Measurements of Screw Threads — Microscopic Measuring Machine having Microm- eter Screw Adjustment — Examples illustrating use of Micro- scopic Measuring Machines 233-252 Chapter X PROJECTION METHOD OF TESTING GAGE THREADS Optical Projection Apparatus and Its Application to Plug and Ring Gages — Tracings of Inaccurate Thread Forms — Photographing Thread Forms — Vertical Projector for Screw Threads — Thread Angle Projection Apparatus 253-271 Chapter XI GENERAL GAGE-MAKING PRACTICE Steel for Gages — Grinding Snap Gages — Gage Grinding Machines — Grinding End-measuring or Pin Gages — Heat- treatment of Machine and Tool-steel Gages — Methods of Hardening Gages to Prevent Distortion and Breakage — Re- storing Worn Gages by Electroplating — Taper and Angle Gages 272-305 - * ^ ^ ^ V. • _ • • «•_■»•« • ^» • ■» • • 1 _ • • • • ; • * • • • « » k » • "# « GAGE DESIGN AND GAGE- MAKING CHAPTER I DEVBLOPING A GAGING STSTBM When a part for a machine or device is made, it must be measured to ascertain that it is of the right dimensions for ful- filling the purpose for which it is intended; this applies equally whether a single piece is made^ as when making a special machine, or whether a hundred-thousand pieces are made, as in inter- changeable manufacturing. Often the simplest method of measuring a part made for ji specific purpose is to fit it to the component part which it is to engage when in use. This was a common practice of measuring or gaging in the early develop- ment of the machine trade and is still used to a large extent in repair work; but, in manufacturing work, this method has been gradually displaced with the introduction of interchangeable manufacturing. The next step was to measure the part being made by various measuring devices — first a scale, and later micrometers and protractors, but often the fitting of each piece to its component part was still required in the assembling of the mechanism. Still later, as interchangeable manufacturing be- came more highly developed, gages were devised, making it possible, in most cases, to so accurately determine the dimensions of each individual piece that, after the parts were made, they could be assembled without further fitting. Principal Groups of Measuring Devices. — There are two broad groups of measuring tools. In one of these, the dimen- sions measured are read ofif on a scale, and measuring instruments of this type may be used for measuring any distances within a • \:i'': •':•.• *:: . )1aCs - i>esign and gage-making certain range. The other group of measuring devices is used only for determining if a certain dimension on a piece of work is equal to a given predetermined dimension. The ordinary ma- . chinists' scaJe, the vernier caliper^ the micrometer, protractors, and measuring machines belong to the first group, while fixed gages constitute the second group. A gage may be defined, in general, as a measuring device made or set to measure one or more certain dimensions, and it is used for determining if manu- factured parts have been made to agree with this dimension. If the gage is provided with means for measuring the maximum and the minimum dimensions to which a given piece may be made and yet fulfill the fimction for which it is intended, it is known as a "limit gage " (sometimes also as a "tolerance gage ")> because it is the means of determining if the part is made within the pre- determined limits set for it. Gaging and Interchangeable Manufacturing. — Generally speaking, therefore, gages are instruments to be used as fixed measuring devices, and consequently each gage, in general, can be applied for a certain purpose only; hence, gages are used only in interchangeable manufacture; that is, in cases where a number of similar parts are to be made, all of which may be measured by the same gage and the accuracy of which, within the prescribed limits, may thereby be assured. Gaging S3rstems and inter- changeable manufacturing are so closely tied together that it would be impossible to discuss one without also dealing with the other. The main object m the introduction of interchangeable manufacturing was to reduce costs by standardizing the manu- facturing operations and reducing the time required in the assembling of a mechanism. A secondary advantage was thereby gained, however; that of a complete interchange of parts in the manufactured article. Indeed, in many instances, this last advantage has become an object equally important with the reduction of the cost in the assembling, and is insisted upon in the manufacture of rifles and heavy ordnance, so that damaged parts may be replaced with other parts without fitting. As an example of the reduction of cost in assembling, due to the adop- tion of an adequate gaging system, it may be mentioned that in DEVELOPING A GAGING SYSTEM 3 one case the cost of assembling a rifle was $2.50. This was reduced to 25 cents by the adoption of a new tool and gage equip- ment^ and resulted in the scrapping of the former tool and gage equipment valued at $80,000. How Far should Interchangeability be Carried? — Inter- changeability is sometimes carried too far. Where the object sou^t is merely to facilitate assembling, any further refinement to insure absolute interchangeability involves useless expense. Very few mechanisms, in fact, are absolutely interchangeable. Even in rifle manufacture there are a few parts where matching of the parts is permissible, this practice being known as '^selective assembling." It is, therefore, necessary that proper judgment be used in determining upon the importance of interchange- ability, and that the various factors be carefully weighed against each other. If it is cheaper to throw away the whole or a group of the mechanism and obtain a new one, rather than to unduly increase the expense of manufacture by aiming at the high degree of accuracy that is required for absolute interchangeability, then the refinement required for such interchangeability is unwar- ranted. As an example of work where interchangeable manu- facturing methods are employed largely for the sake of assembling may be mentioned the case of shell or projectile work, where many of the parts are made interchangeable simply in order to facilitate assembling, because, when once assembled, they will not be again interchanged with other parts. Sometimes one part is made in one shop and other parts in another shop, in which case accuracy of dimensions is necessary in order that these parts later may be propedy assembled without additional fitting. Interchangeability between Parts made in Different Shops* — The great difficulty of obtaining absolute interchangeability between products made in two different shops is due to the fact that, if the two shops work entirely independently, they are likely to use different locating points, different tooling equipment, different machining methods, and different gages and gaging methods. To insure interchangeability, therefore, there must be some kind of cooperation, so that similar methods are used in the two plants. 4 GAGE DESIGN AND GAGE-MAKING It has been found very difficult to obtain properly inter- changeable work when two plants produce the same mechanism, imless proper precautions are taken at the outset to insure inter- changeability. When two plants are to manufacture the same mechanism, the most certain method of insuring interchange- ability is to make sure that the locating and gaging points employed in the machining of the parts in both plants are the same. The details of the machining methods and, to some extent, the tools may vary, but the locating points and the gages ought to be identical; otherwise it is almost certain that the parts will not exactly interchange. The modem gaging system using fixed caliper gages is said to have been introduced into the United States by John Richards, who inaugurated this system in the plant of the present John M. Rogers Works, Gloucester City, N. J., in 1865. As the methods of interchangeable manufacturing developed, these gages, instead of having one fixed dimension, were provided with means for measuring two dimensions, the maximum and the minimum measurements between which the work must come. This was the origin of the modem limit gage in use today. Definitions oi Terms used in Gaging and Interchangeable Manufacturing. — Much confusion is caused by a misunderstand- ing of the terms used in connection with gaging and interchange- able manufacturing, and often disputes and disagreements occur simply because terms are differently interpreted by different men. The terms that will be employed in the following will, therefore, be defined at the outset, so as to prevent any possible misunder- standing. These terms are "tolerance," "limit," "maximmn metal dimension," "clearance," and "initial clearance." Tolerance. — Tolerance is the permissible variation in a given dimension of a part. For example, if it is permissible to make a part with a diameter anywhere between 0.996 and i .000 inch, then the tolerance is 0.004 uich* Limit. — The limits are the maximum and minimum dimen- sions of a piece. For example, when the tolerance is 0.004 inch, as in the case referred to above, the maximum limit is i.ooo inch and the minimum limit is 0.996 inch. DEVELOPING A GAGING SYSTEM Maximum Metal Dimension. — The maximum metal dimen- sion is the dimension which is equal to the limit at which the part contains the most amoimt of metal; hence, in the case of a shaft, the maximum metal dimension is the maximimi limit; in the case of a hole, however, the maximum metal dimension is the minimum limit, as there is more metal in the part the smaller the hole; to enlarge the hole, of course, metal has to be removed. (See A and B, Fig. i.) In the case of shafts and holes, it is very simple HOLE SHAFT ■4— X — ^ — ■ Shz X XWui ': ■ztiizL ± OLOU—H initial CLeARANOE / t "T A 1^ MmMION [] ^ ^ ■ _J. (LOOl— H INITIAL CLEARANCE TOTAL INITIAL CLEARANCE— Q,0Q8' T h h FIs. X. mostration ihowlng Meaning of Some of fhe Tenns oted m Connection wifh fhe limit System of dimensioning Drawings to determine which is the maximum metal dimension, but there are cases where it is somewhat difficult to say definitely which is the maximum metal dimension unless the conditions are carefully analyzed. Clearance, — Clearance is the diflference in dimensions between two component parts which are assembled together, and is pro- vided either to prevent interference, to produce a certain fit, or to provide for lubrication. Initial Clearance. — Initial clearance is the difference between the maximum metal dimensions of two component parts that are ' GAGE DESIGN AND GAGE-^MAKING assembled together. If, for example, as at C in Fig. i, the maximum metal dimension of the hole is i.ooo inch and of the shaft 0.998 inch, then the initial clearance is 0.002 inch. In Fig. 2, the tolerance for the shaft is 0.003 ^^^1 ^^^ ^^^ ^^^ ^^^^ 0.002 inch, which makes the limits for the shaft 0.995 inch and 0.998 inch, and the limits for the hole i.ooo inch and 1.002 inch. By studying these figures in combination with the drawing, it will be seen that, while the initial clearance is only 0.002 inch, the actual clearance between the two parts, if the shaft should happen to be O.OOI-*K INITIAL OtXARANOt OM'^H INITIAL CLEARANOC H or LAROEST POSSIBLE CLCARANCC JfoeMiMry Fig. a. mostration indicatiiig Meftning of Iidtial Cleaniico made to the minimum limit and the hole to the maximum limit, might be 1.002 — 0.995 ^ 0007 inch. From this it is clear that the initial clearance is equal to the difference between the maximum metal dimensions, while the total clearance may include, in addition, the tolerances, so as to become the difference between the minimiun metal dimensions of both pieces. There are a number of other terms that are frequently used th^t /elate to the same dimensions as those defined above, such as '"working tolerance," "permissible tolerance," "necessary tolerance," "high limit," "low limit," "working clearance," DEVELOPING A GAGING SYSTEM 7 "allowance," and "working allowance." The use of these terms tends to produce confusion and should, therefore, be discourag^^;^ The expression " a limit of o.ooi inch " is also objectionable and should not be employed. It is incorrect, because tolerance is meant instead of limit; and it is indefinite in that it gives no indication of whether a tolerance above or below standard size is meant. The expression may mean a tolerance of o.ooi inch above standard size; it may also mean o.ooi inch below standard size; and it might possibly mean either a tolerance of 0.0005 or o.ooi inch above and below standard size. The most important thing with regard to the expressions defined and their use is to employ them only when they mean actually the dimension for which they are used, and express definitehr, without any doubt, the meaning that is to be conveyed. ^ Classification of Gages according to Their Use. — \^th regard to the use to which gages are put in the shop and inspection rooms, three different kinds of gages are used — working gages, inspection gages, and reference gages; the latter are also known as "check gages." As the name implies, the working gages are used by the workmen at the bench or machine in gaging the work as it is being made. The inspection gages are, of course, used by the inspectors in checking the product to determine if it has been properly made to the required dimensions. The reference gages are used for testing or checking the inspection gages from time to time, to make sure that they have not become unsuitable, through wear or otherwise, for the use for which they are intended. / Inspection Gages. — The inspection gages are usually made with minimum and maximum limits, corresponding to the limits given on the drawing of the piece for which they are used. While in the making of small arms there are usually inspection gages for inspecting the work after ev»y operation, such detailed inspec- tion is not generally employed in interchangeable manufacture. As a rule, in the making of small parts, it is not common practice to inspect after every operation, but only at certain points during the process of manufacture; as, for example, when the parts leave one department to pass into another, or before a locating or working point on the piece being made is removed. Generally, • 8 GAGE DESIGN AND GAGE-MAKING therefore, the inspection gages are fewer in number than the working gages, as this facilitates rapid inspection, is generally sufficient for a satisfactory check, and lowers the cost of the product by decreasing the cost of gages and of inspection. Some- times one of the inspection gages may contain the gaging sizes of several working gages; in which case it is preferable, if possible, to have them so made that they could be applied after each opera- tion in instances when this becomes desirable. ^ Tolerances for the Gages. — By having the maximum and ^ minimum limits of the work duplicated in the gage, it becomes possible to determine quickly and accurately if the work is made within predetermined limits. It is evident, of course, that the gage dimensions themselves can be accurate only within certain limits, as it would not be practicable to make gages for inter- changeable manufacturing that would be absolutely to the required dimensions. The general practice, therefore, is to make sure that the maximmn size of the gage is not larger than the maximum limit on the work and that the minimum size of the gage is not smaller than the minimum limit, and, furthermore, that the errors from these dimensions do not exceed, as a rule, 0.0002 inch. This applies to ordinary inspection gages of various types, for interchangeable manufacture, but, of course, it does not apply to so-called "standard " master plug and ring gages which are supposed to be accurate within less than 0.00005 ^^^i the object of these gages being to provide an actual standard of measurement for setting micrometers, originating dimensions, etc. There is, however, no definite standard with regard to the tolerance that may be permitted on the working and inspection gages used in interchangeable manufacture, as these tolerances will naturally depend, to some extent, upon the conditions and upon the amount of tolerance in the work itself. For example, if the tolerance in the work is o.oio inch, it is evident that the gage need not be made as accurate as when the tolerance is only o.ooi inch. Generally, a tolerance on the gages amounting to 10 per cent of the tolerance on the work is allowable. For example, if the tolerance on the work is 0.004 inch, the tolerance on the maxi- DEVELOPING A GAGING SYSTEM mum gage would be o.cxx)4 inch, and an equal amount on the minimum gage. As will be explained in a following paragraph, working gages are made lo per cent of the tolerance inside of the inspection gage and drawing dimensions; hence, a tolerance of lo per cent on the inspection gage would bring the minimum permissible dimension of the maximum limit gage down to the same size as the maximum dimension of the worlqng gage, which is permissible. The working gage, in turn, would have a similar tolerance, but the tolerance would have to be in a direction that i z r i T III o z a z III o ss a. ill o p o w a, 10 MLR CENT OF TOLERANCE /ON WORK SB TdLERANCE OF / L . INSPECTION OAOE IK O z lU o I o z S c »- o z z o S e _ o ii i. J J_ .1 n l HI "^1 u S HO u Z ' -J M O K I- Z u O " *• H >c ee o s*ii J zo o B IfacAiiitfry Fig. 3. DUgrammatic XUastratioii ■howing Differences between Work- ing and Inspection Gages, and Tolerances on these Gages would not overlap the tolerance in the inspection gage. This condition is illustrated graphically at £ in Fig. 3. Locating pins on working and inspection gages should be made at least 0.0002 inch smaller than the minimum size of the hole in the component part for which the gage is used. In the case of reference gages, however, this is not required, as here the locating pins may be made to fit the corresponding working and inspec- tion gages as closely as possible. Working Gages. — In the case of interchangeable manufac- ture, working gages should be provided for every operation, and each operator should have his own working gage. These work- lO GAGE DESIGN AND GAGE-MAKING ing gages are usually limit gages provided with the mftTtmnm and the minimum gage sizes, and, when practicable, as in the case of a snap gage, also with the mean dimension. It is believed by many that, by having the mean dimension in the gage in addition to the maximum and minimum dimensions, it is easier for the workman to keep within the required limits, as he will try to work to the mean dimension, and in that way be certain to adhere to the predetermined tolerance. In the case of a snap gage, for example, having the mean dimension, there will be three stq>s on the gage, as indicated at C in Fig. 4. The mean dimen- sion in the working gage is particularly useful when the part is tMMOnOMOAaS WOmUNaOAQB B WOftKINO OAOC WITH MEAN OfMENtlON c Maehtnenf Fig. 4* Difference between Inspection and Working Snap Gages machined by adjustable tools or by methods where the workman can make independent adjustments, as in that case it is claimed that, by setting the tools to the mean dimension, there is less likelihood of the piece not being between the maximum and minimum dimensions than would be the case if there were no mean dimension to which the tools could be set; in fact, it is only for conditions of this kind that the mean dimension is of any real value in a gage. If the tool were a solid reamer, for example, it is evident that the mean step on the gage would be of no value. The mean size is always omitted on inspection and reference gages. T he working gage s are not made to the limits indicated on the drawing or to the size to which the inspection gages are made, but the minimum size of the working gage is made 10 per cent of the tolerance larger than the minimum size of the inspec- DEVELOPING A GAGING SYSTEM XX tion gage , and the maximum size of the working gage is made lo per cent of the tolerance smaller than the maximmn size of the inspection gage, as shown diagrammatically at A^ Fig. 3. An example will more clearly explain this: Assume that the toler- ance for the diameter of a shaft is 0.005 ^^^ ^^^ ^^^ ^^ mini- mum dimension, as stated on the drawing, is 1.000 inch, the maximum dimension then being 1.005 ^^' '^^^ inspection gage will then also have a minimum dimension of i.ooo inch and a maximum dimension of 1.005 ^^* (^^ -^^g* 4» ^^ ^0 "^^ working gage, however, will have a minimum dimension of 10 per cent of the tolerance (that is, 10 per cent of 0.005 inch) added to the minimimi dimension of the inspection gage. Ten per cent of the tolerance is 0.0005 inch; hence, the minimum dimension of the working gage will be i .0005 inch. The maximimi dimen- sion of the working gage will be 10 per cent less than the maxi- mum dimension of the inspection gage, and will, therefore, be 1.0045 in^* (^^ ^^S* 4> ^^ ^) ^ ^^^ working gage is also made to measure the mean dimension, it will have a step, in the case of a snap gage, equal to 1.0025 ^'^y which is the mean dimension between the minimum 1.0005 ^^ ^^^ ^^ maTiTniim 1.0045 inch. (See Fig. 4, at C.) The object of not making the working gages and the inspection gages alike, but making the woiidng gage to a smaller tolerance than that allowed in the inspection gage, is to make sure that all pieces passing the working gages will also pass the inspection gages. This eliminates disputes and misimderstandings between the inspection department and the shop, as it is quite certain that if a piece does not pass the inspection gages it has dther been gaged carelessly in the shop and does not pass the working gages, or the working gage is so worn that it should be replaced. When the inspection and working gages are made alike, there are fre- quent disputes as to the justification of the inspection department in rejecting work. The working gages may wear more than the inspection gages, and, therefore, work that would pass the working gages as being correct would not pass the inspection gages, if the two were originally made alike. It would be slightly cheaper to make the working gages and the inspection gages to the same 12 GAGE DESIGN AND GAGE-MAKING dimensions, but the saving, in most cases, is too small, as com- pared with the advantages gained by having working gages made to a smaller tolerance than the inspection gages. Reference Gages. — Reference gages are made to test or check the dimensions of inspection gages. The tendency is toward reducing the cost of gaging systems by making reference gages only when standard measuring plugs or other simple and accurate measuring means cannot be conveniently used. When a com- paratively small number of pieces are to be made, it is also more economical not to make reference gages. When the inspection gages and working gages are made to different tolerances, as indicated in a preceding paragraph, reference gages are not provided for the working gages, due to the fact that it would require a separate set of reference gages, which is unnecessary and which would merely involve an additional expense, which is not warranted by the requirements. Working gages may be checked from the reference gage for the inspection gajge by com- parative measurements, using ordinary shop measuring tools. For similar reasons the reference gage for the " not go " inspection gage is frequently omitted, as the "go " gage, which checks the maximum metal dimension, is the more important one. Refer- ence gages are generally made the reverse or opposite to the inspection gages; that is, female reference gages are made for male inspection gages, and vice versa. As a rule, it is best to make the reference gages so that they fit the gaging and locating surfaces of the inspection gage to the same extent that the work fits the inspection gage. In this way, wear of the gage is more easily detected. It is not customary, however, to make a ring gage as a reference gage for a plug inspection gage, but a snap gage is used instead. The reference gage for a snap inspection gage, again, is usually a cylindrical plug gage, not a flat plug gage. By a flat plug gage is meant a plug gage that is rect- angular in cross section. While it is the general practice to make reference gages oppo- site to the inspection gages, that is, using a male reference gage for a female inspection gage, and vice versa, this is not alwa3rs the case. It is, for example, most convenient to compare DEVELOPING A GAGING SYSTEM 13 a plug gage with another plug gage, and this holds true especially with thread gages, because it is much easier to compare the diameters of a plug thread gage with another plug thread gage than to do the checking with a ring gage. While a reference gage for a snap gage will have both maxi- mimi and minimum limits, the general practice is to make only one reference gage for contour gages, flush-pin gages, and similar types, in order to minimize the expense. The reference gage in that case ought to be made to correspond to the basic or, gener- ally speaking, to the maximum metal dimension on the drawing of the component part for which the gages are used. The gage list that is provided with the complete set of gages should men- Description of Inspection Gages Description of Reference Gages Remarks I Dial Gage I Flush Gage I Swing Gage I Ref., Dial Reading Zero I Ref., High Step I Ref., Go Step Tolerance, —0.005 Tolerance, 0.008 Tolerance, 0.003 Fig. 5. Method of giving Information relating to Tolerances on Gage Lists tion for which limit the reference gage is made and should contain a statement of the tolerances, so that the inspector of the gages may test the other limit also, using ordinary shop measuring tools. The gage list with this information would appear as in Fig. 5. The Marking of Gages. — There are three different ways in which the maximum and minimum limits or maximum and minimum sizes of gages may be marked. In the case of a plug gage, for example, the larger end may be marked either "max." (maximum), "high " " + " (plus), or "not go," while the small end would be marked "min." (minimum), "low," " — " (minus), or "go." In the case of a snap gage, the maximum dimension would be marked "max.," "high," "+," or "go," and the minimimi, "min.," "low," "— ," or "not go." The markings "max.," "min.," "high," and "low" refer to the dimension itself, while the markings "go " and "not go " refer to the use of the gage. 14 GAGE DESIGN AND GAGE-MAKING The Pratt & Whitney Co/s practice, at the present time, is to mark the words "maximum " and "minimum," generally abbre- viated "max." and "min.," to indicate the part of the gage that measures the maximum and minimum dimensions. It may, however, be preferable, especially in the case of plug and snap gages, to use the expressions "go " and "not go " when the gages are used by imskilled labor, as to them the latter terms are more definite and expressive. When gages are marked "max." and "min.," it is evident that in the case of a plug gage tried in a hole the "min." size will pass into the hole while the "max." size will not enter. When trying a shaft with a snap gage, however, the conditions are reversed. The "max." size will pass over the shaft, while the "min." size will not. Were the gages marked "go " and "not go," the meaning of these words would, in both cases, be the same. That part of a gage marked "go " would pass over or into the work, while the part marked " not go " would not pass over or into the work. Working and inspection double-ended plug gages should have the "go " end longer than the "not go " end. Working and inspection double-ended snap gages should have the "go " end rounded to a radius of about | inch, while the "not go " end should be beveled for a distance of about | inch. This makes it possible to see at a glance which is the " go " and which the "not go " end. In marking the sizes on gages, the marking, when expressed in inches, should always be carried to at least three decimals, whether the last decimal is a o or not; for example, 0.370 and 0.200, etc. When the exact size requires more than three deci- mals, as, for example, 0.5798, the required number of decimals should, of course, be stamped on the gage. When the size is expressed in millimeters, the marking should be carried to at least two decimals. For example, 6.00 and 7.40; and if more than two decimals are required to express the exact size, the required number of decimals will, of course, be given, as3-7iS- Example of a Set of Gages. — Fig. 6 shows a set of gages for a part of a rifle, exemplifying the working, inspection, and reference I DEVELOPING A GAGING SYSTEM 15 gages used in a simple case. At ^ is shown the working gage which is here provided with three steps — maximum, wsan, and minimum. The corresponding inspection gage is shown at B. This is provided only with a mftyimnm and a jninirnum dimen- ^on, as inspection gages are never provided with mean dimen- aons. The corresponding reference gage is shown at C. This is a plug gage having at one end the maximum and at the other the minimum dimension used for checking the correqxmding dimen^ons in inspection gage B. At P is shown the reference gage used for checkii^ the circular portion of the inspection gage E, and at F the reference gage for the snap gage G. In this case, the working gage is provided at / with two steps for the mayipmm and minimum limits, while at H is shown the mean step, this construction being used to make unnecessary so deep a slot as woidd be required if all three steps were located in one slot It may be mentioned in this connection that, when plug gages such as are shown at C are used for reference gages only, the two ends of the gage are made of equal length. l8 GAGE DESIGN AND GAGE-MAEING is finally to be used. Sometimes, however, they may coosst of a testing mechanism built into a frame in which proviaons are made for placing the object to be tested. An example of such a functional gage is shown in Fig. 7, this gage being used for testing the sights of a rifle in relation to the bore. In Fig. 8, the riEe is shown in position in the gage, indicating how gages of this type are used. Below the gage are shown dummy car- tridges, which are used for testing the length of the cartri<^e chamber with relation to the end of the bolt. The bar shown beneath the gage in both Figs. 7 and 8 is the reference gage. Fl(. 9. Vunclioiul Gas* lot BrMch Hecluuilim of a Gm ' In the case of large guns, functional gages are a practical necessity on the score of economy, as otherwise large gages in ;reat numbers would be required, and these gages would be more expensive than a functional gage. In this case, the gun mecha- nism is generally duplicated id every detail and the parts to be tested are inserted into the mechanism in place of the master part. This shows immediately whether or not the part will function as required. Gages of this type are especially useful when employed for final inspection by customers who merely want to know that the mechanism functions properly and who are not concerned with the details of the dimensions of each part. DEVELOPING A GAGING SYSTEM^ 19 In rifle manufacture, a dummy rifle or mode] is generally sui^lied to the makers of the equipment for producing the rifles. This dummy rifle is, in reality, also a functional gage, which may be used in exceptional instances by a chief inspector in order to settle a dispute as to the accuracy of the parts made. Functional Gage for Breech Mechanism. — An interesting example of what is known as a "functional gage " or "dummy mechanism " is illustrated in Figs. 9 and 10 which show the mechanism opened and closed. The device is a duplicate of the breech mechanism of a large gun, and is used for gaging the vari- ous parts of the breech mechanism. When used as a gage, the component parts are removed from the dummy mechanism and the actual parts to be gaged are substituted. It is evident that, if the working parts substituted fit in the dummy mechanism properly, they will be satisfactory and can be used in the inter- changeable assembly of guns. This dummy mechanism must be made carefully to extremely close limits, and should preferably be made to maximum metal dimensions throughout. This latter requirement is preferably for a functional gage, as it will abso- hitely assure that any part that will work or enter the duaimy 20 GAGE DESIGN AND GAGE-MAKING mecha ai sm will also function properly when assembled in the finished gun. For large pieces of work where ordinary gages would be heavy and cumbersome, the functional gage offers a satisfactory means of gaging; and, in addition, it is considered very valuable because of the assurance it gives that the parts can be properly assembled. The instances where functional gages can be applied are numerous. Sometimes, of course, slight changes from the actual design of the mechanism itself may be required in order to simplify the methods by which the work to be tested in the gage may be inserted or removed. On the whole, however, the functional gage should be a duphcate of the actual mechanism, at least in so far as the design of those component parts that are required to ht each other is concerned. CUssificatioa of Gages as Regards Design and Construction. — Considered with regard to their design or construction, gages may be divided into plug and ring gages, snap gages, pro&le gages, flush-pin gages, and star gages, etc. The plug and ring gages may, in turn, be either plain or threaded, and the plug gage may be either cylindrical or flat. The snap gages may be solid or adjustable, and may be of either the one-piece or the built-up type. The profile gages may be either templets, receiving gages, or dial contour gages. The receiving gage, in turn, may be either solid or used with a maximum or minimimi feeler plug gage. Dial gages may be either dial contour gages or lever-indicator gages of different kinds. Types of these various gages will be described in following chapters. Relation of Gaging System to Type of Work. — The number of gages required and the extent to which gaging and inspection operations should be prescribed depend, to a large extent, upon the type of work to be produced and the interchangeability required, as well as upon the saving in the assembling operations that may be accomplished by an accurate gaging system. On lall arms and in the production of similar mechanisms, provided for every operation, including the forging. one set of working gages is provided for each operator, r of sets of inspection gages required depending upon r of inspectors necessary. Reference gages, of course, DEVELOPING A GAGING SYSTEM 21 can be provided in a still smaller number; in fact, a single set of reference gages is suflScient for all the inspection gages in one plant. In the manufacture of ammimition both for rifles and for heavy guns, the practice of providing working gages for every operation is also followed, and in each case it is customary to inspect the work after practically every operation. The other extreme is met in the case where only a few parts are made for which no fixed gages are employed, except for such parts as cannot be measured by regular measiu-ing tools, but where all other measurements are taken by ordinary measuring instruments, such as scales, micrometers, and protractors, and where the inspection is limited to the measuring of the important dimensions of the work when completed. Between these two extremes there are all kinds of conditions for which gages are used, according to the requirements. In the manufacture of small tools, for example, the operators use limit gages in their work at the machines, but the inspection is mainly limited to a thorough inspection after the completion of the tool before it goes into the stock-room or is shipped to the customer. It would be impossible to lay down a definite rule as to when complete gaging systems should be adopted and when ordinary measuring instruments could be more profitably employed. Each case must be studied on its own merits. As an example of one of the most complete systems of gaging that has been worked out may be mentioned that of the manufacture of modem rifles. A modern rifle has from 60 to 125 parts, according to its design, and requires about 700 machining operations for its completion. For this work, not less than 1750 working gages are required. An equal number of inspection gages is used for work of this kind, and for every inspection gage there is one reference gage, so that altogether there is in one set a total of 5250 gages. Of course, in the equipment required for a complete rifle factory, there may be a large number of sets of working gages and several sets of inspection gages. This is mentioned in order to give an idea of the magnitude of the gaging equipment that is necessary in the manufacture of a thoroughly interchangeable mechanism. 22 GAGE DESIGN AND GAGE-AfAKmG Adjustable snap gages may be profitably employed when a small number of pieces are made. These gages .are provided with adjustment so that they can be set to various dimensions within a limited range, and having been so set or adjusted, they are locked and sometimes sealed in the position required so as to prevent the operator from tampering with the adjustment. Adjustable gages are also employed for taking care of the wear of the gaging points and are being more and more generally employed in the manufacture of parts where these features are of importance. Method of Procedure in Developing a Gaging System. — Briefly described, the method of procedure in developing a gaging system is as follows: A design is first made of the device required; detail drawings are made and dimensioned, but no tolerances are included. Some American manufacturers do not follow this practice, however, but place complete tolerances on the detail drawings from the beginning. European practice in regard to ordnance and small arms material differs in this re- spect, in that the drawings are generally sent to the manufacturer of the tool and gage equipment without tolerances being marked on them. Assuming that the design of the mechanism and the design of the tools and gages are all made under one roof, there is no doubt whatever but that the best practice is not to put com- plete tolerances on the original working drawings. The engineer in charge of the tool and gage design then studies the drawings of the mechanism and consxilts with the designer about the require- ments for accuracy. Locating points for the manufacturing operations are determined upon; these should preferably be such points as are of importance in the functioning or operation of the mechanism itself when completed. When such points cannot be selected, it is well, however, as far as possible, to at least deter- mine upon such locating points as will be left in the finished piece. It is often difficxilt to do so, and it is seldom that every machining operation can be performed by locating the work from one locat- ing point. One of the best methods of locating work while machining, in order to insture accuracy, consists in using two holes fitting over DEVELOPING A GAGING SYSTEM 23 pins in the jigs and fixtures. Sometimes holes required in the piece are utilized, and, when permissible, holes may be drilled in the piece for locating purposes only. Again, ears may be pro- vided on the piece through which holes may be drilled, the ears being later removed. Holes have proved to be a much better locating means than edges. Sometimes cylindrical projections may be provided on the piece, which are later removed; the projections fit into holes in the fixture and thus locate the work. After having studied the mechanism and become thoroughly familiar with it, so that its function and the reqxiired degree of accuracy are perfectly evident to the tool designer, he de- termines upon the general methods by which each operation is to be performed and makes a list of the operations required on each piece. After this list has been completed, a diagram- matical drawing of each operation is made. This drawing shows the piece as it appears after each operation, gives dimen- sions and tolerances for that particular operation, indicates the type of machine on which the work is to be performed, the type of tool used, shows the locating points, and gives, in a general way, in a sketchy or diagranunatical form, the information that is required by the jig or fixture designer in order that he may be able to provide the proper kind of holding device for the work to be machined. The operation drawing, however, does not show the actual fixture, as this is left for a later stage. The best way in which to arrange this operation sheet is to place the drawing for one operation only on a sheet. This makes it possible to rearrange the operations later, should it become necessary, by simply rearranging the order of the sheets in a set. The importance of using, when possible, the same locating point for all the operations should be again emphasized here, as, if that is not done, rearranging of the sequence of opera- tions might be impossible, or might lead to extensive changes in the equipment. These operation sheets also prove of value later diuing the manufacture, as they show the foreman and inspector how the work is intended to be done. Operation Sheets for Developing Tool and Gage Equipment. — The accompanying illustrations. Figs. 11 to 14, inclusive, show a 24 GAGE DESIGN AND GAGE-MAKING set of operation sheets for the trigger of a rifle. There are alto- gether twelve operations required for completing the trigger, but the drawings for only ten of these operations are shown in the illustrations, as Operation Sheets i and 2 are simply for the drop- VERTICAt SURFACE GRINDER ^ SURFACE GfUMO MDES E i OPERATION 3 AUTOMATIC MILUNQ MACHINE T I gyp 'PMT«^*^^^wr^^>*»i^ ^m 99 00 0m *m 0» 00 ^ tm ^» ^ A >-- .mJ OPERATION 4 AUTOMATIC MILLING MACHINE OPERATION 6 Fig. IX. Some of the Opantion Sheets for e Rifle Trifger. These Sheets are used in the Deyelopment of Tool and Gage Equipment forging and the trimming of the drop-forging, respectively. Operation 3 (Fig. 11) consists of grinding the sides, which is indicated on the drawing together with the dimension to which the sides are to be ground and the tolerance. In the illustrations^ DEVELOPING A GAGING SYSTEM 25 the actual figures for the dimensions have been omitted, as they do not serve any specific object in illustrating the method here referred to, but arrow-heads are left to indicate the particular dimensions that will be required. 8PUNE MILLINQ MACHINE I DRILLING MACHINE \ fc'iV. I I OPERA'^ION 6 AUTOMATIC MILLINQ MACHINE OPERATION 8 OPERATION 7 HI^ sa* Oontlinuitioii of Op«ratloiis on Rifle Trigger fndwUng DriDlog Operation 4 consists of rough-milling the entire front side and the top side of the trigger and finish-milling the straight part of the front side and the top. This is performed by a gang-milling cutter. The cutter to the right performs the rough-milling operation on the top, hook, and front side, while that to the left finish-mills the top and the front side. It will be seen that the 36 GAGE DESIGN AND GAGE-UAKING dimensions are giveo to such points as are tised for locating the trigger in the fixture that holds it. Operation 5 consists in rough- and finish-milling the rear side, the work being located for this operation as indicated below the HAND PROF I LI HO OPERATJOr* 9 MAUD PDOFIUNa MASHINE ^^ AUTOMATIC MILLING MACHINE 1 OPERATION 10 N^r Fi(. 13. OpBiaHon 1 Hand PioBUnc and Automatic 1 trigger, while the lines above the trigger show the contour of the cutter. In this way, all the various operations are indicated by separate drawings. It will not be necessary to refer in detail to all the illustrations, as they are self-explanatory as tar as the general principle is concerned. It may be mentioned, however, that Operation 6 (Fig. iz) consists of spotting, drilling, and reaming DEVELOPING A GAGING SYSTEM 27 the pin-hole; Operation 7 is the milling of the top; here part of the trigger is shown on an enlarged scale, in order to make it possible to give dimensions on the drawing; the drawing also shows that two pieces will be milled at once in a duplex fixture; Operation 8 consists of spline-milling the sear opening; Operation 9 (Fig. 13), profiling around the top lug; Operation 10, milling the ddes ; Operation 1 1 , milling the round on the end of the finger AUTOMATIC PROFILING MACHINE OPERATION 12 Fig. 14. Operatioiis on Automatic Profiling Machine hook; and Operation 12 (Fig. 14), automatic profiling aroimd the finger hook, four pieces being machined at once in a profiling machine with a rotary table. This is the last operation and completes the trigger. It will be noted that the illustrations indicate the locating points and the general principles of the tools employed, but do not show any details of the fixtures to be used. This diagrammatical method of laying out the machining operations, indicating the work to be done, the machines on which it is to be performed, the locating points, the principles 28 GAGE DESIGN AND GAGE-MAKING of the tools to be employed, and the dimensions required, with tolerance in all cases where the work is afterward to be gaged, has been f oimd to be of great value in developing tool equipment and gages for interchangeable manufacture, as the work can now be distributed to a great number of draftsmen. When the work has been laid out as indicated, it is comparatively easy to proceed to design the jigs and fixtures reqmred, as well as the cutting tools and gages. Final Steps in Tool and Gage Design. — As these operation sheets are being completed, the limits or tolerances for each operation are determined upon by the tool designer and the designer of the mechanism, in conjunction, and these tolerances are indicated on the piece on each operation sheet. A complete set of operation sheets having been made, the actual fixttures and the gages are designed, in the course of which a rearrangement of the operations and a change in the tolerances may often be necessary. The number of fixtures required for the manuf actture of a given number of parts per day is also determined upon at this time, as well as the number of machines required for a pre- determined output. The fixture designer must keep in mind the maintenance of the locating points as far as possible, and must thoroughly understand the preceding work that has been done by the designer of the mechanism as well as the head of the tool and gage department. Importance of Agreement between Locating Points in Fixtures and Gages. — The first and most vital principle to be observed in the development of a rational gaging system is that the locating points for the machining operations must agree with the points used for gaging the work. When the operation sheets have been laid out as explained, this will automatically insure that this very vital principle of agreement is observed. It is evident that, unless the gaging is done from the same points as are used for locating the work in the jigs or fixtmres, it would be impossible to obtain a product that would meet the reqtiirements of the gages. When the complete tool and gage eqtdpment has been made, it is tested by making a number of pieces for trying out the DEVELOPING A GAGING SYSTEM 29 fixtures and the gages. In the case of small work, like rifle manufacture^ as many as a himdred pieces may be made in order to give the tooling equipment and the gages a thorough try-out as to their suitability for turning out the work both accurately and within the time required. Methods of Dimensioning Drawings. — The methods used for dimensioning drawings are of the greatest importance in inter- changeable work for which a gaging system has to be developed. It is generally assiuned by those who have had no direct experi- ence in this work that a draftsman of fair experience and some knowledge of shop methods can properly dimension the drawings, giving the tolerances or limits, when working out the detail drawings, so that the tool and gage designers afterward simply have to work to these tolerances or limits in the designing of the tools and gages. It should be pointed out very definitely at the outset, however, that there is no engineer or designer — let alone any ordinary draftsman — who is capable, at the time the first detail drawings are made, of dimensioning, with the proper tolerances and limits for efficient interchangeable manufacture, any mechanism that is at all complicated. He may be capable of so setting the limits that the mechanism would function properly, but if the tool and gage designer were to follow the tolerances given without deviation, it might lead to complicated and impractical machining and tooling methods, and would probably increase the cost of manufacture beyond all reasonable limits. This has often been proved in ordnance and small arms work, and is not a theory, but a statement based definitely upon practical experience. In ordinary work, limits or tolerances may be given for com- paratively simple dimensions, such as those of thread diameters, holes and shafts, slots, or anything where it is evident how the work will be located when machined, and how it will be gaged while being machined and when inspected. To go further than this, however, is impossible, or, at least, impracticable, unless the complete machining and gaging methods are determined upon at the time when the first detail drawings are made. Instead, the tolerances on the drawings, except for the simple dimensions 30 GAGE DESIGN AND GAGE-MAKING mentioned above, are added to the drawings after the machining operations have been determined upon and the tooling equipment and the limit gages have been planned, and are then entered on the drawing merely as a record. Procedure in Making Drawings. — As an example of the ideal procedure in the making of drawings for very accurate work, such as small arms, the following outline is given : There are three stages in the design. First, the mechanism should be designed with a view to obtaining proper working or functioning of the parts. In the case of a rifle, for example, the first design includes merely the interrelation of the various parts, so that the mecha- nism will perform the exact function that is required of it. This part of the design could well be called "functional design." The functional design having been completed, the work enters upon its second stage, which may be called **economy-of -manufacture design." At this stage, many of the parts may be redesigned in detail in order to cheapen their manufacture and reduce the mechanism to a commercially practicable manufacturing proposi- tion. After the mechanism has been reduced to what appears to be the simplest design for manufacturing purposes consistent with the proper functioning of the parts, the third stage of the design is entered upon, which is merely a subdivision of the economy-of-manufacture design, and which consists in making such modifications as are required at the time when the tools and gages are designed. When the detail drawings of the mechanism come into the hands of the tool and gage designer, it is nearly always necessary to make certain modifications in the design, in order to adjust it to practical tool design and gaging methods. This is true, irrespective of the fact that the designer of the mechanism has used proper judgment in its design to facilitate manufacture. Cooperation between Designer of Mechanism and Designer of Tools and Gages. — Assuming that the design is being worked out along the lines outlined in the preceding paragraphs, then for very accurate work, such as rifles, assembly and detail drawings are first required showing the general design and containing general dimensions without tolerances; or, at least, with only DEVELOPING A GAGING SYSTEM 31 temporary or provisional tolerances, as these, of course, may give the tool and gage designer a general idea of the requirements, so as to provide him with a tentative basis for his work. Of course, any tolerances that are absolutely reqxiired for the proper functioning of the device should be placed on the drawing; that is, tolerances which, if exceeded, would definitely prevent the mechanism from working properly. The designer of the tools and gages then studies the mechanism so as to fully understand its action, and the designer of the mechanism and the tool and gage designer must work constantly together until the complete tool and gage equipment has been determined upon and the proper tolerances have been adopted. The same man should be at the head of the tool and gage design in order that the tool and gage equipment may be worked out with the same principles in mind as regards locating points, methods of machining and gaging, etc. In the dimensioning of the provisional drawings, one of the important points to be considered is that, whenever possible, the dimensions or arrow-heads should be made from such points as can be used as locating and gaging points. In the past, this has been only partially observed. It has generally been considered that if all the dimensions were on the drawing from some impor- tant surface or center, that was all that was required. The tolerances and limits are then placed on the drawings as the tool and gage equipment for the part is planned on the operation sheets. The designer of the mechanism will decide upon the tolerances that are permissible, and the tool and gage designer then decides upon machining methods and tooling and gaging equipment that will produce the parts with the required accuracy and in a commercial manner. Finally, the detail drawings of the mechanism are completed by placing upon them the tolerances and limits determined upon during the working out of the machining methods and the design of the tools and gages. The drawings then become a permanent record of how the mechanism is actually made with whatever tool equipment may have been provided. The point that the designer of the mechanism and the de- 32 GAGE DESIGN AND GAGE-MAKING signer of the tools and gages must work constantly together during the development of the tool equipment and the gaging system cannot be too strongly emphasized nor too often repeated. Failure to observe this point caused a great deal of delay in the manufacture of ordnance during the war and added hundreds and thousands of dollars to the cost of war equipment. As an example of the difficulties that restdt from an attempt to provide complete dimensions for tolerances or limits on a set of drawings without consulting the makers of the tool and fixture equipment and without considering commercial possibilities of manufacture, it may be mentioned that in one case a design was submitted to the Pratt & Whitney Co. on which complete limits were provided. It was found that before these drawings were submitted to the Pratt & Whitney Co. for the making of a tool and gage equipment, they had been submitted to another factory making a similar mechanism to that for which the design was provided. At this factory, they knew from previous experience that the tolerances given were too close for practical manufac- turing worky and a new set of drawings was made on the strength of their recommendations, giving more liberal tolerances. Still these drawings were made without consulting the tool and gage designers, and, when the drawings were finally submitted to them, it was found necessary to make a third set of drawings, which was completed after the tools and gages had been made, and which contained the correct tolerances, permitting of com- mercial methods of manufacture. Altogether, several thousand dimensions were changed during this procedure, delaying the work and increasing the cost in an imwarranted manner. Detail Instructions for the Dimensioning of Drawings. — The first principle that must be adhered to in the dimensioning of drawings that are to indicate definitely clearances and tolerances is that the maximum metal dimensions should be given as basic dimensions, and from these the tolerances should be subtracted, in the case of male parts, or to them the tolerances should be added, in the case of female parts. The object of giving the maximum metal dimensions is, in the first place, to provide a very simple means for adding up the dimensions to check them with DEVELOPING A GAGING SYSTEM 33 other dimensions, as the adding of all the maximum metal dimensions will give an over-aU maximum metal dimension, as illustrated at i4 in Fig. 15. By adding all the tolerances, the total tolerance is also obtained. It makes the changing of toler- ances much simpler, as the basic dimensions need not be changed, as long as no change is contemplated in the initial clearances. Furthermore, the difference between the two maximmn metal dimensions on two component parts always indicates the amoimt of the initial clearance. For example, if the minimum size of a — (usod — I »i O.t76-O.006r^.'nOH).O06»|(^ - -1.000-0.006^- 4.126-0.016* A V//////////////A Jr. B f T JfOdMlMTf Fig. 15. Methods of dimearioning Drtwingt hole is 1. 000 inch and the maximum size of a shaft is 0.999 inch, then these two dimensions are the maximum metal dimensions, respectively, for the hole and the shaft, and the initial clearance, which is always the minimum clearance, is apparent at a glance. In the present case, it is, of course, o.ooi inch. Now the permissible tolerances will be taken care of by writing the dimension of the hole as i.ooo + 0.002, and the dimension of the shaft as 0.999 "" 0.002, as indicated at B in Fig. 15. The use of the maximum metal dimensions has the added advantage of showing, at a glance, that there is no interference between the parts under any conditions. 34 GAGE DESIGN AND GAGE-MAKING Summing up, the first drawings should generally give the maximum metal dimensions for all parts, but without tolerances. By giving the maximum metal dimensions, the initial clearances are also given, and these clearances it is possible for the designer to determine upon at the time that he makes his first detail drawings, because he must know the amount of the Tm'niTnnm clearance that is permissible in the mechanism, and these mini- mum clearances are also the initial clearances. It should be emphasized that the original design should provide for as few fits as possible, and liberal clearances should be provided for wherever there is no actual fit. Repetition of Dimensions and Unnecessary Dimensions. — Another point to be taken into account is to avoid the repetition of dimensions. One dimension in one view, and in one view only, is sufficient; it is better and safer not to repeat the same dimen- sion i]i another view, and certainly it should not be repeated in the same view in another location. The repeating of dimensions causes a great deal of trouble when drawings are changed, as some of the dimensions are likely to be overlooked. In one instance, a drawing had the same dimeni^on repeated six times. It is evident that, if a change was made in this dimension, it would be likely that one of the five repeated dimensions might be overlooked, as the designer or draftsman making the change would hardly expect to look for the dimension in all the views, and certainly not twice in the same view. Another fundamental rule is that no more dimensions than are necessary to make a piece should be placed on the drawing. If only the basic dimensions, without tolerances, were given, there would be no harm; in fact, it would be useful to give additional dimensions, such as over-all dimensions, etc. But when tolerances are expressed, such additional dimensions are not only unnecessary, but very confusing. They are unneces- sary because dimensions which are interdependent on others already gaged will take care of themselves. How confusing they become is best shown by an example. In Fig. 1 6 is shown at ^4 a piece that is properly dimensioned, and which gives the gage designer the full information he requires DEVELOPING A GAGING SYSTEM 35 for making gages. If he makes two snap gages, one that measures the over-all dimension k and one the dimension h, as indicated at Dy all conditions are fulfilled. Should the draftsman, in making the original detail drawing, however, attempt to put on dimen- sions for all three lengths, g, A, and k, it is most likely that he would dimension the drawing as at C Correctly interpreted, this drawing means that all three length dimensions must be made according to the drawing, and this can be accomplished only by two gages that gage dimensions g and k, but not by O.TGO- 0.005 i — 2.686-0.010" H <1.010H).OOB^ 0.750- -0.006 -1.687^.002* i S.686H).003 B --H 0.750-0.006* 1 1 f< - - 1.625-0.005 -->4< 010^.006H I< 2.686-0.010" -♦I c 0.760^.006" 0- k- 7^ § I 0.76O*-H).O06* i m I .010^.010^ >\ 0.760''-^.005'' i 1 - l.e85''REF. >H.oio^.oio!H 2.686(!^.OlO* >1 F JVaoMnery Fig. z6. Illustration showing Importance of giving Tolerances for those Dimensions Only which are to be gaged gaging k and h, or k and g. As an example, assume that an attempt is made to gage, say, k and g, and that it is believed that if these were found correct, this would cause dimension h always to be correct also. Analyzing the case, k may be 2.635 inches and g may be i .620 inch. Subtracting g from k, we have h equal to 1. 01 5 inch, which is o.cx)5 inch too large, according to the drawing. Again, k may be 2.625 inches and g may be i .625 inch. In that case, the actual dimension of h would be i.ocx) inch, which i^o.cx)5 inch too small. But g is the very dimension that should have agreed to the tolerance specified at A. Hence, it is dear that by adding the dimension g, and then gaging only two di- 36 GAGE DESIGN AND GAGE-MAKING mensionsy an error has been introduced that would not have occurred if the drawing had been left as shown at A, and h and k gaged as there directed. However, should it be found more convenient for some reason to gage dimensions k and g instead of k and k^ still maintaining the required limits for dimension k, which is assumed to be the most vital dimension, then the drawing must be dimensioned as at B. Here, if k measures 2.635 inch^ ^^^ S 1-625 ^^> ^^^ ^ will be i.oio inch; and if k measures 2.632 inches and g 1.627 inch, then k will measure 1.005 i^cht which agrees with the limits given at A ; but in accomplishing this result, it will be noted that it has been obtained at the cost of a material reduction in the tolerance otherwise permissible on dimension k. This shows how important it is that only those dimensions that are to be actually gaged are given with tolerances on the drawing. In a case where it would be convenient to have a figure on the drawing,which is not to be used for gaging and for which, therefore, no tolerances are given, this figure may be surroimded by a line, as indicated at £, or the abbreviation ^^Ref." (reference) may be added after the dimension, as indi- cated at F. The latter method is now used by the United States Ordnance Department. These indications show at a glance that the dimension is added for reference only, and is not intended as an actual ga^ng dimension. Dimensioning Tapers. — All dimenaons must be given so that they can be understood in but one way. A difficidty of this kind is met with in the dimensioning of tapered work, as indicated in Fig. 17. If a piece such as that shown were dimensioned simply with the tolerances for diameters and lengths, as shown at the top, the gage designer would be at a loss to know how to produce a proper gage for gaging the an^e, as he is guided not by the tolerance on one dimension, but by the tolerances on two diam- eters and two lengths in determining the proper dimensions for gaging the tapered part. One way of dimensioning a drawing to indicate the accuracy required in a tapered part is as indicated at the bottom in Fig. 17. A diametral dimension is given some- where on the tapered part, this diameter having no tolerance. DEVELOPING A GAGING SYSTEM 37 Then a length dimension provided with the required tolerance is given from some point with reference to which the tapered part must be properly located. When a piece is dimensioned in this manner, there can be no doubt as to the limit gage dimensions. Tolerances, where a taper depends both upon diametral di- mensions with tolerances and length dimensions with tolerances, may be called "compound tolerances." Such tolerances should always be avoided on drawings. The case shown m Fig. 17 is one of the most common cases of compoimd tolerances. They T t.000~0.0»* ojoo*-o.ooe" um-Mtf I G.w!) - •jM-ojm" [2.000 MaeMmtr p Fig. Z7. Method of ghring Tolerances for Tapered Work I occur, however, in many other instances and present one of the confusing conditions that are met with by the tool and gage designer in interpreting drawings. Dimensionixig Contours. — In the dimensioning of contours when tolerances are given, it is sometimes difficult to make the drawing perfectly clear if, for example, a radius is given with a tolerance in the same way as the tolerance would be expressed in the case of a diameter of a hole or the length of a stud. In cases of this kind, the best method is often to draw a dotted line out- side or inside the full line that gives the maximum metal dimen- sions, and to give a dimension between these full and dotted lines, indicating the tolerance as shown at il in Fig. 18. This is par- ticularly useful when the tolerance varies at different points of 38 GAGE DESIGN AND GAGE-MAKING the contour. Screw thread tolerances may be shown in the same way, as indicated at B and C. Methods of Designating Tolerances on Drawings. — There are a number of different methods for designating tolerances on drawings. The most common way, no doubt, is that of writing the tolerances as a certain amount above or below a basic size, as, for example, 0.998 d= o.ooi inch. This method, however, will not apply directly when maximum metal dimensions are given, as in that case the tolerance will be given either with a JfodMiMry Fig. x8« Method of showing Tolerancot on Contovn and on libreads plus sign or with a minus sign, but both signs will not be used at the same time. This is dear, of course, from what has been said in the foregoing about maximum metal dimensions; hence, the tolerance in cases of that kind would be given as 0.999 — 0.002 inch, or 0.997 + 0.002 inch, according to whether a male or a female piece were dimensioned. In order to reduce the number of figures that have to be written on the drawings, the Pratt & Whitney Co. has developed a simplified method of expressing tolerances on tool and gage drawings. If it is desired to express the following dimension, 1.37s ■" 0.004, the Pratt & Whitney Co.'s practice is to write this 1.37s — 4, it being understood that the last figure (4) indi- DEVELOPING A GAGING SYSTEM 39 cates the tolerance in the same units as the last decimal figure in the basic dimension; hence, when the basic dimension has three decimals, the tolerance will be expressed in thousandths of an inch. If the basic dimension has four decimals, the tolerance figures represent ten^thousandths of an inch. For example, 1.2736 — 3 is equivalent to 1.2736 — 0.0003 inch. This method permits of using more than one figure to express the tolerance, if required, but it is always understood that imits in the value expressing the tolerance are equivalent in value to the last unit value of the decimals in the basic dimension. For example: 1.23 — 2 =a 1.23 —0.02 1.400 + 5 = 1.400 +0.005 1.60 + 3 = 1.60 +0.03 1.460 +25 = 1.460 +0.025 1.60 + IS = 1.60 + 0.15 3-6967 - 5 = 3-6967 - 0.0005 ^•395 - 5 = 1-395 - o-ooS 0-9675 + 15 = 0.9675 + 0.0015 This method is exceedingly convenient when the draftsmen as well as the men in the shop have become used to it, but it takes some time to train the force to understand in all instances >he proper meaning of the tolerance expressed in this manner. Of course, it is evident that where tolerances are either above or below a basic size, the method is equally applicable, as, for example, 1.748 d= i, which means the same as 1.748 d= o.ooi. Furthermore, the system is understood to imply that, when no tolerance is given, a tolerance of + or — one-half of the last decimal unit is permitted; for example, 1.728 would imply a dimension of 1.728 rb 0.0005; in the same way, 1.20 would mean 1.20 d= 0.005; and 1.7963 would mean 1.7963 it 0.00005. Practical Tolerances for Interchangeable Manufacture. — Perhaps the most important subdivision of the whole subject of establishing a gaging system is the determination of suitable tolerances. Had this subject been thoroughly understood at the beginning of the present war, a great deal of time and expense could have been saved in the manufacture of war materials, and in the making of the tools and gages for this purpose. The first principle that must be considered — and the one that, unfortunately, is most often violated — is that tolerance 40 GAGE DESIGN AND GAGE-MAKING should not be determined upon with reference to what would be the minimum tolerances that can be obtained by modem machining methods, but should be established with reference to what are the maximum tolerances that are permissible in the mechanism without interfering with its proper purpose and action. Were this principle always kept in mind by designers, the expense of manufacture could be greatly reduced. Tolerances which are permissible. — In general, there is a misconception as to the tolerances ordinarily permissible in the building of machines and other mechanisms. Even engineers and designers, as well as other men engaged in the machine- building field, ordinarily believe that the tolerances on what are considered high-class mechanisms are much smaller than they actually are; and, hence, in designing such mechanisms, unless they thoroughly study the subject, they are likely to require tolerances that are by no means necessary. A rifle is a highly accurate mechanism. Generally speaking, the tolerances for a rifle might be considered as small as the toler- ances required in any ordinary machine or device, and yet it probably will surprise many to find that the average tolerances on the important dimensions on a first-class rifle are about 0.004 inch. The country has been educated for years and years, particularly by salesmen, to the beUef that many firms are working, in practically every case, to a tolerance of a thousandth of an inch; hence, there is a widespread belief that in commercial interchangeable manufacture there is no difficulty in maintaining tolerances that do not exceed o.ooi inch, but nothing could be more erroneous. This point cannot be too strongly emphasized. A revision is necessary of the ideas regarding accuracy in interchangeable manufacture. It is possible, of course, by the application of expensive machining operations to obtain an accuracy, even in interchangeable manufacture, of o.ooi inch, and even less than this, but such accuracy is not necessary for most purposes, and is highly undesirable from the point of view of economy in manu- facture. As an example of how greatly the ideas as to the re- quired tolerance vary, it may be mentioned that the bolt of a rifle DEVELOPING A GAGING SYSTEM 41 is required by one nation to be accurate to a tolerance of o.ooi inch, while the rifles of another nation have a tolerance of o.oio inch. In general, it may be said that the tendency when using limit gages, particularly in the manufacture of war materials, has been toward too small tolerances, which has reduced the output and increased the expense. Tolerances on Work and on Drawings. — Another point that should be thoroughly understood is that the drawings of the detail parts of the mechanism, when giving limits or tolerances, give the size of the maximum and minimum limit inspection gages; hence, it is evident that if the gages are made inside these dimensions with a slight tolerance in the gage itself, as, of course, is always required, the actual tolerance of the work usually is somewhat less than that indicated by the dimensions on the drawing; and, in addition, it must be remembered that the greater percentage of the parts made will come well within the limits. Furthermore, when working gages are made as outlined in a preceding paragraph, with dimensions 10 per cent of the tolerance below or above the maximum and minimum inspection gages, this also reduces the tolerance on the work as produced in the shop. Hence, the tolerance on the drawing must be liberal enough to allow for this decrease in the actual shop tolerance. Tolerances for Different Machining Operations. — To lay down in definite figures the tolerances that are obtainable in inter- changeable manufacture is something that has seldom before been attempted. In the following paragraphs, however, are given figures based upon the experience and the practice of the Pratt & Whitney Co. in making equipment for rifle manufacture, and while some of these figures may vary with the circumstances, they may be laid down as denoting average minimum values for this and similar classes of work. It should be understood that, when the function of the part to be made allows of greater tolerances than those specified in the following, the tolerances should be made as liberal as possible, as already referred to. It is pre- supposed that the machines used are in good condition and that reference is made only to interchangeable manufacture — that is, the tolerances given should be obtained day after day and month 42 GAGE DESIGN AND GAGE-MAKING after month with the proper tooling equipment in the hands of men with a fair knowledge of its use. Under tool-room condi- tions, in tool- and gage-making, and when machines are "built " rather than "manufactured," of course, much closer tolerances are obtainable, but the figures in the following are not intended for work of this kind. Automatic Screw Machine Work. — For threading in auto- matic screw machines, a tolerance on the outside diameter of 0.003 inch and on the pitch diameter of 0.002 inch can be main- tained; in turning with a box- tool, 0.003 i^^ch. In drilling, the tolerances depend upon the diameter of the drill. For drills from Nos. 60 to 30, a tolerance of 0.002 inch can be maintained; for drills from Nos. 30 to i, 0.003 inch; for drills of from t to i inch in diameter, 0.004 inch; for drills of from ^ to f inch in diameter, 0.005 inch; and from f to i inch in diameter, 0.007 i^^ch. The tolerances that may be maintained on shoulder work depend largely upon the design of the tools. The accuracy may also be greater when one tool is used than when a number of tools are used for several different cuts. The condition of the machine and the feeding mechanism also are of importance. In general, it may be said that shoulder work on automatic screw machines requires tolerances of from 0.003 to 0.005 i^ch. In forming with a forming tool, the tolerances depend upon the width of the tool. For widths less than f inch, a tolerance of 0.003 i^ch can be maintained; for widths between f and i\ inch, 0.004 inch. HoUow-milling is, at best, an inaccurate operation, and should be used for roughing only. The tolerances depend upon the diameter of the cut. For hollow-milling from -^ to \ inch, a tolerance of 0.006 inch can be maintained; from | to f inch, 0.008 inch; and from f to i inch, o.oio inch. Reaming permits of tolerances of o.ooi inch for sizes up to J inch in diameter, and 0.0015 inch for sizes of from | to i inch. Power Milling. — A flat surface on small work such as in rifle manufacture may be milled to a tolerance of from 0.002 to 0.003 inch, provided only a single surface is milled at a time. If two or more surfaces are milled simultaneously, one surface may be DEVELOPING A GAGING SYSTEM 43 milled to a tolerance of 0.002 inch, but the other surface could hardly be held to a closer tolerance than 0.005 inch. In general, however, a tolerance, for milling, of from 0.004 to 0.005 i^^ch should be permitted on all surfaces, because, while a tolerance of 0.002 inch is possible, it is not practicable if economy of manufacture is an item to be taken into consideration, which, of course, it always should be. Straddle-milling permits of a tolerance of 0.003 inch, and contours milled by form cutters require a tolerance of at least 0.005 inch. The tolerances for slots milled by end-mills in one cut depend upon the diameter of the mill, which, of course, equals the width of the slot. Assuming that the mill nms as true as commercially possible, and that the depth of the slot is not materially deeper than the diameter of the mill, then for vddths between J and J inch, a tolerance of 0.004 hich can be maintained; for widths between J and f inch, 0.006 inch; and for widths between J and I inch, 0.008 inch. Hand Milling, — For hand milling, it is advisable to provide slightly greater tolerances than for power milling, on account of the less even feeding motion and the jerky action that is, there- fore, likely to take place. Profiling, — Profiling operations performed in hand profiling machines require a tolerance of 0.004 inch on simple contours and 0.008 inch on more complicated contours. (These are radial and not diametral tolerances.) For work done in automatic profiling machines, tolerances of from 0.005 to 0.015 inch are required according to the nature of the surfaces milled. The tolerance of 0.015 ^i^ch applies to extreme cases only. Spline Milling, — Spline milling operations performed in spe- cial spline milling machines with fish-tailed cutters require rather large tolerances, depending upon the width of the spline. On the width dimension of the slot, a tolerance of 0.005 i^^^h can be maintained. If the slots are to be shaved afterward, tolerances between 0.005 and 0.0 10 inch are used, according to the width of the slot. In milling shallow keyways, a tolerance of 0.002 inch is maintained. 44 GAGE DESIGN AND GAGE-MAKING Vertical Shaving. — For operations performed in vertical shav- ing machines, which are, in reality, small slotting machines and which are used for squaring the ends of slots, a tolerance of 0.004 inch may be maintained both as regards the width and the end of the slot. Thread MiUing. — While it is possible, with a machine ex- tremely well taken care of and a very accurate form of cutter, to maintain a tolerance of o.ooi inch for short pieces on the pitch diameter, and a tolerance of 0.002 inch on the outside and bottom diameter, it is impracticable to give tolerances for interchange- able manufacture more accurate than 0.002 inch for the pitch diameter and 0.004 inch for the outside and bottom diameter. Tolerances on the outside diameter refer only to Whitworth or other threads with a formed top of thread. It is evident that here the larger tolerance given for the outside diameter does not affect the accuracy or working of the thread, because the apex of the thread is of little value and the important dimension is the pitch diameter. Lathe Work. — In rough-turning, the minimum tolerance on the work for diameters of from J to J inch should be 0.005 inch; for diameters of from i to i inch, 0.007 inch; for diameters of from I to 2 inches, o.oio inch; and for larger diameters, 0.015 inch. For finish-turning, the tolerance on the work for diameters of from i to I inch may be 0.002 inch; for diameters of from ^ to I inch, 0.003 inch; for diameters of from i to 2 inches, 0.005 inch; and for larger diameters, 0.007 inch. When tolerances requiring the work to be more accurate than this are given, the work should be ground; generally, grinding is, whenever possible, the most accurate as well as the cheapest method of finishing cylindrical work in any case. In boring holes in the lathe, diameters of from i to 2 inches may be rough-bored with a tolerance of 0.008 inch, and holes of larger diameter, with a tolerance of from o.oio to 0.015 inch. In finish-boring, tolerances of 0.005 inch may be maintained for diameters less than 2 inches, and of from 0.007 to b.oio inch for larger diameters. Closer tolerances, when required, are taken care of by reaming or grinding. DEVELOPING A GAGING SYSTEM 45 For thread cutting in the lathe, whether outside or internal threading, tolerances on the pitch diameter of from 0.0015 ^0 0.002 inch may be maintained. Drilling. — For drilling in a drilling machine using suitable jigs and fixtures, the tolerances depend upon the diameter of the hole — the larger the hole, the larger should be the tolerance. The following minimum tolerances may be maintained: For drills from Nos. 60 to 30, 0.002 inch; for drills from Nos. 30 to 1, 0.003 inch; for drills of from } to | inch, 0.004 iuch; for drills of from i to i inch, 0.005 inch; for drills of from f to i inch, 0.007 inch; and for drills of from i to 2 inches, 0.010 inch. Planing and Shaping. — In planing and shaping compara- tively large pieces, ^uch as the base and slides of machine tools, tolerances of from 0.005 to o.oio inch may be maintained. Grinding, — Grinding tolerances for interchangeable manufac- ture may be assumed as follows: Cylindrical grinding, 0.0005 inch; surface grinding, 0.0005 inch; grinding in vertical surface grinding machine, 0.002 inch. liJnder very favorable circum- stances, tolerances of o.ooi inch are maintained in vertical surface grinders, but to maintain so small a tolerance requires a machine in first-class condition and great care on the part of the operator to prevent dirt or chips from coming between the work and the machine table. For practical purposes, in inter- changeable work, 0.002 inch should be considered the minimum tolerance for vertical surface grinding. In gSLge grinding, it is possible to maintain tolerances of 0.00025 inch for both cylindrical and surface grinding. Turret Lathe Work. — In the turret lathe, the tolerances on diameters may be assimied as a minimum of 0.004 inch, and on shoulder work, as 0.003 "ich. In this case, of course, a great deal depends upon the tooling equipment, the care with which the machine is maintained in a first-class condition, as well as upon the operator. Reaming. — In hand reaming, for diameters up to i inch, a tolerance of 0.0004 inch may be maintained; for diameters above i inch, 0.0006 inch. For machine reaming, the tolerance for diameters up to § inch may be assimied as 0.0005 inch; for 46 GAGE DESIGN AND GAGE-MAKING . diameters of from i to i inch, from 0.00075 ^ o.ooi inch; and for diameters above i inch, o.ooi 5 inch. Die Work. — On small work using high-class sub-priess dies, it is remarkable what close tolerances piay be maintained. It is stated that with the pimch and die kept perfectly sharp, with the fits of the sub-press die in the best condition, and with small work, tolerances of 0.0005 inch may be maintained. Tolerances for Center Distances. — On important work, a tol- erance of from o.ooi to 0.002 inch may be maintained forcenter distances between two holes. Such dose tolerances, however, are very difficult to maintain, and can only be obtained by the very best tooling equipment and machining methods. Tolerances for Screw Threads. — Li connection with the work that has been done for the interchangeable manufacture of rifles at the Pratt & Whitney Co., tolerances for screw threads have been determined upon, suitable for work of this kind, and insuring interchangeability as well as limits obtainable in commercial practice. The accompanying table, "Tolerances for Tapped Holes and Screws," shows the tolerances that have been adopted for the U. S. and the Whitworth forms of thread, for pitches of from 20 to 50 threads per inch. Li the illustration, the line marked' H indicates the maximum dimension for the hole, while the line marked S indicates the minimum dimension for the screw. The heavier line marked H and 5 shows the basic diameter, which is the maximum diameter for the screw and the mmimum diameter for the threaded hole. It will be seen from the dimen- sions given on the drawing in connection with the table, that the tolerance on the outside diameter of tapped holes is equal to +0.004 inch on the outside and the root diameters, while it is equal to the basic diameter +0.002 inch on the pitch diam- eter. For screws, the tolerances are —0.004 inch for the out- side and the root diameters, and —0.002 inch for the pitch diameter. Relation between Initial Clearance and Tolerance. — There should be a certain relation between the initial clearance and the tolerance. It is evident that, if there is a large initial clearance, considerable tolerances should also be permitted for the com- DEVELOPING A GAGING SYSTEM 47 I 1 CO — "i U-j800*0 ^ *-J U zoo'o '-*\ W-JEOO'O 5500'0-J U-_ II * s c 8 .§ ? 88 • • • o o o I I I o o o PQPQPQ o o o CS CQ PQCQCQ 1-1 1 i (2 a I f 88 • • ■ o o o I I I o o o •W* 'W* -IM 310 (0 PQPQPQ u o o •«-• '^^ 'fH M M CO cd cd ciS CQPQPQ 1-1 lie 8 I |2 G 6 ct o o o (0 M CO cd CO 4 PQPQPQ 888 • ■ • o o o +++ o o o 'f^ "^ "IM CO M CO CQ Cd (g PQnCQ 1-1 « u ^- o o o CO CO to cd ctf CIS nCQPQ in • • • o o o +++ o o u »w^ 'r* »w^ (0 CO (0 cd c4 cS nnpQ 1-1 0) QS.S J 48 GAGE DESIGN AND GAGE-MAKING ponent parts, in order to reduce the manufacturing expense. For example, assume that there is an initial clearance between two parts of 0.005 ^^- ^ ^^ case, it would be entirely out of the question to demand a manufacturing tolerance of 0.001 inch on each of the component parts. It is more likely that in a case of this kind the initial clearance might be reduced to 0.003 inch, and that a manufacturing tolerance of 0.003 inch would be per- missible on both of the component parts; or if an initial clearance of as much as 0.005 ^<^h is desirable, it is likely that tolerances of 0.005 ^^^ ^^ ^^^h ^^ ^^^ component parts are also permissible. In general, when the manufacturing tolerance is less than the initial clearance, it may be assiuned that there is an error in judgment on the part of the designer. There are, of course, exceptions to this statement, but for general practice it will be found to be true. Great difficulty is experienced with designs that do not pro- vide for initial clearance in the original drawings. While the designer cannot and ought not to determine upon the toler- ances at the time that the detail drawings for the component ■ part of a mechanism are made, he should, as a general rule, determine upon the kinds of fit that he requires, which, in turn, makes it possible for him to dimension the drawings with the maximum metal dimensions, thus providing for the initial clear- ance at the time the design is made and the first or provisional detail drawings are made. For mechanisms that are assembled and taken apart various times ^during their use, more initial clearance is required than in the case of parts that go together in a permanent assembly, in which case little or no initial clearance is necessary. By reducing the initial clearance to a minimum amount, whenever this is possible, there is a chance for giving greater tolerances on the component parts, thus facilitating the machining operations, reducing the cost of manufacture, and increasing the rapidity at which the completed mechanisms may be made. CHAPTER n SNAP AND PLUG GAGES Snap gages are among the oldest and doubtless the simplest and cheapest of all gages that are used for interchangeable manu- facture. As a general rule, they should be used whenever possible, and in nearly all cases where external measurements of diameters or widths of pieces are to be gaged, the snap gage is most easily applied. The earliest form was the "one-size" type; that is, a gage generally of a horseshoe shape, intended to measure or gage the exact dimension to which it was made. Later, the limit gage was introduced, having two steps, one for the maximum and one for the minimmn dimension of work for which a certain tolerance had been previously determined. At first, all limit gages were made with solid jaws and with a horse- shoe-shaped handle or holder, but later adjustable snap gages were introduced having measuring points similar in principle to those of a micrometer, and adjustable within a certain range, so that, if the tolerance were changed on the work, the gage could be easily adapted to the new tolerance; or if the gage points should wear, this wear would be compensated for by an adjust- ment. Locking means were provided so that, when the gage had been set or adjusted, the measuring points could be positively locked in the required positions. The horseshoe-shaped one-size snap gage, when worn, may also be brought back to size by peening and subsequent lapping of the gaging surfaces, an opera- tion that requires considerable care. Snap Gages for Small Interdiangeable Work. — In the case of small interchangeable work, where the quantities made are very large, as in rifle manufacture, it has been found most satis- factory to make the snap gages non-adjustable, as in this case a number of gages may be easily combined into one piece, or held in one holder. The old-fashioned way was to take a flat piece of 49 GAGE DESIGN AND GAGE-MAKING SNAP AND PLUG GAGES 51 steel and cut a number of gage slots or openings in it. Gages of this type are shown in Fig. i, in each of which are combined four gage sizes. Of the two gages shown, one is the inspection gage and the other the working gage. Fig. 3 shows an example of a gage containing eight gaging slots. The advantage of gages having a number of gaging slots is that it makes it unnecessary to use a great niunber of individual gages; time is saved both in the shop and inspection room, because it is easier to handle one gage than to pick up successively a niunber of gages; and, furthermore, mistakes are more readily guarded against, as the operator and inspector quickly become used to the positions of the various gage slots in the combination gage, whereas it is more likely that the wrong gage might be used when a great number of individual snap gages are employed. However, the types of gages shown in Figs, i and 3 are objectionable, because, where all the snap measurements are cut from one solid piece, if, in making the gage, one gage size is spoiled, the whole gage becomes useless and must be scrapped. Furthermore, if one gaging size becomes worn, the whole gage also becomes useless. Snap Gages having Removable Inserts. — A method for par- tially overcoming the defeats of snap gages having a large niunber of slots or ** snaps " is shown in Fig. 2; as will be seen, blocks or filler pieces are put into the gage openings, the dark piece shown in the gage opening being a detached piece or block of steel. This block is held in place by screws passing through the side of the gage jaw. When worn, the blocks may be packed out by a thin sheet of mica placed between the side of the solid jaw and the block, or an entirely new block may be put into the gage. This method of making a combination snap gage also eliminates any danger of spoiling the whole gage in the making, by a mis- take in one gaging size. It also facilitates changing the gage if a change is made in the size or tolerance of the work. Incidentally, the gage illustrated in Fig. 2 shows the applica- tion of small marking disks or buttons driven into the body of the gage and on which the gage sizes are marked. The advantage of using these buttons instead of stamping the sizes directly on the gage as is done in Fig. i is that, in case the gage size should be 53 CAGE DESIGN AND GAGE-MAKING changed, the button may be removed and a new button with the new marking inserted. Another method for keeping together snap gages that are generally used at the same time is to put a number of individual snap gages on a ring similar to a key-ring. In this way, all the snap gages that belong to one piece or operation may be kept together without having all the gage sizes cut in one solid piece of steel. Built-up Snap Gages. — The ktest development in snap gage con- struction is embodied in the built-up snap gage, an example of which is shown in Fig. 4. Here the snap gages are made in mdividual pieces of steel which are afterward assembled into convenient units, held together by a clamping strip and screws as shown. This type of gage combines the advantages of the types previously described, and eliminates the SNAP AND PLUG GAGES S3 V3- a % ] I i HI 13 T JL T 1 M I i T I I i ^ \ H'^ » «» ■! ) • I a k^ NT* t— « \ - ^ f^'» a i- -IS [^ i ^ WT I ±« %l — T T [ I Q u « •I P m M 9 «9 I GO ■S I GO S4 GAGE DESIGN AND GAGE-MAKING disadvantages. All the gages for one operation are combined in one holder, but if one gage size is spoiled during, the making of the gage or changed after the gage has been put in use, or if the gage becomes worn, the particular gage affected can easily be replaced by another without changing the m. ^■ -* *-H IJTlf ^" :m-i: ^: n'§ •»'• MacMnerf Flf. 6. Method of boildinc up Snap Gages by Means of Clamping Strip and Screws Other gages in the set. Should the order of operations be changed, it is easy to rearrange the gages in the various holders, as they may be removed and replaced at will and arranged in any required combination. As indicated in Fig. 4, not only snap gages but also ring gages, profile gages, and other types of gages can be combined in one holder, so that all the gages SNAP AND PLUG GAGES 55 fOT one operation may be held in the same holder, provided, of course, that they are of such kind and type that they can be made from flat pieces of steel. Gages made in this manner are much Kg, 7> End-muaniiiiK Rodi iiied >■ Rafarsace Gagei cheaper than those made from solid blocks of steel, because the strips, screws, and blanks may be standardized and kept in stock. A set of gage blanks which covers a range from the smallest size up to snap sizes of zj inches is shown in Fig. 5, this being a stand- ard line of blanks used by the Pratt & Whitney Co. Fig. 6 shows the construction of the built-up gage in detail, indicating how the clamping strips are made and how the blanks are held S6 GAGE DESIGN AND GADE-MAKDJG by it. It will be noted tbat there is a V-groove on the top of the gage blanks into which a V-projection of the upper c l a mp ing strips fits, thus holding the blanks firmly in place. It will be noted that, at the snap opening, the edges are beveled off to a 45-degree angle; the object of this is to permit the work to enter the gage ea^y. In some cases, the edge is rounded instead of being beveled. In Fig. 4 is shown a complete set of gages for one operation. In the lower right-hand comer are shown the workmg gages Fif. 9. Snap Ga(« mad* bj attecUng Strip* to SidM of Blank which have three steps in the limit snap gages — maximuro, mean, and minimum. To the left are shown the inspection gages which have only two steps for maximum and minimum dimen- sions. The plug gages at the top are the reference gages. This illustration, therefore, shows a complete set of working, inspec- tion, and reference gages such as would be furnished for inter- changeable manufacture. When the snap gage dimensions are comparatively small, plug gages are used as reference gages, but when the snap dimensions are large, end-measuring rods are used as references for the inspection gages, as indicated in Fig. 7. This illustration shows to the left the three-step working gage SNAP AND PLUG GAGES 57 and to the right, the two-step inspection gage, together with the mfiiimiim and minimiifn reference end-measuring rods. Reference Gages for Snap Gages. — Cylindrical plug gages, as shown in Fig. 4, are generally preferred as reference gages for snap gages of smaller sizes. Flat plug gages are sometimes used, but are not recommended, because it is easiei^ to determine wear with a cylindrical plug gage, as there is a line contact between the plug and the surface of the snap gage. For smaller sizes, the cylindrical plug gage is cheaper to make, while, for the larger sizes, the end-measures are cheaper, it being evident that it is out of the question to use plugs as reference gages for very large dimensions. Standard blanks are kept in stock for these end- measures, a line of Pratt & Whitney standards being shown in Fig. 8. These end-measures are made from round stock, because it is cheaper to make them in this way. Modifications in Making Snap Gages. — In Fig. 9 is shown a special construction of a snap gage to be used in standard holding strips, as indicated in Figs. 4 and 6. Here the gage slot is very narrow and, therefore, a strip is screwed onto the side of the 58 GAGE DESLGN AND GAGE-MAKING Standard gage blank and the gage measurement is taken between this strip and the side of a notch cut into the corner of the gage blank. The object of this construction is to facihtate the grind- ing of the gage surfaces; it is used when the gage size is | inch or i Fig. II. Complete Set of Workioi, Inspection, uid Refetenca Gagea ahowing how Different Type* of GagcB m>T be cam- blued by the Built-up Gage Principle less. In tills case, the working gage is shown to the left, the inspection gage in the middle, and the reference gage to the right. The working gage has the mean step to the left and the maximum and minimum steps to the right. The inspection gage has the maximum step to the right and the minimum step to the left. The reason that the two or three steps are not combined in one slot is to avoid the very deep cut that would then be necessary. SNAP AND PLUG GAGES 59 Fig. lo illustrates another example of the same construction of gage, except that the snap gage is the reference gage and the plug gages are working and inspection gages. Another example of the built-up type of gage is shown in Fig. II, which illustrates how many dilTercnt types of gages may be combined in this manner, and also shows what constitutes a complete set of working, inspection, and reference gages for a given operation. Fig. 12 shows how a flush-pin gage may be combined in the same holder with snap gages, indicating the 1 h 1t " ^ Tii o.iao-fs i!r -DRILL ROD «.«»•« "TZIIld I OJK+2 — 1* J 0.2B0+2 B»> cd 0.«+8 "r::iF i.wk+B I U— — IH i — ''^-i ^ 1.010+8 I Machinery Fig. X4. Standard Plug Gage Blanks for Limit Gagec OJUaS REAM- a 0J56IIEAM— •! I ! I Pr-$ JL R. l*-J<-J 0l4S18 ream !• — A i J I 2X % !* — "< — n [g^ 1 QiaSO REAM—* ri^ _^ k-X- OifOO REAM f* *i I ^1 ^ KR. -2X- I •IX H R. I H-X-*l I I -H 6o Fig. Z5. Hexagon Handles used for limit Ping Gagea SNAP AND PLUG GAGES 6l flexibility of the built-up system of holding gages with standard strips. It is evident, of course, that the standard gage blanks can be used without being held in a holder, as indicated in Fig. 13, where individual gages are shown made up from standard gage blanks, but not used in combination with other gages or held in a holder. Plug Gages. — The Pratt & Whitney Co.'s practice in making plug gages for interchangeable manufacture is to use standard Fif . 16. Comidete Set of WorUng, Inapaction, knil BefetancB Oaie* in whicb ue ihowa Working and Inapectioii Plni GagM with Enda ot Vneqaal Length, and Seference Plug G«gei with Sndt ot Equal Length plug gage blanks as shown in Fig. 14, and from these blanks any sizes that are required arc made. The blanks shown to the left have a longer gaging surface than those shown to the right and are intended either for the "go " size in a working or inspection gage or for the "go " and "not go " sizes of a reference gage. The blanks shown to the right, having a shorter gaging surface, are intended for the "not go " ends of working and inspection gages. In Fig. 15 are shown the standard hexagon handles that are used for holding the plug gage ends, the "go " size being 63 GAGE DESIGN AND GAGE-MAKING inserted at one end of the handle and the "not go " at the other. It is evident that it is both cheaper and more convenient to make gages in this manner. By using hejuigon stock for the handles, the gage may be laid on the bench or table without rolling off. Another advantage of the hexagon handle is that it is easier to stamp the gage dimensions on it, as it has a better place for marking than would a round handle which, if knurled, would have to be provided with a milled flat for marking. The hexa- gon handle is also easier on the user's hand. It is evident that, by having the two limit gages inserted one in each end of the handle, repairs or changes are greatly facili- tated, as if only one end is defective only one gage needs to be SNAP AND PLUG GAGES 63 scrapped. As the "not go " gage does cot wear out as soon as the "go " gage, generally the former only need be replaced when Fl<. 10. I«ag "Go" Plug Gage* ui«d tor Rifla Baneli worn. Also, in case the tolerance should be changed, it is likely that only one of the gages need be replaced, and in any case the handle is saved. 64 GAGE DESIGN AND GAGE-MAKING The gages are pinned in place in the handles by taper pins, the ^>^aTi^^ of the gages being straight. The hexagon handles are made from low-carbon machine steel, while the gages are made from drill rod for the sizes up to and including J inch, and from 0.50 per cent carbon steel for larger sizes. Fig. 16 shows a set of plug and snap gages. The two plug gages at the top are working and inspection gages having the "go " ends longer than the " not go " ends. The two plug gages shown in the lower part of the illustration are reference gages in which the plugs at both ends of the gage are of equal length. The reference gages are often made with long necks so that they Fig. 31. Flat Plug Gages of the Built-up Type will reach down into the snap gage slots when these consist of three steps. Referring again to Fig. 16, it will be noted that the reference gage for the plug gages is a snap gage, as shown in the up{>er left-hand comer. A snap gage is cheaper and simpler to make as a reference gage for a plug gage than would be a ring gage, and as a plug is not likely to be out of round, the snap gage is generally as satisfactory as a ring gage. Fig.» 17 shows a simple set of working and inspection plug gages with a reference snap gage. Fig. 18, again, shows an ex- ample of working and inspection plug gages with a snap reference gage at the top, while in the middle is shown an example of plug gages having a ring reference gage. The reason for using the SNAP AND PLUG GAGES 65 ring reference gage in this case is that the plug gages measure the depth as well as the diameter of a hole, and a ring reference, of correct length, checks both of these dimensions. At the bottom are shown working and inspection snap gages with a reference Hol«t and Babs plug gage. The reference gage has an equal length of plugs at each end, while the lengths of the "go " and "not go " ends of the working and inspection gages in the upper part of the illus- tration are unequal, as already mentioned. Special Types of Plug Gages. — Sometimes plug gages are provided with two concentric steps of different diametere in order 66 GAGE DESIGN AND GAGt-iMiVKING to enable the concentricity of two holes to be gaged. Again, a tapered plug gage may occasionally be used for gaging cylindrical holes. In that case, the gage is provided with lines, as shown in Fig. 19, that indicate the maximum and minimum dimensions, and sometimes, as in the illustration referred to, the mean dimension. The gage in the lower part of the illustration b the reference gage for this tai>ered plug gage. The reference gage is also provided with three lines which must coincide with the three lines on the working and inspection gages. Gages of this type are used for special cases only, as a taper gage, of course, does not indicate that the hole is straight nor that it is not tapered, but merely shows the size at the end. The gage shown is used in drilling gun barrels to indicate definitely how near the barrel is to the exact size. Fig. zo shows long straight plugs that are also used for gaging gun barrels. The tapered gage, it will be seen, therefore, is simply an auxiliary gage used in connection with the regular plug gages. The "go " ends of the plug gages in Fig. 20 are very long, in order that they may indicate the straightness of the barrel lor a considerable distance. In this SNAP AND PLUG GAGES 67 case, ring reference gages are used for the plug gages instead of snap gages, as is the usual practice. Flat Plug Gages. — Plug gages made from a flat piece of steel instead of from cylindrical stock are sometimes used, especially for measuring the width of slots, as they provide for greater wear than cylindrical plug gages, the flat plug gage having a surface contact and, hence, greater wearing surface. These flat plug gages are generally made from the regular snap gage blanks, as indicated in Fig. 2 1 , where flat plug gages are shown held by strips and screws the same as regular snap gages. Fig. 22 shows a modification of this type of gage which is used for gaging the length of slots or splines having round ends. In this case, the edges of the gage are rounded so that there is a line contact with the ends of the spline. It is evident that a plug gage with flat edges could not be used. Combination Plug and Snap Gages. — Fig. 23 shows a com- bination plug and snap gage which may be used for gaging the diameter of a hole and at the same time measuring the distance of a projection from a hole or the concentricity of a hub with a hole. The gage shown in Fig. 23 is double-ended, having limit gages at each end and being used for two hubs. The upper one of the gages is the working gage, while the lower one is the in- spection gage. Fig. 24 shows a similar type of gage, but this is used for one hub only, and, therefore, is single-ended. The gage at the bottom is the working gage, which has three steps — maxi- mum, mean, and minimum. The one at the top is the inspection gage which has only maximum and minimum steps. A Practical Ring, Plug, and Snap Gage System. — The follow- ing is a description of a gaging system embodying the principles set forth in Chapter I on "Developing a Gaging System." While the methods of applying these principles may vary in some of their minor details, they would alwa)^ be essentially the same in their more important features. To insure the accuracy of the product, three classes of plug, ring, and snap gages are necessary. Each of these three classes will be referred to in the order of their application. Gages of the first class are used by workmen at the machine who inspect the product from time to time. The / 68 GAGE DESIGN AND GAGE-MAKING 1 a» ui O < o oc 111 < lU -I < Z Id u. M m O u 9 I d o a a a u O n a o ■ d 9 B d « • d o lO m SNAP AND PLUG GAGES 69 tolerance allowed by a gage depends to a certain extent upon the length of tune the gage has been in service, but when the work- man's gage is new it allows about 80 per cent of the actual toler- ance of the product. (See "Female Working Gages," Fig. 25.) By allowing but 80 per cent of the actual tolerance on the work- man's gage, it can wear considerably before being resized or replaced, besides doubly insuring the acceptance of the product 0.791 HUOOOO +0.0006" ] •HGT-OO J. ±'-'»*-'iS MIN. INS. DIA. OF PRODUCT 0.79 AMOUNT AOOCO 0.001 •*00" OAOE OIMENWON 0.791 T MAX. INS. DIA. OF PRODUCT OJ AMOUNT SUBTRACTED 0.001 •*NOT QO» OAOE DIMENSION 0.799 MALE WORKING QAQE 0.7906 —0.0000 +0.0006 t . •■ «o- NOT-OO 1 +0.0000 0.0005 MIN. INS. DIA. OF PRODUCT 0.79 AMOUNT ADDED 0.0006 '*0O " OAOE DIMENSION 0.7906 JoO~n 0.7996+ MAX. INS. DIA. OF PRODUCT 0.8 AMOUNT SUBTRACTED 0.0006 "NOT OO" OAOE DIMENSION 0.7996 MALE INSPECTION QAQE 1 0.79 -0.0000 +0.00flr" T -OO- «OT-00- L 0.8+t2SJ — 0.0006 f MALE MASTER INSPECTION QAQE Fig. a6. Working and Inspection Gages by the inspection department. A workman's gage is accordingly called a "working gage." After leaving the production department the product enters the inspection department. Here each part is inspected by the inspectors whose gages, when new, allow about 90 per cent of the actual tolerance allowed on the part to be gaged. Female gages of the class used by the inspectors are shown in Fig. 25. Gages of this class are called "inspection gages." ^0 GAGE DESIGN AND GAGE-MAKING It is often found necessary to reinspect the product to de- cide disputes arising between the producer and the purchaser. Especially is this the case on government or other large manu- facturing contracts where parts are manufactured in one plant and assembled elsewhere. In the case of a dispute, a gage of the third class is indispensable. This class consists of gages which allow precisely the full tolerance allowed on the part. A set of this class is held in reserve for an emergency, and these gages are called "master inspection gages." (See Fig. 25.) Determining Gage Dimensions. — Calculations showing how the respective gage dimensions are determined are given in con- junction with Figs. 25 and 26. Working gages allow but 80 per cent of the actual tolerance, a 10 per cent reduction being effected by a change in the maximum dimension, and an additional 10 per cent reduction by a change in the minimum dimension. In- spection gages allow 90 per cent of the actual tolerance, a 5 per cent reduction being effected by changes in each of the two gaging dimensions. In determining the "go" and "not go" gage dimensions, the respective per cent of reduction (if working gages, 10 per cent and if inspection gages, 5 per cent) is added or subtracted, which is determined by the following rules: Rule I. — For "go " male and "not go " female gages, add the respective per cent of the actual tolerance to the part dimension. Rule 2. — For "not go" male and "go" female gages, sub- tract the respective per cent of the actual tolerance from the part dimension. Rules I and 2 cannot be applied to the outside and root diam- eters of "not go" thread gages. In this case, the outside and root diameters are the same as the "go " gages, the effective or pitch diameter only being subject to Rules i and 2. Gage Manufacturing Tolerances. — It is essential that the manufacturing tolerance for gaging dimensions be given on gage drawings so that those who order gages have a definite basis on which to reject them, if necessary, and also to insure the amount of service to be received from the gages. The day is past when the gage-maker's veracity is allowed to be questioned. Atten- tion is called to the gage manufacturing tolerances as shown in SNAP AND PLUG GAGES 71 Figs. 25 and 26. One-thousandth inch is the maximum gage manufacturing tolerance allowable, but, for all ordinary toler- ances on the work, the gage manufacturing tolerance is 5 per cent of the tolerance allowed on the product. On the "go " gages it is applied so that, if the manufacturing tolerance is entirely taken advantage of by the gage-maker, the life of the gage is length- ened. On the "not go " gage, however, the tolerance is applied . INCORRECT APPLICATION l-H 1.00025 — 0.W475 — >J 0.0Q66 TOU I ' h- Fig. 27. Gage Manufacturing Tolerances applied in Different Directions in the opposite direction from that of the "go " gage. If the "not go*' gage has its manufacturing tolerance applied plus, the manufacturing tolerance will be applied minus on the "go" gage. If the tolerance were applied in the wrong direction on the master inspection gages, they would pass parts which should be rejected. The results of applying the manufacturing tolerance in differ- ent directions are shown in Fig. 27. In this illustration, the " go " female gage is shown placed on top of and concentric with the 72 GAGE DESIGN AND GAGE>MAKING '^ not go " female gage. The solid circles represent the maximum and mininmim dimensions allowed on the part. The dotted circles represent the gages made to drawings on which the manu- facturing tolerances have been applied correctly. • The dot-and- dash circles represent the gages made to specifications where the tolerance has been applied incorrectly. It is evident that the incorrect application of this tolerance can result in the passing of parts the actual dimensions of which are not within the prescribed limits of accuracy. 0.6 PITCH DIA.« 0.466 +J'.S6 11 PER INCH U.S. 8T0. THREADS 80*±0*-6l^.i^ I ENLARGED VIEW OF THREAD SECTION 1 0.606 — 0.0000 +o.ooo»-1 '^innnrrrrr r 'liiillJljljUtji - -oo- -hoj-ao- -- M m i o.«>6;S:SS T PITCH DiA.=r 0.466 :;;;«« PITCH DIA.= 0.46 +5: Jgj IS PER INCH U.S. STD. THREADS IS PER INCH U.S. STD. THREADS ALLOWABLE VARIATION IN IXAOs ±0,Wa m 0.5 Fig. 28. A Nut and its Gage If the following rules are adhered to in appl}dng the manu- facturing tolerances to drawings of any of the three classes of gages, no difficulty will be experienced. Ride 3. — Apply the gage manufacturing tolerance minus to "go" female and "not go" male gages, the outside and root diameters of "not go" thread gages excepted. Rule 4. — Apply the gage- manufacturing tolerance plus to "go" male gages and "not go" female gages, the outside and root diameters of "not go" thread gages excepted. SNAP AND PLUG GAGES 73 Thread Gage Manufacturing Tolerances. — In applying the manufacturing tolerances to thread gages, Rules 3 and 4 apply with the exception of the root and outside diameters of "not go " thread gages which should be the same as the corresponding diameters of the "go " gages. The great improvements made in thread gage making and measuring facilities have made it possible to hold the angle and lead within tolerances that easily suffice for all practical purposes. A variation in lead of ±0.0002 inch in a distance equal to the effective length of the thread on either the male or female part to be gaged, depending upon which is the shorter, will suffice for all practical purposes. The nut shown in Fig. 28 is to be gaged by the gage represented in the same illustration. Here the application of lead tolerance is illustrated. It should be noted that plain plug gages (which do not appear in the illustration) will gage the nut at its root diam- eter. Also that the function of female thread gages is to test the part mainly at the pitch diameter, leaving the outside diameters to be gaged by plain ring gages. For the angle of thread, a gage manufacturing tolerance of ±5 minutes on the 30-degree angle formed by the side of a thread with a line perpendicular to the axis of the thread, as shown by the enlarged sectional view, is sufficient for all practical purposes. Roots of threads on plug gages and bottoms of thread grooves in ring gages are preferably made to sharp 60-degree vees, as this will not reduce the value of the gage but will facilitate making the gage, especially in the grinding operation. Elaborate and Simple Gage Systems Compared. — No refer- ence has been made to gages used for setting up machines or for inspecting gages of any of the three classes referred to, because the former are not, strictly speaking, production gages and the latter are now substituted by measuring devices used by the gage inspectors to whom all gages should be submitted at regular intervals after they have been in service. A careful study of some of the most elaborate as well as the most simple of gaging systems has led to the conclusion that the elaborate systems usually include a superfluous lot of gages for checking other gages, and the consequence is that the slightest 74 GAGE DESIGN AND GAGE-MAKING revision in dimensions is very costly from a gage equipment standpoint. On the other hand, if too few gages are available, many pieces will go to the scrap heap, justly or unjustly, dq)end- ing upon the acciuracy of the workman's gage in the first case, and whether or not a good set of master inspection gages are used for reinspecting in the instance of disputes, in the second case* Manufacturers who are at present using the three-class system as outlined in the foregoing, and those who will adopt this sys- tem, will benefit by the experiences of others. Provision for Escape of Air in Plug Gages. — An important feature in connection with gage construction, which applies only to plug gages, is the provision for the escape of air when such o I Fig. 39. Plug Gage with Hole for the Escape of Air, and Fofm for Large Gages gages are to be used in blind holes or those having closed ends. Of course, in most cases where plugs are used, such provision is not necessary, as the hole to be tested passes through the work, and, therefore, gives the air plenty of chance to escape; but sometimes a hole has to be tested which is closed at one end. In this case, the solid plug is rendered absolutely useless, as it is almost impossible to insert the plug before the hole is about 0.001 inch or more too large, because the air cannot escape. Trouble of this kind may be avoided by putting a hole through the center, as shown by the upper view. Fig. 29. Form for Large Plug Gages. — The lower view. Fig. 29, shows a good way of making large plug gages. They may be made in this style for sizes of one inch and up. Gages made in this way are light, which is an important feature, and, besides, it is possible SNAP AND PLUG GAGES 75 to tell with this form of gage whether or not a hole is round, which may be impossible with a solid one. It is good practice to make one side of the gage with about 0.005 ^^^h taper, especially when it is used for grinding, as it gives the user a chance to tell when he is near to the gage size. After a little practice, one will know just how much to take off by observing how far the taper part goes in the hole. Assembling Plug and Ring Gages of the Same Dimensions. — It is well known among toolmakers, inspectors, and users of accurate gages that plug and ring gages of the same diameter may be assembled by an expert, and the plug will move freely NOT 00 FtNWH Machinery Fig. 30. Biagram illustrating PHnciplo of UnndmiU or Wing Gage within the ring so long as it is kept in motion. But the moment the plug is allowed to come to rest the ring will grip it tightly^ and much force will be required to push the plug out. The question is: How is it possible to put a plug measuring, say, exactly one inch into a ring gage measuring exactly one inch with an oil film between the opposing surfaces? The theory advanced by one concern that has made accurate gages for many years is elasticity of the plug and ring. The ring is supposed to stretch and the plug to compress, when assembled, sufficiently to provide the necessary space for the intervening oil film. The theory is apparently borne out by the fact that as long as the plug is kept moving in the ring the gages fit smoothly, but the moment move-' ment is stopped the oil is squeezed out and they freeze together. 76 GAGE DESIGN AND GAGE-MAKING Another theory advanced to account for the phenomenon is that the surfaces of the plug and ring, even though highly finished, are quite irregular when viewed under the microscope, and that the inegularities act much the same as fur on an animal when smoothed by the hand. Under pressure, these minute excrescences are pressed down, giving way before the pressure of the oil film, but as soon as motion between the opposing parts ceases, these irregiilarities tend to return to thdr ori^nal posi- tions, thus interlocking and causing seizure. When measuring a plug or ring, the measurement, of course, represents the di- men^on over the tops of these minute hills. Flog and Ring Gage Allowances. — There appears to be no agreement of gage-makers on the matter of allowances for plug and ring fits. One well-known concern claims that it makes the plug and ring of the standard size, i.e., a one-inch plug measures one inch and a ring to fit also measures one inch. These can be assembled oidy by an expert, and so close is the fit that the plug must be kept moving constantly or it will be seized by the ring. Other makers of gages make the plug to standard size and fit the ring to the plug, making an allowance of about o.oooi inch on the one-inch size. The Johansson gages are made to a uniform limit of o.ooooi inch per inch of length. This is proportional in aU ^zes, the one-tenth inch gage being accurate to the o.oooooi inch or one one-millionth inch. Having this in mind, the follow- ing experiment indicates how small the clearance may be in an accurately fitted plug and ring gage: Four Johansson gages were taken, two half-inch and two of other sizes; the other gages were SNAP AND PLUG GAGES 77 locked to the half-inch gage, one on each side overlapping so as to form a snap gage. The three were held together alone by their adhesion to each other. The other half-inch gage or block was forced through the snap gage opening without breaking the adhesion between the three assembled gages. Here we have the condition of the snap gage opening being exactly the same size as L B- ■rrCZZm y^ pji >-*o E Ur=i '^-' 4-* I! o s; ! iy i I :^ I — f-» F^f^ V • I «, — ji I I I r I I dt — ^— ♦"!— "^1 I I !! j: H 1^, I I Ma4Shinenr Fig. 33. Gage for Testing Depth and Width of a Slot the gage fitting it, and still the one can be moved freely within the other. Windmill or Wing Gages. — Windmill or wing gages are used in some instances where snap gages cannot be conveniently applied to the work. The name is derived from the fact that the gage is provided with some kind of swinging member or wing, resembling, in some instances, the wings of a windmill. These gages are limit gages, and the principle is simply that one wing 78 GAGE DESIGN AND GAGE-MAKING will pass by the work while the other wing will not. Fig. 30 shows, in diagrammatical form, a simple case where a gage of this kind would be applied. The part -4 is to be gaged, and the length marked "finish " is the distance that is required to be within certain limits. It is evident that it is not convenient to apply a snap gage to a piece of this shape; hence, a gage having a swing- ing member B is used. One end of this member will pass by the work j,t C, while the other end will not. In this way, the work can be very rapidly and accurately gaged. Fig. 31 shows a practical application of this t)^ of gage. Here the swinging member does not swing clear around, but merely has two surfaces A and J5, one of which will pass by, and one of which will not pass by, the end of the work when it is inserted into the gage. Fig. 32 shows a more elaborate design of this kind of gage. This gage is provided with a swinging member A and gages the depth and width of a slot in a piece of work. One end of the gage must pass through and one must not pass through the slot in order to determine the depth of the slot. At B is provided an ejector for the work with an eccentric on one end of a stud which pushes the work out of position. The work to be gaged is shown in position. The "go " end of the swinging member used for measuring the depth of the slot is here shown in position for measuring the thickness of the side of the piece on one side of the slot. This end of the swinging member is provided with a pro- jection on one side which bears against the side of the slot, and the end of the block C is provided at D with steps as in the case of a flush-pin gage, by means of which it may be determined whether or not the thickness of the work at £ is made to the required tolerance. CHAPTER m CONTOUR OR PROFaE GAGES CONTOUHS W profiles are among the most difficult surfaces to gage properly, and, as a rule, it is seldom possible to hold these within as small limits as diameters or length dimensions, which can be measured by plug and snap gages. However, q>ecial types of gages have been developed which make it possible to check, the Fig. t. EuUast and Amplest Type of Contour Gage accuracy of curved surfaces within limits of accuracy not thought practicable a few years ago. These newer types of gages have been developed to a high degree of perfection, and involve a new principle that is not generally known to gage users. Early Types of Contour Gages. — The earliest and Amplest type of contour gage is that shown in Fig. i, where working, inspection, and reference gages for a curved surface are shown. In a set of gages of this type, the reference gage is generally made 8o GAGE DESIGN AND GAGE-MAKING of opposite curvature to the infection gage; that is, if the in- qwction gage is convex, the reference gage is concave, and vice versa. In exceptional instances, the reference gage may be made identical with the inspection gage so that, when laid on top of it, the checking of the inspection gage is made by seeing that the two gages coincide. As a rule, however, this way of mafeing the reference gage is not as satisfactory as the method shown. Gages of this kind are generally used by holding up the gage and the work toward the light and noting how closely the contour n^ a. Gnldod Cmtoof Oaf* of the gage fits the contour of the work. The main objection to this type of gage is that it is impossible to give tolerances on the work, and the judgment of the inspector is depended upon en- tirely for determining whether or not the two profiles £^ree closely enough to meet the requirements. Hence, gages of this type are rather uncertain in their use, and are employed only where acciuacy is not of great importance and where the toler- ances can be rather large. As a rule, these gages are applied in such a way that a concave gage measures convex work, and vice versa. Sometimes they may be used as "matching " gages; that is, the gage may be laid CONTOUR OR PROFILE GAGES 8l directly on top of the work and the two outlmes matched. While it is apparent that it is impossible to provide a ga^e having limits when convex or concave work fits a concave or convex gage, respectively, it would be possible, although hardly practi- cable, to provide limits on a gage used as a matching gage laid on top of the work, as in that case it might be possible to provide Hatching Tjpa of Contour Gag* two steps on the gage, the work being gaged by noting that no part of the work extended beyond one step nor receded below the other. Another objection to the plain templet gage is that, as a rule, it does not gage the position of the contour on the work but merely its shape. Should the contour be of the correct shape, but incorrectly located, it would seldom be practicable to provide the gage with means for indicating this. Tig. 4. Hatching Oags for InspacUiig Shape of Spring Gtiided Contour Gage. — In order to overcome the fault inherent in the ^mple contour gage of not gaging the accuracy of the position of the profile, a further refinement of this type of gage provides for guides in which the contour slides so that the position of the gage is always the same with relation to a fixture in which it is mounted. Now, if the work is also mounted and properly located in this fixture, it is evident that the portion as well as the 82 GAGE DESIGN AND GAGE-MAKING shape of the contour may be correctly gaged. A device of this kind is shown in Fig. 2 , where the piece to be gaged is shown at A , while B shows the gaging fiztnie. This fixture consists of a base Fig. 6. nztiir* fM Oatfac or Testios Strmftta of Spring block C upon which is mounted the profile gage Z); the hole E behind the gage is provided simply so that the gage with its work may be inspected against the light to see whether the profiles CONTOUR OR PROFILE GAGES 83 match closely. The locating blocks F hold the piece being gaged in the proper position. Fin G is attached to the contour gage and slides in an elongated slot so that the gage may be moved in and out by it. This gage is also made to indicate limits of size of the piece being gaged, steps being ground on the end of the block / Pts. 8. S«t of Uatching Gag** so that the indications may be noted in the same maimer as in a flush-pin gage; in other words, the end B of the gage D must come between the two steps or shoulders on /. Matching Type of Contour Gages. — Contour gages of the matching type are sometimes used for work where very close accuracy is not required, and where gages of this kind may be 84 GAGE DESIGN AND GAGE-MAKING much more coavenient than any other type. Fig. 3 shows an example of this kind. The work to be gaged is shown at A , with the working gage at B and the inspection gage at C. To gage the work, it is merely slipped onto the plug on the ead of the gage nc 0. S«l of Ty^cal Recelring Gaiet and the projection on the work is matched with the projection D on the gage, the judgment of the workman and the inspector being 'depended upon to determine that the work is nearly enough like the gage in shape and dimensions to answer the requirements. f-j f% Oo®PoO M-uH *«r» Fi«. Another type of matching gage is shown in Fig. 4, which is used for inspecting the sha[>e of a spring made from flat spring steel, and bent to the outline shown in the illustration. The spring is shown to the right in Fig. 6, The lines on the face of the gage (both the working and inspection gages are shown in CONTOUR OR PROnLE GAGES 85 Fig. 4) are etched into the surface of the gage-plate; or shallow grooves may be cut, if desired, with a small end-mill. The ^riog is then matched up with this shallow groove to determine whether or not it is of the r^t shape. Incidentally, it may be of interest to show the ^ture used when hardening this sprii^. This fixture is shown in Fig. 5, and is employed to give the proper shape to the spring. The spring, after having been shaped to the required outline, is heated to the hardening temperature, and is then clamped in the hardening fixture, in which it is cooled by contact with the jaws. In this way, it is possible to retain the required shape. The jaw A of the fixture is movable by means of the handle and lever arrangement shown; so tbat the spring may be placed into this jaw when in the outer position and then pressed gainst the inside jaw for cooling. An interesting fixture is also used for gaging or testing the strength of this spring, this fixture being shown in Fig. 6, where 86 GAGE DESIGN AND GAGE-MAKING the completed sprii^ is also shown, 'llie spring b placed under the lever A on the end of which weights are placed at B. On the upright C are two lines. When the spring is placed in the testing fixture, the use of the two weights shown at B should bring the Fie. >}■ RecslTiiii Oftge of Om Buflt-np Tnw lever A down so that its upper surface coincides with the lower Bne on the upright C; then, when the upper weight B is removed, the spring should raise the lever so that its upper surface coincides with the uppf r line on the upright. An important feature of the fixture is the use of the swinging joints at D and E, which produce CONTOUR OR PROFILE GAGES 87 as £ree a movement as possible, so that the ^ring is able to take its natuial position while being tested. Another exan^jle of a matching gage is shown in Fig. 7. This is used for testing the graduations on a fuse ring by comparison with the graduations on the gage. In a case of this kind, a refer- Fig. i6> Simpla Type of Dltl Indicator Contour Gtge ence gage is hardly necessary, except that it may be kept as a record in case the inspection gage should become lost. It is evident that an inspection of this kind needs no further checking after it has once been approved, as the graduations cannot wear in the sense that a snap gage or plug gage might wear. Fig. 8 shows a matching gage used for gaging -a contour of 88 GAGE DESIGN AND GAGE-MAKING the shape shown to the right. The complete set of matching gages — one working, two inspection, and one reference gage — is shown. In this case, the reference gage is shown to the Fig. 17. Method of stamping or marking Tolerance on Dial Contour Gages 12.0' /^ r 6.0^ -8.0' 10.0' -4.0' -3.0^ > "X J- \ al I C4I V C:;a J A > o o 0> lAo" J ^^ ^ I i^ O.SR. JCodMnery Fig. z8. Standard Baseplates for Gaging Fixtures — Small Sizes extreme right and is made of the same shape as the piece to be gaged. The testing of the inspection gages is made by matching the gages. Receiving Gages. — The simplest type of receiving gage consists simply of a flat templet in which a hole or opening is cut, CONTOUR OR PROFILE GAGES 89 corresponding to the shape of the work;- the work is then fitted into this opening to determine whether it is of the required dimen- sions. Receiving gages can, of course, be made to act as limit gages, as two gages may be used, the work passing through one and not through the other. However, this is a rather unsatis- factory method of gaging, as there is nothing to indicate at what point the work is incorrect, in case it should not pass through the maximum gage, and, furthermore, it is possible that the work will pass through one gage and not through the other, and yet be defective; because the gage will be expected to measure or gage so many diflferent dimensions on the work that satisfactory gaging t^^ "-^^ j^^^JiR. ■^yr-X" MacMnery Fig. 19. Standard Baseplates for Gaging Fixtures — Large Sizes of very accurate work in this way is impossible. As a rule, the simple templet receiving gage is merely used to pass over the work with a view to seeing that it corresponds approximately to the shape of the gage, and the judgment of the inspector is depended upon the same as in the case of the plain templet contour gage. In Fig. 9, at By is shown a receiving gage which is typical of this class of gages. The work is shown at A and a reference gage is shown at C. The work is located in the gage by the two pro- jecting stems which fit into the bushings in the gage. The com- plete set of gages for the work shown at A consists of working and inspection gages, like the one shown at 5, and one reference gage, as shown at C. The design of the working and inspection GAGE DESIGN AND GAGE-MAKING gages is more clearly illustrated in Fig. lo. Another example of receiving gages is shown in Fig. ii, and the work for which they are used may be seen in Fig. 13. The working gage is shown in Fig. 11, at ^, the inspection gage at B, and the reference gage at C. Sometimes receiving gages may be made of the built-up type as in Fig. 13, which shows a gage for a round piece of work, turned partly to a taper, and made m hve different parts held together by binding strips, screws, and dowels. This method of making Fig. 30. Bue Btoch tor Dial Gtga the gage, of course, is used simply to make it posable to grind the gaging surfaces. Iiiq»roved Form of Receiving Gage. — Fig. 14 shows an im- proved type of receiving gage which makes it possible to gage work made within prescribed limits, and which also makes it practicable to determine exactly where an error is located in the contour in case the work is not within the required limits. The work to be gaged in this case is of the shape shown at .i4. It is placed in the opening m the center of the gage and is located by the studs B pasdng through the holes in the work. The out^de CONTOUK OR PROFILE GAGES 9I of the work, however, does not fit the oblong hole in the gage- plate C closely, but there is a space between the outside of the work and the in^de of the hole, this space, when the piece is correctly made, bdng of practically uniform width all around the Trp« ol Dial Indicator Contour Gaga I aean diieeOf piece. The maximum and minimum plug gage D is then used to determine whether the required tolerance has been observed in making the piece. The minimum plug E must pass in the opening between the outside of the work and the inside of the hole in the gage all a'round the work, while the maximum plug F 92 GAGE DESIGN AND GAGE-MAIUNG must not enter tiiis slot at any point. If it enters, it is evident that the work is too small at the point where it enters. On the other hand, if the minimum plug E cannot pass clear around the work, it is evident that the work is too large at the point where the minimum plug cannot pass by. The snap gages shown at G and H are the reference gages for the plug gages. Fig. 33. DUl Indicator Contatu Ga(a for a Riflo ReceiTar As compared with the simple receiving gage, this gage has the advantage that it shows where the error is located, and indicates, to a certain degree, the amount of the error. It also gages the portion of t^e contour with reference to the locating point. The ordinary plain receiving gage generally is understood to require an exact fit, and should not be used except when the component part is required to have a corresponding fit in its mating part. However, when an exact fit is required, the plain receiving gage may be of advantage, as it then insures that the part really will CONTOUR OR PROFILE GAGES 93 fit its component part in the mechanism for which it is to be used. If limits are required at some points, additional snap gages may sometimes be used at those points. In most instances, however, the receiving gage using maximum and minimum plugs, as shown in Fig. 14, is superior to the plain receiving gage that does not indicate both limits. In gages of the type shown in Fig. 14, ball points instead of cylindrical plugs are sometimes used. The maximimi and minimum testing-plug type of receiving gage is LOCATE WITH REFERENCE GAOE IN PLACE FOfLBOTH WORKINO AND INSPECTION OAGES XCOUNTERBORE Uaohinery Fig. 34. Design of Dial Contour Indicator Gage shown in Fig. 23 sometimes found necessary to use when other means of gaging are impracticable. The principle of the receiving gage using maximmn and mini- mum plugs may also be applied for the gaging of the contours of holes, as indicated in Fig. 15, where the gage for testing the cart- ridge opening in a rifle receiver is illustrated. The^determining shape in the gage is shown at A , the receiver being slipped over this shape and the opening of the hole in the receiver being in- spected by passing a maximum and minimum plug around the shape A , and between it and the inside of the opening to be gaged. 94 GAGE DESIGN AND GAGE-UAKING The rifle receiver is held in the gage by plugs at both ends, and means are also provided for holding it in position so that it cannot rotate or move while being gaged. Dial Indicator Contour Gages. — The highest development in the gaging of formed surfaces is doubtless the dial indicator con- tour gage, the elements of which are shown in Fig. 16, and a brief explanation of the principles of action of which follows. This type of gage consists in its simplest form of a baseplate, which has mounted on it means for holding the piece to be gaged as well as ■(■iiut the Mastai Fonn the master form with which the piece being gaged is compared. In Fig. 1 6, this form is shown at ^ , the piece to be gaged being of exactly the same shape and having two holes by means of which it is located over studs B, A disil indicator mounted on a base- plate C of its own slides on the baseplate D of the gaging fixture in such a way that the point on the indicator baseplate C is in contact with the form A , while the point of the indicator itself is in contact with the work to be gaged. It is important, of course, that the point of the indicator be exactly above and coincident with the point £ of its base. At F is shown a setting CONTOUR OR PROFILE GAGES 95 block, used for setting the dial to read zero. This block is high enough so that, when the base block of the dial with the dial mounted on it is brought against the setting block, both the point of the base block and the pointer of the dial indicator bear against the block. Hence, the dial can be set to read zero when the two points coincide before beginning gaging. Advantages of Dial Indicator Contour Gage. — The advant- ages of the dial indicator contour gage are numerous. In the first place, this type of gage makes it posable to determine quickly and accurately whether the work is within the prescribed limits or not, as the variations of the indicator band will show 96 GAGE DESIGN AND GAGE-MAKING the deviations; the gage is easy to use and apply; and the toler- ances or limits may be varied at different points along the con- tour, as the indicator will show the different limits on its dial at any point. The usual practice is to stamp the tolerances on the basq)late oppoate the part of the contour that should be made with a certain tolerance. Not only is the gage ample in its ^pli- cation, but it is also comparatively inexpensive to make when made from standard parts, as is the usual practice. The dial indicator itself is a commercial product, and the only expensive part is the master form A, but as this form would be expen^ve Kg. J for any contour gage, it will not increase the cost of this type as compared with other gages. The wear of a gage is small, because the master form is in contact only with the smooth hardened point of the block on which the dial is mounted, and is not subjected to the grindii^ wear to which an ordinary contour gage is exposed — when pieces of work are constantly passed through it. The gage b also well adapted for the adjustment of machines and tools when setting up work, because it indicates the exact amount of error, and hence the amount of adjustment required. In some in- stances, there are no other means practicable for gaging work, as, for example, for slots cut into a piece of work and having a bottom that is not flat but shaped to a contour. In that case CONTOUR OR PROFILE GAGES 97 ■^JH^^ ^ the only accurate means of gaging is by an indicator point that can reach into the bottom of the slot and follow it while the point of the baseplate underneath the dial follows a similarly shaped contour on the gaging fixture. As already mentioned, the point of the indicator gage and the point of the base block on which it is mounted must be exactly in line and of exactly the same shape, so that no error can be in- troduced in case the base block is not held squarely against the surface being gaged. The two points are, therefore, lapped to- gether against a lap- ping block. The only requirement for ac- curacy on the part of the setting block F is that its setting sur- face is square with the top of the plate D on which the base block C of the indi- cator slides. Indicating Toler- ances for a Contour. — Fig. 17 shows the method of stamping or marking the tolerances for a contour that is made to variable limits. The arrow-heads indicate the direc- tion in which the indicator needle should swing from the zero position. The figure beneath the arrow-head gives the tolerance in thousandths of an inch. Thus, for example, an arrow-head with the figure 5 beneath it indicates a tolerance of 0.005 inch. The direction of the arrow-head indicates whether the tolerance is -f- or — . If there are arrows pointing in both directions, as also shown in Fig. 17, this indicates that the tolerance is, in the case shown, ±0.005 i^ch. The arrows and figures are stamped on ,&■ §n r-t-— ! t" r-3 . 6 -I, «ff-:L: m> -C;? cq" -^^-t--- J. L ' ' 'I Machinery Fig. 28. Construction of Base Block for Indica- tor Dial used in the Gaging Fixture shown in Fig. 27 98 GAGE DESIGN AND GAGE-MAKING the baseplate of the fixture, opposite the respective parts of the contour form. Baseplates for Gaging Fixtures. — Figs. i8 and 19 show stand- ard designs of baseplates for gaging fixtures as made by the Pratt & Whitney Co. These plates are made from cast iron, and the smaller sizes are practically soUd except that they are relieved on the bottom side. Fig. 18 shows standard sizes varying from 2 by 3 inches to 6 by 12 inches. When the dimensions are larger, the design shown in Fig. 19 is followed. These are ribbed base plates, having ditnenstons 8 by 12 mches and 12 by 16 mches, respectively. Fig. 30 shows a drawing of the standard base block used for mounting the dial gage. These baseplates are made in different types according to the kind of work that is being gaged. It is evident that the point of the dial gage may have to be raised higher or be held at a lower level, according to the character of the piece to be tested ; hence, the construction shown. Examples of Dial Indicator Contour Gages. — Fig. 21 shows a dial indicator contour gage in the working position and the kind of fixture that often is required in connection with it, provided with holdit^ and locating means for the work to be gaged. In CONTOUR OR PROFILE GAGES 99 this case, the shaped bottom of a slot is to be tested, the work bemg held on the cylindrical plugs at the ends and resting upon and being located by the two plugs A in the baseplate of the fixture. The setting block is shown at B, and the master con- tour form at C. Fig. 22 shows the same gage directly from the top and the rifle receiver that is gaged in it. The slot, the bottom of which is gaged, is indicated at ^. As this slot is very narrow, the movable point on the indicator gage is flattened so that it will enter the slot to be gaged. Fig. 33 shows another gage of the same kind, which is used for gaging the contour at ^ in a rifle receiver. The details of the construction of this gage are shown in Fig. 24. The contour at A, Fig. 23, is also curved in a vertical direction, and this vertical contour is tested by the gage B shown at the end of the receiver; this gage is shown in place for gaging on the fixture in Fig. 24. This auxiliary gage does not actually gage the contour of the receiver, but locates it in the proper position so that the indicator too GAGE DESIGN AND GAGE-MA£ING gage will ride along the contour at the proper height or along the proper line of the work. A Special Application of the Dial Indicator Contour Gage. — Fig. 35 shows a special application of the dial contour gage. This construction is required when the surface to be gaged is not in one horizontal plane, so that the gage point could not follow the outline as it moves in a horizontal plane. In that case, the position of the indicator pointer and the point on the base block upon which the indicator is mounted are reversed. The base is provided with a projection extending upward, so that the point of the base can bear against the work, while the movable indicator point bears below against the master form. Fig. 26 shows the construction of the baseplate for the dial gage when used in this manner, and also indicates its application to the work and to the contour form. This construction greatly increases the range of work to which this gage may be applied. When the principle upon which the dial indicator contour gage is understood, it is evident that it is not limited to the gaging of - .' • » . • , . -. -.»•■'. CONTOUR OR PROFILE GAGES loi contours, but it may also be used for the gaging of steps, recesses, and other cases where it may often be simpler and cheaper than would be a great number of snap gages. Fig. 27 shows a special case in which a gage of this type is used for indicating the concentricity of an internal recess of a cylin- drical piece, as well as its diameter. In this case, the base is pushed against a flat locating surface and hence has no pointer. A bellcrank arrangement, mounted on the indicator point as shown, makes it possible to reach into the inside of the work to be gaged, which, while gaging, is held upon the arbor A . Fig. 28 shows the construction of the base block for the indicator dial, together with the dial pointer, but does not show the bellcrank attachment. Fig. 29 shows a special type of dial contour gage which is only used for gaging the depth of a slot; hence, the base upon which the dial gage is mounted is guided as indicated. The pin A gages the location of the slot, the depth of which is measured by the dial. The work is mounted on the plugs at the ends, so that it can be turned around into two positions for gaging first the loca- tion and then the depth of the slot. Figs. 30 and 31 show the application and use of a guided dial gage which is used for testing in both a vertical and a horizontal position. It will be noted that the gage is first applied in the vertical position, sliding down along a guide so that it will be held Exactly vertical, and is then applied horizontally in the same manner. (An interesting design of contour- and radius-measur- ing instrument is illustrated in Chapter IX, which includes precision measuring machines of the microscopic type.) CHAPTER IV FLUSH-PIN, SLIDING-BAR, AND HOLE GAGES Flush-pin gages are largely used for tolerances that are not less than 0.002 inch, especially in cases where snap gages cannot be conveniently applied. The simplest type of flush-pin gage is that shown in Fig. i. A hexagon handle is provided in which a pin known as the *' flush-pin " is inserted. The hole in the womi r TO 6Ur »^ 1" 4:^^^^^^^^i: nitrji X - IH"— -2% i T >4n— " g OOWtL t: -A ■1%- TOftUIT •2%' i. -IS T rOMIT DOWEL r ;_ ¥gi: IH"— JVarMfifry Fig. X. nosh-pin Gages of a Simple Type handle is provided with a shoulder and the pin inserted in it is also provided with a shoulder, so that a spring can be inserted between these shoulders, to keep the end of the pin that is in contact with the work in the outward position. A small dowel-pin is driven through the handle and fits into an annular 102 y FLUSH-PIN, SLIDING-BAR, AND HOLE GAGES 103 recess cut around the pin. The dowel-pin permits a limited movement of the flush-pin, but prevents it from falling out of the handle. At the right-hand end of the handle, as shown in Fig. i, there are two ground steps. When the gage is used, the right- hand end of the flush-pin must come beyond the lower step but must not pass beyond the high step on the end of the hexagon handle. These steps, therefore, indicate the limits on the work «— » V. I»ll»»M1>aBUU»»1«WWWl o® 0O 5 iwn ■tifajiafinnnnB S ■tr^ O0 ^H:' ®o §s n >= lii) UNEl a; rTt |~7 IfaoMtiery Fig. a. Gage employing the Flush-pin Principle and the difference between them is equal to the tolerances per- mitted. The gaging is generally done by the sense of touch, the gage being applied to the work, and the position of the end of the flush-pin with relation to the steps on the handle being deter- mined by passing the finger over the end of the gage. It was mentioned that flush-pin gages are generally used for tolerances that are not less than 0.002 inch. It is possible, however, to use them for smaller tolerances, but it is seldom practicable in that case to depend upon the sense of touch, but I04 GAGE DESIGN AND GAGE-MA£ING the use of a straightedge, laid over the end, is generally necessary, and the gage must be held up against the light; hence, the use e Tnw ihown in Fig. i of a flush-pin gage for tolerances so small that a straightedge must be employed is not generally recommended. Simple Application of Flush-pin Principle. — Fig. i (and also the view to the right in Fig. 3) shows the simplest application of FLUSH-PIN, SUDING-BAR, AND HOLE GAGES II I » o ;lLi U.ai io6 GAGE DESIGN AND GAGE-MAKING the flush-pin principle and indicates how flush-pin gages may be made for measurit^ the depth of holes and recesses. Three standard sizes, as indicated, are made of gages of this design, Centareil, for Depth and Concentricit? er position, the work being indicated by the dotted lines at ng. ^, in Fig. 5. Steps are ground at the four points B to correspond with the limits on the work. The gages shown in Fig. 6 are used for gaging a piece immedi- ately after it has been centered. The gage shown at the bottom is first used for determining the depth of the center, and then the piece is placed in the fixture at the top, where the two flush-pins at A and B determine whether or not it runs out. This type of gage is Ampler than a dial gage, and when a standard gage-plate is used, to which are attached the small parts on top of it, it is FLUSH-PIN, SLmiNG-BAR, AND HOLE GAGES ill cheap to make. Standard gage-plates ^milar to those described in Chf^>ter III are employed whenever possible as the base for gages or ga^ng fixtures of this kind. The gage shown in Figs. 4 and 5 is also buHt up on a standard gage-plate. The construction of the lower gage in Fig. 6 is also shown in Fig. $ (upper right-hand view), where the appear- ance of the piece being gaged and the position of the locating surfaces are also indicated. The gaging fixture shown in the uj^er part of Fig. 6 is illustrated in Fig. 8, where the dotted lines at A show the part being gaged and clearly indicate the method in which the gage is applied. Flfr 13. C*s* for Teitlni DlrecUon of ■ Dbsp Drilled Hole Fig. 7 shows a swinging-leaf flush-pin gage, the swinging leaf being used because it is necessary to remove the flush-pin from the gaging poMtion when the piece being gaged is put in position or removed; hence, by mounting the flush-pin in a leaf, it can be swung out of the way for insertion and removal of the work, and then swung into place for gaging. The gaging surface is at ^4, and the small pin at B holds the gage m position, but permits it to move the required amount for gaging. Fig. 9 shows a different design of swinging-leaf flush-pin gage embodying the same principle. Here four flush-pins are employed. Those at A and B are bent at the ends and measure the location of the ends of a slot milled GAGE DESIGN AND GAGE-MAKING ■3 g? FLUSH-PIN, SUDING-BAR, AND HOLE GAGES II3 and one horizontal flush-pin, indicating the different constructions possible and the great variety of gaging fixtures based in practice upon the flush-pin principle. In Fig. 1 1 is shown a flush-pin fixture in which the flush-pin is mounted vertically and is moved by the pin A. This same fixture is shown in Fig. 12 where the bayonet lock for the pin is shown, making it possible to hold it in its upper position while the work is put in place. At the side of the fixture, n<. 15. ntuh-^ Plxtnra for Ga^g Depth and PositJOD of a Slot a pin B is shown that operates the ejector by means of which the piece is easily removed after gaging. The knurled-head pin C, shown in Fig. 11, is merely used for holding the work in position. In Fig. 12, the construction shown is slightly different, but the object of the pin in the hole at C is the same. The end of the flush-pin plunger is shaped to fit the contour of the work. Testing the Direction of a Deep Drilled Hole. — Figs. 13 and 14 show an interesting gage used for testing the direction of a deep hole drilled in a piece of work. This gage or fixture is used for 114 GAGE DESIGN AND GAGE-MAKING determining, while drilling, if the hole nxns true or if it runs out and how much. Of the two fixtures shown in Fig. 14, one is the working gage while the other is the inspection gage. The auxili- ary arbors are shown below the fixtures themselves: i4 is the working arbor; B^ the inspection arbor; C, the inspection gage for the size of the hole; Z), the working gage for the size of the hole; E, the reference piece for checking the gage; and F, a reference snap gage for inq;>ection gage C. The method of using the gage is as follows: The arbor A or B, according to whether the working or inspection gage is used, b c r=l Wn 4 :=:rJ"r MackUurp Fig. 16. Work Gaged in Fixture shown in Fig. 15 first inserted into the work to be gaged. This arbor has, at the end, an expanding part at a, which enters into the hole, and a conical part at 6, which engages the outer end of the hole. On the outer projecting end of the arbor, there are two cylindrical portions c and d which, when the piece to be gaged is placed in the V-blocks of the fixture with the arbor in place, extend out in front of the flush-pin gages at G and H, In this way, it is possible to determine whether the hole in the piece being gaged is straight within the prescribed limits, as otherwise the projecting ends c and d will be out of alignment and force the flush-pin gages beyond the limits prescribed for them. The expanding part a is held in contact with the sides of the hole into which it is in- FLUSH-PIN, SLIDING-BAR, AND HOLE GAGES I15 serted simply by the springmg action of the arbor itself, the diam- eter over a bdng slightly larger than the diameter of the hole in the piece being gaged. Fig. 13 shows the construction of this gage in detail, indicating the action of the flush-pin gages which act in a vertical direction and which are held in the upper position by pins K and a bayonet lock. Flush-pin Gage for Depth and Central Position of a Slot. — Fig. 15 illustrates an interesting application of the flush-pin gage Kg. 17, Fliwh-plD Gu« luTiiigAdjiiiUble FMtura to take Cue of Wur principle. The gage shown in this case is used for ga^g the depth and central portion of a slot in the piece shown in Fig. 16, and also indicated at ^ in Fig. 15; B, in Fig. 15, shows the refer- ence gage used for the inspection gage. In application, the part shown in Fig. 16 is pushed onto the arbor of the gage and the flush-pins enter at the ends of the slot C, and indicate thereby both the depth of the slot and its location in the piece being gaged. The plates screwed onto the gage at the right-hand end, as shown Ii6 GAGE DESIGN AND GAGE-MAKING in Fig. i5,arethelocatiiig[neaQs which hold the piece in position while being gaged. Fig. 17 shows another application of the flush-pin gage prin- ciple, in which an adjustable feature is incorporated to take care of wear. The work here is placed over the upright A, while the Fig. iS. SliiUng-lur Gag* with Tlu«e Sliding Bua gage is swung around over the work, as shown to the right, for gaging. When the work is taken off the upright A or placed in position on it, the gage can be swung out of the way as shown to Fig. 19. AnoUiM Ezunple of ■ SUdlag-bar Gsge the left. As the top of the upright or the end of the head of the screw which comes in contact with the work when gaging will gradually wear, the screw can be adjusted to the proper portion, and then clamped in place by the two check-nuts shown,- one at the top and one at the bottom of arm B. FLUSH-PIN, SLIDING-BAR, AND HOLE GAGES 117 Advantages of Flush-pin Gages. — The advantages of flush- pin gages may be briefly summarized as follows: The flush-pin gage is the simplest form of gage for measuring or gaging the position of one surface with reference to a locating point, when the relation is such that a snap gage cannot be used. Flush-pin WORK ^ I y ^ 8UOINQ I BARS W/\ m J WW' SLOT MocWnery Fig. ao. Diacimmmatical mustratioii of Application of Sliding- bar Gage shown in Fig. 18 gages are subject to a comparatively small amount of wear; if they do wear, the repairs necessary are cheap. They are reliable, and mistakes in reading the indications on them are not likely to Fig. 31. Work for which Gage shown in Fig. 19 is used happen. If the tolerance on the work is changed, the limits on the gage may easily be altered, merely by regrinding the steps. Except for the snap gage and the plug gage, the flush-pin gage is probably more widely used than any other types of gages, especially for small arms work. GAGE DESIGN AND GAGE-MAKING FLUSH-PIN, SLmmC-BAR, AND HOLE GAGES 1 19 Sliding-bai Gages. — Among the gages employing sliding members, the siiding-bar gages occupy an important place. In principle, they are somewhat similar to flush-pin gages, but differ in regard to the method of taking the readings or indications. Fig. 18 shows a sliding-bar gage provided with three gaging points. Two of these measure the length of a slot, as indicated in Fig. 20, while the middle one indicates the location of one V\g. 33. radin<-bu Gag«« with CTUndrlcal Bar side of the slot. All three bars are provided with "go " and "not go" steps on their ends; hence, the sliding bar will slide into the slot to a certain point, but after the "not go " step has been reached, it must not slide in any further. Lines may or may not be provided at the outer end of the sliding bars at A , Fig. 18, to indicate when they have slid in to the proper depth. The plug B supports the work to be gaged. The small V-block at C locates the work, and the plugs D support and locate the work with regard to its axis. GAGE DESIGN AND GAGE-MAKING 5-9 1 i3i '^ •a j3 g _ «, ^ — f^ S ** 3 a * .§ -S « I s » -a a i which gage rom the end hown in Fij ace that is 1 at an angle ding bar is i larked on the i i^-^ ""c.a"^ B £ Fs a sliding-bar ga| 5 distance of a slot irt being gaged is >w indicates the su ice of this surface piece; hence, the fixture. Lines are 1^ ^s':^! FLUSH-PIN, SUDING-BAR, AND HOLE GAGES where the work being gaged is shown in position by dotted lines. Fig. 23 shows three examples of sliding- bar gages, where the sliding bar is cylin- drical and provided with lines as shown at A. Gages ol this kind can be used to advantage when the tolerances are suffi- ciently large so that the eye can easily distinguish if the line Fii. as- Example of a Hola Oagiiig Flztora Hf. 36. Anotliar Typ« of Hole Oai^g Fixtma falls between the steps indicated on the end of the hub B. This gage is, strictly speaking, a Sush-pin gage having a line marked on a pin, instead of a flat end surface, as the gaging GAGE DESIGN AND GAGE^MAEING FLUSH-PIN, SLIDING-BAR. AND HOLE GAGES 123 I HH I 2-^ la • ^ ffl ^ -^ .a fl s C. ja s 'B I S .s » i E a £3 -J •a 3 1 •'= f ca *j -a « * I 3 1 . 3 -2 e „ 124 GAGE DESIGN AND GAGE-MAKING Bole Gages. — By " hole gages " are meant such gages as gage the position or location of holes. Sometimes, and in fact fre- quently, they may gage the diameters of the holes at the same time as they gage positions. They may, for e:(ample, measure the distance between two holes, and at the same time the plimgers or other means used for gaging may act as a check on the hole diameters. Hole g^es do not, however, measure center dis- tances directly. What they measure is the distance between Pit- 30. Oat« tor ChtcUiig LoMUon of Six HoIm >t On* Tlae the outer and imier points of the peripheries of the holes; but while the gage does not measure the center distance, it does insure that the parts may be assembled properly, provided the gage is designed in a satisfactory manner. Figs. 26 and 27 show a hole gage which is used for determining if the holes or circular ends of a spline-milled slot are correctly located. The piece of work is FLUSH-PIN, SLIDING-BAR, AND HOLE GAGES 125 placed upon the plugs or studs at the ends of the fixture, and rests upon the studs driven into the bottom plate of the fixture, while the two movable gaging points A and 5, Fig. 27, are pushed into the ends of the slot to determine that the holes at the ends are properly drilled. Fig. 25 shows another simple type of hole gage with the piece to be gaged to the left. This piece is also of the same appearance Fig. 31. An Indicating Hole Oage as the reference gage, so that the illustration shows how the complete set of working, inspection, and reference gages would appear. The work has a large hole at A and a small hole passing through it at J?. When tested in the gage, the large hole is passed over plug C while the two plungers D and E enter into hole B. The gage thus determines the location of the small hole with relation to the large hole, and also determines the location of the small hole with regard to the bottom of the piece, as the 126 GAGE DESIGN AND GAGE^-MAKING bottom of the piece rests upon the top surface of the base of the fixture while gaging. Fig. 28 shows the complete set of gages used for gaging the location and accuracy of projecting bosses of a piece of work. The hole-gage principle is applied, except that, instead of a hole, a projection is being gaged. At il is shown the reference gage or reference piece, the af^earance of which is practically the same as that of the piece to be made. The projecting bosses, the location of which is to be gaged, are shown at B. The piece is placed in the gage, which has hollow plungers at D and £, so that the plungers will fit over the projections. The height of the projections, or rather the location of the milled surface at F and G, is also measured by these plimgers, as there is a flush-pin gaging arrangement at the outer end H. The work is located in the gage in the same way as it would be located in a jig. The snap gages shown in the center are used for checking the diam- eters of the pins and the width of the milled surface at K. The plug gages shown are the reference gages for the inspection snap gages. The illustration, therefore, shows the complete set of gages used for measuring the milled recess in the piece, and the location and size of the projections. Fig. 29 shows a gage used for checking the location of three holes with reference to each other. A swinging leaf is used as a locating means for the work, while the three plimgers, by entering the holes in the work, determine that they are accurately located. The plugs at the ends of the jSxture are for holding the work while it is being gaged. Fig. 30 shows a highly developed and complicated hole gage, which gages six holes at one time, five of which may be seen in the illustration. Six plug gages are used in connection with the gaging. The work is placed on the studs at the ends of the fix- ture and then the plugs are passed through the bushing holes in the fixture, and must enter the holes in the work. The diam- eters of the plugs used with this gage, as well as the diameters of the plimgers used in hole gages generally, must be a suitable amoimt imder the actual size of the hole being gaged to provide for the permissible tolerances, as otherwise the gage would FLUSH-PIN, SLIDING-BAR, AND HOLE GAGES 127 require that the work be made to the absolute dimensions, which, of course, would be impracticable. Generally, the plugs or plungers are made 0.002 inch under the size of the hole, which allows for a tolerance of ±0.001 inch on the center distance. Fig. 31 shows a special indicating hole gage used for indi- cating the accuracy of the recentering in a rifle bolt. After the hole in the bolt has been drilled, it is necessary to recenter the bolt in order to insure that the center will be properly in line with the hole. This is of great importance, as later operations are made from this new center and is the only means by which it can be insured that the outside of the bolt will be true with the hole. In finishing the outside of the bolt, it is not possible to work from the hole directly on accoimt of its small diameter, and hence it is necessary to ascertain that the center is correct. The dotted lines in Fig. 31 show how the bolt is placed over the upright stud A of the gage, which it fits at the upper diameter B and the lower diameter C. The bolt is then rotated, and the indicator working against the side of the center will show on the scale whether or not the center runs out. The indicating gage levers are mounted on a swinging leaf so that this part of the gage may be swung out of the way when the work is placed in, or removed from, the gage. A standard commercial test indicator is used. CHAPTER V THREAD GAGES AND THEIR PRODUCTION TBe subject of thread gages is one of the most important in connection with gaging systems. Gages of this general class are among the most difficult to make accurately, and their measure- ment has resulted in the development of many different types of gaging tools and instnunents designed for detecting errors or measuring the different elements of a screw thread such as the diameter, lead or pitch, and thread angle. The type or design of thread gage to use is an essential point to consider, as well as the means for obtaining thread gages that conform to whatever standards of accuracy are required. A common idea of a gage is that it should be and is an exact duplicate of the part to be produced in aU essential details. A thread plug gage, for instance, is regarded as being properly a threaded and hard steel plug, the thread shape conforming exactly to that of a standard. Accord- ing to this idea, a United States standard thread gage has a thread section of 60 degrees included angle, and with the top and bottom flattened an amount equal to one-eighth the pitch, whereas a Whitworth gage would have an included angle of 55 degrees and the top and bottom rounded to a radius equal to 0.1373 times the pitch. The manufacturers of gages supply this type, in response to common demands, although it is well known that a gage so made is not suitable for checking an internal thread accurately. It is true, however, that the ordinary thread plug gage is satisfactory for many classes of work, and, furthermore, it is perhaps impracticable to devise a low-priced gage in which the faults of the plug gage are eliminated. The plug gage of the type referred to satisfies the conmion demand for a standard form that can be used as a check for all dimensions, angles, and shapes. It is well to recognize, however, that a gage which measures all the dimensions of even a simple part is likely to be imsuitable when a very accurate test is required. 128 THREAD GAGES 1 29 A plug gage, which is a duplicate of the thread, may be entirely practicable and satisfactory as a working gage for use in testing the accuracy of threaded holes in manufacturing duplicate parts, but not suitable for testing ring gages or those having internal threads. The ring gage, in order to serve its purpose properly, should be as accurate as a corresponding plug gage, and it is desirable to check the different elements independently, the same as is commonly done in the manufacture of plug gages. If a ring gage is checked by using a plug gage which is a duplicate of the standard thread, the plug may seem to have a good fit into the ring gage when the bearing is, in reality, on the full or outer diameter only. Owing to the difficulty of checking ring gages by direct measurement, the same as is commonly done with plug gages, the accuracy of the former is often tested simply by the fit of a plug gage. In order to obtain a more accurate test, the following set of plug gages was used by the British Ministry of War in the United States in checking ring gages for Whitworth threads. There are six gages in the complete set, which are so modified as to check the different elements in the ring gage separately. Gage No. i is a "not go" plug gage for checking the full diameter. This is a check of a full diameter equal to that of the ring gage on the high limit. The angle between the slopes of the thread at the crest is made 2 d^rees less than the included angle of the normal thread. The rounding at the top of the thread is of small radius, and the core diameter is that corre- sponding to a V-shaped root. Such a gage can only bear on the full diameter, and it should only have about two threads to minimize the effect of lead errors. Gage No. 2 is a "not go " plug gage for checking the effective diameter. This gage has the effective diameter equal to the high limit of that element of the ring screw gage. The crest of the thread is made flat and the root is V-shaped. This gage can only bear on the effective diameter of the ring gage, and it should only have about two threads. Gage No. 3 is a "go " plug gage for checking the effective diameter. This gage is similar to gage No. 2, except that the I30 GAGE DESIGN AND GAGE-MAKING effective diameter is made to the low limit of the ring gage effective diameter. Gage No. 4 is a full form **go " gage. It should have the dimensions of the low limits of the ring gage on the full, effective, and core diameters, and also the same nimiber of threads. The rounding at the crest and root should conform to the standard. This gage tests the three diameters on the low limit, the round- ness at the crest and root, and also proves that the pitch is correct if the plug gage enters the ring gage properly. Gages Nos. 5 and 6 are " go " and " not go " gages for checking the core diameter. These gages may be in the form of a double- ended plain plug with two diameters corresponding to the high and low limits of the ring gage core diameter. The ruling diameters of each of these plug gages should not have a greater tolerance than +0.0001 inch for the "not go" gages and — o.oooi inch for the "go " gages. The error in pitch should not exceed d=o.oooi inch. This set of plug gages enables the manufacturer to test the accuracy of the ring gage at every stage of the finishing processes and enables the mmaber of spoiled gages to be greatly reduced. A set of gages including Nos. i to 4 is also desirable for inspection purposes. While the form of a ring screw gage is checked by gage No. 4, the accuracy of the thread form may be further verified by taking a cast or impres- sion of the thread so that an enlarged image can be projected onto a screen, as described in Chapter X. This chapter, and also Chapters VIII and DC, deal with the checking of plug gages for detecting errors in the diameter, angle and lead. Set of Thread Gages for Screws. — The thread gage system developed by the Pratt & Whitney Co., will be described, the gages first referred to including a complete set for external threads or screws. This complete set is shown in Fig. i, and consists of the following inspection, reference, and working gages: One soft master plug A ; One inq)ection limit snap gage B for the outside diameter of the thread; One reference gage C used as a reference for the inspection gage jB; THREAD GAGES 132 GAGE DESIGN AND GAGE-MAKING One limit working snap gage D used for the outside diameter of the thread; One maximiun inspection thread templet F for testing the pitch diameter (or, for Whitworth thread, for testing the full form); One minimum inspection thread templet G used for testing the pitch diameter; One maximiun reference gage or setting plug J for the maxi- mum inspection templet; One minimum reference gage or setting plug K for the mini- mum inspection templet; One maximum working thread templet N used for testing the pitch diameter (or, for Whitworth thread, for testing the full form); One Tninlnnim working thread templet O used for testing the pitch diameter; One maximiun reference gage or setting plug R for the maxi- mum working templet; One minimum reference gage or setting plug S for the mini- mum working templet. This constitutes the complete set of gages required for a screw. The soft master plug is made to the basic size of the screw which is also the maximum screw size. This master plug is the ulti- mate reference for the whole set of thread gages. It is not intended for general use in connection with the gaging of the product, but only for the re-establishing of the worn reference gages J, K, Ry and 5. The soft master plug is not even intended to be screwed into a hole, as this would cause wear, and for that reason it is provided with hexagon ends larger than the outside thread diameter. This construction also protects the thread when the master plug is laid down upon a bench or machine table. The master plug is used as an ultimate reference gage for the other gages in the set only by using comparative measurements. A thread micrometer or ball micrometer is used for transferring measurements from the master plug to the thread gages. It will be noted that in Fig. i enlarged views of the thread forms of the various templets and gages are shown in conjunction THREAD GAGES m. II i ■ 1 i . i . i I - s= ,v .1 ^£ il i' i-Q ^ LpHj r i| i mi ii 134 GAGE DESIGN AND GAGE-MAKING with the gages themselves, the thread form at E being for the inspection gage F\ that at H, for the inspection gage G\ that at /, for the reference gage J\ and that at L, for the reference gage K. In the same way the thread forms at Af , P, Q, and T are for the working and reference gages Ny O, i?, and 5, respectively. It will be noted that dimensions are given in connection with some of these, and also in connection with some of the gages. These dimensions are based upon the practice of the Pratt & Whitney Co. and cover gages intended (or the U. S. standard and Whitworth form of threads from 20 to 50 threads per inch, these being the number of threads required in connection with small arms work. These dimensions agree with those given in the table " Tolerances for Tapped Holes and Screws/* in Chapter I. Complete Gage Set for Tapped Holes. — A complete gage set as furnished by the Pratt & Whitney Co. for tapped holes is shown in Figs. 2 and 4. This consists of the following: One maximum and minimum inspection plain plug gage A for the diameter of the plain drilled hole which is to be tapped; One reference gage for inspection gage A ; One maximum and minimum working plain plug gage B for the diameter of the plain drilled hole to be tapped; One maximum and minimum inspection thread gage D for the pitch diameter of the threaded hole (or, for Whitworth thread, for testing the full form) ; One reference gage F for the outside diameters of the maximum and minimum ends of inspection gage Z?; One minimum reference gage G for the ipinimum thread plug of inspection gage D\ One maximum reference gage I for the maximum thread plug of inspection gage Z?; One maximum and minimum working thread gage L for the pitch diameter of the threaded hole (or, for Whitworth thread, for testing the full form) ; One reference gage N for the outside diameters of the maxi- miun and minimum ends of working gage i; One minimum reference gage O for the minimum thread plug of working gage L\ Gigaa carraapondliig to Kg. : 136 GAGE DESIGN AND GAGE-MAKING One maximum reference gage P for the maximimi thread plug of workmg gage L. It will be noted that the gages shown both in Figs, i and 2 are for the U. S. standard thread, but the thread is made sharp in the bottom of the groove in all cases, both for templets and plugs. The form of the thread and the tolerances for the various templets and plug gages are shown in conjunction with the gages in Fig. 2, the same as in Fig. i ; thus, C Is the thread shape for the min- imimi inspection gage Z?; £, for the maximimi inspection gage D\ H, for the minimimi reference templet G; /, for the maximum reference templet /; and similarly for the working gages. In all cases the adjusting screws in the outer ends of the templets are not shown, as it is not the intention to show the specific construction of the gages, but merely to indicate what constitutes a set of gages and tolerances to which they are made. Illustrations of a Gage Set. — Fig. 3 shows a set of gages such as is made by the Pratt & Whitney Co. for its customers. At A is shown the soft master plug corresponding to plug A , Fig. 1 ; B shows the inspection snap gage for the outside diameter of the thread, corresponding to gage J?, Fig. i ; D shows the working snap gage for the outside diameter of the thread, corresponding to the snap gage Z?, Fig. i . These two snap gages are held with other gages in the standard snap gage holder used by the com- pany. It will be noted that the gage B, being the inspection gage, has only two steps, while gage Z?, being the working gage, has three steps, maximum, mean, and minimum. At C is shown the reference gage for inspection gage jB, corresponding to gage C, Fig. I. In the same way, gages F and G, Fig. 3, correspond to inspection gages F and G, Fig. i, and gages N and O in both figures are also the same. The reference gages, /, K^ if, and 5, are also lettered the same in both illustrations. In Fig. 4 are shown the gages that correspond to the dia- grammatical lay-out in Fig. 2, these gages being lettered to correspond in both illustrations. The reference gages F and N are not made with two steps in Fig. 4, as indicated in Fig. 2, but, in the case shown in Fig. 4, separate snaps are made for the maxi- mimi and minimum sizes. THREAD GAGES 137 ' Special Forms of Thread Gages. ^ When thread gages are made in large sizes, it is customary to use round forms instead of the templet forms shown in the preceding illustrations. Fig. 5 shows an example of this kind, in which handles are provided on the gage for convenience. The gage is^lit on one side and is provided with an adjusting screw. A large thread gage for an internal thread, also provided with handles, is shown in Fig. 6. nc- 5- A Large ThtMd Oaftt for u BHwimI Thread It is evident that it would be difficult to handle large plugs of this size if they were not constructed in a special manner as indi- cated. By boring out the center of the plug, the gage is made much lighter, and by eliminating the solid handle and instead using two ordinary handwbeel handles, as indicated, the gage is much more convenient to use. Special types of thread gages sometimes used are known as "qualifying" thread gages. These are employed when the thread must be cut in a certain position relative to a shoulder, as, for example, in a rifle barrel and receiver. In this case, lines are 138 GAGE DESIGN AND GAGE-MAKING scribed on the gage and a corresponding zero line is scribed on the work. The gage is then screwed home into the work untU the lines check with each other. TooIb for Catting Oage Threads. — The making of thread gages involves, first, cutting the thread either by using a angle- point tool or a milling cutter, and, second, finishing the thread after the gage is hardened, dther by grinding and lapping or Ftf. 6. k LMt« Tlmad Gate lor ui Internal Tbtead simply by lapping it to size, the practice varying according to the nature of the work or its size, and the ideas of the tool- or gage-maker. Since the final object in view is to produce as ac- curate a thread as possible, it is evident that there is an advantage in cutting an accurate thread to begin with, so that the grinding and lapping operations which may follow will not be required to correct excessive and imnecessary errors. The machines used for gage threading include bench lathes for the smaller gages, engine lathes, thread milling machines, and regular milling machines equipped with some form of thread milling attachmmt THREAD GAGES 139 R^ardless of whether the thread is cut with a single-point tool or milled with a revolving cutter, it is essential to use a machine having a lead-screw which has been tested and is known to be reasonably accurate. Some of the special thread milling attach- ments which are used for this work have a short precision or master screw which forms a part of the attachment and is long enough to take care of ordinary thread lengths. This screw is a duplicate, as to lead, of the screw or thread gage to be milled, and it may be revolved for traversing the gage blank past the Fig. 7. DUgnun of Thread Tool cutter either by hand or by power. Most gages are threaded by using a single-point tool and some form of lathe, in which case a very important part of the work is the grinding of the tool so that it conforms to the standard shape of whatever thread is to be cut. A number of different methods of grinding and gaging thread tools will be described. Gaging the Ends of Thread-Cutting Tools. — When grinding the cutting end of a thread tool intended for precision work, means should be provided for measuring or checking the angle and for determining when the point of the tool is properly formed. In the case of tools for such threads as the U. S. standard. Acme, or the French and International standard, it is necessary to deter- mine when the width of the flat is correct, whereas, for a Whit- I40 GAGE DESIGN AND GAGE-MAKING worth or British Association thread tool, the radius at the point must be considered. As a tool is held so that the top face lies in a plane intersecting the axis of the screw thread being cut, the angle of the tool in this plane, or the included angle between the cutting edges, should equal the standard angle of the thread. This is the angle of the tool in a plane a-a (see Fig. 7). When grinding and gaging the tool, it is convenient to work to the angle in a plane b-b at right angles to the front edge. This angle, which depends upon the clearance angle of the tool, is somewhat greater than the t □ a on u ^J ^ o I — ^ t-j -YT-- II II i-i <^: -^•iV-^- Fig. 8. Gage for MeMoring Pi& 9- Gage for Detennifling Included Ang^e of Thread- width of Flat or Radiua on cutting Toola Thread Tools angle in a plane a-a, and may be determined as follows: Divide the tangent of one-half the thread angle by the cosine of the clearance angle on the tool. The quotient equals the tangent of an angle which may be determined by referring to a table of tangents. This angle, multiplied by 2, equals the included angle of the tool in a plane at right-angles to the front edge. Take, for example, a U. S. standard thread tool having a clearance of 15 degrees. The tangent of one-half the thread angle, or 30 degrees, divided by the cosine of 15 degrees, equals 0.5977. The corresponding angle is found in the table of tangents to be 30 degrees 52 minutes, so that the total angle equals, approximately, 6if degrees. THREAD GAGES 141 Table z. Dimenskms for Determining Flat at Point of U. S* Standard and Radius of Whitworth Thread Tools * Dimension x. Fig. 7 Dimension x. Fig. 7 Dimension x. Fig. 7 No. of Threads No.bf Ihreads No. of Threads per Inch U.S. Thread 1 Whit- worth Thread per Inch U.S. Thread Whit- worth Thread per Inch U.S. Thread Whit- worth Thread 2M 0.0465 0.0687 9 O.OI16 0.0172 34 0.0031 0.0045 2H 0.0440 0.0651 10 0.0105 0.0155 36 0.0029 0.0043 2H 0.0418 0.0619 II 0.0095 O.OI41 38 0.0027 0.0041 2H 0.0398 0.0589 12 0.0087 0.0129 40 0.0026 0.0039 2H 0.0380 0.0562 13 0.0080 O.OI19 42 0.0025 0.0037 2H 0.0364 0.0538 14 0.0075 O.OIIO 44 0.0024 0.0035 3 0.0349 0.0515 IS 0.0070 0.0103 46 0.0023 0.0034 3M 0.0322 0.0476 16 0.0065 0.0097 48 0.0022 0.0032 3W 0.0299 0.0442 18 0.0058 0.0086 SO 0.0021 0.0031 4 0.0261 0.0387 20 0.0052 0.0077 S2 0.0020 0.0030 4H 0.0232 0.0344 22 0.0048 0.0070 S6 0.0019 0.0028 5 0.0209 0.0309 24 0.0044 0.0064 60 0.0017 0.0026 sw 0.0190 0.0281 26 0.0040 0.0059 64 0.0016 0.0024 6 0.0174 0.0258 28 0.0037 0.0055 68 0.0015 0.0023 7 0.0149 0.0221 30 0.0035 0.0052 72 0.0015 0.0021 8 O.0131 0.0193 32 0.0033 0.0048 80 0.0013 0.0019 * For a clearance angle of 15 degrees. Table a. Dimensions for Determining Radius at Point of British Associa- tion Standard Thread Tools * British Ass'n No. Dimension x, Fig. 7 British Ass'n No. Dimension x. Fig. 7 British Ass'n No. Dimension x. Fig. 7 British Ass'n No. Dimension s, Fig. 7 I 2 3 4 S 6 0.0102 0.0092 0.0083 0.0075 0.0068 0.0060 0.0054 7 8 9 10 II 12 13 0.0049 0.0044 0.0040 0.0036 0.0032 0.0029 0.0025 14 IS 16 17 18 19 20 0.0023 0.0021 0.0019 0.0017 0.0015 0.0014 0.0012 21 22 23 24 25 ■ • • • • « • ■ O.OOII O.OOIO 0.0009 0.0008 0.0007 * For a clearance angle of 15 degrees. Gages for Thread Tools. — The two thread tool gages shown in Figs. 8 and 9 are intended for measuring the angle and the width of the flat on thread tools of the U. S. standard form. While these gages are intended for 60-degree thread tools having a clearance angle of 15 degrees, the same general type of gage may be used for any thread or clearance angle. The angle gage 142 GAGE DESIGN AND GAGE-MAKING (Fig. 8) has an angle of 6i degrees 45 minutes, which, as pre- viously explained, corresponds to the angle of a tool having 15 degrees clearance, the angle being measured in a plane at right angles to the front edge. The gage shown in Fig. 9, which is used for testing the width of the flat, is provided with a microm- eter for measuring the dimension corresponding to dimension x in Fig. 7. This dimension varies according to the pitch of the thread, and it indicates whether the flat is groimd to the correct width. This gage is not only used for testing the width of the flat on tools for U. S. standard, Acme, and the French and Inter- national standard threads, but the same type may also be used for checking the radii of the points or tools for cutting Whitworth Fig. zo. Ani^e Test for Acme Threed Tool Fig. II. Plug Gages used for In- specting Accuracy of Angle of Gages and British Association threads. The angle of the gage for Acme thread tools is 29 degrees 58 minutes; for Whitworth thread tools, 56 degrees 38 minutes; and for Bridsh Association thread tools, 48 degrees 58 minutes, assuming that the clearance angle is 15 degrees in each case. The micrometer is so graduated that it is in the zero position when the end of the spindle is at a point corresponding to the extreme edge of a tool if the latter were ground to a sharp V-form. The dimensions x (see Fig. 7), corresponding to threads of different pitch and different standards are given in Tables i to 4, inclusive. To illustrate the use of the table, assume that a tool is being groimd for a U. S. standard thread having eight threads per inch. In this case, the point of the tool should be ground away until the gage, Fig. 9, gives a reading of 0.013 1, which figure is obtained from Table i. If the tool is for cutting a Whitworth thread having eight threads per THREAD GAGES 143 inch, the point should be ground back until the micrometer read- ing is 0.0193. "^Us surface will then be tangent to the point of the tool, which is afterward ground to a circular shape. Fig. II indicates how plain plug gages may be used for testing the accuracy of the gage angle if a plain plug gage 0.1894 inch in Table 3. Dimensions for Determining Flat at Point of French and Inter- • national Thread Tools* Pitch. MDH- meters Dimension at, Fig. 7 Pitch, Milli- meters Dimension *. Fie. 7 0.0154 Pitch. MiUi- meters Dimension X. Fig. 7 Pitch, MUIi- meters Dimension *, Fig. 7 S-O 0.0206 3.75 2-5 0.0103 I. 25 0.0051 4 9 0.0201 3-7 0.0152 2.4 0.0099 1.2 0.0049 4 8 0.0197 3.6 0.0148 2.3 0.0095 I .1 0.0045 4 75 0.0195 3.5 0.0144 2.25 0.0093 I .0 0.0041 4 7 0.0193 3-4 0.0140 2.2 0.0091 0.9 0.0037 4 6 0.0189 3 3 0.0136 2.1 0.0087 0.8 0.0033 4 5 0.0185 3.25 0.0134 2.0 0.0082 0-75 0.0031 4 4 0.0181 3-2 0.0132 1.9 0.0078 0.7 0.0029 4 3 0.0177 3.1 0.0128 1.8 0.0074 • 0.6 0.0025 4 25 0.0175 3.0 0.0124 1-75 0.0072 OS 0.0021 4 2 0.0173 2.9 0.0119 1.7 0.0070 0.4 0.0016 4 I 0.0169 2.8 O.OI15 1.6 0.0066 03 0.0012 4 0.0165 2.75 O.OI13 1-5 0.0062 0.25 O.OOIO 3 9 0.0161 2.7 O.OIII 1-4 0.0058 0.2 0.0008 3 .8 0.0156 2.6 0.0107 1.3 0.0053 * For a clearance angle of is degrees. Table 4. Dimensions for Determining Flat at Point of Acme Thread Tools* Threads per Inch Dimension z. Fig. 7 Threads per Inch Dimension x. Fig. 7 Threads per Inch 9 10 II 12 Dimension x. Pig. 7 I 2 3 4 0.6828 0.3365 0.2210 0.1635 5 6 7 8 0.1287 0.1057 0.0893 0.0768 0.0674 0.0596 0.0532 0.0480 * For a clearance angle of 15 decrees. diameter is used. For a U. S. standard thread gage of the type shown in Fig. 9, the micrometer reading will be 0.090 inch. If a plug gage 0.097 i'^ch is used, the micrometer reading should be 0.046 inch. The accuracy of the angle of an Acme gage may be checked by the same method For example, if the plug gage is 144 GAGE DESIGN AND GAGE-MAKING 0.1254 inch in diameter (see Fig. 10), the micrometer reading should be 0.180 inch. U. S. Standard Thread Tool Gage. — As the U. S. standard thread has a 60-degree included angle and a flat at the top and bottom equaling | of the pitch, it will be evident that a thread of i-inch pitch has a |-inch flat at the top and bottom, and this is taken as the basis in constructing the gage to be described. The decimal equivalent of | is 0.125. Suppose it is attempted to divide this number by 8 in order to obtain the flat on a U. S. standard thread tool for eight threads to the inch. The length of the flats in this case will be found to be 0.01562 inch, but it would be very difficult to make an accurate gage for a thread tool jroekAMnr] Fig. 13. U. S. Standard Thread Tool Gage and Templet need for Setting intended for this pitch. This difficulty may be entirely avoided by using a thread of i-inch pitch for the basis in making the gage. Referring to Fig. 12^ it will be seen that the gage consists of a body which has one jaw secured to it by a screw and pin. The other jaw is secured by a screw which passes through a slot in the jaw, so that this jaw may be adjusted longitudinally to adapt the gage for different pitches. The jaws run in a slot machined in the body of the gage so that they are kept in the desired align- ment, and the end of the tool is brought up against the body. The gage is set for different pitches by measuring the distance between the outside ends of the jaws with a micrometer. In order to obtain the proper location of the angular sides of the two jaws when they are together for a one-pitch V-thread or open for a U. S. standard thread tool of the same size, a templet was made of the form shown at the right-hand side of the illus- tration. This templet was first made an accurate equilateral i THREAD GAGES MS triangle with sides i inch long, and one point was then cut off to make the height h exactly 0.75775 inch. The jaws were then adjusted so that they fitted against this templet with one of the sharp points in position, which corresponded to the setting for an ordinary V-thread, and with the cut-off point in position when the setting is that for a U. S. standard one-pitch thread tool. When set for a one-pitch V-thread, the distance between the ends of the jaws, as obtained with the micrometer, is 1.875 u^ch; and for a one-pitch U. S. standard thread, 2 inches. j2k I I I I .^ ^■^ ^sa. Fig. 13. Gage for 39-degree Thread Tools A convenient method of setting the gage is to open it a small amount more than is required and then partially tighten the screw which holds the movable jaw. The micrometer is then put over the ends of the gage and screwed up to the reqxiired dimension, carrying the movable jaw with it Gage for Twenty-nine Degree Thread Tools. — The fixed gages used for Acme or worm-thread tools do not provide for the degree of accuracy that is often necessary, for they do not give the width of the point of the tool exactly. The gage shown in Fig. 13 is for 29-degree tools which must be accurately ground, and consists of a main piece A on which the slides B and C are made adjustable. The head of A carries a damp for C, the piece 146 GAGE DESIGN AND GAGE-MAKING C being adjustable so as to extend along the side of a long tool, thus insuring a square point on a tool of coarse pitch. This blade C is held to the correct angle by the front face of A which is ground to the proper angle with its base. The adjustable slide B is controlled by a clamp and an adjusting screw. The front edge of £ is ground in such a manner that, when the vernier reads zero, the space enclosed by B and C forms a sharp 29-degree angle. The gage is easily set for the different widths of tools by Fig. 14. Accurmte Method of Grinding Thread Toots means of the vernier, which is one inch long and provides suffi- cient adjustment for all pitches ordinarily met with. The end of blade C is ground to the proper angle and it can easily be removed and used as a gage for the purpose of setting the tool in the lathe. This gage is used not only for measuring thread tools of the Acme or worm-thread type, but for measuring cutters for the thread milling machine. In this connection, the method of figuring the width of point for the tool for these two styles of THREAD GAGES 147 threads might be mentioned. The Acme is not so deep as the worm thready and so, for a given pitch, it is wider on the top of the thread, there being a difference of 0.0357 inch in the case of a one-pitch thread. For the Acme thread, the width of the tool point equals 0.3707 times the pitch minus 0.0052 inch. In the case of a worm thread, the width of the tool point equals 0.310 times the pitch of the thread. Grinding Thread Tools to the Required Angle. — Thread tools intended for precision work may be ground to conform to the standard angle of whatever screw thread is to be cut, either by using a sine bar to set the tool on a surface grinder, or by means of a special holder. Fig. 14 illustrates how a sine bar and angle- QRINOINO WHEEL Fig. 15. Holder for Grinding Tliread Tools plate are used. The tool is clamped on top of the sine bar which is clamped to the angle-plate and set to the angle required. Special holders are sometimes used so that it is not necessary to set the tool by means of a sine bar. A simple form of holder which may be applied to the magnetic chuck of a surface grinder is illustrated in Fig. 15. The sides A and B are placed in con- tact with the chuck for grinding 60-degree tools, and the sides C and D are intended for Whitworth thread tools, the latter being inserted in end E of the tool-holding slot. The angles a between the sides of the holder and center line of the tool are modified to conform to the angle of the tool in a plane at right angles to the front edge. The tangent of angle a is found by dividing the tangent of one-half the thread angle by the cosine of the front clearance angle. The tool-holding slot is inclined 148 GAGE DESIGN A^fD GAGE-MAKING an amount corresponding to the clearance angle and the ends of the block are parallel with this slot. The holder stands on one end when the top face of the tool is being ground. An adjustable holder for grinding thread tools is illustrated in Fig. 16. The sides of the holder are pivoted or hinged, and they ng. 16. AdjiHtabls TtaiMd-lool Giindint FiUnie are held against an adjustable wedge by a coil spring. This wedge is adjustable in a direction parallel with the center line of the tool to be ground. This adjustment is obtained by turning a screw, and graduations are provided to show for what angle the holder is set. The hinged parts are so located that the tool is THREAD GAGES 149 given the required clearance. This holder is made by the Lafay- ette Tool Works, New York City. A grinding fixture or block intended for holding thread tools of the wide flat form is shown in Fig. 17. This tool-holding block has a groove which is ground to the correct angle, and it is pro- vided with a clamp (seen at the right-hand side of the illustration) which holds the tool in the groove. The angle a of this groove should be 30 degrees 52 minutes. GLAMP Fig. 17. Another Form of Holder for Grinding Thread Tools Tools for Cutting Whitworth Thread Gages. — One of the important features connected with the making of Whitworth master thread gages is the making and u^e of the thread-cutting Fig. 18. Blank for Female Radius Tool; Male and Female Radius Tools; Thread of Rough-threaded Gage tools. After the blank has been roughed out, a thread having the correct lead may be cut with an ordinary sharp-pointed tool having a 55-degree angle. This thread should not be cut quite as deep as required for the finished thread. Tools for cutting the finished radius at the top and root of the thread are then made as follows: A drill with a diameter equal to twice the root or top radius is used to drill a hole in a piece of soft tool steel, as ISO GAGE DESIGN AND GAGE-MAKING shown to the left in Fig. i8. This piece of tool steel is then cut straight across the diameter of the drilled hole, along line A- A. After this, it is cut to the shape of a thread tool with an angle of 55 degrees, as shown by the sketch in the center. The tool is then hardened, ground and lapped. The 55-degree angle and horizontal cutting surface should be Iapp)ed. If taken directly from the grinding wheel and used, the angle of the tool may be correct and the radius, which is the most important part in cutting the thread, may be in error, due to deformation during grinding. The length of this female radius tool is not of importance. After ( < >V, nnni'^^inr"'! jjjjjiay'i^ii UJiililililiiiUa LCAD-8CREW ^ TTrrrraa! ,iJJJUUuL.JJ., can also be used for checking the core diameter. These points are so made that they can be removed from the anvil and spindle of the micrometer, and they are more convenient to use than the wedge blocks in roughing. With micrometer points, a hardened plug should be ground down to a diameter correspond- ing to the size of the core diameter, and the micrometer points set from it. The use of a thread micrometer is illustrated at JE, and at F are shown methods of testing the radius of the root and crest. A wire of the correct radius is laid in the root, and a templet is used for the crest. In measuring the angle and effective diameter of the thread while grinding, three sets of different size wires should be used. Assiuning that the gage has 14 threads per inch, as in this par- ticular case, the 0.0375-inch, 0.040-inch, and 0.055-inch wires should be employed. As 0.003 or 0.004 inch has been left on the full diameter for finish-grinding, and wires 0.0375 inch in diam- eter extend only 0.00437 ^^ch beyond the full diameter, the larger wires should be used until the full diameter is ground nearly to size. The wedge blocks for measuring the root are inserted and held in the thread groove the same as wires. After rough-grinding the gage suflSciently to true it, a measure- ment will usually show that the amount of stock left for finishing has been reduced from 0.004 to 0.005 inch, leaving approxi- mately 0.007 or 0.008 inch on the effective diameter and usually from 0.008 to 0.009 inch on the core diameter. The wheel is WHITWORTH THREAD GAGES 167 now redressed to a sharp vee as before, and the root is ground until the relation between the root and effective diameter is right and the size is plus 0.003 ^^ 0.004 ^^h. Heat-treatment to Prevent Subsequent Distortion of Gage. — The gage is now passed through another heat-treatment con- sisting of an immersion in boiling soda water for a period of at least thirty minutes. This is necessary, especially at this stage of the work. The remainder of the material to be removed from the gage by machining or lapping is extremely slight, and the precautions to be taken to insure a permanent set of metal and to eliminate as much as possible any likelihood of distortion of the gage after it is completed, make it necessary for it to be subjected to this additional heat-treatment. Since the gage has passed through the hardening, there has been approximately 0.008 inch of stock removed from almost all the surfaces. This removal has reduced the case or binder of the gage sufficiently to affect its interior structure in such a way as to cause it to shrink in some spots. As it is impossible for the gage to assiune immediately the shape which it tends to take as the result of the strains caused by the removal of metal, this change of form should be quickened as much as possible by a heat-treatment that subjects the gage to more extremes of temperature than it will imdergo while in use. This effect is obtained by the heat-treatment referred to. After the gage has been rough-ground, enough metal has been removed to alter the internal strains and leave the structure of the metal in an imbalanced state. As previously mentioned in connection with carburizing and quenching, the hole in the gage blank closes in from 0.006 to 0.008 inch. If the outside diameter is carefully measured before and after carburizing, it will be seen that there is an increase of from 0.003 ^^ 0.004 inch. When the gage blank is quenched, the threads cool first, so that the interior and the hole have a marked tendency to contract. As the out- side surface and the faces of the gage are gradually reduced by the machining processes, these parts of the gage are gradually weakened, and the result is a slight contraction due to the interior strains. The gage remains comparatively cool throughout the ZIH l68 GAGE DESIGN AND GAGE-MAKING • grinding operations, so that the hardened surface does not become very elastic, and, therefore, is not affected directly by the forces acting upon it. For this reason, it is necessary to subject the gage to a high enough temperature to insure its taking a per- manent set. A pack-hardened thread gage that has not been seasoned or subjected to some rapid seasoning process will change in diameter and lead so that it becomes practically useless. If such a gage is finished at the normal temperature and is allowed to remain near a furnace or other warm place, it will decrease in size and lengthen in lead. If a gage is heated to the equivalent of a light straw color and allowed to cool ofiF, accurate measure- ment will show that it has reduced in size from 0.0002 to 0.0003 inch. Frequently in shops where gages are made, rejected gages are foimd which have changed considerably since the inspection previous to shipment, owing to the lack of proper seasoning. The effect of the lack of proper heat-treatment is illustrated by the following es^rience of a gage-maker: All the operations on the gage, such as grinding and lapping, were finished without any special heat-treatment of the kind previously referred to, and the gage was made to the required size and passed inspection. This particular gage was kept as a master to be used in making dupli- cate gages. In less than two weeks, this gage had distorted to such an extent that it was from 0.0002 to 0.0004 uich out of round and the lead had shortened about 0.0002 inch. Changes due to Removal of Metal. — The removal of metal will also cause appreciable changes of form or size which are often sufficient to entirely spoil the gage. For instance, a gage that had been finished to size was afterward altered by cutting away the threads at the ends back a distance of about one inch farther than they had been previously. This gage at the time was the right size and passed through a female master gage perfectly and without interference of any kind. In less than a week after cutting away the end threads, the gage had changed so much that it would not even enter the master gage one and one-half turn. The gage had a longer lead at each end and the outside diameter also decreased from 0.0002 to 0.00025 ^^^- WHlTWOR-ni THREAD GAGES 169 Knish-griading and Lapping OperationB on Gage. ^After the last heat-treatment previously referred to, some time should be allowed to elapse before the finish-grinding operation^ three or four days usually being sufl&dent. The finish-grinding is prac- tically the same as the rough-grinding. The gage is wrung onto an arbor and the sides are finished to the required ^2e. The full diameter is also ground to size within 0.002 inch. After the ^des of the gage are ground, the hole in some cases will be found to have closed in from o.oooi to 0.0002 inch on the ends so that they should again be lapped to size. The sides of the thread are finish-ground on the machine used for rough-grinding. It will be understood, of course, that the finishing operation requires more care than the rough-grinding. In finishing the root of the thread, a little skill and experience are required. The actual root diameter is from 0.001 to 0.002 inch less than the nominal root diameter, so that at the beginning a wheel having a fairly sharp edge may be used. No attempt is made to dress the edge of the wheel to the radius of the root, since the root can be finished accurately enough for practical 170 GAGE DESIGN AND GAGE-MAKING purposes by using a wheel which has an included angle of 55 degrees and a sharp edge or apex to begin with. As the grinding proceeds, this sharp edge naturally crumbles away and is finally rounded off sufficiently for finishing the root of the gage thread. After a wheel has removed, say, o.ooi inch, the effective diameter of the gage may be 0.003 inch over size, whereas the root diam- eter is, perhaps, only o.ooi inch over size. After another 0.001 inch has been removed, a measurement will usually show that the relation between the effective diameter and the root diameter has changed slightly* For instance, when the effective diameter is +0.002 inch, the root diameter may be +0.0005 inch, owing to the fact that the apex of the wheel has worn down slightly. One experienced in this work can reduce the gage by this method to the lapping size with one dressing of the grinding wheel and maintain the proper relationship required between the effective diameter and the root diameter. In the case of the particular gage selected as an example, when the effective diameter is from +0.0003 ^^ +0.0004 inch, the root diameter should be from 0.0012 to 0.0015 ^^h ^^^ than the nominal root diameter. When beginning to grind a gage, it is weU to consider the effect of expansion on the position of the wheel due to the heating of the wheel-spindle. Owing to the slight change due to this cause, it is advisable to keep the wheel running continuously when taking the finishing cuts. K it is necessary to stop the wheel for measuring or inspecting, when the wheel is again started it should be allowed to run idle long enough for the temperature of the spindle to rise to the normal working temperature before taking another cut. After the angular sides of the thread gage are ground, the same machine is used for grinding the crest or top of the thread. By moimting the wheels as mentioned in connection with Fig. 9, Chapter VII, it is possible to replace one wheel with another of different form with the maximum error not exceeding 0.0015 inch. The methods of forming the wheel for grinding the crest and of finishing the thread by lapping, are described in detail in Chapter VII (see pages 186 to 200, inclusive). A group of fin- ished gages is shown in Fig. 7. CHAPTER Vn GRINDmG AND LAPPING THREAD GAGES When making thread gages, especially external gages of the plug form, it is quite general practice to grind the thread after the hardening operation and prior to lapping it to the exact size required. Some gage-makers, however, cut the thread as accurately as possible and then, after hardening, finish it by lapping and without a preliminary grinding operation. The success of one method as compared with the other may be gov- erned largely by the kind of steel used, the method of heat- treatment, and the size or pitch of the thread itself. The prin- cipal object of grinding is to correct any errors which may have resulted from hardening of the gage ; hence, there is less reason for grinding when a steel is used which changes very little in hardening. In any case, the method of heat-treatment which will give the best results should be determined and standardized d3 f ar as possible in order to reduce the hardening errors to a minimum. The size of the thread or its pitch is another point which has a bearing upon the method of finishing. It is difficult to maintain a sharp enough edge on the wheel for grinding threads of fine pitch. If an attempt were made to grind the thread to a sharp V, the edge of the wheel would quickly wear away, but in the case of the U. S. standard, Whitworth, and some of the other threads, the flat or curved root allows for more or less wear on the edge of the grinding wheel, and the root of the gage may be ground smaller than the theoretical root diam- eter, thus forming a little clearance space. While a small amount of wheel wear at the extreme edge may not be objectionable in the case of a thread of medium or coarse pitch, the allowable wear diminishes with the pitch, and in the case of fine threads of, say, the U. S. standard form, the width of the flat at the root is very slight, and does not provide much of a margin for wheel wear. 171 172 GAGE DESIGN AND GAGE-MAKING The amount of stock to leave on a gage for grinding usually varies from 0.003 or 0.004 to 0.007 or 0.008 inch, depending upon the size of the gage. The allowance should be just enough to provide sufficient stock for correcting errors in the lead, thread angle, or diameter after the gage is hardened. Advantages of Grinding Thread before Lapping. — Before taking up the subject of rough-grinding the thread, the advant- ages of a preliminary grinding operation as compared with lapping the thread to size will be considered. A thread that is to be finished after hardening by lapping alone must be very accurately machined in order to leave as little stock as possible for lapping. This threading operation requires a longer time than when cutting the thread for grinding, because, if the thread is to be rough-groimd, it need not be nearly as smooth nor accurate as to size and angle as would be required if lapping were the only means employed to bring it to the required size. As lapping is a very slow process as compared with grinding, several times the allowance for lapping can be left for grinding, and the surplus stock can be removed more rapidly than the smaller amoimt left for lapping. The amoimt of stock left for grinding also in- sures that all threads will clean up when reduced to the finish size. Another great advantage of grinding threads is that they can be finished within 0.0004 or 0.0005 inch of size on the effective and full diameters, and the core diameter can be ground right down to size, leaving just enough to remove the slight wheel marks and lap the thread to a high finish. If threads are ground extremely accurate as to form and lead, the lapping operation is simplified and fewer laps are required, because, as the angle and lead are accurately groimd, the laps will continue wearing accurately and will need little correction. On the other hand, if the thread is finished exclusively by lapping, a number of laps may be needed for roughing and finishing, because the thread, after hardening, is distorted and inaccurate as to lead, thus requiring continual correcting and renewal of laps, especially where many thread gages are made. The making, of thread laps is accurate and tedious work and may greatly increase the cost of production. GRINDING AND LAPPING THREAD GAGES 1 73 General Types of Machines used for Thread Grinding. — The grinding of thread gages is done on engine lathes, bench lathes, and on special machines designed for this work. What- ever type of machine is used, it is very essential to have it equipped with an accurate lead-screw ^ce the ground thread ia Giindlng thB ThTMd of « Flog Oaga a duplicate of the lead-screw so far as the lead or pitch is con- CTmed. When a standard engine lathe is used for thread grind- ii^, some form of attachment is mounted upon the carriage. This attachment is arranged to hold the wheel at an angle cor- responding to the helix angle of the thread, and it ^ould have some form of diamond wheel-truing device, since the truing 174 GAGE DESIGN AND GAGE-MAKING of the wheel is a very important part of the thread-grinding operation. A motor-driven thread grinder of the type which is applied to a lathe is shown in Fig. i. The grinding wheel spindle can be adjusted vertically as well as longitudinally, and it can also be tilted to an angle of lo degrees from a horizontal position for Fi(. 3. Grinding tha Thratd of ■ Ring Gage aligning the wheel with the thread being ground. The motor is mounted directly on the spindle housing and drives the spindle by means of an endless multiple belt, the tension of which is maintained automatically. This grinder has a diamond wheel- truing attachment which may be set at any angle. The grinder is sometimes used in conjunction with a relieving attachment for grinding relieved threads on taps, hobs, and dies. This grinder I ■3 S Si 8 8 s I 3 t Us, '^ 0) bo u ^ < ce PM u S-i I7S lyfi GAGE DESIGN AND GAGE-MAKING the grinding fixture. It will be noticed that this lathe is not provided with back-gears, which are objectionable on grinding lathes. The lead-screw of this lathe is placed in the center of the bed so that the pull on the carriage will be in the center. This obviates any possible twisting of the carriage and uneven feeding. The lead-screw has to be well covered, as it is neces- sary to protect it. The lead-screw was cut on a master thread-cuttii^ lathe and was made only long enough to provide feed for the work to be ground. It is not necessary to support one end of it, as it has Fig. 4. Thiud-crindinf FlztaT« three bearings; namely, the two brackets underneath the head- stock and the nut under the carriage, which takes the place of one end bearing. This nut is solid and not made in halves, as is the custom on other lathes, because it has been found that it is best to reverse the lathe when going back to take another cut. The solid nut also makes the carriage feed more rigid. The lead- screw can be disconnected from the drive shaft at C, so that, when it becomes slightly worn, it can easily be replaced. The tailstock and tailstock sleeve are cut away to allow the grinding fixture to pass when grinding very small plugs. The wheel that is used is only about 3 inches in diameter when new. The carriage is provided with a cross-slide and a compound slide, the cross-slide being adjusted with a fine-pitch screw of 20 threads per inch, so that the dial on the screw can be graduated to ten-thousandths. GRINDING AND LAPPING THREAD GAGES 177 The compound slide has no graduated dial, as it is used only for adjusting the wheel to the thread groove. The gearing for driving the lead-screw on this lathe should receive especial attention. Being a lathe of about 12 inches swing, it is advisable to make the gears of very fine pitch, say, 20 diametral pitch, and as the gears will be small in diameter, they are made of steel. The gears must be turned and milled with the utmost care, because, if they are not true, they will not drive the screw evenly, and drunken threads will be the result. As this lathe has no feed except the screw feed, it does not require an apron. The machine is simple and inex- pensive to build. The grinding wheel spindle and sUde are shown in the end elevation and plan view in Fig. 4, The wheel can be set at any angle to conform to the helix angle of the thread, and it can also be adjusted up or down to bring the center of the wheel in line with the center of the work being ground. The most important part of this fixture is the spindle A and its bearings B. The spindle, being made of tool steel, is hardened and ground. It is well to make it of some oil-hardening steel that will not change much, if any, when hardened. The spindle is hollow, so that it may be cooled, when running, simply by the air passing through it. The bearings B are made of some good bearing bronze, and it is necessary that they be absolutely concentric. The casing C is a cast-iron sleeve with a taper hole in each end for receiving the bearings. These taper holes should be in line with each other. To obtain the accuracy desired, first grind one hole to fit the bearing; then put a plug in the chuck and grind this to fit the Fig. 5. Fixture for Dressing Grinding Wheel 178 GAGE DESIGN AND GAGE-MAKING taper hole that is ground. Then place the casting on the plug and tap it lightly. If the other end is ground in this position, these two taper holes should be true with each other. A block for dressing grinding wheels is shown in Fig. 5. This block should be hardened and ground all o ver^ and it should be lapped at Cy Z>, and E and tested with a sine bar. The hole through the block which is to receive the arbor must be true with C, Dy and E. This fixture is simple in design, but can be made very accurately and will dress a wheel so that the only lairing necessary on the plug or ring gage is merely to remove the vibration marks. These vibration marks are so small that they will not be seen until a lap is applied. When using a block or fixture of this type for truing the wheel, it is possible to grind a plug or ring gage almost to size. This will increase production, because it requires less time to grind than it does to lap. To use this block, simply put it between centers. A small jack should be put underneath it to keep it from turning. By sliding the small block A that carries the diamond along the angle of the large block £, taking care to press slightly against it, the wheel can be dressed. Thread Grinder of Type having Traversing Master Screw. — The thread-gage grinding machine illustrated in Fig. 6 is equipped with a master screw which is revolved by hand when grinding a thread. The master screw is revolved by means of the crank handle seen at the extreme right-hand end of the machine. This crank is attached to the spindle, which carries an interchangeable master screw of the same lead as the thread gage that is being groimd. This screw runs in a nut, so that, as the spindle is rotated, the work receives a corresponding rotary motion through connection made between the work and spindle by a driving dog, and the work is traversed past the grinding wheel at the proper rate. In order to provide for this traverse movement, the dead center at the left-hand end of the work is held in contact with it by means of a compression spring; consequently, as the work is traversed, the center is pushed back against this spring tension. As soon as the traverse movement has been completed in one direction, the machine operator simply reverses the direction in GIUI4DING AND LAPPING THREAD GAGES 1 79 which he has been'rotating the aank, thus feeding the work back past the wheel with the work rotating in the opposite direction. It is important for the nut in which the master screw runs to be a close fit, and before starting to describe the provision made for assuring a tight fit of the nut, attention is called to Fig. 7, in which there is shown a close view of (me of the interchangeable master screws and nuts. The screw thread is cut on a steel sleeve that is held in place oa the ^indle by lock-coUars; and the nut, which is made of bronze, is tapered on the outdde with l8o GAGE DESIGN AND GAGE-MAKING a Straight threaded section below the taper. An adjustmg collar goes on the outside thread, and by tightening this collar, the nut is pulled into its seat in the machine frame, thus assuring a close- working fit between the screw and nut. To avoid redresang the wheel to different shapes each time a change is made on the class of work that is being ground, inter- changeable spindles are provided for use on the machine, on which the different forms of wheels are permanently mounted. One of these qiindles with its grinding wheel is shown in Fig. 7, where it will be seen that at each end the spindle is provided with a 60-degree tapered center. These centers fit into corresponding sockets carried in the wheel-head, and when it is de^ed to change wheels, it is merely necessary to loosen a binding screw and move one of the sockets back sufficiently so that the spindle can be removed and another one substituted in its place. This change of spindles can be made with very little loss of time. For truing the grinding wheel, there is an attachment per- manently secured to the machine that b furnished with means of adjustment to provide for truing wheels for grinding different forms of thread. Extending out of the attachment at each side l82 GAGE DESIGN AND GAGE-MAKING thread. This axljustment is accomplished by piiUing out pins Cf which control the position of the diamond-bars in the attach- ment and entering these pins into the proper holes for the par- ticular type of thread for which the wheel-truing attachment must be set. This machine is the design of the Commimity Machine & Tool Works, Inc., New York City. Master Screw traversed by Belt I>rive. — The thread grinder shown in Fig. 8 is another design having a traversing master screw, but it is revolved by power instead of by hand, as in the case of the machine shown in Fig. 6. This machine (Fig. 8) has a cast-iron base C upon which is moimted the frame for supporting shaft X. This shaft carries three pulleys. The outer pulleys are loose and the central one tight on the shaft. Two bearing brackets A and B support the master screw shaft E, upon the outer end of which is mounted the gage G to be ground. The cross-slide D carries spindle F and grinding wheel R. The whed-spindle may be adjusted to locate the wheel in line with the thread. The belt pulleys for driving the master screw E should be so proportioned that the gage will have a surface speed of about 40 feet per minute. The surface speed of the grinding wheel should be about 8000 feet per min- ute, imless a lack of balance in the wheel or other conditions make it unsafe to revolve the wheel so rapidly. When this machine is in operation, the master screw E is revolved in first one direction and then the other by means of open and cross belts, which are alternately shifted onto the central or tight pulley Q. The drive from the pulley shaft to the master screw is through faceplate F, stud Z, and a dog W. The threaded section of the master screw E passes through a babbitted bearing in the bracket A . This screw should be cut in a precision lathe that has been thoroughly tested for accuracy or one that is equipped with some device that will compensate for slight errors in the lead-screw. The spindle of the grinding machine is driven from a bench motor by a belt running over the grooved pulley shown at the right-hand end of the spindle. When the machine is first started, shaft E travels forward and the grinding wheel is adjusted in a lengthwise direction imtil it GRINDING AND LAPPING THREAD GAGES l93 is in alignment with the thread groove. After shaft E has made a certain number of turns, the gage is carried past the grinding wheel and then the rotation of shaft E is reversed by shifting the belt; the gage then traverses past the grinding wheel in the opposite direction, so that the machine is grinding almost continuously and is more rapid in its operation than a machine ar- ranged to grind in one direction only. The gages ground on this machine are duplicates of the master screw, so far as the lead of the thread is concerned. The grinding wheels used for this work should be moimted permanently on bushings as shown in Fig. 9. The face A of this bushing that comes into contact with the groimd shoulder of the whed-spindle is finished square and true with the hole; consequently, when the wheel is placed on the grind- ing machine spindle, it will always nm true and be in the same position rela- tive to the master screw. Thread-gage Grinding Machine having Traversing Wheel. — Fig. 10 shows a thread-gage grinding machine, together with the overhead works which are required for driving it. In connection with the following descrip- tion, a better idea of features of the design will be gathered by referring to Fig. 11, which shows a view of this machine with the belts removed from the driving pulleys. To provide for rotating the work and traversing the grinding wheel past the work, power is transmitted from a pulley A through a worm and worm-wheel, which serves the double purpose of giving the desired speed reduction, and at the same time afifording a smooth transmission. The thread gage S which is to be ground is carried between centers, and as the work Fig. p. Method of mounting Grinding Wheel to insure Accurec? and reduce Dreta* ing of wheel Pace laa X84 GAGE DESIGN AND GAGE-MAKING rotates, grinding wheel C is traversed along it at tlie required rate to give the proper lead to the thread. The carriage which supports the grinding wheel head slides on ways on the bed of the machine, and this movement of the carriage is controlled by a master screw (the end of which can be seen at D) which has the same pitch as the thread which is being ground. An ann on the back of the carriage carries a nut which nms in mesh with this master lead-screw to provide for feeding the grinding wheel in a lengthwise direction along the work. TTiread gages ground on this machine are supported between GRWDING AND LAPPING THREAD GAGES ■8S centers, and to facilitate the changing of work, a lever F is pro- vided which enables the tail-center to be pulled back against a compression spring, instead of depending upon a screw for securing this adjustment. Grinding wheel C, which is employed to grind the thread gage, is driven by an endless fabric belt G that provides for increasing the speed of rotation at which the wheel runs, to give the necessary peripheral cutting speed for the abrasive. The driving pulley over which belt G runs and also the pulley that is driven from the countershaft are supported on a swing- ing bracket connected to a tension ^ring, which will be seen at the right-hand side of the cross-slide. This spring provides for maintaining the required belt tenaon. The grinding wheel is trued by means of a truing device provided with two diamond points which are carried in the holders shown at H. It will be apparent that one diamond engages the wheel at each side, and by swinging lever / back- ward and forward, these diamond points move across the in- clined faces of the wheel, thus truing up the wheel to exactly the desired shape. At the back of the truing device there is a i86 GAGE DESIGN AND GAGE-MAKING small handwheel /, which operates a screw employed for feed- ing the wheel into the work as its diameter is reduced. The grinding wheel is set to correspond with the helix angle of the thread which is being groimd, by means of a graduated head Kj which turns a worm L that runs in mesh with a worm- wheel segment secured to the grinding wheel spindle head. This head is carried by curved slides which have their center at the center of the grinding wheel. As a result of tiuming head K^ the wheel is rocked slightly about its own center to provide for setting it to correspond with the helix angle of the threaded work. Graduated handwheel M operates the cross-slide .to Fig. 13. Bquipment for PormlAC Concave Orooye in Periphery of Wheel for Grinding Greet of Wbitworth Thread obtain the required pitch diameter for the work. The thread gage is supported on two dead centers and driven by the usual form of dog. The tail of the driving dog engages the rotating faceplate to provide for revolving the work while it is being groimd. This machine is made by the Blair Tool & Machine Works, Inc., New York City. Forming Wheel for Grinding Crest of Whitworfli Thread. — Before grinding the crest of a Whitworth thread, a concave groove may be formed in the periphery of the wheel, by using a wire of the right size, after the groove has been partly shaped by a diamond tool. The diamond-holder shown at -4, Fig. 12, was designed for use in conjimction with the fixture illustrated in Fig. 5, Chapter VT. The V-shaped tongue on the diamond- GRINDING AND LAPPING THREAD GAGES 187 holder is placed into engagement with the slots in the fixture, and the sliding bar to which the diamond is attached is moved forward to bring the diamond into contact with the wheel, thus forming a slight groove in its periphery. The diamond-holder is next removed and is replaced by the fixture shown at By Fig. 12. This fixture also has a V-shaped tongue for engaging the slots in the fixture previously referred to, and is provided with projecting arms on each side, contain- ing holes through which a wire is inserted. This wire is used in finishing the concave groove in the grinding wheel to the required form. The cross-slide of the grinding machine is ad- justed in far enough to come into contact with the wire which TEMPLET Madkinent Fig. 13. Section of Wheel dressed for Grinding Crest of Thresd, and Templet for Testing Concave Groove in Wheel passes through the arms of the fixture and is held taut. The radius of this wire should correspond to the radius of the crest of the thread or be slightly larger. This wire should extend at least 12 inches beyond each guide bushing in the fixture, and the ends are held lightly by hand when forming the groove in the wheel. As the wheel revolves, the wire is pulled tight enough to bring it into contact with the edge of the wheel, and then the wire is moved in first one direction and then the other a number of times. A piece of thin hardened steel is next held against the edge of the wheel, thus forming it to the shape illustrated in Fig. 13. The depth x of the concave groove is measured, and if it is not deep enough the wire is again used. On the contrary, if the groove is too deep, the top of the wheel 1 88 GAGE DESIGN AND GAGE-MAKING may be dressed oflF with an ordinary diamond. When the groove has been properly formed, it is advisable to dress the angular sides of the wheel, so that they will not come into contact with the gage thread when grinding the crest. When starting to grind the crest, the revolving wheel is adjusted inward mitil it comes into contact with the revolving gage. The position of the wheel in a direction parallel to the axis of the gage is determined by noting from which side the sparks come, and the wheel-spindle slide is adjusted accordingly. As those who have had experience in grinding know, the sparks indicate inaccuracy of adjustment with greater precision than an indicator, and the wheel may be adjusted by this method until it is central with the thread within 0.00015 i^^^- When grinding the crest of the thread a good micrometer should be available, and it is preferable to have one of the heavy type having an anvil and spindle ^ inch in diameter, because such a micrometer gives a better bearing surface on the top of the gage threads. When the top of the thread has been ground until the gage is within o.ooi inch of the full diameter, it is ad- visable to test the radius of the crest by using a templet of the form shown in Fig. 13. After testing the radius with the templet, the grinding is continued until the full diameter is plus whatever amount is to be allowed for lapping. This completes the grinding of the gage, and the next operation is that of lapping in order to remove all grinding-wheel marks and reduce the gage to the final dimensions within the limits specified. Wheels used for Thread-Gage Grinding. — The grain of the wheel used for grinding thread gages should be fine enough to allow the wheel to be dressed to a point or edge sufficiently sharp to meet the necessary requirements. The abrasive should be free-cutting and the bond sufficiently tough to prevent the edge of the wheel from crumbling too rapidly, but not so hard as to retain the abrasive long enough for the wheel to become glazed. Conmdiun wheels of good quality are used for gage grinding and also genuine Naxos emery, especially if a high finish is desired. The artificial alumina abrasives such as aloxite and alundum are extensively used for this work, as they cut faster than the natural GRINDING AND LAPPING THREAD GAGES 189 abrasives and the wheel may be run at higher peripheral speeds. In one shop where thread-gage grinding was done extensively, alundum wheels of 120 grain, grade J, and 120 grain, grade K, proved satisfactory. The 120- J wheel was used for rough- grinding and the 120-K wheel for either roughing or finishing, but it is best adapted for finishing. Another grinding wheel that has given satisfactory results for roughing is a 180-M aloxite wheel. If the threads on the gage are coarser than 12 per inch, a 120- J alundum wheel may be used for finishing. If the threads are finer than 16 per inch, a 200-M alundum wheel should preferably be used. Another wheel that has given satisfactory results is the 180-J carborundum for grinding gages having not over twenty-five threads per inch. If a finer thread is to be ground, an India oilstone wheel may be used. Wheels of coarser grain than those previously referred to are used by some gage-makers, the grain varying from 60 to 80. The grade mark depends upon the system adopted by whatever manufacturer made the grind- ing wheel. Lapping Thread Gages. — To obtain the best results in making thread gages, the operations prior to lapping should be so per- fected that the gage thread is quite accurate as to shape and lead before it is lapped. It is a mistake to rely upon the use of laps for correcting errors which might be avoided if greater care were taken with the previous operations of thread cutting, hardening, or grinding. The allowance for lapping should also be reduced as far as is practicable so that the lap merely removes a very slight amount of metal for reducing the gage to size and impart- ing a smooth finish to the threads. While laps may be and are used to correct errors, if these errors are reduced to a minimum before the lapping operation, the total cost of producing accurate thread gages may be reduced considerably. The cost may also be reduced by the use of equipment that will enable the gage to be ground very close to size so that the lap simply finishes it. As to the material for laps, some gage-makers prefer cast iron and others soft steel. The practice also differs in regard to the general form of the lap used, and the method of applying it to the work. It is essential to use laps which are accurate as to lead I90 GAGE DESIGN AND GAGEr-MAKING and thread fonn, although some laps intended for correcting errors have thread angles which are slightly greater or less than the standard, the object being to more readily change the angle of the gage thread. For instance, a group of four laps may be used with angles varying above and below the stand- ard angle. Allowance for Lapping. — The allowance for lapping usually varies from 0.0002 to 0.0005 ii^ch, although, in some cases, the allowance may be as high as o.ooi inch or more, the amount left for lapping increasing with the size of the gage. The larger the allowance for a gage of given size, the greater the care and skill required to remove the larger amoimt without introducing errors; the amoimt of time required for lapping may also be greatly in- creased. Assuming th^t a gage is accurate after having been groimd, the allowance for lapping should be just enough to re- move the rough surface left by the grinding wheel, and, for this reason, the allowance depends to some extent upon the degree of finish left by the wheel. In any case a ground surface, if exam- ined by a microscope, will appear rough and imeven, and lapping is necessary to reduce the minute ridges left by the grinding wheel. If a gage is not given a fine, smooth, mirror-like surface, it will wear much more rapidly and may soon be below the mini- mum size. Lapping a Plug Gage. — One method of making laps for plug gages and of lapping them is as follows: First make a tap and grind it on a precision lathe to sizes corresponding to the plug before the plug is lapped. Good practice requires that there should be about 0.0005 ^^^ ^^ ^^P ^ over. After the tap has been made, inspected, and found to be all right, the lap can be made as shown in Fig. 14. This lap is made of very soft cast iron. It has a guide pin A , adjusting screw 5, and a screw C for locking. To use the lap, simply adjust it to the plug thread, put on some lapping compound, and turn the plug back and forth a few times; then remove the plug and reverse the lap. Care must be taken not to run the lap over the plug loose, as this will ruin the shape of the thread, and if it is too tight, it will cause over- heating. \\ will often be found that the plug has a tendency to GRINIDING AND LAPPING THREAD GAGES igi become snutller at each end. To remedy this, a narrow lap must be used so that the high points can be lapped off. A good ]a.ppiii% compound is made by mixing fl is used for reducing the root of the thread to the required size, which in this case is about 0.0015 inch less than the nominal root diameter. The lap A which was first used is again placed on the gage and is charged with rouge. After the lap has been given a number of turns in each direction, it is re- moved for testing the size of the gage, and this is repeated until the gage has been reduced to the required size. The first abra- sive used, which is coarse as compared with the rouge, tends to creep up the sides of the gage thread and remove more material at the top, which accoimts for the fact that the gage thread angle is sometimes a trifle wider than it should be. When rouge is used, the lap has more of a rubbing than a cutting effect and the abrading action on the side of the gage thread is more even than when using a coarser abrasive. In some cases, the gage thread is groimd to an angle which is slightly less than is required, in which case the obtuse form of lap shown at C is employed. The time required for lapping one of these gages should not exceed two and one-half hours, and where a niunber of duplicate GRINDING AND LAPPING THREAD GAGES 195 gages are being made, it is possible, under favorable conditions, for one man to lap twelve gages a day. As previously mentioned, the laps for use on the sides of the thread are made so that they will clear the crest and root. As a lap wears down, however, the clearance space decreases and it may be necessary to remove a little metal from the top of the lap thread. The lap may be tested for clearance by first placing it on a thread gage and ad- justing the screws imtil it turns with a slight drag; the lap is then removed and a plug gage o.ooi inch smaller than the nominal core diameter is tried into the gage. If this plug passes through, the lap still has sufficient clearance, but if the gage cannot be inserted, a reamer o.ooi inch larger than the core diameter should be run through the lap. Reversing Position of Lap. — It is not only necessary to have the gage thread of the correct angle, but the angle between each side and the axis of the gage should also be the same. A gage thread that is tilted with relation to the axis may be due to the use of a lap cut with a tool that was not set properly. The angle of the tool may be correct, but, of coiurse, it is essential to set it so that each cutting edge has the same inclination relative to the axis of the lap. It is good practice when using a lap to reverse its position relative to the thread gage. When this is done, if the lap thread is tilted this will soon be apparent, owing to the obtuse angle given to the thread gage. Causes of Concave and Convex Sides. — Sometimes the sides or slope of a gage thread are lapped concavely instead of ex- tending in a straight line from the crest to the root. This is usually due to the use of a lap that is too acute. Sometimes the opposite effect is produced and the sides of the thread are lapped convexly instead of concavely. This may be due to the use of a lap that is not properly adjusted and one that works too freely or wabbles. If the obtuse lap C, Fig. 17, is used too long, it will, of course, increase the angle of the gage thread beyond the standard of 55 degrees, and if the root lap D is used too long, the thread will be too deep and the angle too small. Lapping Crest of Whitworth Thread. — The form of lap used for finishing the crest is shown by the sectional view, Fig. 18. 196 GAGE DESIGN AND GAGE-MAKING This lap has a helical concave groove corresponding to the pitch of the gage thread and of the same radius, in cross-section, as the crest of the thread to be lapped. This lap is bored out to a diam- eter D equal to the full diameter of the gage thread, minus twice the vertical distance from the top of the thread to the point where the crest is tangent to the side of the thread. The depth of the concave groove in the lap for different pitches is given in the accompanying table and also the radius of the thread crests. When using the crest lap it should be adjusted so that it bears very lightly on the thread being lapped- Many gages have been spoiled by adjusting the lap too tightly. When lapping the angular ades of the thread, there is at least six times as much surface being lapped as when lapping the crest. Notwith- standing tMs fact, many mechanics apply the same amount of pressure, and the result is that the lap itself is cut instead of the work. The lap should be adjusted for this operation so that one is just able to feel the drag or pull. If used in this way, the lap will cut properly and retain its shape while lapping a number of gages. This lap should also be re- versed frequently the same as when using the angle laps. If the lap for the top radius or crest b not properly made, this section of the thread may be finished slightly off center. It is important to remember that the full diameter is reduced quite rapidly when la[^ing the crest. The radius should be tested occasionally by using a templet gage ^milar to the form illus- trated at F, Fig. 6, Chapter VI. Timing Lapping Periods. — When lapping a thread gage or any other tool or part which must be reduced by the lap^ng GRINDING AND LAPPING THREAD GAGES 197 • process to a given dimension within small limits, the work can be done with less chance of error and more quickly by timing the lapping period, provided the relation between the lapping time and the reduction in size during that time has been determined previously. It is generally recognized, of course, that if a cutting tool removes a given amoimt of material in a given time, the same tool or one similar to it operating under corresponding conditions will approximately duplicate the amount of work done during equal periods of time. It also holds true that, if a lap removes a certain amount of metal in a given time, the same lap or another one like it used under similar conditions will repeat the per- Radli of Crest and Root and Vertical Distance to Point of Tangency Threads Radius of Rcx^t Vertical Depth Threads Radiol of Root Vertical Depth per Inch and Crest of Crest per Inch and Crest of Crest 36 0.00381 0.00206 ^3 0.01056 0.00568 32 0.00429 0.00231 12 0. 01 144 0.00616 30 0.00458 0.00247 II 0. 01248 0.00672 24 0.00572 0.00308 10 0.01373 0.00739 20 0.00687 0.00370 9 0.01526 0.00821 18 0.00763 0.0041 I 8 O.01716 0.00924 16 0.00858 0.00462 7 O.01961 0.01056 14 0.00981 0.00528 6 0.02288 0.01232 formance. For instance, if a lap freshly charged with a certain abrasive is used continuously for, say, ten minutes, and careful measurements show a reduction of 0.0004 inch in size, it is reasonable to assume that this relation between the lapping period and the amount of metal removed may be used as a basis for determining the time required for making further reductions in size. To illustrate, suppose in this case that a lapping period of ten minutes reduced the size of the lap 0.0004 inch and that a measurement showed that there was 0.0002 inch more to be removed; in that case, the next lapping period might be five minutes in order to remove the 0.0002 inch, although it would be advisable to reduce this time somewhat in order to be on the safe side. Thus, instead of lapping continuously for five minutes, the lapping period might be reduced to four minutes, after which the part would again be measured. This simple principle as applied 198 GAGE DESIGN AND GAGE-MAKING to lapping, eliminates largely the uncertainty connected with the lapping process^ and makes it possible to reduce a precision tool, such as a thread gage, to the required size without taking so many measurements in order to avoid spoiling the work. In fact, by carefully timing the lapping periods, it is possible to reduce a thread gage to a specified dimension in three or four lapping periods. Abrasives for Lapping. — Flour of emery is extensively used for lapping gages, and artificial abrasives are also used. The abrasive is mixed with some oil such as lard oil, sperm oil, or possibly kerosene oil. When a very slow cutting abrasive is Fig. 29. Lap for Angle of Thread. Exaggerated Thread Form after lappiii^ Top of Oage Thread. "Swelling" on Top of Thread required and the amount to be removed by lapping is small, rouge and lard oil may be used. Emery, being tough and shaip, will cut for quite a long time without recharging and is preferred by some gage-makers. Rouge is used when a high polish is desired. Satisfactory results may be obtained by using flour emery (grades F to F4), for lapping gages after grinding and then using rouge for the final finishing touch. Information regarding the different kinds of artificial abrasives adapted for lapping may be obtained from the manufacturers. After lapping a gage, it should be washed in gasoline before measiuing it. If the gage has been heated appreciably as the residt of lapping, it should be cooled in water down to the room temperature before measuring. Lapping a Whitworth Master Gage. — The lapping of Whit- worth master gages by means of a set of four laps will now be described. The first lap is made as shown to the left in Fig. 19. In this lap the angle must be correct. There need be no top or bottom radius on the lap thread. The root must be cut away so that it does not touch the top radius of the gage. The top of the lap thread should be cut away so that it does not touch the root GRINDING AND LAPPING THREAD GAGES 199 radius of the gage. The fonn of the gage thread (exaggerated) after this lap has been used is shown by the central view. The next lap is used to lap the root radius of the gage to size. The Wj.3 angle of the thread of this lap is 54 degrees. The root of the lap is cut away and the top is cut with the female radius tool used in cutting the top radius of the gage. This gives a perfect top radius, so as to lap the root radius of the gage to the proper fonn 200 GAGE DESIGN AND GAGE-MAKING and depth. Another lap with a 54-d^ee angle thread and with a perfect root radius is made by using the male radius tool used to cut the root radius of the gage. The top of the thread of this lap is then cut away. This lap finishes the top radius of the thread of the gage. The laps, if made as described, will lap only one part of the thread at a time, which enables the gage-maker to control the various dimensions. It has been found by ei^eri^nce that, after lapping the angles and root radius of the thread, a swelling is formed near the top of the thread by the distortion of the metal, as indicated by the right-hand view. The ridges are removed from the thread by means of a lap cut with a very shallow thread, that is, using the top male radius tool to cut a lap just as deep as the radius of the curve at the top of the thread. Machines used for Lapi^ng Thread Gages. — The rotary motion required for lapping thread gages may simply be a hand-turning movement, or it may be derived from a power- driven machine. Laps are frequently turned by hand, but where a large amoimt of this work is done, power-driven lapping machines are often employed. The machine used for this work may be a standard machine tool such as a lathe, or a special design. Special lapping machines used by the Greenfield Tap & Die Corporation, Greenfield, Mass., are designed to give the gage a reversing rotary movement through a fractional part of a revolution, as this is considered the best method of lapping. One machine is driven by open and cross belts which operate on pulleys that are alternately connected with the spindle by a dutch for reversing the direction of rotation. The gage to be lapped is held in a chuck and the lap itself is held by hand. Another design of machine is double-ended so that two gages can be lapped at the same time. The spindle of this machine is given a rocking movement by means of a crank and connecting- rod. Fig. 20 shows how an ordinary engine lathe may be used for lapping thread plug gages. The lap in this case is in the form of a split cast-iron ring which has a handle projecting from one side. While the lap is held with the left hand, the right hand is used for reversing the rotation of the lathe ^indle. CHAPTER Vm LEAD AND DIAMETER MEASUREMENTS ON THREAD GAGES Wbile precision methods of cutting and finishing thread gages are essential, it is equally necessary to provide adequate means for measuring the sizes of thread gages and for detennining the magnitude of whatever errors may exist. The effective or pitch diameter is the most important diameter measurement, and the usual method of taking this measurement is by using wires in conjunction with some form of micrometer. An ordi- nary micrometer and the three-wires are often used, but where gage measuring is done frequently, the " floating micrometer " has proved very successful. The common method of using wires in conjunction with an ordinary micrometer is to place two in contact with the screw thread on one side, and a third wire in a position diametrically opposite. Frequently, the wires are held in place by elastic bands which are placed over the outer ends. If the vires are small, howevN, they may be 202 GAGE DESIGN AND GAGE-MAKING bent somewhat, thus causing an inaccurate measurement to be obtained. Floating Micrometer for Diameter Measurements. — Some form of micrometer moimted upon a compound sBde so that it can move freely, either in a direction parallel or at right angles to the axis of the screw, is preferable for measuring thread gages by the wire method. The floating micrometer, designed by the National Physical Laboratory of England, is illustrated in Fig. i. The screw to be measured is held in a horizontal position between ANVIL ANVIL 1^ i 1 !■ ePINOLE VJ M SPINDLE A B Fig. 3. Method of measuring EflFectiTe and Core Diameter when uaing a Floating Micrometer adjustable centers, and the micrometer is mounted upon a com- pound slide so that it is free to traverse parallel to the axis of the screw and at right angles to this axis. The micrometer is also held constantly in the right-angle position. The slides are equipped with roller bearings consisting of small cylinders rest- ing in V-ways so as to prevent excessive friction, but at the same time allow sufficient friction to keep the carriage in any given position. Measuring EflFectiye and Core Diameters. — Whoi using the floating micrometer for measuring the effective or pitch diam- eter, it is considered preferable to use only one wire on each side LEAD AND DIAAIETER MEASUREMENTS 203 of the screw instead of having two on one side and one on the other, as is the usual practice. The diagram -4, Fig. 2, shows these wires or " needles " and their positions relative to the anvil and spindle of the micrometer. The two wires may be used with this instrument because the micrometer is securely held at right angles to the asis of the screw. These wires are sus- pended from the light supporting member located just above the gage being tested. The gap or space between the spindle and anvil may be adjusted to suit the diameter of the gage, the zero of the reading being fixed by using Johansson blocks. When the core diameter of a plug gage is to be measured, small triangular-shaped pieces are used instead of wires, as illustrated by diagram 5, Fig. 2. This particular illustration shows how the core diameter of a Whitworth thread is measured. The triangular pieces are of such a form that they bear at the bottom of the thread groove, and as their width is known, the core diameter equals the micrometer reading minus the sum of the width of the two pieces. The following formulas may be used to obtain the correct micrometer readings for different standard threads. In these formulas: M = the micrometer reading or measurement over the wires; D = the standard outside diameter of the screw; P = the pitch of the thread; and W = the diameter of the wires used. U. S. Standard Thread: M =' D - 1.5155 P + ^W. Whitworth Thread: M = D - 1.6008 P + 3.1657 W. British Association: M = D — 1.7363 P + 3.4829 W. Lowenherz: JIf = Z) — 1.75 P + 3.2359 W. Sharp V: M = Z>- 1.732 P + 3W. The formula given for the U. S. standard thread may also be used for the A. S. M. E. standard and the French and Inter- national standard. Determining Error in Thread Angle by Three-wire Method. — The angle of a gage thread may be tested by comparison with a standard gage which is known to be correct, or it may be determined by measurement and calculation. In either case, 204 GAGE DESIGN AND GAGE-MAKING wires of two diameters are employed. The diameter of the small wires for a U. S. standard thread is usually approximately 0.6 times the pitch of the thread to be measured, and the diam- eter of the large wires about 0.9 times the pitch. The small wires must, of course, be large enough to extend above the top of the thread and, on the other hand, the large wires shoiild bear at some point below the top or upper corner. If a standard plug gage is available, the difference between the measurements over the two sets of wires is first determined; the part to be tested is then measured to see if there is the same difference in measurement. When no standard plug is at hand and the accuracy of the thread is to be determined by measure- ment and calculation, the total difference between the measure- ments over the large and small sets of wires is first determined. If the thread is a U. S. standard or any form having an angle of 60 degrees, the difference between the two measurements referred to should equal three times the difference between the diameter of the wires used. Assume in this case that the wires are o.i 16 and 0.076 inch in diameter, respectively. The difference between the diameters equals o.iiiS — 0.076 = 0.040. Hence, the differ- ence between the micrometer readings for a standard angle of 60 degrees equals 3 X 0.040 = 0.120 inch. If the angle is incorrect, the amoimt of error may be deter- mined by the following formula, in which A = difference in diameters D and d of wires used (see Fig. 3) ; B = difference in measurements M and tn over the wires and is equal to twice the dimension x; a = one-half the included thread angle. A Sin a = B -A This formula applies to any thread, regardless of angle. To illustrate the use of the formula, assume that the diameter D (see Fig. 3) of the large wires is 0.116 inch and of the small wires, 0.076 inch, as in the preceding case. It will be fm'ther assumed that a measurement over the two sets of wires shows LEAD AND DUMETER MEASUREMENTS 20S a total difference of 0.122 inch instead of the correct difference (0.120 inch) for a standard angle of 60 degrees when using the sizes of wires mentioned, as was previously determined. The difference between the wire diameters equals 0.116 — 0.076 = 0.040; therefore, Sin a = 0.040 0.040 = 0.4878. 0.122—0.040 0.082 By referring to a table of sines, it will be seen that this value 0.4878 is the sine of 29 degrees 12 minutes, approximately; Fig. 3* FActon required for Determliiliig Angle of Thread by Three-wire Method therefore, the angle of the thread is 58 degrees 24 minutes, or I degree 36 minutes less than the standard angle. Comparator for Pitch Diameter Measurements. — Fig. 4 shows a machine on which the Prestwich fluid gage is employed. While this machine is intended for testing the pitch diameter of threaded work, it is only a comparator which affords a rapid method of ascertaining whether an error exists, and, if so, how much its magnitude may be. It will be seen that the thread gage which is under test is supported below by means of two 3o6 GAGE DESIGN AND GAGE-MAKING accuratdy lapped wires which raise the gage from the tables and at the top there is a contact point with two V-shaped pro- jections which straddle a thread on the work. In using this measuring machine, the thread gage to be tested is put in place as indi- cated, after which the reading of the fluid gage is 'taken. If there is any error in the diameter, this will cause the column of fluid in the gage to assume a position above or below the zero point, as the case may be, thus indicating the amount of error by the graduated scale on the fluid gage. When so desired, this machine may be used for taking actual measurements of the pitch diameter by arrang^g a combination of Johanascoi gage-blocks equal to the pitch diameter of the work and setting the fluid gage to zero from these blocks. After a setting has been made in this way, read- ings are taken from the n|. 4. Pitcb iHunattf MeuDiing thread gages, with the re- sult that the reading of the fluid gage above or below the normal position in which it was set by the gage-blocks indicates the amoimt which the pitch diameter is above or below the required size for the work. Illustration A, Fig. 5, shows the method of setting the com- parator for measuring threaded work of a given pitch diameter. LEAD AND DIAMETER MEASUREMENTS 207 A lapped wire is placed between the gage-blocks and the 60- degree vee which tranamits pressure to the Prestwich fluid gage. After adjusting the height of the column of the fluid gage to the zero point for the setting shown at A, two lapped wires are placed under the thread gage, as indicated at B, and one of the threads of the gage is passed through the vee. Then, if the work is accurate, the column of liquid in the fluid gage should return to the zero point, as it was for the setting at A. Testing Thread Gages for Lead Errors. — As accuracy of lead or pitch is very important in a thread gage, various methods have been developed for measuring the lead or for detecting lead errors. A simple form of lead gage consists of a sheet- metal templet having V-shaped projections which conform to the shape of the thread groove and are spaced in accordance with the irftch or some multiple of the pitch. A amilar type of 2o8 GAGE DESIGN AND GAGE-MAKING gage is provided with disks having V-shaped edges. When using these gages, the test is made by simply observing (possibly through a magnifying glass) how closely the screw thread fits the gage. Sometimes a gage is formed of two blocks having V-shaped ends. These blocks are accurately finished and are spaced the right distance apart by using Johansson gage-blocks. Even when gages of the general type referred to are very accu- rately made, there is more or less imcertainty in regard to the results on accoimt of the personal factor. In order to obtain a more sensitive and reliable method of testing the lead, and one that is less dependent upon the judg- ment or perhaps the vision of the worknaen, many lead-testing instruments have been developed which have some form of multiplying device or sensitive indicator combined with contact points that engage the thread. One general method of arrange- ment is to have one contact point fixed and the other movable. Any motion of the movable point is considerably magnified by one or more multiplying levers and is indicated by the position of a pointer or indicating hand relative to a graduated scale. In connection with this subject of lead testing, it is well to remember that errors in screw threads may be either variable or progressive. If the pitch from thread to thread varies in an erratic manner, possibly on account of distortion in hardening or lack of proper seasoning, the error will be variable. On the other hand, if the error is practically uniform and gradually increases throughout the length of the screw, it will be a pro- gressive error. While the principal error may be either variable or progressive, both kinds of error are combined in every screw thread, although in some cases they may be so small as to be negligible. When a thread gage has a decided variable error, it is apparent that an error between two threads may be offset by an error of the opposite kind occurring in other threads farther along the screw, so far as the total error in a given length is concerned; consequently, if the lead is only tested at points some distance apart, there may be variable errors which are not detected. For this reason, different forms of lead-testing appa- ratus have been devised for checking the accuracy of the entire LEAD AND DIAMETER MEASUREMENTS 209 screw in case such refinement is necessary. One general method of testing the lead of a gage from thread to thread is to mount the gage on centers and then bring into contact with the differ- ent tiu'eads, a point or stylus, the axial motion of which is indi- cated in different ways, as explained later. One type of pitch- measuring instrument designed on this principle has work- holding centers moimted on a stiff bed and a saddle which is free to move parallel to the axis of the screw. The motion of this saddle is indicated by a micrometer head. On the saddle Fig. 6. Mlciometar Tjpe of Lead-toMiuc MmUh* there is a pointer accurately ground to fit the thread groove and arranged for adjustment at right angles to the screw. The pointer is first set to coincide with the thread and a reading is taken. It is then withdrawn and the saddle is adjusted by the miaometer until the pointer is opposite the next thread. In this manner a series of readings are obtained. As there are many different types of gages and instruments for measuring the lead or pitch, no attempt will be made to describe them all, the idea being to refer to a relatively small number of designs which illustrate different general types. Micrometer Type of Lead-testing Machine. — The first lead- testing machine to be described in detail is of the general type having two contact points which may be engaged with the thread. 2IO GAGE DESIGN AND GAGE-MAKING These contact points are mounted upon an adjustable slide, the movements of which can be measured either by a micrometer head or by means of gage-blocks. This machine, which is shown in Fig. 6, is not only used for testing the lead but the angle of the thread. It has a heavy bed -4, and a stationary head 5, which carries a fixed center O and is permanently attached to casting A. The sliding tailstock C can be clamped positively to the bed Ay by means of the two clamping screws Q^ and carries a spring center P, which can be withdrawn by the knob E or clamped in position by the screw F. The gage H to be tested is put between centers by placing one center upon center O, bringing up the tailstock C, clamping it by screws Q, and allowing the center P to enter the opposite center of the gage, as it is forced outward by a spring inside of C. Center P is clamped in position by handle F. The gage between the centers is thus held in correct alignment with the measuring devices, which are carried on the slide D. This slide is carefully fitted to the bed A and is clamped into the posi- tion required by the thumb-screw G. It consists primarily of a compound slide which is carefully and accurately made, so as to be in correct alignment with the axial line through the centers and P of the measuring machine, all planes and surfaces being either accurately perpendicular or accurately parallel to this axis. The lower member of the compound slide K is adjustable in toward and out from the line of centers through and P, and may be clamped by screws L in whatever position is suitable for the diameter of the thread gage to be measured. This slide carries a micrometer head /, which operates against a hardened stud R in the upper member N of the compoimd slide. The upper part N is free to move parallel with the axial line through the centers O and P. This movement can be measured by the micrometer head /, or by the use of distance blocks or Johansson gages placed between the point of the micrometer (which is clamped in position) and the point i?, on the upper member N. The latter carries two angle test-pieces, which are held by the spring clip M. This spring clip permits adjustment by finger pressure toward or away from the gage or thread to be tested. The center-to-center distance between the angle test-pieces / is LEAD AND DIAMETER MEASUREMENTS an just one inch, which is a basis for checking error of lead in gages that are to English measurement and are over one inch long. For metric screw threads or threads less than one inch long, dther the micrometer or proper distance gages are used between the point of the micrometer and the stud R on the ^ide. Fig. 7 which represents a view through a magnifying glass, shows the points of these angle test-pieces on an enlarged scale, and it can be seen how they fit the angle of the threads on the gage, which is shown in position. Method of Testing Lead. — When using the machine shown in Fig. 6 for testing the lead, the first operation is to turn the thread g^e on the centers of the machine unUl the thread will allow testing point a (see diagram A , Fig. 8) to engage it as shown at B, thus shutting out light on both sides of the conical point This is done merely by rotating the gage by hand in between the centers of the machine, and gives it a very fine adjustment. After point a has been pressed down lightly in the direction of the arrow, point b is brought down in the same direction until it 212 GAGE DESIGN AND GAGE- MAKING touches the thread. If the lead on the thread gage is short, the effect will be as shown at C, which has been exaggerated to illus- trate the point clearly. If the lead on the thread gage is long, the effect will be as shown at D. The problem now is to determine the amount that the lead on the thread gage is long or short. With the micrometer adjusted against point R (see Figs. 6 and 7) and set at zero, point a is drawn back from the threads as shown at £, Fig. 8, Fig. 8. Diagrams illnttratiiif Methods of adjuatiiig Testing Points and Ikow Lead Brrors are detected and measured which permits moving point b in either direction. If the lead on the gage is long, as at Z>, point h is moved in the direction of the arrow as at £, by means of the micrometer screw, and when it is opposite the thread it is moved down in the direction of the arrow as at F. The amount that it has moved over (as indicated by x) is the extent that the lead of the gage is long in one inch. If the lead is found to be short, the order of operations is reversed. The amount x could be measured either by the mi- crometer direct or with a gage-block placed between the microm- eter point and the point R. If the thread on the thread gage is LEAD AND DIAMETER MEASUREMENTS 213 not over one inch long, only one of the measuring points is used, and in this case either point a or 6 is engaged with the screw thread wherever the lead is to be checked. The distance x as measured either by the micrometer or precision gage-block is compared with the nominal or correct distance, and the differ- ence is the error in lead for that number of threads. Testing Angle of Thread. — The methods of testing the angle of the thread with the machine shown in Fig. 6 are indicated by Fig. 9. These views simply show the ends of the angle test gages Fig. 9. Vaxloas Screw Thread Del ecto and the contour of the thread in the gage where it is being tested. Sketch A shows a standard test angle gage fitted into a thread that has too large an angle. After testing it with the standard plug, a plug with a greater angle is tried, as shown by the dotted line, and in this way the amount of angular error can be ascer- tained. Sketch B shows a gage having a thread angle that is too small. A thread with unequal angles is shown at C, and one having concave sides, at D. These errors show up prominently by the amount of light that passes between the angle test gage and the thread. Threads are often rounded over near the top, as at £, as the result of excessive lapping with an old-style lead lap. 214 ^AGE DESIGN AND GAGE-MAKING Sketch F shows how a thread having unequal angles, as at C, can be tested to ascertain the amount of angular error, by placing different angle test-pieces against each side of the thread walls. ' The angle test gage shown by the fidl line serves to test one side of the thread wall, while the angle test gage shown by the dotted line tests the angle on the other side. If the standard angle were 60 degrees and the test-bar shown in full lines were 58^ degrees, whereas the test gage as shown in dotted lines were 6i| degrees, this would show that the thread was "tipped " or leaned over J degree, while the included angle of the thread might be or might not be standard. Diagram G shows how the angle test gage dis- closes that the bottom of the thread groove is too wide, the root diameter being too large. A Whitworth form of thread is illustrated at H, which is correct on the angle and at the bottom. Sketch / shows a Whit- worth thread that has too large a radius at the bottom of the thread, and J shows a Whitworth thread that is correct as to angle, but is cut too deep and has too small a radius at the bottom of the thread groove. Fig. 9 is merely intended to show, in a somewhat exaggerated degree, a few of the diversified defects that this angle and lead-testing machine will detect. This apparatus was developed by the H. E. Harris Engineering Co., Bridgeport, Conn. It has been used by the Bureau of Standards and in gage manufacture. Lead Tester equipped with Micrometer and Dial Gage. — The lead-testing machine, the front and rear views of which are shown in Figs. 10 and 11, has a single pointer which engages the thread, and a micrometer head and dial gage which are used in combination to indicate measurements and errors. The thread gage is supported between centers and a plug which is accurately ground at the point to fit the threaded work is supported by an arm at the back of the machine. This arm is carried on a slide, so that its position may be adjusted for handling any work which comes within the range of the machine, and the holder in which the plug is carried has rack teeth milled on its under side to engage a pinion that is turned by means of a knurled thumb- screw. In this way, the position of the holder may be adjusted LEAD AND DIAMETER MEASUREMENTS 315 to bring the point of the plug into position for testing threaded work of any diameter up to 6 inches; the machine also has a capacity for handling work up to 6 inches in length. When the lead tester is in use, the arm which supports the pointed plug that engages the thread to be tested is securely clamped to the slide, and the holder in this arm is also clamped in position. When it is desired to change the pointed plug from one thread to another on the work, this is accomplished by draw- ing the plug back in the holder by means of the small knurled extension which will be seen at the top of the holder in the rear view of the machine. L«4d Taitei eqiiipp«d with BUciometer and Dial Gage At the end of the machine there is a micrometer head which has a large graduated wheel moimted on it so that readings may be accurately taken to o.oooi inch. The spindle carried by the micrometer head is extended to ei^age the plunger of a dial test indicator, and after the pointed plug has been set in position in a thread, the micrometer spindle is advanced to engage the indicator, after which the reading of the indicator is carefully noted. It may be considered desirable to set the indicator needle back to the zero position on the dial after the plunger has been engaged by the micrometer spindle, although this is not really necessary. The next step is to withdraw the pointed plug from the thread and move the plug along so that it may be entered 3X6 GAGE DESIGN AND GAGE-MAKING into the adjoining thread or the second or third thread from the one in which the point was origiiially placed. Referring to the rear view of the machine, it will be seen that the aim which carries the micrometer head and the arm which carries the pointed plunger that engages the threaded work are both at- tached to a sliding bar. The bar itself moves on the bed of the machine, carrying these two members with it, just as if the bar and the anns were a single piece. After the pointed plimger has been reengaged with the threaded work, the micrometer spindle is again advanced into contact ng. II. SflU new of Hachlae •howti In Hg. to with the indicator point and screwed up until the needle of the indicator comes back to the same portion which it occupied after the first setting was made. Then, if the pointed plug was ei^aged with the next thread on the work, the difference in readings of the micrometer head between the first and second settings should correspond with the lead of the thread, provided there is no error. In case the point was entered into the second or third threads from the one in which it was originally placed, the micrometer reading should be two or three times the lead, as the case may be. The large graduated dial on the micrometer head has gear teeth cut around it meshing with a long-faced pinion that transmits movement to an automatic counter. This coimter is arranged so that it records each interval of 0.0001 LEAD AND DIAMETER MEASUREMENTS 217 inch, thus making the apparatus direct-reading and saving the time which would be required in taking readings of the microm- eter. As a result, it is possible to test threaded work very rapidly with an apparatus of this kind, and the method of opera- tion is so simple that the use of this lead-testing machine re- quires little experience. Use of Gage-blocks for Measuring Lead. — When so de^red, Johansson gage-blocks may be used in connection with the ma- chine shown in Figs, lo and ii for checking the accuracy of the lead of any threaded work. Figs. 12 and 13 show two posable methods of procedure. As shown in Fig. 12, a gage-block is used, the thickness of which represents some exact multiple of the lead of the thread. The pointed plunger is introduced into one thread on the work, after which the micrometer head is screwed up against the gage-block which is introduced between the micrometer spindle and the plimger of the dial test indicator. The reading of the indicator is then taken, and if it is considered necessary, the needle may be set back to the zero point. The next step is to lift the pointed plimger out of engagement with 2l8 GAGE DESEGN AND GAUE-MAKING the work and move it along so that it may be entered into a groove separated from the groove in which the first setting was made by a number of threads that is equal to the multiple which the gage-block is of the thread lead. The micrometer spindle b then directly in contact with the plunger of the indicator, and it should bring the needle back to the same position which it occu- jaed for the original settii^. If there is any error in the lead of the thread, this is indicated by the micrometer when the indicator nzed stop in n«c« of iDdicatw has been brought back to the required portion. It will be apparent that, by this method, the Johansson gage consUtutes a distance piece, and saves time which would otherwise be required to screw the micr(nneter spindle through a considerable distance each time the change of setting was made. By the other method of usmg a Johansson gage-block, the dial test indicator is dispensed with and a fixed stop substituted in its place, as shown in Fig. 13. The pointed plunger is intro- duced into a groove in the threaded work, and the micrometer spindle is then screwed up to just come into contact with a gage- block held against this fixed stop. After this has been done, the LEAD AND DIAMETER MEASUREMENTS 219 micrometer reading is noted and the pointed plui^er is moved over and introduced into the proper thread, so that, with the Johansson gage- Fi(. 15. AootliM Lead-teitliiB Uachlna of the Trp« aliowii la Fi(- 14 block removed, the fised stop and micrometer spindle will be in contact. If there is any error in lead, this is indicated by the micrometer reading when screwed up into contact with the gage-block. As an alternate 220 GAGE DESIGN AND GAGE-MAKING method, the micrometer spindle can be locked and the testing done with Johansson gage-blocks, using the dial indicator to determine the amount of error. This lead tester is made by West & Dodge Co., Boston, Mass. Lead-testing Machine having Multiplying Indicator and Size- block Adjustment. — Tl^ lead-testing machine illustrated in Fig. 14 is provided with a multiplying indicator carried by a slide which may be adjusted in a direction parallel to the axis of the screw, this parallel movement being measured by the use of size- blocks. The cast-iron bed of this machine has two parallel dovetail bearings on the top. Upon the rear and larger bearing are mounted the centers for holding the gage to be tested, and upon the front bearing is fitted the sliding block which carries the indicator slide. The indicator is held in place by a small hard- ened stud, which is clamped in position with a headless set-screw. By mounting the indicator in the proper bushing, the operator IS assured of a full bearing of the slide in the block when testing a gage thread. The indicator slide has a knurled stop-screw and lock-nut to insure that the indicator ball point will make contact with the angle of the thread at corresponding points after being set in contact with the first thread. A better idea of the machine may be obtained by considering how it is actually used. The gage to be tested is mounted be- tween the centers as shown, the heads being clamped in the required positions by means of knurled binding screws at the rear. A Johansson block is next inserted between suitable contact points to obtain a zero reading. One contact point is on the sliding block and there are two others in the steel plate seen projecting up from the end of the bed. The indicator slide is now moved toward the gage until the ball point of the indicator comes into proper contact with the ade of the thread; the stop- screw is then set and locked. The gage is next turned slightly, or is adjusted longitudinally by releasing one center and advanc- ing the other. The adjustment of the centers is obtained by means of the threaded center spindles which have knurled heads. This turning or lengthwise adjusting movement of the gage is continued until the indicator needle is in the ssero position. LEAD AND DIAMETER MEASUREMENTS 221 Assume, by way of illustration, that the gage has eight threads per inch, and that another Johansson block measuring 0.125 "^ch is added to the one between the gage pomts. Then, if the lead of the g^e thread between these points is perfect, the indicator needle should stand at zero when the baU end is again brought Fig. 16. Flnld-caie Type of L««d-te*tliif Hachliw into contact with the thread. If there is even a slight error, the change in the portion of the indicator needle will make this apparent. By using other combinations of gage-blocks, the thread may be tested at various points. The indicator is graduated to read to o.oooi inch with standard length ball points and o.ooc»s inch with short points. Several sizes of ball points are provided in both lengths, smaller points being used for the finer pitches. The hardened and groxmd test- 322 GAGE DESIGN AND GACE-MAEING bar seen at the front of the machine in Fig. 15 is provided for checking the alignment of the two parallel dovetail bearings to detect any wear which might affect the accuracy of the tests. This machine is the de^ga of the Sheffield Machine & Tool Co., Dayton, Ohio. Fluid^^e Tjrpe of Lead-testing Machine. — Fig. 16 shows a lead-testing machine which is equipped with a Prestwich fluid gage for indicating errors. In this particular illustration, the machine is sliown as arranged for testing the accuracy of the ring thread gage, and Fig. 17 illustrates how it is used for measuring the lead of plug gages. The fluid gage is supported on a bracket carried by horizontal bar A (Fig. 16), the arrangement bdng worked out in such a way that the entire gage can be pushed in LEAD AND DIAMETER MEASUREMENTS 223 a direction parallel to the machine bed, this movement being against the pressure exerted by a compression spring. In using this lead-testing machine, the method of procedure is briefly as follows: Carried on bar B there is a steel stylus point C, which is shaped to fit into the thread on the work. Pin D engages a notch cut in bar B, and by rocking bellcrank E about pivot Fy pressure is imparted to the diaphragm of the Prestwich fluid gage. With point C set in position in a thread of the gage, the inspector carefully notes the level of the liquid in the gage, and after this has been done, he lifts point C out of the thread and places it in either the next thread or in any convenient position. Then a Johansson gage-block G is selected, which is either equal to the lead of the thread or to some multiple thereof, according to whether the pin was placed in the next thread or several threads along the gage. After the Johansson gage has been put in place, the compression spring pushes sliding bar A back so that it is firmly in contact with the gage. It will now be apparent that if the lead of the thread is accurate, the reading of the Prest- wich fluid gage should be exactly the same as it was before, when the stylus is again brought into contact with the thread. In the case of any error in lead, the level of the liquid in the fluid gage will be changed due to pressure applied by bellcrank £, which will be rocked about pivot F because of the changed position of point C; and the graduations on the fluid gage will then indicate the amoimt of error which exists in the lead of the thread gage. When testing a plug gage, the method of procedure is exactly the same as that which has been described, except that a point carried by the bellcrank £, enters directly into the thread on the work, instead of entering a notch cut in the extension bar that is required to reach into a ring gage. This machine is made by the Blair Tool & Machine Works, Inc., New York City. Lead Tester having Sliding Contact and Magnifying Reflector. — A machine that has been used by the National Physical Laboratory of England for detecting lead errors is so arranged that the magnitude of the errors is indicated by the position of a spot of light relative to a scale. The thread gage, or whatever screw thread is to be tested, is moxmted on centers and the ma- 224 GA^'E DESIGN AND GAGE-MAKING chine has a saddle that is free to slide parallel to the axis of the screw. This saddle carries a bent lever at one end of which there is a small spherical ball that comes into contact with the thread to be tested. This spherical shaped end is held against the thread by a light spring, and it always remains in contact with the screw as the saddle is traversed along the bed. The ball is too large in diameter to touch the bottom of the thread, and it slides down one slope or flank aiyi up the next one as the saddle moves along. The change of motion as the downward move- ment of the ball is arrested by contact with the opposite flank, is sharply defined and the pitch is measured by noting when these changes of motion occur. A sensitive indication of the move- ments of the ball or stylus is obtained by attaching a mirror to the arm or lever upon which the spherical end is formed. The move- ments of the ball are transmitted to the mirror from which a spot of light is reflected onto a scale. This spot of light moves in one direction, then stops, and moves backward, after which the motion is again reversed. These points of reversal, which occur as the ball passes from one slope of the thread to the adjacent slope, are clearly defined and enable the pitch to be measured by means of a micrometer head which is used for measuring the movements of the saddle. Lead-measuring Machine equipped with Sliding Contact and Magnifying Pointer. — The principle of using a contact point or stylus which slides over the threads and indicates by the move- ments of a spot of light when the stylus is at corresponding points on different threads, is embodied in the lead-measuring machine to be described. This machine, however, differs from the light reflecting type in that it has a magnifying pointer which indicates the different positions of the stylxis relative to the screw thread. This machine is another development of the National Physical Laboratory of England. A design capable of measuring screw threads up to 3 inches in diameter and 8 inches in length is shown in Figs. 18 and 19. The bed of this machine carries a sliding bar A , to which is attached a micrometer head at £, and a saddle which supports the indicator. The gage or other screw thread to be tested is mounted between the centers, the center LEAD AND DIAMETER MEASUREMENTS 225 I 1^ ■-••■'■itv.v, 226 GAGE DESIGN AND GAGE^-MAKING LEAD AND DIAMETER MEASUREMENTS 237 to the left being adjustable. The sUding bar B is mounted upon two groups of three balls each, which are located at C and D. The spindle E of the micrometer head has a roimded point which may be brought into contact with the anvil at the end of the right-hand center. The indicator saddle may be adjusted rela- tive to slide A by screw P, and provision is made for clamping it after adjustment. The indicator is attached to a cross-slide Plf. 10. Cro M wctiopal View ol Umchln* ■bown In Vlff. iS tnd 19 which is moved at right angles to the line of centers by turning screw G. llie stylus H (see cross-sectional view, Fig. 30) is attached to one end of lever /, which is supported by a thin strip of steel J and the pivot K. The of^wate end of leva: / is forked and embraces lever or pointer S a short distance above its pivots. The pivots of pointer 5 are carried by a cylinder which, in turn, is pivoted on a line parallel to the centers. The steel strip / not only acts as a pivot for lever /, but allows it a little lateral morion. This strip of steel is straight when the stylus is 228 GAGE DESIGN AND GAGE-MAKING in a central position, and it simply bends when the stylus is moved backward. If the stylus is shifted to the right or left, the steel strip twists and acts as a vertical fulcrum. The pointer magni- fies this movement about three hundred times. The diagram, Fig. 21, shows the stylus in three different posi- tions relative to the thread and illustrates how these positions are indicated by the pointer. The direction of movement for the sliding bar A (Fig. 18) is indicated by the arrow a in this illustration. When the stylus is in contact with the left-hand a """-^-<^^ i "" "~!t Fig. 21. PositioiiB of Stylus reUtiye to Thretd, and DiAgram thowing Path followed by Pointer slope of the thread, as at Z, the pointer is moved to the left, whereas contact with the right-hand flank, as at Ry causes the pointer to be deflected to the right. When the stylus is in con- tact with both slopes, as at C, the pointer is zero and the pressures on both slopes of the thread are equal. When this occurs a reading is taken. Then, as the micrometer movement continues, the stylus slides up one thread over the crest and down the follow- ing slope until it again comes to the central position when an- other reading is taken. In this way the lead is checked. The machine previously referred to is operated on the same general principle, except that the positions of the stylus relative to the thread are indicated by a light spot reflected from the mirror. The diagram at the lower part of Fig. 21 shows the LEAD AND DIAMETER MEASUREMENTS 229 rectangular path followed by the pointer S (Fig. 18). Thus, when the stylus slides down the right-hand slope, the pointer follows the path 1-2; the path 2-3 indicates the period when the stylus is in contact with both slopes; the line 3-4 indicates the movement up the left slope; and the line 4-1 indicates the movement of the stylus over the crest of the thread. Different contact points are provided for Whitworth threads of various pitches, and are indicated by letters. When testing the lead or pitch of a screw, the micrometer head is first turned to the left to take up the lost motion, and then far enough to the right to bring the stylus into engagement with the first thread to be tested, the adjustment being such as to locate the pointer in the zero position. A reading is then taken, and the same operation repeated for each thread to be tested. The results may be plotted on squared paper and the diagram for a perfect screw would show all points on a horizontal line at equal distances apart. In actual practice, the curve will be somewhat irregular, there being an upward slope when the pitch is too long, and a downward slope when it is too short. Improved Design of Optical Lead-measuring Apparatus. — The construction of the optical indicating apparatus to be de- scribed is based upon designs furnished to the Bureau of Stand- ards, Washington, D. C, by the National Physical Laboratory of England. The illustrations Figs. 22 and 23 show machines set up for testing a plug thread gage and a ring thread gage, respec- tively. It will be seen that thread gage X which is to be tested is supported between centers carried by a fixed frame L, these centers being adjustable to carry thread gages of different lengths. A stylus A enters the thread groove, this stylus being carried by a spring support secured to carriage T which is free to move longitudinally. When such a movement of the carriage occurs, the stylus swings on its spring support about a fulcrum at the point of the stylus which is in contact with the thread groove. At its rear end, stylus A carries a small projection lens Dy through which light is transmitted from a straight line fila- ment F. The image of this incandescent filament passes through lens D and then, by means of two suitable prisms located beneath 330 GAGE DESIGN AND GAGE-MAKING tlie table, this image is projected up onto ground gl&ss G. Hme is a line scribed on this ground glass with which the image of the filament may be brought into coincidence by moving canine T, Mounted at the right-hand end of carriage T there is a microm- eter head which reads to 0.0001 inch, and carriage T is controlled by a couaterwdght so that the micrometer ^indle is always held in contact with an accurately ground ball S at the rear end of the Fit' II- ImproTSd Daalgn of Optical L««d-teitin| Appantoa right-hand center. In testing the accuracy of lead of a thread gage with this apparatus, the gage is first set up between the centers and then the stylus point is entered into a thread groove, after which the image of the incandescent filament is brought into coincidence with the line scribed on groimd glass G. When this result has been accomplished, the reading of the micrometer head is noted, after which the stylus point b withdrawn from the thread and placed in either the next thread groove or any known number of threads from the one in which the stylus was originaUy LEAD AND DIAMETER UEASUSEHENTS 331 located. Belore this change can be made, the micrometei head must be turned back so that the spindle is sufficiently released from the ball S to allow the required travel of carriage T. After setting the stylus in the required thread groove, the micrometer spindle is turned up into contact with ball S, and then by care- fully turning the micrometer, carriage T will be moved so as to bring the image of the incandescent Oament back into coincidence Fig. aj. Taittug Acciu«ct or Ring 0«(8 irith Optlctl Lead' tMtlQg Appantu with the line on ground glass G. If the lead of the thread is accurate, the difference in the miaometer readings between the first and second settings will be equal to either the lead of the thread or some multiple of the lead, according to whether the stylus was placed in the next thread groove to the one originally occupied or several threads farther along the gage. If there is an inaccuracy of lead this will, of course, be determined by subtract- ing the difference of the micrometer readings. 232 GAGE DESIGN AND GAGE-MAKING In using this appaxatus for measuring the lead of ring gagdSf an attachment has to be used which provides for reaching into the interior of the gage. The a^^aratus for this purpose is shown in Fig. 23 and consbts of a stylus £, which has a bent point K which will enter the thread groove. The ring gage is clamped to a faceplate by means of straps U. It will be seen that stylus E is pivoted and connected at its right-hand end to the end of a rod carrying the projection lens through which light from the in- candescent lamp is transmitted. Aside from the special means of adapting the apparatus for testing the lead of internal threads, the method of procedure is exactly the same as that described for a plug gage. For testing both internal and external thread gages, the ratio of magnification of the error is 350 to i (as shown by the failure of the image from filament F to coincide with the line on ground glass G), so that it will be apparent that this machine may be used with a very high degree of precision. This lead-testing apparatus is made by the Arthur Knapp Engineering Corporation, New York City. Electrical Contact Type of Lead-measuring Machine. — The Vidal lead-measuring machine, which is another English design, is so arranged that the position of the stylus or spherical point which comes into contact with the thread is indicated by elec- trical means. The ball-shaped point which bears evenly on the thread, regardless of the inclination of the spindle to which it is fixed, is split and has a layer of mica between the two sections to insulate them. An electrical circuit between the half sections of the ball and the screw is closed only when the ball is touching the slopes of the thread on both sides, which enables the position of the ball relative to the thread to be determined for measuring the pitch. The spindle with the ball is connected to a carrier which, in conjunction with a spring, keeps the ball continually in light Contact with the thread. The contact is so light that the carrier may be easily blown away from the contact position by the breath. The carrier is attached to a micrometer head for giving the pitch measurements. With this type of instrument, any oil or grease is objectionable on the gage as it will interfere with the closing of the circuit. r CHAPTER IX MICROSCOPIC MEASURING MACHINBS FOR TESTING ACCURACY OF CONTOUR AND THREAD GAGES Microscopic measuring machines provide a convenient and accurate means of measuring certain forms of templets and gages which, because of their shape, cannot readily be measured by other forms of precision measuring instruments. The micro- scopic measuring machine is also used to some extent for testing the accuracy of screw threads or gages, etc. Measuring machines of this general type have a microscope provided with cross-hairs or lines which make it possible to accurately locate the microscope with reference to an edge or line on the part being measured. Means are also provided for adjusting this microscope not only in the direction of its axis for focusing, but laterally for setting it to coincide with different points on the work from which meas- urements are to be made. The method of making adjustments for taking measurements varies on di£Ferent types of machines. The adjustment in some machines depends entirely upon the accuracy of a micrometer screw having a graduated dial or disk, whereas other machines are so designed that the adjustment depends either entirely or principally upon the accuracy of gage- blocks or end-measuring rods which are inserted between contact surfaces formed on some fixed part and on the adjustable member. Microscopic Measuring Machines of Type havmg Gage Ad- justment. — The microscopic measuring machine shown in Fig. i is the type that is used in conjunction with end-measuring gages. This machine is shown with a hob between the centers, as it is used for testing tools of this kind and plain screw threads, as well as templets or gages, form tools, or any edge or contour, the accuracy of which cannot be tested by ordinary means. It has a rigid box-shaped bed upon which is mounted a table A , fitted with centers for holding all tools having centered ends. The 233 234 GAGE DESIGN AND GAGE-MAKING center seen at the left has a crosswise adjustment which enables the part being tested to be lined up accurately. When taking length measurements, table A is adjusted along the bed whatever distance may be required. This distance is determined by plaC' ing standard measuring rods or gage-blocks between hardened contact pieces. One of the contact pieces is attached to the end nc. I. TMUng Accniacj of Rob with UicroMopic HmswIbc Hachlna of table A and the other one is fixed to block B. The microscc^ is mounted on a compound slide-rest having movements parallel with, and at right angles to, the table slide. The microscope may be adjusted vertically, of course, for focusing, and it is fitted with two hair-lines. One of these may be rotated with the outside tube, whereas the other one rotates with the eye-piece. The outer tube has a large dial C, which is graduated in half degrees. The eye-piece carries a vernier graduated for taking readings to MICROSCOPIC MEASURING MACfflNES 3$$ one minute, so that the angle between the two hair-lines may be determined with considerable accuracy. The machine described in the preceding paragraph is shown in Pig. 2 fitted with an apparatus for illuminating whatever edge or contour on the work is under observation. The object of using this apparatus is to obtain a better illumination than is afforded by uncontrolled lighting. The rays of light from an electric bulb are directed onto the surface of a mirror located directly beneath the ^de or edge of the work under observation. The li^t rays are reflected upward by the mirror, thus showing the profile of the work In distinct relief when viewed through the microscope. This illuminating attachment is carried by a bracket bolted to the rear side of the bed. The electric bulb is located inade and at the outer end of the tube shown. At the other end of this tube there is a collecting lens, and midway between the bulb and the lens is a metal partition having a small hole through which the light passes and is then diffused by a piece of groimd glass fitted over the hole. The light tube can be adjusted in a length- 236 GAGE DESIGN AND GAGE-MAKING wise direction for varying the strength of the illimunation. The rod which carries the mirror has a rack-and-pinion adjustment for locating the mirror in the correct position relative to the work. Length Measurements. — When making a plain length meas- urement with the machine shown in Fig. i, the work is clamped to the table, which, in turn, is clamped to the bed after a meas- uring rod or one or more gage-blocks has been placed between the two compact pieces. The microscope is then focused on the end or edge of the part to be measured and is adjusted imtil the hair-lines coincide with the particular point from which the measurement is to be made. The table is next adjusted a dis- tance equal to the required measurement, by means of other measming rods or blocks. The position of the hair-line relative to the work then shows whether the distance between the points on the work corresponds to the actual measurement or to the difference between the lengths of the two measming rods. If there is any variation, the amount of error can be determined readily by adjusting the compound slide of the micrometer imtil the hair-lines exactly coincide with the second point on the work; the graduated dial of the micrometer slide then indicates the error. As the hair-lines can both be rotated, they can be set to coincide with whatever edge or profile the measurement is to be taken from. Measurement of Templets. — The gage or templet shown in Fig. 3 is an example of the class of work which may easily be measured by this machine. The particular measurements indi- cated in the illustration, however, would be rather diflSicult to check with any other form of gage or measuring instrument. This templet is clamped to the table with its long sides parallel to the edges of the table and with one of the angular edges or ends of the recess imder the microscope. One hair-line is ad- justed to cover the long side of the templet and the other is located at right angles, the intersection coinciding with the outer corner from which the measurement is to be made. (As- sume, for example, that two end-measuring rods are used having a difference in length of 2^ inches.) The table is next moved along the bed and redamped with the longer measuring rod in MICROSCOPIC MEASURING MACHINES 237 position. Since this gives a movement of 2^ inches, which is somewhat greater than the distance to be measured on the gage, the compound slide is used to obtain the exact adjustment required. For instance, if the measurement is between the outer corners of the high-limit side of the gage, the compoimd slide will have to be adjusted an amoimt equal to 2.5 — 2422 = 0.078 inch. The hair-line should first be set to coincide with the work before taking the reading. If the micrometer is first adjusted to the required dimension and an observation is then made to see if the hair-lines coincide directly with the work, slight errors k M ■2.407- J/ aiT" A T A &885 ?r SL4»' ■X JfoeMiMrir Fig. 3. Gage or Templet measured by Mlcroscoiiic Meararlng Machine in the reading are more likely to occur. By repeating the ad- justments previously referred to, the length of the notch or recess at the bottom can also be determined. The depth of the recessed portion is tested by simply setting one hair-line to cover the outer edge and then adjusting the compound slide imtil this hair-line coincides with the inner edge, the reading between the two positions being noted. When setting the long sides of the gage or templet parallel to the table, the hair-lines may be used to advantage. One of the hair-lines is set parallel with the table edge, and at the same time the templet is held by hand so that it also coincides with this edge. The table is then moved slowly in an endwise direc- tion, and by adjusting the hair-line and templet with the table 238 GAGE DESIGN AND GAGEr-MAKING movement, a parallel setting can be obtained. After the templet is clamped to the table, its position should be tested again. The form of templet shown in Fig. 4 is another interesting example of the class of work for which the microscopic measur- ing machine is adapted. In this case, a measurement equiva- lent to 2.6043 inches is taken near the upper end of the templet. The measurement is taken between two points along the curved sides instead of attempting to measure from one comer of the gage to the other, which is difficult, especially when, as in this case, the corner is formed by the intersection of a curve and a straight line. As the illustration shows, the measurement is jfodMiMfy Fig. 4. Anotiier Ezample of Gage or Templet Measurement taken at points located 5 inches from the base of the templet. It is necessary first to determine this dimension (2.6043 inches) by calculation. The templet is then carefully set with its two straight sides parallel with the edges of the table and with the shorter side under the microscope. The hair-lines are set at right angles to each other, and with one of them covering the edge or short side. The microscope is then moved over 0.190 inch in this particular case, and an ordinary length measure- ment is taken from one ciurve to the other by the method pre- viously described for plain length measurements. The width of the base is measured without any calculation, as the tangent to the curve is at right angles to the base line, so that it is easy to set the hair-lines to coincide with these edges. One hair- line is adjusted to coincide with the base or lower edge, and the MICROSCOPIC MEASURING MACHINES 239 Other one is set at right angles. The length is then measured in the usual way. The machine illustrated in Fig. i is made by Alfred Herbert, Ltd., Coventry, England. It will measure lengths up to a piflTrimiim of 12 inches, and the microscope has a magnihcation of 25 diameters. Contour- and RxuUus-measuring Instrument. — The accurate measurement of contour and radius gages or templets is often nc. s- PI*" Vlaw of Conlrar- and Badina-ineaiuriiig Instnunent very difficult, if not impos^ble, when ordinary measuring appli- ances are utilized. The contour- and radius-measuring instru- ment to be described is designed especially for the accurate measurement of irregular profiles, circular edges, or various combinations of strai^t and curved sections. This instrument was deigned by J. H. Wilhelm for the Frankford Arsenal. An instrument of this general type was required for measuring the contour of rifle bullets (which must conform within close limits to a given dze and shape), but the instrument as finally made 240 GAGE DESIGN AND GAGE-MAKING is a universal type that is applicable to all kinds of profile measurement. This instrument (see Figs. 5 and 6) is equipped with a micro- scope having cross-hairs or lines which are adjusted to coin- cide with the different edges and intersecting points on the part being measured, and these hair-lines, in conjunction with the graduations and Johansson gages used, indicate the amount of error or enable measurements between different points to be taken. The microscope A^ Fig. 5, is mounted upon a cross- . arm or carriage B^ which is pivoted at each end to holders C. The fulcrum pins are adjustable along the holders, and they are accurately located in any desired position by Johansson blocks. These pins are reduced one-half their diameter, and by inserting different combinations of gage-blocks, any radius from o up to 6 inches can be measured. For instance, if 0.020-inch blocks are inserted in each holder, a radius of this dimension can be measured accurately. The gage-block holders are pivoted to brackets D attached to slide £, and the outer ends of the holders are connected by a tie-rod. The horizontal slide E provides a lateral adjustment of six inches, the micrometer and the straightedge at F (which is integral with slide E) moving in parallel planes. This adjustment is effected by a rack and pinion operated by handwheel G. The screw ff is used for making fine adjustments after a split nut is engaged with it. This fine adjustment is indicated by a scale and vernier at J, and Johansson blocks may also be inserted at K when a very accurate movement of the slide is necessary. The microscope is carried by a slide which can be adjusted at right angles to straightedge F. There is a total movement of two inches in this direction, and either Johansson blocks at iV or a scale and vernier may be used when making the adjust- ment, which is effected by screw Af. The circular movement of the microscope and its carriage is indicated by dial O, which is graduated to 90 degrees on each side of the zero mark, or to 180 degrees in all. By means of a vernier scale, direct readings to 5 minutes can be taken. The disk is shown in the zero posi- tion in Fig. 5 and in the 90-degree position in Fig. 6. It is held MICROSCOPIC MEASURING MACHINES 24 1 positively in the zero position by lock-pin P, and it may be damped in any other position by damp screw Q, When the lock-pin is in place, the hair-line on the microscope coinddes with a plane which is at right angles to the straightedge F and intersects the center line R on this straightedge. These different adjustments and means of measurement make it possible to determine very accurately the movements of the microscope, whether the direction of motion is parallel to straight- edge F, at right angles to it, or along a circular path. The microscope is provided with six objectives, the three shown in position being interchangeable with three others. By using these different objectives, the magnification may be varied from 20 to 100 times. In some cases, high magnifying power would not be suitable, because irregularities in the edge or contour of the part being inspected would be enlarged to such an extent as to make it difficult to follow readily the general outline; moreover, the constantly changing focus due to pronoimced irregularities would interfere, so that a relatively low magnifica- tion is desirable for the rougher classes of work, and a more powerful objective for exceedingly fine work. Before using the contour- and radius-measuring instrument, a glass scale having graduation lines spaced o.ooi inch apart is placed on the surface plate and the microscope is focused onto it. The microscope is fitted with an adjustable eye-piece (not shown in the illustration) and the hair-line is traversed from one of the graduations on the glass scale to the next one, and the reading of the graduated dial of the adjustable eye-piece is noted. If this is a fractional reading, the microscope is adjusted vertically and focused again; another reading is then taken, and, if necessary, other readjustments are made, until a move- ment of the hair-line from one graduation line to the next one gives a reading on the eye-piece dial equal to an even or whole number of divisions. The dial then gives direct readings and may readily be used for measuring errors in gages or other parts imder observation. How Contour- and Radius-measuring Instrument is used. — The work to be measured with the instrument shown in Fig. 5 343 GAGE DESIGN AND GAGE-MAKING is placed on the surface plate which fonns the base of the in- stnimeat and is located, in some cases, with reference to the straightec^e F. For instance, it may be necessary to have a strught section of a contour gage parallel with this straightedge. The work can easily be set in this parallel position by simply adjusting it until the intersection of the hair-lines follows the straight section when slide E is traversed laterally. If a contour gage or templet is being measured, the distance between parallel edges or surfaces lying in different plaries is, as a rule, measured by moving the slide upon which the microscope is directly Fit. (■ Kiciowop* uid ita CaniaBe ut in 90-decr»« Podtion mounted and also by adjusting the main slide E, as may be required. As previously mentioned, the extent of these move- ments may be determined accurately rather by vernier scales or the Johansson gage-blocks. For instance, in testing the acciu'acy of the profile gage shown diagrammatically in Fig. 7, the measurements indicated by the letters a, b, c, etc., would be taken first, since they represent distances between important surfaces and intersecting points. It might be necessary also to check angle *. This could be done readily by the method shown on an enlarged scale at the lower part of the illustration. The hair-lines are first set to coincide with the angular edge as at j4, and then the microscope is adjusted to some portion B. An MICROSCOPIC MEASURING MACfflNES 243 adjustment at right angles is next made, the movement y in this direction equaling dimension z times the tangent of the required angle. Radial Measurements. — When measuring the radius of an arc or circular edge, the pivots of the cross-arm 5, Fig. 5, which carries the microscope, are set in the zero position or in line with the fixed pivots 5. The intersecting point of the hair- lines is then set in line with the center of the arc, this center ^ .4 ^'T\ Jf acMMry Fig. 7. DUgram showing Method of using Contour- snd Radius- measuxing Instrument being located by adjusting the microscope longitudinally and laterally with reference to locating points on the work and according to dimensions on the gage drawing. After the micro- scope has been set in this way, the cross-arm pivots are adjusted to the required radius by inserting Johansson blocks in the holders C; then, as the holders swing about pivots 5, the intersecting point of the hair-line should follow the circular edge of the gage. This test not only shows an error in the radius, but also an error in the location of the circular edge. The amount of error may be measured by the graduated dial of the adjustable 244 GAGE DESIGN AND GAGE-MAKING eye-piece previously referred to. If the angle included by the arc is required, it is simply necessary to note the reading on dial as the cross-arm traverses from the beginning of the arc to the point of tangency or to the end of the circular section. Contour Tests on Parts of Circular Cross-section. — When this instrument is to be used for measuring or testing the accu- racy of parts of circular cross-section (such, for example, as rifle bullets), the microscope is first focused on the point of tangency or with reference to a horizontal plane intersecting the Fig. 8. Two Methods of Mntnrliig Rifle BaUets center of the object to be inspected. As it would be difficult to focus on the point of tangency or on the side of a cylindrical part, the proper focus is obtained by placing on the surface plate Johansson blocks having a total height equal to one-half the diameter of the part to be tested. The microscope is then focused on the surface of the upper block. Fig. 8 illustrates two methods of measuring the contours of rifle bullets. The method shown at A consists in measuring from the base of the bullet to the point of tangency, then measuring the radius of the arc after locating the microscope directly over the center of the arc, and adjusting the cross-arm pivots the required amount. The dotted lines indicate how errors either in radius or in the proportions of the bullet may be detected. What MICROSCOPIC MEASURING MACHINES 245 might be defined as the ''step method" is illustrated at B. After measuring from the base to the point of tangency, the contour is checked by a series of lateral and longitudinal move- ments as the illustration shows. The universal adjustment of this instnunent makes it possible to test the accuracy of practi- cally any profile for comparing it either with a master gage or with prescribed dimensions. Measurement of Screw Threads. — The microscopic measur- ing machine shown in Fig. i may be used for testing the pitch, depth, and angle of screw threads on gages, etc., or the teeth of hobs, taps, or chasers. If the threads are finer than six per inch, the pitch and angle can be measured in one operation. The coarser threads require two operations, one side of the thread being used while measuring the pitch. To test the angle of a thread, the microscope is focused on the thread profile, and one hair-line is rotated until it coincides exactly with one side of the thread; the other hair-line is then adjusted to coincide with the other side of the thread. In order to do this, an adjustment of the compound slide may be necessary. The thread angle is indicated by the reading on the graduated dial C. In order to test the pitch, the table is next moved endwise any whole number of inches up to twelve. After this adjustment, if the hair-lines again coincide exactly with the thread, this indicates that the pitch is accurate, although to detect local errors or those that might exist between adjacent threads or in relatively short lengths of the screw would require repeated observations. The depth of thread is easily determined by superimposing one hair-line on the other and then setting the hair-lines to coincide first with the tops and then with the roots of the thread. The micrometer dial of the compound slide-rest indicates the depth and whatever error there may be. When measuring a hob or tap, the microscope can be focused directly on the sharp profile or cutting edge of- the tool, but if a plain screw thread is to be tested, the focusing is a little more difiScult. A simple method of obtaining the correct focus is to use a small block of steel or other material having two parallel faces lapped to a polish. The distance between these parallel 246 GAGE DESIGN AND GAGE-MAKING faces should be equal to the height of the work-holding centers of the measuring machine. On one of the faces a fine line should be scratched, and then the microscope can be accurately focused on this line before placing the work between the centers. Microscopic Measuring Machine with Micrometer Screw Adjustment. — The precision screw measuring machine to be described is of the general type which is provided with a micro- scope for accurately setting the saew thread to be measured m different positions and micrometer screws for determining whether or not the dimensions of the screw thread conform to required standards. A general view of this machine Is shown in Fig. 9 and a side elevation in Fig. lo. The fiied miaoscope A, Fig. lo, through which the screw thread is viewed, is mounted so that it can be adjusted in a lengthwise direction for focu^ng, and provision is also made for turning the microscope about its ails for making tests that will be referred to later. Adjustment of Microscope Relative to Screw Thread. — When a screw thread is to be inspected, the microscope is inclined either to the right or to the left (depending upon whether the thread is MICROSCOPIC MEASURING MACinNES 247 right- or left-hand) so that the axis of the microscope is in line with the screw thread instead of being perpendicular to its axis. The position of the microscope relative to the screw thread is indicated in Fig. 11. The object of this adjustment is to enable a correct view of the profile of the thread to be obtained. The microscope may be set perpendicular to the axis of the screw if it is desired to measure the thread angle in the axial plane, although Fl|. 10. Side Elsratioii of Scnw Heamrioc KacUns ahowii in Vig. g in the case of standard screw threads, the difference between the angles in the plane of the axis and at right angles to the thread will be very slight, and if the helix angle of the thread is quite large, it will be impossible to see the true profile in an axial plane. Therefore, the microscope is usually set in line with the helix angle of the thread. The angle a equals the helix angle of the thread, when measured with reference to a plane perpendicular to the axis, and the position of the microscope is indicated by an arc graduated to half degrees. Adjustment for Observing Different Parts of Screw Thread. — TTie screw to be tested is held in a self-centering split chuck. A 248 GAGE DESIGN AND GAGE-MAKING set of these chucks is provided to take all sizes of saews within the regular capacity of the instnuneat. The chuck in use is held at one end of a cyUndrical spindle that is supported in vees. These vees are adjustable so that the axis of the spindle may be set true. By sliding the spindle in the vees, any desired part of a screw thread may be inspected, and by turning the spindle about its axis, it is possible to locate under the microscope as many different sections or profiles of the screw as may be considered desirable. This chuck spindle is turned or traversed by means ng. II. AucnUr Podtion of HlcroKopc Kolati** to Screw Thread •nd HeUiod of focoilag It of a milled screw located on the opposite side of the instrument from that shown in Fig. 9. The outer face of this milled screw head is divided into 72 divisions of 5 degrees each to indicate the angles through which the spindle is turned. Readings are taken with reference to a pointer which is delicately pivoted and has most of its weight below the point of support; consequently, this pointer is maintained by gravity in the upright poation with as much accuracy as is necessary for the adjustments of the spindle. The Micrometer Screw Adjustments. — In addition to' the movements which have just been referred to that enable the screw MICROSCOPIC MEASURING MACfflNES 249 thread under test to be set in a position suitable for observation, there are two fine adjustments controlled by micrometer screws. One of these adjustments enables the screw being tested to be moved in a direction perpendicular to its axis. The micrometer screw at B, Fig. 10, serves to control this adjustment. In con- junction with this micrometer there is a shor t scale for registering complete turns of the micrometer screw. The range of move- ment provided for is 15 millimeters (0.5906 inch). The microm- eter screw is of J millimeter pitch (0.0197 inch) and the screw head is divided into 50 parts, enabling readings to be taken directly to o.oi millimeter (0.0004 inch). The screw under test may also be traversed in the direction of its axis by means of another micrometer screw which is similar to the one just referred to and is clearly shown in Fig. 9 attached to the right-hand side of the instrument. Focusing Microscope upon Side of Screw Thread. — Before taking any measurements, the microscope must be focused properly so that the profile of the thread under observation is clearly defined. This is done by first focusing on that part of the screw thread which is nearest to the microscope. The micro- scope will then be in some position A, Fig. 11, with its axis in the same plane as the axis of the screw to be tested. When the micro- scope is in correct focus for this position, it is then adjusted to some position B so as to be in focus with the profile of the thread in a plane co-x. In order to do this, the microscope is moved downward in a direction perpendicular to the axis of the screw, a distance y equal to the radius of the screw. Since the micro- scope is inclined an amount a equal to the helix angle of the screw thread, the actual adjustment y of the microscope in an axial direction is equal to the radius of the screw multiplied by the secant of angle a. The microscope is first focused on the upper- most part of the thread, as previously mentioned, and is then adjusted by measurement to bring it into focus with the profile, because it is easy to focus it upon the upper part of the thread by noting the clearness and sharpness of the thread as the micro- scope is adjusted axially in one direction or the other. If an attempt were made, however, to focus the microscope in this 250 MICROSCOPIC MEASURING MACHINES 251 Testing Accuracy of Pitch. — The method of setting the screw preparatory to testing the pitch of the thread is illustrated at D, Fig. 12. The intersection of the two cross-lines coincides with the slope or side of the thread. After the screw is accurately set in this position, it is traversed azially until the cross-lines again intersect the slope of the adjacent turn of the thread. The pitch, of course, is equal to the difference between the micrometer readings for the two positions. This test for pitch may be applied to different parts of the length or circumference of the screw thread, owing to the adjustments provided, so that any lack of uniformity in the pitch can be detected and measiured. The cross-lines of the microscope may also be set as indicated at E for testing the pitch. Measuring Angle of Thread. — For measuring the angle of the thread, one of the cross-lines is set to coincide with the slope of the thread as indicated at £, Fig. 12, by turning the ntiicroscope about its own axis. The inclination of the cross-line to the mean position may be read directly on the graduated disk C, Fig. 10, which is located near the objective end of the microscope. In order to determine the effective or pitch diameter of a screw thread, the intersection of the cross-lines is set to coincide with the thread, the same as indicated at D, Fig. 12, for testing the pitch. The screw is then adjusted laterally until the cross-lines intersect the slope of the thread on the opposite side, and the difference between the readings for the two positions represents the effective diameter. The Illuminating Apparatus. — This instrument is equipped with an illuminating apparatus so that the profile of the screw thread under observation will be clearly defined. For ordinary work, an electric lamp may be used, but to insure accuracy when measuring screws of different angles, it is necessary to use a parallel beam of Eght and to provide means for varying its angle. For this reason, the special illuminating apparatus shown applied to the instrument in Figs. 9 and 10 is recommended. A Nemst lamp D in conjunction with a prism E illuminates the screw from below and throws a parallel beam of light up the microscope tube parallel to the optical axis. This illuminating apparatus can be 252 GAGE DESIGN AND GAGE-MAKING adjusted to the right or left in accordance with the angle at which the microscope is inclined. If the light rays are at right angles to the axis of the screw, some li^t will be reflected from the slope or side of the thread, as indicated by the left-hand diagram, Fig. 13, and there will be a amilar effect if the beam of light is not composed of rays which are at least approximately parallel. This reflected light spoils the .de^tion, but when the light rays are in line with the helix angle, as indicated by the right-hand dia- gram, a good definition is obtained, and it is then possible to set the microscope accurately relative to the work. As the helix angle of the thread varies for different pitches and diameters, means are provided for adjusting the angle of the parallel beam of light. This instrument was designed and constructed by the Cambridge Scientific Instrument Co., Ltd., of Cambridge, Eng- land. It was made for the small screw-gage committee of the British Association, for testing the accuracy of commercially produced screw threads and taps. CHAPTER X PROJECTION METHOD OF TESTING GAGE THREADS A MODERN and very eflfective method of testing screw threads on gages or other precision work, is by the use of an optical pro- jection apparatus. The principle is very simple: An image of the thread is projected upon a suitable screen and is then com- pared with the profile or outline of aii accurate thread of standard proportions. It is necessary, of course, to determine previously the magnification or the extent to which the thread under obser- vation is to be enlarged upon the screen, so that the profile or thread outline used as a standard in making comparisons may be drawn to a corresponding scale. This projection method shows errors in the pitch, angle, or form of the thread. Optical Projection Apparatus. — The projection apparatus to be described was developed by the National Physical Laboratory of England and is employed in conjunction with other precision measuring machines for verifying the accuracy of thread gages of both the plug and ring forms. The apparatus will first be described and then some examples illustrating its practical application will be referred to. Two views of the apparatus are shown in Figs, i and 2. There is a projection lens at -4, an arc light at 5, and adjustable conical centers at the rear of the lens for holding the gage C to be exam- ined. Just beneath these centers there is a clamp which may be used for holding any gage or other part that cannot readily be mounted between centers. The slide which carries the work- holding clamp and centers may be adjusted along its base, thus varying the distance between the object to be projected on the screen and the lens. The purpose of this adjustment is to obtain the proper focus. The focusing slide is attached to a rod F, which connects with a small eccentric at Z?, mounted on a shaft carrying 253 254 GAGE DESIGN AND GAGE-MAKING the double-ended lever E. The focus is first adjusted approxi- mately by unclamping the slide from the rod and moving it along the base, and lever E is then used to obtain the fine adjustment. This lever has cords attached to it so that adjustments may be made from the screen where it is posable to see more clearly just when the focus is correct. Fig. I. Horizontal Piojaction Appuatui for TsillaE Scrsw Tlir«ad* It is very essential to provide for this apparatus a lens of good quality and one that has been corrected for distortion, to secure an accurate reproduction of the thread profile on the screen. A strong source of light b also necessary, so that a clear and sharply defined outline or shadow will be projected. The carbons of the arc light used with this apparatus are located 90 degrees apart instead of being placed one above the other. With this arrange- ment, the crater or concave depression which forms m the end of the positive carbon is pointed directly toward the lens; con- sequently, a maximum amount of light is reflected and shadows PROJECTION METHOD OF TESTING THREADS 255 of the crater wall or edge aie not projected upon the screen. When one carbon is placed above the other, the formation of the arc at differ^t points around the edge of the crater sometimes results in casting a shadow of the crater wall upon the screen, thus interfering with the image of the part under observation. The projection apparatus referred to is arranged for a mag- nification of 50, the thread section being enlarged that number of times, which enables even slight inaccuracies to be observed readily. The magnification may be verified before testing a screw thread by placing between the centers back of the lens a cy&i- drical part such as a plug gage, and then adjusting the focal distances until the shadow upon the screen is of the required Mze as determined by measurement. The focal distances mentioned are the distances between the screen and projection lens and the 256 GAGE DESIGN AND GAGE^MAKING distance from the screw to the projection lens, the measurements in each case being along the line of projection. Between the lens and the arc light there is a condenser or lens G which gathers the rays of light from the arc and changes their direction so that all the rays between the condenser and lens are parallel. These parallel rays are obtained by locating the con- denser at the right distance from the arc. The axis of the gage or screw thread to be observed is located at right angles to the axis of the lens, so that the image projected on the screen repre- sents an enlarged section of the thread in the plane of its axis, or in the plane in which the thread angle should be measured. As is quite apparent, if the gage were adjusted from this perpendicu- lar position an amount equal to the helix angle of its thread, the projected image would represent a section normal or at right angles to the thread. In order to secure a clear, sharply defined projection on the screen, it is essential to have the light rays between the con- , denser and lens in line with the thread on the side that is to be r^ observed and projected. This alignment of the light rays with the helix angle of the thread is of particular importance when the pitch is large in proportion to the diameter or when the thread inclines considerably relative to its axis. Therefore, the arc light and condenser are moimted upon a frame which is pivoted directly beneath the lens and is free to swing in a horizontal plane. When a gage is placed between the centers preparatory to projecting its image upon the screen, the frame referred to is adjusted xmtil a well-defined projection is secured. The appara- tus is connected with an ordinary iio-volt lighting circuit, but the voltage is reduced considerably by means of a rheostat H, Fig. I. Screen upon which Thread Profile is Projected. — The screen upon which the shadow or image of the thread is projected is practically a large drawing-board faced or covered with heavy drawing paper or bristol board. The particular screen seen in Fig. 3 is provided with rollers at the bottom and is mounted upon a horizontal track. The screen is also supported by a vertical screw, so that it xmy be adjusted either in a horizontal or a PROJECTION METHOD OF TESTING THREADS 257 vertical diiection for locating any standard thread outline which may be drawn upon the screen in coincidence with the projected image for observing any differences of form, angle, or pitch. This adjustable feature, however, is an unnecessary refinement, as it is much more convenient to have the standard thread outlines drawn on sheets of cardboard which are held against the screen in line with the projected image of the thread when testing it for accuracy. Fi(. 3. SciBMi apoo which Scfew ThraadB are ptoj«ct»d Inddectally, it is important to locate the screen so that it is perpendicular to the line of projection, and in order to test its position, the lens and any other part that may be in the holder are removed. The parallel beam of light from the condenser then Uluminates a small circle on the screen. A mirror is held against the screen in such a position that the beam of light is reflected back onto the apparatus. The screen slide is then adjusted until this reflected beam coincides with the projected beam, or until it passes back through the opening which ordi- narily contains the projection lens. as8 GAGE DESIGN AND GAGE-MAKING Testing Plug Gages by Projection Metiiod. — When the profile of a thiead gage of the plug type is to be examined, the gage is placed between the centers back of the lens and the slide carrying these centers is adjusted vertically until the thread profile on the lower ^de of the gage is in line with the center of the lens. This vertical adjustment is, of course, necessary for examining gages Pl|. 4. SnuU Plat Gase projected upon Screen of different diameters. The pivoted frame which supports the arc light and condenser lens is then swiveled in a horizontal plane as previously described, in order to align the light rays with the thread. The observer then goes to the screen and, after care- fully adjusting the focus by means of the cords connecting with the apparatus, compares the projected image with a standard thread profile. A comparison with a standard thread outline will show readily minute errors of form as well as any appreciable inaccuracy of pitch or angle. A true form at the crest and root PROJECTION METHOD OF TESTING THREADS ZJp of a Whitwocth thread is very important, except when clearance spaces at these points aie provided, as, for example, in the case of plug gages of the type used for checking the effective diameter pnpMatoij to tMtlos by ProjectloD only of a ring gage. The nature of the errors detected by the projection method vary considerably. For instance, the radius at the crest or at the root may be incorrect, or if it conforms to the standard radius, the rounded part may be off center relative to 26o GAGE DESIGN AND GAGE-MAEING the remainder of the thread. All errors of this kind as well as any irregularity along the slc^, or decided variation either in pitch or angle, are readily detected by the projection method. The illustration, Fig. 4, is an actual phot<^aphic reproduction of the image of a small plug gage projected upon the screen. This gage is one of the small sizes shown in Fig. 2 upon the table which supports the projection apparatus. When the angle of the thread is tested, a piece of cardboard is used upon which lines are drawn to the standard included angle and to several greater and smaller anglea with variations of 30 minutes. An error of J d^ee may be detected easUy by simply comparing the projected image with the standard referred to. An error in the pitch will be detected by a lack of coincidence of Fig. 6. spindle contaloioc Cut made u ihown in 71c. 5 the succes^ve threads of the image and standard thread outUne, but it is not advisable to attempt to measure such differences ^ce the scale of magnification is insufficient to obtain the result accurate to tenths of a thousandth of an inch, an error of 0.0001 inch in the gage being indicated by 0.005 ^^^ "^ ^^ screen with the 50 m^ni&cation used, which is a quantity too small for accurate measurement. Application of Projection Method to Ring Gages. — When a plug gage is being examined, the profile of the thread is projected directly upon the screen, but obviously this method could not be employed when testing a ring gage, although the accuracy of the latter may easily be verified by the use of the projector. This is done by making a small cast of a section of the thread. This cast or impression (which corresponds to a segment of a plug gage of corresponding diameter and pitch) is then mounted in the projector the same as a plug gage. The cast is made of PROJECTION METHOD OF TESTING THREADS 261 a composition containing 93 per cent of sulphur and 7 per cent of graphite. This composition is poured into a mold formed by a slot in a spindle which is held against one side of the ring gage. Fig. 5 illustrates how a cast is made. The spindle A, which contains a slot and serves as a support for the cast, is screwed into baseplate B and is perpendicular to the surface of this plate. The ring gage C, the thread of which is to be examined, is held •against .the spindle by screw D, and then the composition is poured into the mold thus formed, as the illustration indicates. A detailed view of the spindle with the cast in place is shown in Figs. 6 and 7. The latter illustration also shows the baseplate of the casting fixture and three cast segments which have been removed from the spindle to show clearly their form. The spindle and cast segment is placed between the centers of the projection apparatus, the same as though it were a plug gage. Tracings of Inaccurate Thread Forms. — In order to secure a permanent record of the thread outline, the edge of the shadow projected on the screen may be traced in pencil upon a sheet which is filed for future reference. These outlines are sometimes sent to gage-makers to show them the exact nature of the error, or to manufacturers when the outline is of a thread on the prod- 262 GAGE DESIGN AND GAGE-MAKING uct instead of a gage thread. Figs. 8 to lo, inclusive, show a number of these tracings which were obtained by means of the projection apparatus. The dotted line in each case shows the standard thread form, and the full line, the actual shape of the thread. The thread illustrated at A^ Fig. 8, is on a plug gage intended for testing the fuse holes in the ends or noses of shells. This thread is accurate as to pitch, but the crest is flat instead of being rounded, whereas the root extends considerably below the dotted arc representing the correct depth and radius. The out- Fig. & Tndngs made in Conjunction with Projection Apparatus which fldiow InaccnracieB in Plug and Ring Gages line shown at B is the thread of a ring gage and was projected from a cast. In this case, the pitch is also very accurate and the principal defect is at the root which has too large a radius, so that the sides of the thread are under-cut. Another ring gage thread is shown at C. The radius of the crest is small and the radius of the root large, the result being a thread that is too narrow. It will be understood that these thread profiles were not selected as typical examples of gage work, but merely to illustrate the nature of some of the common defects. Tracings are made only when the errors are suflBdently pronounced to require an investigation. Most of the gages examined conform more closely to the standard outline than is indicated by the PROJECTION METHOD OF TESTING THREADS 263 examples shown in Fig. 8, but when the gage is snfStdently accu- rate to pass the inspection test, the outline is not traced, since there is no need of such a tracing. Fig. 9 shows two examples of threads on the product or work. The upper view A illustrates the thread on a J-inch bolt forming part of an airplane motor. The actual thread form in this case deviates only slightly from the standard outline, and this is regarded as a very good example of work. The other extreme is illustrated at 5, which represents the thread of a nut. This thread was supposed to conform to the British Association Fig. 9* {A) Tradng of Thread on Airplane Motor Bolt. (B) Thread aupposed to conform to Britiah Aasociation Standard standard indicated by the dotted line, and it illustrates what it is possible for a tap to do when it is not in good condition or is improperly applied. While the projection apparatus is of especial value for examin- ing Whitworth threads, it may also be used to advantage in connection with the U. S. standard thread or any other form. Two tracings from U. S. standard threads are reproduced in Fig. 10. The outline shown at A was projected from a plug gage having eighteen threads per inch. The pitch of the thread is very accurate, but the top is a little too wide and the root is finished almost to a sharp vee. The thread illustrated at B was projected from a turnbuckle. There are 30 threads per inch in this case, and it is supposed to conform to the U. S. standard, 17 ■ 264 GAGE DESIGN AND GAGE-MAKING although there is considerable deviation, as indicated by the full and dotted lines. Photographing Thread Forms. — A photograph of the outline of a projected thread may be obtained without much difficulty instead of tracing the contour in pendl, although the latter method is simpler, more rapid, and less expensive. One method of securing a photograph is as follows : A part similar in form to the focusing bellows of a camera is attached in front of the pro- jector lens and is arranged to hold a plate slide. The exposure is made either on a rapid plate or directly upon bromide paper. ¥igi xo. . Two Examples of Defective U. S. Standard Threads A ground-glass screen is inserted in the plate slide prior to making the exposure and is used for focusing. For comparative pxirposes, the outline of a standard thread form may be secured in conjunction with the photograph of the thread to be examined by first making an artificial negative. This should be a dupli- cate of the standard thread except that the outline is enlarged in proportion to the magnification of the projected thread image. A magnification of 20 is considered about right for making photo- graphic reproductions. The artificial negative may be made by drawing as accurately as possible the thread contour on tracing paper. This outline, which should be twenty times the actual size of the thread, is next filled in with India ink to obtain a solid PROJECTION METHOD OF TESTING THREADS 265 266 GAGE DESIGN AND GAGE-MAKING black thread section. Several coats of the ink should be applied, and to prevent the paper from wrinkling and buckling, it should be moistened before applying the ink and then be smoothed out thoroughly after the ink is dry by using a hot flat-iron. This artificial negative is placed in contact with the sensitized plate preparatory to making the exposure, and it serves as a standard for comparing the photographic reproduction with the actual thread. The artificial negative must be set so that it will be parallel and close enough to the projected image to enable the actual and true outlines to be compared readily. The optical projection apparatus is not restricted in its use to testing screw threads, but may be employed to correct the accuracy of the profiles of gear teeth, the shapes of cams or gages of the contour type, or any other parts having an outline that may be projected and compared with a standard form. Vertical Projector for Screw Threads. — The vertical projec- tion apparatus which is used for testing the accuracy of screw thread forms is a development of the horizontal type of projector previously described. This apparatus, which is also a design of the National Physical Laboratory of England, is arranged to project an image of the thread onto a horizontal table located in a convenient position near the projector. A front elevation of the apparatus is shown in Fig. ii. A horizontal beam of light from an arc-lamp at A is reflected by a prism at B vertically upward past the screw which is supported in a horizontal position on centers C and D. The shadow image of the screw is projected through a lens E onto a plane mirror which is held in a horizontal position about six feet above the lens, as illustrated by the dia- gram A, Fig. 12. From this mirror the image is reflected down- ward onto a small horizontal table, where the observer is able to take measurements or to check the shape of the thread profile by means of standard thread form diagrams. The arc-lamp is a special design, and the carbons which are carried on slides are adjusted by means of rod F connecting with a bellcrank lever. Above the lamp there is a reflector G with ground and colored glass through which the observer may watch the arc. The beam of light is made parallel by a condenser, and PROJECTION METHOD OF TESTING THREADS 267 means are provided for adjusting the beam of light reflected from the 45-degree prism in the base of the machine, so that the light rays are in hne with the helix angle of the thread under obser- vation. The machine is equipped with three micrometers (B, J, and K), two of which are used for diameter measurements and one n .« II 1 1 1r ' ii !i«""» c , 4 Hicromalei KelatlTa to Lowat S for pitch or lead measurements. The bedplate of the machine carries two slides. The lower slide is free to move horizontally in a direction at right angles to the axis of the screw. The arrangement of this slide and of the two micrometer screws is shown by the diagram B, Fig. 12. The micrometer nut N is fixed to the lower slide and a stop or anvil O is attached to the bedplate. The other micrometer is arrstnged in the reverse order, the micrometer nut P being fixed to the bedplate and a stop Q being attached to the lower slide. When measuring screw 268 GAGE DESIGN AND GAGE-MAKING diameters, the lower slide need not be moved across from one ade to the other by rotatdng the micrometer screws. The slide can be adjusted first to one side and then to the other by a "throw-over " gear, and the distance that it moves is indicated by the sum of the two micrometer readings. An adjustable dashpot R, at the rear of the machine, serves to prevent any jar on the micrometer spindles when they come into contact with the stops. The upper slide which rests on the lower one may be moved in a direction parallel to the axis of the gage. This move- ment is controlled by micrometer K (Fig. ii). This micrometer is used for measuring the pitch of the thread. The magnification with this apparatus is 50 diameters, and this may be adjusted by varying the height of the reflecting mirror. PROJECTION METHOD OF TESTING THREADS 269 Thread Angle Projectton Apparatus. — In order to determine the accuracy of various types of master gages, such as are re- quired as reference standards in checking gages used for measure ing the dimensions of interchangeable parts of manufactured products, the Bureau of Standards, Washington, D. C, has established what is known as the "Gage Section." The equip- ment with which it is furnished includes complete facilities for use in determining with absolute accuracy the shape or thread angle of thread gages, their pitch diameter, and the lead of their thread. The thread angle projection apparatus (which is manu- factured by the Arthur Knapp Engineering Corporaticfli, New 27© GAGE DESIGN AND GAGE-MAKING York. City) is shown in Fig. 13. (The equipment for testing the lead and pitch diameter is illustrated in Chapter VIII.) An image of the thread is projected onto a suitable stage, and its form is measured by compstrison with a master gage or templet of standard form. The apparatus is ol the type designed by the Bureau of Standards. A feature of the design is that all adjust- ments of the lantern, gage-holder, and bevel protractor which supports the master templet are within easy reach of one operator. The source of light for the projection apparatus consists of an arc-lamp contained in the lantern L, an arc-lamp being used for this purpose because it is the nearest practical approximation of a perfectly white light. Rays of light from the arc pass through a condensing lens A and are then projected onto the thread of the gage which is to be measured. In preparing to use this appara- tus, two preliminary adjustments must be made; namely, the thread gage must be set so that the side on which the thread is to be projected is located opposite the center of lens B, and the lantern bracket must be swung around its pivotal support so that the rays of light pass over the gage in a direction tangent to the ■ helix of the thread. After passing the thread gage, the light PROJECTION METHOD OF TESTING THREADS 271 rays go through projecting lenses B and C, and then through the prism G, which deflects the light up to the surface of a mirror, as indicated by the arrow. This part of the apparatus is shown more clearly by the detail view, Fig. 14. Care is taken to so adjust the angles of prism G and of the mirror that the light rays projected from the surface of the mirror will fall upon the stage N in exactly a perpendicular direction. This is important because, if the rays did not fall perpendicular, there would be danger of inaccuracy in comparing the image of the threads thrown on stage N with the form of master templet M with which the form of the thread is com- pared. This. master templet (see also Fig. 15) has an included angle of 60 degrees so that it may be used direct in checking the accuracy of form of U. S. standard and other 60-degree threads. Where it is desired to check an Acme thread or any other thread than one with a 60-degree included angle, this can still be done with templet M. The method of procedure is to first align one side of the templet against one side of the thread image on the stage and then note the reading of the vernier swivel on which the templet is supported. After this reading has been taken, the vernier is swung around to bring the opposite side of templet M into alignment with the other side of the image of the thread. A second reading of the vernier is then taken, and from the diflference of readings, bearing in mind the fact that the templet has a 60-degree included angle, the accuracy of the thread may be determined. So far, the discussion of the use of this apparatus has been based upon its application for testing male thread gages. The same equipment may be employed, however, for testing the accuracy of the thread form of female gages. This result is accomplished by the use of a special core check composition with a known coefficient of contraction and expansion. With this composition, a cast is made of a section of the internal thread and then, after the cast has been hardened, the accuracy of the thread form of this cast is determined by setting it up on a special support on the gage-holder and proceeding in exactly the same way as in testing the accuracy of a male thread gage. CHAPTER XI GENERAL GAGE-MAKING PRACTICE As GAGES are made in an endless variety of forms, gage- making practice also varies accordingly, and frequently it is necessary for the gage-maker to resort to the use of special tools and methods, p)ossibly because a gage is required that is unusual as to form or in regard to the degree of refinement jind accuracy necessary. The making of gages not only includes the machine work and lapping operations, but the heat-treating processes, and much may also depend upon the selection of suitable mate- rials. The use of accurate methods and instruments for checking and measuring gages under construction is, of course, absolutely essential. While many of the measuring tools are simDar to those commonly employed in toolmaking or for other kinds of precision work, and are familiar to tool- and gage-makers, many appliances especially adapted to gage work have been developed, and some of the more important designs have been described in preceding chapters. This chapter deals with the selection of steel for gages, heat-treating processes, and gage grinding, and includes some miscellaneous examples of gage-making practice to illustrate different methods of procedure. Steel for Gages. — Gages may be made either of machine steel, plain carbon steel, or of special alloy steels. Machine steel, having a rather low carbon content, is now used extensively for gages, as it is much cheaper than tool steels and is also easier to machine. Machine steel containing from 0.15 to 0.25 per cent of carbon is generally used, although it may have a content as high as 0.50, especially for ring or plug gages. The outer surface of a machine steel gage is carburized so that the parts subject to wear can be hardened. What is known as a 0.20-per-cent carbon steel is generally considered very satisfactory for gages; it con- tains from 0.90 to 1. 10 per cent of manganese and about 0.05 per 272 GAGE GRINDING 273 cent of phosphorus and sulphur. This steel should not contain silicon, as this causes warping in hardening. The carbon in the so-called 0.20-per-cent carbon steel may vary from 0.15 to 0.25 per cent, and many gage-makers consider it preferable to use steel having the smaller amount. This general class of steel is extensively used for making snap gages which are drop-forged. Steel containing approximately 0:50 per cent of carbon is often used for making plain plug gages, ring gages, or other forms which may easily be ground after hardening. While a steel containing 0.50 per cent of carbon is Uable to greater distortion in hardening than steel containing a much smaller percentage, such changes are not so important when they may readUy be corrected by grinding. When machine steel Is used for gages which are rough-turned and afterwards ground on centers, it is advisable to recenter them before hard- ening. The reason for re- centering is that the heavy ^ „_,.,, , o ^ , . ° , ., / Fls- t- Orindim Jam of Sup Oag» roughing cuts, and possibly such operations as knurling, distort the centers more or less, which causes trouble during the grinding operation. The cen- ters should also be lapped after hardening to remove the scale. Some gage-makers prefer a high-carbon or tool steel, partly because it can be hardened in much less time than is required for pack-hardening gages made of machine steel. In fact, tool steel is often used for gages which must be made quickly. Another disadvantage referred to in connection with pack-hardened machine-steel gages is that the thin hardened surface is removed by re-grinding worn gages so that heat-treatment is again neces- 374 GAGE DESIGN AND GAGE-MAKING saxy, thus increasing the eqiense for gage repair. When a high- carbon steel is used, the carbon content generally is about 0.90 per cent. A steel having this amount of carbon can, of course, be hardened by simply heating it above the decalescent point, and then cooling, preferably in oil. This high-carbon steel should have about 0.30 per cent of manganese, a phosphorus and sulphur content not ex- ceeding 0.025 per cent, and about 0.15 per cent of silicon. Special alloy steels may now be obtained which are adapted to fine gage woA, partly because the changes, due to hardening, are exceedingly small, the dimensions of the gage being practically the same after hardening as before. This feat- ure is of especial value where thread gages and some other designs are of such a form that it is especially desirable to leave as small an allowance as pos^le for grinding or lappii^. Grinding Snap Gages. — The general method of grinding snap gages is to hold the roughed out gage blank in a horizontal position and grind both surfaces parallel at one setting by u^g the side of a recessed wheel. Care should be taken to clamp the gage so that it is not sprung out of shape, thus keeping the ground jaws parallel after the gage clamps are released. Fig. i shows a machine arranged for grinding a snap gage. In this particular case, the gage is held in a vise attached to an angle-plate. After GAGE GRINDING 275 one side or jaw has been ground, the machine table is moved over for bringing the opposite side of the grinding wheel into con- tact with the other jaw or gaging surface. Fig. 2 illustrates how a large snap gage was ground at one setting by using the face or Fig. 3. Lftppiiis Jaws of Snap Ga(e oa Plat Caat-lron Lap periphery of an ordinary disk wheel. After the gaging surfaces are finished parallel and the right distance apart, the front edges should be ground square with the jaw faces and so that they lie in the same plane. When a gage is ground as illustrated in Fig. 3, 276 GAGE DESIGN AND GAGE-MAKING a cup-wheel may be used for finishing the end surfaces. The gage jaw should be ground slightly under size, an allowance bdng left for lapping. This allowance is usually about 0.0003 or 0.0003 inch. One method of lapping a snap gage is shown in Fig. 3. The lap is in the form of a cast-iron plate having a flat or plane sur- Kg. , face. This lap, which is charged with a fine abrasive, is held in a vise and the gage jaw is rubbed over it, care being taken to hold the gage flat on the lap to prevent any tilting. The set of end- measuring gages seen on the vise just back of the lap are used for testing snap gages while lapping them, A suitable combination of gage- or size-blocks is frequently used for this same purpose. GAGE GRINDING 277 Gage Grinder. — Most gage grinding is done on machines designed for general work, but grinders arranged especially for gage grinding are desirable, particularly where this work is bang done continually. A universal gage grinder (made by the Steel Products Engineering Co., Springfield, Ohio) is shown in Fig. 4, grinding the jaws of a snap gage. The gage is clamped in a tit- S- Grioiliiig an End-measnrlas Rod on Gage Orlodlng Machine horizontal position on the work table which has a cross-feed and may be adjusted longitudinally for bringing the wheel into contact with either gaging surface. This cross-feed is operated by a hand lever at the left-hand side of the machine. This lever provides a quick movement for removing the surplus stock. A finer feeding movement may also be obtained by means of a hand- wheel which operates a screw of fine pitch. The grinding wheel 278 GAGE DESIGN AND GAGE-MAKING is carried by a slide which is mounted upon a horizontal bearing surface at the rear, so that the grinding wheel may be set in any position relative to the work table. The latter may be tilted upward or downward through an included angle of 10 degrees, which permits of locating the center of gages of different thick- ness in line with the center of the wheel. The traversing move- ment of the work table is regulated by a feed-screw and hand- wheel with graduations reading to 0.0005 ^^ch. This same type of machine arranged for grinding an end-measuring rod is shown in Fig. 5. Combination Gage Grinding and Lapping Machine. — A number of different special designs of machines have been built for the grinding and lapping of gages. One of these special machines has a vertical column which supports two wheel-spindle heads, one carrying the grinding wheel and the other a steel disk which is charged and used for the finishing operation of lapping. The grinding wheel and lapping wheel spindles are not connected and may be operated at different speeds. The gage to be ground and lapped is held by a slide which, in turn, is mounted upon a horizontal cross-rail located below the wheel heads. A feed- screw and crank are used for traversing this slide upon the cross- rail and locating the gage in the proper position relative either to the grinding wheel or the lapping wheel. The cross-rail slide also has an adjusting screw of fine pitch, which is used for feeding the gage up to the wheel for grinding or lapping operations. The vise which holds the gage is carried by a slide mounted on vertical ways. This slide is given a vertical reciprocating motion by means of a heart-shaped cam driven through worm gearing from a horizontal shaft running lengthwise of the cross-rail. When a gage is being lapped, it is brought up against the revolv- ing disk with just enough pressure to give a suitable cutting action. The work-holding slide does not have a uniform vertical movement, the cam being designed to accelerate the motion at the ends of the stroke to avoid lapping the gage bellmouthed. The work-holder or vise is pivoted so that it can swing out to a 4S-degree position for convenience in gaging the work. When measuring the gage, the vertical reciprocating movement is GAGE GRINDING 279 Stopped by operating a clutch which controls the rotation of the cam driving the shaft. This machine is one of the special designs built by the Waltham Machine Works, Waltham, Mass. Grinding End-measuring or Pin Gages. — End-measuring or pin gages are preferred to plug gages by many mechanics, es- pecially for large holes that must be round and straight. Taper and out-of -round errors can be easily detected with the pin gage, and they are also used for setting or adjusting micrometers. The ends of this type of gage should be ground perfectly spherical. •CRCW8 FOR HOLDING OAQES Qw-qAvox'^ aPHCRICAL-END PIN GAOE^ MJ GRIND ON THESE CORNERS SPARK AT TOP AND BOTTOM Maekinerf Fig. 6. Method of grinding Spherical-end Pin Gages witii a Cup-wheel although many mechanics just file the ends to a sharp point, making a fair gage, but rather unsatisfactory for permanent use. Satisfactory results can be obtained by the following method and with but one special tool. The arbor A, Fig. 6, is made of tool steel and has several holes drilled and reamed through the center to take the various diam- eters of stock used for making the different lengths of gages, screws being provided to prevent the gages frofn slipping. A cup-wheel B is mounted on a toolpost grinder and used in a lathe. After roughing the gage and hardening the ends, the gage is placed in the proper hole in the arbor A , placing the arbor on the centers of the lathe as illustrated, and adjusting the wheel as z8h 28o GAGE DESIGN AND GAGE-MAKING nearly as possible by the eye to agree with center lines D and £, and having the face of the wheel parallel with the arbor. By rotating the gage slowly by hand past the wheel and noting the sparks, it is easy to see if the wheel is too high or too low. When the wheel just sparks on the top and bottom, the wheel is right for center line E. Center line D must be determined by measuring the pin gage and adjusting the lathe carriage sidewise until the contact points of the micrometer, when swung back and forth parallel with the arbor, strike at all points on the gage. The wheel revolving in the direction indicated by the arrow and the work being swung up and down past the wheel by hand, generates a sphere. The accuracy depends upon the alignment of the wheel face to the center of the arbor and the center lines of the wheel to the center of the arbor and the center of the gage. Care should be taken to see that the wheel cuts only at the inside edge of the wheel. If care is taken in grinding, little or no lapping is required. The principle of this method of grinding spherical-end pin gages is geometrically correct, but it may be somewhat difficult to see without some explanation. The inner edge of the cutting face of the cup- wheel lies in a plane that inter- sects an imaginary sphere the center of which is at the intersec- tion of the axes of the wheel-spindle and the work-arbor, and the diameter of which is equal to the length of the pin gage being ground. The cup-wheel characteristic in this case is that it grinds with the inner edge only. The end of the pin gage ground in the manner described lies within the section of the spherical surface intercepted by the plane coinciding with the end of the cup-wheel, and because of the characteristic of the wheel referred to, it must have a spherical end when finished. If the cup-wheel had an internal diameter equal to the length of the pin being ground, it would have to be moved toward the work until its grinding plane would intersect the axis of the arbor in which the pin is moimted, which, of course, is impossible; but when con- sidered as being in this extreme position, this method of generat- ing spherical surfaces on the ends of the gage is more readily understood. GAGE GRINDING 381 Aiboi for Grinding End-measuring Gages. — A special arbor is shown in Fig. 7, for grinding end-measuring gages having spherical ends. This arbor is mounted between the centers of the grinding machine. One end of bar A is turned down to suit the ordinary driving dog. The other end is turned down for a spring and provided with a stop-collar. The central portion is bored and ground to fit the swiveling plug B, and has a slot cut through it at right angles to the plug. This slot should be of the same width as the diameter of the gage to be ground. The swiveling plug is drilled and reamed to fit the gage and provided with a clamping screw D. Fig. 7. Derice tor Griodlns OagM with Sphoiical Endi A piece of round steel of the same diameter as the gage to be ground, and with a length equal to the diameter of the bar A, is inserted through the slot and clamped in the swiveling plug. The outside of the bar and the ends of the plug are now ground • up tt^ether to suit the bore of sleeve C. This sleeve keeps the plug in position when the g^e is removed, and, being provided with a cam surface on its end, gives the required motion to the gage being groimd. The right-hand end of this sleeve is knurled so that the operator can grip it and hold it stationary while the remainder of the device rotates. The gagas to be made are rough-turned, cut off to the required length with proper allowances, and the diameter ground to suit the slot and the bore of the swivel. Then the centers are removed, the gage is inserted in the fixture and clamped central by screw D, and finish-ground on the ends. Only a slight grip on the sleeve is required to hold it stationary so that the 282 GAGE DESIGN AND GAGE-MAKING gage to be ground will swivel to and fro against the cam surface on the end of the sleeve. This arbor was made for grinding gages varying from 2^ to 6 inches long. Grinding a Precision Angle-testing Gage. — The gear-testing fixture or gage illustrated in Fig. 8 is an example of gage-making i:-X: — «- r \ --4 --4- I ! ^t- .-|5J) i UocMntT^ Hg. 8. Elevation and Plan of Gear-teitins which required an unusual degree of accuracy throughout and made it necessary to provide special tools and methods. This gage is a special design made by the Nelson Tool Co., Inc., New York City, and is used for testing the accuracy of the bevel gears A and JS. As the axes of these gears are located at right GAGE GRINDING 283 angles, the body C of the fixture is in the form of an angle-plate in which there are horizontal and vertical holes. Gear A is supported in close-fitting bushings D by its shaft, and gear B is held in place by plug £, which enters bushing F. llie ga^g members consist of two blocks G and B., which are held in posi- tion by the bolts shown and are free to revolve. Each of these ga^g blocks has two anns which project outward at an angle. If the gears are correctly made, one of these arms will just clear the top of a gear tooth when the gage is turned about its pivot, whereas the other will strike the tooth. In other words, these are "go " and "not go " arms; they are both finished, of course, to the same angle, but one gaging surface is about 0.006 inch hi^er than the other one. It was necessary to finish these arms very accurately both as to the angle and location of the angular surfaces relative to the center lines of the gaging mem- bers. In addition to the extreme accuracy in regard to the angles, it was necessary to finish the body of the fixture pre- viously referred to so that the horizontal and vertical ^des were as near square as it was possible to make them, and the tolerances specified for a number of the more important dimen- ^ns were only 0.000J and 0.0002 inch. 284 GAGE DESIGN AND GAGE-MARING The most interesting and important part of the work is grinding the angular axms of the gaging blocks. The general method of doing this part of the work is illustrated in Figs. 9 and 10. The block to be ground is moimted on an angle-plate A, Fig. 9, and it is held in position by a bolt and a spring B. The object of using the ^ring vs. to hold the work securely against the face of the angle-plate and at the same time permit it to be revolved easily wWle grinding. The grinding attachment, which is of the push-kindle type, is moimted upon a horizontal slide C, and both the angle-plate and slide are mounted on the faceplate D. The grinding of this gaging block involved, first, setting the horizontal slide and the angle-plate with reference to the angle required on the work; then it was necessary to adopt an accurate method of testing the inclination and location of this ground GAGE GRINDING 385 surface. Fig. 9 shows clearly how the slide and angle-plate were set relative to each other by using the sine bar £ and the Johansson gages. The sine bar bears directly against the ^de of the horizontal slide upon which the grinding attachment is mounted, and the gages are used to measure the distance from the buttons on the ^ne bar over to the side of the angle-plate. The method of mampulating the grinding attachment and work Mathod of toiting Accuracy of Ocound Surfaces is indicated in Fig. 10. As the gage-maker pushes the grinding wheel ^indle back and forth with the right hand, he ^ves the work an osdllating movement, so that the entire gaging surface is ground and finished to a convex form. The motor for driving the spindle is held in place by the vertical arm shown, and the entire arrangement is very simple, especially in view of the complicated nature of the operation. The method of testing the angle and location of these ground surfaces is illustrated m Fig. 11. This test is made without 286 GAGE DESIGN AND GAGE-MAKING removing the gage from the grinding fixture. The particiilar gage arm to be tested is simply turned around so that it is at the lowest position, as Fig. ii indicates. Johansson gage-blocks are then inserted at A and B, and an accurate plug is pushed in between the horizontal and vertical surfaces of these blocks and the angular surface of the gage being tested. When this test has been made with the plug bearing close to the outer end of the angular arm, a different combination of gage-blocks is used and the same test is repeated at the inner end of the arm. Before this test could be made, it was necessary, of course, to calculate what the horizontal and vertical distances, as obtained by the Johansson gages, should be for a plug of given diameter. Since the accuracy of the gear-testing fiaiture referred to depends largely upon the accuracy of the body of the fixture, it was necessary to adopt some method of testing the squareness of the horizontal and vertical faces, which were required to be as close as possible to 90 degrees. The ingenious form of "square " used for this purpose is illustrated in Fig. 12. This GAGE GRINDING 287 is amply a very accurate cylinder, having bearing surfaces at the ends which are exactly perpendicular to the axis of the cylinder. This "cylindrical square" is made of machine steel and is pack-hardened. It is bored out to decrease the weight Fis- 13- RooehlDS out Gagea on Filing Michluo somewhat, although a fairly heavy cylinder is preferable, so that it will not move or change its position easily when meas- urements are being taken from the cylindrical surface to an angle-plate, as is sometimes desirable. Each end of the "square " is recused so that the bearing surface is only about J inch wide. 288 GAGE DESIGN AND ' GAGE-MAKING This bearing surface is also ground slightly concave so that the bearing is around the outer edge or circumference. When this "cylindrical square " is placed in contact with a surface as shown in the illustration, even a slight deviation from the vertical will be indicated by the passage of light. Roughing out Gages on Bench Grinder and Filing Machine. — Standard machine tools, such as lathes, milling machines, etc., are used for machining different kinds of gages to the ap- proximate size, the method of roughing out the different gages depending more or less upon their form. This part of the work, however, is often facilitated in many cases by the use of the bench grinder and filing machine. The bench grinder may be used on various forms of gages made of flat or sheet stock. The gage is held on a suitable rest and s guided by hand for grinding it to the approximate shape and size required after rough-machining. The filing machine shown at work in Fig. 13 is also very useful in connection with the work of the gage de- partment. This illustration shows a stack of T-shaped limit gages being filed. These gages are soldered together tempo- rarily, and they are filed on this machine to within about 0.010 inch of the required dimensions. Heat-treatment of Gages. — In the heat-treatment of gages, the important points are to prevent warping, to harden those parts which require hardening, and at the same time to leave any sections of the gage soft, if this is necessary, either as a means of preventing warping or to increase the toughness of the steel wherever breakage is likely to occur when the gage is in use. The heat-treating process must be governed by the kind of steel used for making gages. For instance, in the case of machine or low- carbon steels, the gages are pack-hardened, whereas the high- carbon steels are hardened by the general methods common to steels in this class. The temperatures required and the exact method of procedure depend upon the particular steel used, and the fundamental principles governing the heat-treatment of gages are similar to those which apply in heat-treating similar steels used for other purposes, although gage work may require some modification as to the details of the process. HEAT-TREATMENT OF GAGES 289 When gages are made of machine steel containing^ say, from 0.15 to 0.25 per cent of carbon, the steel is first carburized, is then permitted to cool, and after being reheated for hardening, is quenched in oil. The hardening is followed by a tempering process, in the case of thread gages, which should be drawn to a temperature of approximately 375 degrees F. A temperature of about 1450 degrees F. should be about right for carburizing the low-carbon steels referred to. The time for carburizing depends upon the size of the gage and depth of penetration desired, but for small gages it is about five or six hours. When the gages have cooled after carburizing, they may be repacked with material previously used for carburizing, and reheated to a temperature of 1400 degrees F., which is followed by quenching in oil. In some cases, a gage made from low-carbon steel is quenched in oil immediately after the carburizing heat. This method hardens only a thin outer skin, but tends to prevent distortion, and for this reason is sometimes employed in the case of profile or contour gages which cannot be ground after harden- ing. If grinding is practicable, then the gages may be heat- treated by carburizing, cooling, and then reheating for hardening, as previously described. This latter method is also employed in the case of plug gages or other forms which may be made of steel having possibly 0.40 or 0.50 per cent of carbon. Tool steel gages containing from 0.85 to 0.95 per cent of carbon can be hardened, after rough-machining, by heating above the decalescent point and quenching in oil, the process being carried on in the usual manner. The rough-machined blank is usually heated to from 1450 to 1475 degrees F., and it may afterwards be drawn to 300 or 400 degrees F. The tempering temperature may depend upon the method of using the gage. For instance, in one shop where many gages are made and used, the gages used by machine operators are drawn to about 230 degrees F., or just enough to remove the hardening strains. It has been found by experience that tougher gages are needed for use in connection with bench work, etc., to prevent breakage, and the drawing temperature is about 530 degrees F. The hardening tempera- tures may vary somewhat, depending upon the manganese and 2go C5AGE DESIGN AND GAGE^AIAKING the silicon contents. The temperatures given are for steels having from 0.25 to 0.35 per cent of manganese, and from 0.12 to 0.18 per cent of silicon. Drill rod is often used for making very small plug gages which are heat-treated the same as any high-carbon steel, and are drawn after hardening to about 300 degrees F. Preventing Distortion in Heat-treating Machine Steel Gages. — In manufacturing solid gages of the plug or ring variety, gage-makers often experience difficulty in overcoming warpage of the material. Machine steel generally comes out of the fire after carburizing considerably distorted and shnmk, and, in order to prevent this, manufacturers have endeavored to treat it in various ways. One method is to anneal the steel previous to machining. This has been tried with more or less satisfactory results, but it does not entirely eliminate the possibility of the gage shrinking during the quenching operation. A prominent manufacturer of gages employs the following method of heat-treating machine steel gages: First, they are rough-machined all over to within approximately one-sixteenth inch of the finished size. Then they are heated in an ordinary muffle furnace to 1400 degrees F., and quenched in oil, the same as in hardening tool steel. Owing to the low carbon content of open-hearth steel — between 0.15 and 0.20 per cent — the sted does not harden sufficiently to prevent free cutting. After quenching in oil, the gages are allowed to cool oflf. Then they are finish-machined all over to within the desired limit and packed in bone-dust for pack-hardening. The box in which the gages are packed is then placed in the furnace, heated to a tem- perature of 1500 degrees F., and allowed to remain for not less than an hour, the exact time depending upon the depth of pene- tration desired. After the box has been in the furnace for the correct length of time, it is removed, and the gages are quenched in oil, cleaned off and ground to the finished size. This method has proved satisfactory, resulting in practically no losses from shrinkage; the gages can be machined to within very close limits of the finished dimensions, and then pack-hardened without danger of warping beyond the amount left for finish-machining. HEAT-TREATMENT OF GAGES 291 Caxeful measurements of several gages after carburizing showed that they had not shrunk 0.0001 inch. It is general practice to make large cylindrical plug or thread gages from rings and mount them on a hollow handle in order to reduce the weight. In making thread gages of this type, trouble has been experienced in preventing warpage of the rings, and owing to the time required to lap down large plug gages, these are generally machined within close limits of the finished diam- eter for lapping. When a small amount had been left for lapping, the ring shrunk in some cases so that the gage would not clean OAQE. 1= c .-^- — 5=^5r^ ] -1- •f —LEAD- _ -I L -r r.BATH- ~ZI^_ ^ t--^ -4nMiB«««va^ B OAQE - -•HIELO ■. r ^ -irTi^ri±---f4r^BATHr-.zir — J LL npx -z=^^i^a 3£^:tEi^ Uaehinery Fig. 14. Two Methods of hardening Gages up, and, in order to avoid this trouble, the following method was tried: After the gage had been finish-machined, the hole was fitted with a cast-iron plug which was made a fairly good drive fit. The gage with the plug inserted was packed in bone-dust, heated to 1500 degrees F., and quenched in oil. A gage car- burized in this manner did not shrink o.oooi inch, which is quite remarkable, considering the usual shrinkage of machine steel. Hardening Gaging Surfaces Only. — The lead bath is con- venient to use for hardening gages, especially when only certain parts of the gage need to be hardened. The object of partially annealing sections of some of the gages is to toughen them and 292 GAGE DESIGN AND GAGE-MAKING prevent breakage. One method of hardening a T-shaped limit gage is illustrated diagrammatically dX Ay B^ and C, Fig. 14. The gage is first dipped in the lead bath to the depth indicated at Ay the bath having a temperature of 1450 degrees F., approxi- mately, for this particular steel. The gage is then quenched in water. The end of the gage is next dipped in the lead bath to the depth shown at £, thus partially annealing this end. The -=_--!- ^=^^=^ -zo: B ^*»^ n D n QAaE \ ^ LEAD BATH x: I -» T r JfMMiMry Fig. 15. Diagrmms illiittnitiiig how Long FUt Gages are bardened to preyent Distortion and Breakage lower gaging end is finally dipped in the lead, as shown at C, and is quenched. In this way, the central part of the gage is left comparatively soft and tough. Ring gages having projecting handles are first hardened all over and then the handles projecting from each side are softened in the lead bath so they cannot readily be broken off. These gages are made from 0.85- to o.gs-per-cent carbon steel, and they are at first hardened all over by heating to about 1450 degrees F. i HEAT-TREATMENT OF GAGES 293 and then quenching in water. The temper is then drawn to 3cx> degrees F., in the oil bath, and finally each handle is com- pletely inmiersed in the lead, leaving the ring section hard. Hardening Long Gages. — The long flat gages such as are used for checking over-all lengths, should be hardened locally or on the gaging ends only, in order to prevent warping and at the same time make the gage less brittle and better able to withstand shocks. The three diagrams -4, B, and C, Fig. 15, illustrate the method of procedure. One end of the gage is hardened at a time by holding the gage at an angle of about 45 degrees and immers- ing the end as illustrated at A . The heated end is quenched in water and is then polished so that temperature colors will be visible. The hardened end is next partially submerged in the bath as at 5, and it is held in this position imtil a slight color begins to show on that part of the gage end which is above the lead bath. In order to prevent this gaging surface from becom- ing overheated, the position of the gage is reversed and it is submerged in the water bath as illustrated at C. Use of Shield Tongs in Gage Hardening. — For some forms of gages, special shield tongs are used when quenching in order to prevent the portion covered by the tongs from hardening. The use of these tongs as applied to a flat gage Z?, Fig. 15, is illustrated at £. This gage is made of -^^-inch flat steel. After the gage is preheated, it is placed edgewise on a flat cast-iron plate in the gas furnace and heated to a hardening temperature of about 1450 degrees F. The gage is removed from the furnace by means of the shield tongs which cover most of the body, as illustrated at E\ consequently, when the gage is quenched, that part protected by the tongs is not hardened to any extent. The next step is to soften about half of the projecting gage end by immersing it in a lead bath as illustrated at F. If the entire projecting end were left hard, it might easily be broken, but by dipping this end in the lead bath to the depth shown, the outer portion, which is not used for gaging, is softened and made tough without inter- fering with the hardened gaging surface. Another form of gage requiring the use of the shield tongs is illustrated at G, Fig. 15. This is also made of flat stock and it is 294 GAGE DESIGN AND GAG&-MA£ING a difficult gage to harden without distortion unless the work b done properly. .This gage, like the preceding one, is placed edgewise on a flat cast-iron plate while heating to the hardening temperature. This plate should extend the full length of the gage, and it prevents the latter from warping while in the fur- nace. The gage is first inserted with the taper point inward, and when it has been heated to about 1300 degrees F., it is turned around so that the point is toward the furnace door in order to HEAT-TREATMENT OF GAGES 295 ( insure uniform heating. Fig. 16 shows how the shield tongs axe I applied to this gage for quenching it. Each edge is exposed to ; the water and the central portion is left soft. After hardening gages of this kind, the strains are immediately relieved by heating the gage slightly. This is done by simply holding the gage for a short time close to the furnace, and then allowing it to cool in the air. Use of Tin Shield for Gage Hardening. — The diagrams Z), E, and F, Fig. 14, illustrate a second method of hardening T-shaped limit gages when it is desired to harden them along the edges as indicated by the shaded area. A tin shield which fits closely against the central part of the gage on both sides is placed in the position illustrated at E. After the usual preheating the gage is placed in k lead bath with the shield in place, as illustrated at F. When thoroughly heated, the gage is quenched in the water and is allowed to cool imtil it is just below the sizzling point, after which it is quickly immersed in the oil bath. The shield prevents the central part of the gage from hardening. Heat-treatment for Machine Steel Snap Gages. — Snap gages made of machine steel should be pack-hardened on the gagiiig ends or jaws only. In one shop making these gages, the handle of the gage is covered with fireday so that it will not become car- burized. The temperature for the carburizing process is about 1500 degrees F. After the gage ends have absorbed enough carbon, the pack-hardening box and the gages it contains should be allowed to cool considerably below the decalescence point before being heated again for hardening. Ordinarily, the pack- hardening boxes are allowed to stand over night. The jaws are next heated to about 1450 degrees F., by dipping them in a lead bath, after which they are quenched in water. In order to toughen the jaws and prevent them from breaking oflf just back of the flat measuring surfaces, each side of the gage is partially submerged in the lead bath, as indicated in Fig. 17, so that the hardness of the gage is reduced at the weakest part of the gage section. Restoring Worn Gages by Electroplating. — Methods of mak- ing worn thread gages, plug gages, etc., larger, after they have 19 B 396 GAGE DESIGN AND GAGE-MAKING been reduced below the mipiTwii m ^ze by wear, are of consider- able importance to every manufacturer. Many of these e]q)en- ave tools can eaaly be saved by plating on them a film of hard, close-grained nickel and then lapping the gages to size. In depositing nickel on gages, it is necessary to remove all grease due to handling, as the bath used is very nearly neutral. This may be done by placing the gages, for 15 minutes, in a bath consisting of 8 ounces of caustic potash to 2 gallons of very hot MARKING FINISHED GAGES 297 water; this bath should be renewed quite often if many gages are to be plated. When removed from the bath, the gages should be thoroughly scrubbed with pumice-stone and water, rinsed, and then placed in a bath consisting of 8 ounces of hydrochloric acid to I gallon of water; this bath is used cold. From 15 to 30 minutes immersion will remove any oxide that may have formed. The gages should then be thoroughly rinsed, and dried without touching them with the hands, so that greasy spots will not be made on them. The plating bath is made of from 12 to 14 ounces of double sulphate of nickel and ammonia (pure) per gallon of bath, dis- tilled water being used. The nickel salts are put in a wooden tank and dissolved by pouring hot water on them and stirring with a clean wooden stick. After the salts are completely dis- solved, cold water is added to bring the solution to the right pro- portion. Common table salt is then added to decrease the resistance. The bath should be a little bit add, as it makes the nickel somewhat harder; the acidity of the bath can be tested with a piece of litmus paper. The anode should be made of pure cast nickel, as it gives better results than rolled nickel. The surface of the gages should be calculated and the same amount of anode surface exposed. The current used should be regulated very carefully, as the results are highly dependent upon this. The plating should be begun with a current of from 9 to 10 amperes per 100 square inches of plating surface, and be reduced after the deposit starts to from 0.75 to 1.25 ampere. The voltage used depends some- what upon circumstances, but should begin at 5 volts and be diminished to from 1.5 to 2 volts. The amount of metal de- posited on the gages may be determined by the use of a pair of mi- crometers in the case of plug gages. With thread gages, a piece of steel stock of known size may be immersed in the bath and measured to determine the amount of metal deposited on the threads. The tools do not have to be annealed, which is an advantage that every toolmaker can appreciate. Etchograph for Marking Finished Gages. — The size, number, or any other figures or lettering required, may be marked on the 298 GAGE DESIGN AND GAGE-MAKING gage by means of the etchograph shown in use in Fig. 18. The gage or other part to be marked is held on a plate to which one of the leads or wires is attached. The marking pencil which is connected with the other lead consists of an insulated handle and a pointed copper wire which does the marking. As this point is moved along the surface of the gage, the higher electrical Flf. 18. Marklas Siit, ate, on Gat* hj Hsan* of Etchofrapli resistance of the steel results in burning the surface and thus leaving a dark oxide mark. This apparatus was developed by William Brewster & Co., Inc., New York City. It consists of a transformer tor connection with an ordinary electric lighting circuit to provide for obtaining the required current and voltage for operating the etchc^aph. Two leads are taken off from the secondary of the transformer, one of which runs to a steel plate upon which the work to be marked is placed. The other lead runs to the " jiencil " of the TAPERED GAGES @ ® @ ® OC 3a ..311:=^' is :i Pig. ig. Taper Gagei md DevicM vied in uiembUng them 300 GAGE DESIGN AND GAGE-MAKING etchograph. This apparatus is adapted for use in connection with standard alternating current of no volts, 60 cycles, and the consumption of current from the line is from i J to 2 amperes. To provide for obtaining the desired depth of marking, a rheo- stat is furnished which provides for adjusting the amount of current delivered to the pencil of the etchograph. This pencil is used as an ordinary lead pencil would be handled. The etcho- graph is adapted for. marking either iron or steel, and all classes ■«= X ^ CL 1 m- i § I By L JCcMfelMrr Fig. 30. Gage for Measuring Tapered Work of high-speed carbon and machine steel may be marked with equal facility, no matter whether it is in the hardened or soft condition. Sectional Taper Gages. — A common way of making taper gages is to machine a tool-steel blank until there is just enough material left for grinding and lapping after hardening. The degree of skill required and the trouble that develops are widely known, and many of the finished gages, when tested for hardness, will be subject to rejection. How this trouble may be avoided by making the gage in four parts — two gage-plates and two k TAPERED GAGES 301 Maakinenf Fig. 21. Block for Holdixig Plug Gages used for Setting Gage shown in Fig. ao cover-plates — is shown in Fig. 19 by the upper drawing No. i. The machining is an easy matter. Before hardening, the gage is assembled just as if finished, except that the holes for the taper pins are not reamed, but are drilled straight. After that, the gages are taken apart and holes are drilled in the gage-plates large enough to insert soft plugs after hardening. The remain- der of the work simply consists in hardening, grinding, and lapping only straight surfaces, and assembling. The taper pins are riveted over slightly on the small end and secure the parts in position. To assemble these gages with accuracy, the attach- ment shown by drawing No. 2 is used. Standard plugs are employed and the distance A is determined with the help of trigonometry. (Rules applying to all calculations relating to taper gages may be found in Machinery's Handbook. See " ]!»Ieasuring and gaging angles and tapers," in index.) A limit taper gage may be easily made by etching two lines across the gage, as shown by drawing No. i. The positions of these lines are determined with the help of a limit plug, shown by drawing No. 3. The two diameters of the plug must corre- spond with the required limit of the finished part. It is only necessary to etch the lines exactly over the center of these plugs, as the distance between the lines will give the limit. This plan will render unnecessary the making of a double-end gage and also will save considerable time when inspecting the work. Drawing No. 4 shows a more costly design of taper gage which, in the long run, will be more economical, particularly when the amount of wear on the gage is taken into consideration. When it is once made, all that needs to be done when the surfaces are worn is to take out the gage blades, regrind or relap them, and put them back into place; no adjusting or inspecting will be necessary. This gage may also be inspected directly with the micrometer. 302 GAGE DESIGN AND GAGE-MAKING Adjustable Taper Gage. — The taper gage shown in Fig. 20 is a type that has often been used for testing accurate plug gages, etc. In the construction of this particular gage, the two yokes A were rough-turned from machine-steel forgings and the sur- faces B were then ground on a surface grinder to insure having them parallel. The pieces C were made of tool steel; they were hardened, after which the surfaces D were ground and the sur- faces E, ground and lapped. The slots were made -^ inch larger than the screws, in order to provide for the required amount of adjustment. This tool was used for g^ing the tapered plugs while grinding, the method of setting for the proper taper in inches per foot being as follows: The block shown in Fig. 21 was made of machine steel V\t. n. TTntTerMl Form of Sins B« and An^a Gag* with the holes of accurate diameter and accurately spaced be- tween centers. Assuming that a plug is required with a taper of ij inch per foot and i inch in diameter at the large end, it would be necessary to use accurate -JJ and i-inch plug gages in connection with the block. As the center distance of the holes in the block is 3 inches, it will be evident that the difference in diameter of the plugs should be ^y inch to correspond to a taper of i^ inch per foot. In setting the gage shown in Fig. 20, all that is necessary is to insert the standard plugs in the block, and then set the parallels to engage the protruding ends of the plugs. In case other tapers than ij inch per foot are required, it would be necessary either to use plugs of a different size or to vary the center-to-center dbtance, by making another block. A block arranged for adjustment would be desirable. This gage will be found useful in any grinding department, as it is a sunple matter to dupUcate a special taper for a reamer or similar tool. ANGLE GAGE AND SINE BAR 303 Universal Angle Gage and Sine Bar. — The universal angle gage and sine bar to be described may be used for setting gages of the general type shown in Fig. 20, and for other taper and angular work. This combination angle gage and sine bar^ which is shown in Fig. 22, is a sine bar provided with two slots and two screws for clamping disks of various diameters to obtain differ- ent distances between their centers. The bar is provided with one angular end which is foimd convenient in many cases. The slots are also foimd useful for clamping the bar to an angle-plate. While the length of the bar may depend upon the natiure of the work, a distance of 10 inches between the buttons makes the calculations easier. The rule for determining the center distance between the setting disks of an angle-gage is as follows: The distance between centers must be equal to the difference in the radii of the disks divided by the sine of one-half the taper angle. CaDing the distance between centers L, the difference in the radii of the setting disks 1? — f , and the whole taper angle a, then : smf a To illustrate the use of the ride for determining the distance L between the centers of the setting disks, the following examples are given: Suppose it is required to set the disk for an included taper angle of 60 degrees, using disks i inch and 5 inches in diam- eter, respectively. One-half the taper angle is 30 degrees and the sine of 30 degrees is 0.5. The difference in the radii of the disks is 2 inches. L = -: — \ — = — = 4 inches, sm ^ a 0.5 Spline Gages. — The use of shafts splined with four or more splines of the general type shown in Fig. 23 has become quite common in the manufacture of automobiles and airplane motors. The close limits shown are often required, and gages for this class of work have to be of unusual accuracy. Fig. 24 shows a design of gage which was first suggested for this work. This consists 304 GAGE DESIGN AND GAGE-MAKING of a solid one-piece ring of hardened steel ground all over, but, as indicated in the illustration, the inside surfaces of the gage could not possibly be ground in the usual way. Therefore, the +O.OOQB h V.O r I I il i*-^.875lS-J5ft^ TT I a ¥«eftecial thread-grinding, 175 Lead and diameter measurements on thread gages, 201 ) Lead errors, test ing thread gages for, 207 Lead-measuring apparatus, improved design of optical, 229 Lead-measuring machine, electrical con- tact type of, 232 equipped with sliding contact and magnifying pointer, 224 Lead-screw errors, attachments for cor- recting, 151 Lead tester, equipped with micrometer ' and dial gage, 214 having sliding contact and magni- fying reflector, 223 I..ead-testing machine, fluid-gage type, 222 having multipljring indicator and size-block adjustment, 220 micrometer type of, 209 Limit, definition, '4 Locating points in fixtures and gages, importance of agreement between, 28 Machining operations, different, toler- ances for, 41 Master gage, Whitworth, lapping, 198 Maximum metal dimension, definition, 5 Micrometer, boating, for diameter measurements, 202 Micrometer type of lead-testing ma- chine, 209 Microscope, adjustment of, relative to screw thread, 246 focusing upon side of screw thread, 249 Microscopic measuring machines, for testing accuracy of contour and thread gages, 233 illuminating apparatus, 251 type having gage adjustment, 233 INDEX 309 Microscopic measuring machines, with micrometer screw adjustment, 246 Operation sheets for developing tool and gage equipment, 23 Optical lead-measuring apparatus, im- proved design of, 229 Optical projection apparatus, 253 Pin gages or end-measuring gages, grinding, 279 Pitch diameter measurements, com- parator for, 205 Pitch, testing accuracy of, with micro- scope, 251 Plug and ring gage allowances, 76 Plug and ring gages of the same dimen- sions, assembling, 75 Plug and snap gages, 49 combination, 67 Plug gages, 61 flat, 67 lapping, 190 large, form for, 74 provision for escape of air in, 74 special tjrpes of, 65 testing by projection method, 258 Precision & Thread Grinder Mfg. Co/s thread grinder, 175 Prestwich fluid gage, 205 Profile or contour gages, 79 Projection apparatus for thread angle, 269 Projection method, application to ring gages, 260 of testing gage threads, 253 of testing plug gages, 258 Projector, vertical, for screw threads, 266 Radius- and contour-measuring in- strument, 239, 241 Receiving gages, 88 Reference gages, 12 for snap gages, 57 Ring and plug gage allowances, 76 Ring and plug gages of the same dimen- sions, assembling, 75 Ring gages, application of projection method to, 260 laps for, 191 Screws, set of thread gages for, 130 Screw threads, adjustment of mi- croscope relative to, 246 focusing microscope upon side of, 249 measurement of, 245 measuring diameter with microscope, 250 projection method of testing, 253 testing pitch and angle with micro- scope, 251 tolerances for, 46 Sectional taper gages, 300 Sheffield Machine & Tool Co.'s lead tester, 220 Shield, tin, use of, for gage hardening, 29s Shield tongs, use of, in gage hardening, 293 Sine bar and angle gage, universal, 303 Sliding-bar, flush-pin, and hole gages, 102 Sliding-bar gages, 119 Snap and plug gages, 49 combination, 67 Snap gages, built-up, 52 grinding, 274 having removable inserts, 51 reference gages for, 57 Spline gages, 304 Square, cylindrical type of precision, 286 Steel for gages, 272 Steel Products Engineering Co.'s gage grinder, 277 I aper gage, adjustable, 302 sectional, 300 Tapers, dimensioning, 36 Tapped holes, complete gage set for, 134 Templets, measurement of, 236 Thread angle, determining error by three-wire method, 203 Thread angle projection apparatus, 269 Thread-cutting tools, gaging the ends of, 139 3IO INDEX Threaded blanks, carburiziiig, 157 Thread forms, maccurate, tracings of, 261 photographing, 264 Thread-gage grinding machine having traversing wheel) 183 Thread gage manufacturing tolerances, 73 Thread gages, for screws, set of, 130 grinding and lapping, 171 lapping, i8g lead and diameter measurements on, 30I machines used for lapping, 200 microscopic measuring machines for testing accuracy of, 233 production of, 1 28 special forms of, 137 testing for lead errors, 207 wheels used for grinding, 188 Whitworth, lapping, 191 Whitworth, making, 152 Whitworth, tools for cutting, 149 Thread grinder of type having trav- erang master screw, 178 Thread grinding, general types of ma- chines used for, 173 Thread-grinding lathe, special, 175 Threads, gage, projection method of testing, 253 gage, tools for cutting, 138 screw, adjustment of microscope rela* live to, 246 screw, focusing micrometer upon side of, 249 screw, measurement, 245 screw, measuring diameter with mi- croscope, 250 Thread tool gage, U. S. standard, 144 Thread tools, gages for, 141 gages for 29-degree, 145 Thread tools, grinding to the required angle, 147 Three-wire method of determining error in thread angle, 203 Timing lapping periods, 196 Tolerance and initial clearance, rela- tion between, 48 Tolerance, definition, 4 Tolerances, for center distances, 46 for different machining operations, 41 for gage3^,8' for screw threads, 46 gage manufacturing, 70 indicating, for a contour, 97 on drawings, methods of designating, 38 on work and on drawings, 41 practical, for interchangeable manu- factiure, 39 thread gage manufacturing, 73 Tongs, shield, use of, in gage hardening, 293 Universal angle gage and sine bar, 30 U. S. standard thread tool gage, 144 iValtham Machine Works, grinding and limping machine, 278 West & Dodge Co.'s lead tester, 214 Wheels used for thread-gage grinding, 188 Whitworth master gage, lapping, 198 Whitworth thread gages, lapping, 191 making, 152 tools for cutting, 149 Whitworth threads, forming wheel for grinding crest of, 186 lapping crest of, 195 laps used for, 192 Wing or windmill gages, 77 Working gages, 9