Internal (ID) grinders
Bores, bearing seats, and internal diameters — the surfaces you can't reach from the outside. ID grinding is a specialized discipline, and the workholding decision matters as much as the machine.
What they're for
An internal grinder spins a small grinding wheel on a quill inside the bore of a rotating workpiece. Typical work: bearing bores and seats, gear bores, hydraulic cylinder bores, spindle housings, die components. Where the bore's roundness, straightness, and surface finish determine how the assembly performs — which is nearly always — ID grinding is the process.
ID work is less forgiving than OD work. The wheel is small, the quill is long, and deflection is the enemy. Machine rigidity and spindle quality show up directly in the part.
Workholding configurations: I/I, I/O, I/I-O
Builders describe internal grinders by how the part is held. This is the first decision, and it follows directly from your parts:
- Chuck work (often called I/O or internal-chuck type): the part is gripped in a chuck and the wheel grinds the bore. Best for parts where the bore is the critical feature and the OD is just stock — housings, rings, sleeves.
- Between-centers (I/I): the part rotates between centers while the wheel works the bore. Best when bore and OD must be concentric to each other — the centers establish the relationship.
- Combination (I/I-O): machines that handle both, so one grinder can take chucked parts one day and between-centers parts the next. More flexible, more money — worth it if your mix genuinely spans both.
If every part you grind is chucked, don't pay for between-centers capability you'll never use. If your mix is split, the combination machine usually pays for itself in avoided second setups.
What to compare when you're shopping
- Bore range. Minimum bore diameter (this is the hard limit — the quill has to fit) and maximum bore diameter and depth. Bring your smallest bore, not just your largest.
- Spindle/quill rigidity. Ask about quill diameter options and spindle bearings. On ID machines this is the accuracy story.
- Workholding flexibility. Which of the configurations above the machine supports, and how long changeover takes.
- Dressing inside the bore. How the wheel gets dressed and whether it's programmable. Small wheels wear fast; dressing strategy is cycle time.
- Measurement. In-process gauging options. On bore work, measuring after the fact means scrapping after the fact.
- Control. See our controls comparison — same logic applies here.
- Service and parts. ID spindles are wear items. Ask about rebuild/exchange programs and lead times before you need one.
When ID grinding is the right call — and when it isn't
Right call: bores with tight roundness and finish requirements, bearing seats, parts where bore-to-OD concentricity matters.
Wrong call: large-diameter bores in heavy parts — a vertical grinder with the part sitting on a table is often the better answer. Occasional bore work in a high-mix toolroom — consider a universal grinder that does ID and OD. Honing or hard-turning may beat grinding for some bore applications on cost — we'll say so if that's what your parts suggest.
What we don't know yet. Detailed public specifications for some builders' internal lines (for example, the second internal family in certain catalogs) aren't published. When we match you with a builder, we confirm bore range, configurations, and options against their current spec sheets — not against this page.
Related
- Cylindrical (OD) grinders — the outside-diameter counterpart
- Vertical grinders — for large bores in heavy parts
- Grinding for bearings — raceways are ID grinding's home turf