Blind Bores, Keyways and Thin Sleeves: Internal Grinding Countermeasures

Most internal grinding rules assume a clean through bore. What actually arrives is often blind, stepped, broken by a keyway or a cross hole, or the whole part is a thin sleeve. These forms produce defects a through bore never shows, and almost none of the answers live in the grinding parameters - they are changes to work holding, to how the wheel contacts the bore, and to the order of operations. This article covers the four cases and how to measure the result.

1. Blind and stepped bores: four points

The danger in a blind bore is the wheel face hitting the bottom, so the stroke is set twice: rough it in on the left stop, then fine tune with the screw on that stop so the wheel face just enters the relief groove and reverses. Before that, mark the quill from the bore depth and dry run the stroke with neither wheel nor part turning.

🔴 Note the overrun goes the other way from a through bore: a through bore takes one third to one half of wheel width, a blind bore takes less at the mouth. Where the bore face has a squareness requirement, use a cup wheel and keep its diameter modest so contact on that face stays one sided. For the through bore rules, see Why Internal Grinding Is the Hardest: Get the Wheel and the Quill Right First.

2. Interrupted bores: three forms, three defects

When a wide slot, a keyway or a cross hole breaks the bore wall, wheel contact is interrupted and form error follows easily. The three forms fail in different ways and take different answers:

For dubbed keyway edges there is a far more direct method: where accuracy matters, pack the keyway with hardwood or a phenolic before grinding.
It does not adjust a parameter, it fills the interruption back into a continuous surface. That is the root answer; parameters only reduce the symptom.

One thing is common to all three: at finish, reduce depth of cut and add spark out passes.

3. Plunge grinding: short bores only, four things to watch

Plunge grinding is productive but only suits parts whose bore is not long. The wheel feeds in continuously under heavy load, so it loads and dulls quickly:

Poor alignment is easy to recognise: the internal face comes out convex or concave. Where two bores share a concentricity requirement, for example 40mm and 20mm, grind both in one setting and change quills rather than resetting the part.

4. Thin sleeves: round while held, not round once released

A thin sleeve is the classic part you only discover is wrong at the inspection bench. Clamping force, grinding force, grinding heat and residual stress all distort it, and the most deceptive of them is the three jaw chuck:

The part is held with elastic distortion already in it, so the bore really is round when ground; but the moment the jaws release, that distortion springs back and the bore becomes a curved triangle.
The problem is not that it was clamped too hard. It is that the force arrives at three points.

5. Expanding sleeve or axial clamping: two directions

Three more things belong with it: separate roughing from finishing so roughing distortion is corrected at finish, use the lowest clamping force and ample coolant at finish, and choose a wheel that is coarser and softer, since a wheel that will not self sharpen raises grinding force and pushes the part out of shape. Where stock removal is large, slacken the jaws slightly and re-clamp gently before the finish pass so the part returns to shape. Heat treatment before and after roughing relieves stress. A part whose end faces are lapped and flat also responds better to axial clamping.

Thin plates are a different subject - thermal and clamping distortion must be treated separately - see Thin Parts Bow After Grinding: Separate Thermal Distortion from Clamping Distortion.

6. Coolant: cast iron bores usually go dry

This one runs against the usual assumption and is worth remembering on its own. Coolant helps temperature, distortion and burn, but a bore clears chips badly, and fluid turns swarf into a paste that loads the wheel face and scores the wall.

Grinding the bore of a brittle material such as cast iron is normally done without coolant.
If a customer insists on flooding a cast iron bore, the scoring they are complaining about may be exactly that - ask what the material is before discussing flow rate.

For other materials it depends on material behaviour, accuracy requirement, bore form and size, and what the machine is set up with. There is no blanket answer.

7. Measuring a bore is easier to get wrong than an OD

The general rule for bore size: find the maximum across the diameter and the minimum along the axis, and the real size is where those two agree. For roundness and cylindricity, read three or four positions around the bore and several along the axis, then compare.

⚠ In a bore you cannot see the angle at which the gauge touches the wall, unlike an outside diameter. An indicator tip must be perpendicular to the surface; with a lever type indicator, the smaller the stylus angle the smaller the error. The error you measured may be one you created, not a fault in the part. For concentricity between bore and outside diameter, mount on a mandrel between centres and read radial run-out, then halve the reading.

To trace a defect back to its cause, see Grinding Defects: Tracing Chatter, Burn, Cracks and Size Errors. For internal grinding work holding, motions and parameters, see Cylindrical, Internal and Surface Grinding: Workholding, Motions and Grinding Data.

This article is part of Grinding Wheels and Machines: The Complete Guide - Reading the Wheel, Mounting It, Dressing It, and Tracing Defects; that guide shows how the whole topic fits together.

Published: 2026-09-19｜Last updated: 2026-10-24