How to Measure a Bore

Read the drawing first

Start from the feature and its tolerance: the bore size, its limits and how deep it must be measured (see the measurement plan). Example from the lab: the brass bushing bore is Ø28 +0.05/0, so any reading from 28.00 to 28.05 mm passes.

Measuring a through bore with a caliper

1. Clean and zero: wipe the inside jaws and the bore, close the jaws and check the zero (ORIGIN on a digital caliper; see the vernier and dial caliper pages for their zero checks).
2. Get the jaws in: insert the inside jaws as far as they go, square to the face, and open them until both touch the wall with light, even force.
3. Across the centre: take the largest: rock the caliper gently sideways; off the centre line the jaws span a chord, which is shorter than the diameter.
4. Along the axis: take the smallest: a caliper tilted along the bore spans a diagonal, which is longer than the diameter.
5. Repeat in a second direction 90° away and near both ends of the bore; differences show out-of-round and taper.

On a caliper with separate knife-edge inside jaws the scale gives the bore directly; on models that measure bores with the outer faces of the lower jaws, add the jaw width marked on them.

Measuring blind-hole depth

Clear chips from the bottom first. Put the end of the beam flat on the reference face, push the depth rod down until it touches the bottom, keep it square to the face and read. A beam end that is not seated reads large; a chip on the bottom reads small.

Judging the bore against tolerance

Compare each reading with the limits (tolerances and fits). In the lab the 24-tooth gear has a bore of Ø20 +0.03/0 that reads 20.06 mm: above 20.03, so it is rejected. A blind hole specified 15 +0.05/0 deep that reads 14.93 mm is too shallow.

Common mistakes in a bore

The lab demonstrates the first four on the bushing and the blind-hole part; zero error is shown on outside diameters and shifts an inside reading the same way.

When a caliper is not enough

Choose by the tolerance and the 10:1 rule (choosing instruments). The measurement plan uses a caliper for tolerances of about 0.2 mm or wider; tighter bores need an inside micrometer, a bore gauge or a plug gauge.

Two inside micrometers are in the lab: a 5–30 mm inside micrometer with measuring jaws, which reads the Ø28 bushing bore, and a 62–75 mm three-point bore micrometer, set on a Ø62 ring gauge. A three-point head centres itself in the bore, so it is easier to use than two-point contact (see types of micrometer).

Recording the result

Write each reading with the section and direction it was taken in, the instrument used and the decision, as in the measurement plan and the first article record.

Planned for the lab

Plug gauges, pin gauges, dial bore gauges, telescoping gauges, two-point inside micrometers and depth micrometers are not in the lab yet. Their shared basics — clean faces, a checked zero, a square position and light, even force — can be practised in the caliper bore scenes below.