Common micrometer mistakes

Misreading the half millimetre

The sleeve has whole millimetres on one side of the index line and half millimetres on the other. Missing a visible half-millimetre line costs exactly 0.5 mm (NPL Good Practice Guide 40). Example: sleeve 7 mm with the half line showing and the thimble at 37 reads 7.87 mm, not 7.37 mm.

Read the index line from directly above: viewed from the side, the thimble line appears shifted (parallax).

Zero not checked

Check zero before measuring and again from time to time, closing through the ratchet exactly as when measuring. An unchecked zero error goes into every reading (see zero setting).

Dirt and burrs

Dust, coolant or a burr between a face and the part makes an outside reading too large. Wipe the faces and the part with lint-free paper, or close the faces lightly on a sheet of paper and pull it through; then check the faces for scratches and burrs, which are often near their outer edge (NPL GPG 40).

Tilt

A micrometer that is not square to the part gives a wrong reading; NPL GPG 40 warns of it for inside micrometers tilted in either direction. On a shaft, a tilted outside micrometer spans a longer line than the diameter and reads large: rock it gently along the axis and keep the smallest steady reading.

Force

Close the last part of the travel with the ratchet and read on the first click; spinning the ratchet fast gives false readings (NPL GPG 40). Do not screw the thimble down hard: the extra force deflects the frame and the anvils, and in the lab’s demonstration the reading comes out small.

Heat

Do not measure a part still warm from machining, and leave the micrometer and the part in the room long enough to reach its temperature, at least an hour: a 100 mm iron rod changes length by about 0.012 mm for 10 °C (NPL GPG 40). Hold the micrometer by its insulated grips or briefly.

Judging a part

Judge only a reading you can trust, against the limits. Example: a journal Ø17 k6 has limits 17.001 and 17.012 mm; a digital micrometer with 0.001 mm resolution reads 17.015 mm, above the upper limit, so the part is rejected. A 0.01 mm micrometer is too coarse for a tolerance of 0.011 mm: choose the instrument for the tolerance first (see choosing instruments).

In KELIBRON

On the 0–25 mm micrometer the lab demonstrates dirt on a face, tilt, too much force and an unchecked zero; none of these readings is judged. The Ø17 k6 journal is there to practise a rejection: on the 0.01 mm micrometer of its scene it reads 17.02, still above the limit, but so close to a tolerance of 0.011 mm the 0.001 mm digital micrometer is the right choice.