Calibrating a micrometer to ISO 3611

The metrologist of an inspection laboratory has to check a 25–50 mm outside micrometer before handing it over to a grinding station. Under ISO 3611, the instrument must be checked for screw accuracy, parallelism of the steel anvils and flatness of the measuring faces.

Calibration procedure and tolerances for an ISO 3611 micrometer

Micrometer range	Maximum permissible error MPE (ISO 3611)	Maximum anvil parallelism
0 to 25 mm	±0.004 mm	0.002 mm
25 to 50 mm	±0.004 mm	0.002 mm
50 to 75 mm	±0.005 mm	0.003 mm
75 to 100 mm	±0.005 mm	0.003 mm

All values are given for the reference environmental conditions of 20 °C.

Accuracy check on gauge blocks and anvil parallelism

The check uses at least 5 grade 0 or 1 gauge blocks chosen to test several full turns of the micrometer screw (e.g. 25.12 mm / 30.25 mm / 37.50 mm / 43.62 mm / 50.00 mm). Anvil parallelism is measured with an optical parallel. To go further into uncertainty calculations, read our piece on measurement uncertainty with the 5M method.

Zero setting and the measuring force of the ratchet

The ratchet or friction thimble must apply a steady measuring force of between 5 N and 10 N. A faulty ratchet throws readings out by several microns.

Example: verification record for a 25–50 mm micrometer

Micrometer 25–50 mm (MPE = ±0.004 mm). Readings on gauge blocks: 25.000 mm -> 25.001 mm / 37.500 mm -> 37.503 mm / 50.000 mm -> 50.005 mm. Verdict: at 50 mm, the error (+0.005 mm) exceeds the MPE by 0.001 mm. The instrument is withdrawn for screw overhaul.

Classic pitfalls when calibrating micrometers

The first pitfall is heat passing from the operator’s hand into the micrometer frame during the check. The second is dirty measuring faces. The third comes from a badly set thimble. To manage your measuring equipment, explore our CoteCote tooling page.