A length measuring device consisting of a fixed and a movable measuring surface, which is adjustable with a fine thread, is called a micrometer screw.
The measuring sample is held against one of the two parallel measuring surfaces and the thread is turned up or down by an adjusting screw until both measuring surfaces touch
the testing sample.
The thread is turned by means of a ratchet in order to avoid falsified measured values due to different measuring forces.
If the measuring force is too high deformation of the testing sample or the micrometer can happen.
The occurring measuring forces should be constant and reproducible. The ideal measuring force is between 5 and 10 Newton [N].
The roundness of a testing sample cannot be measured with a micrometer screw.
Almost all micrometers are equipped with plastic handles or shells for insulation.
In order to obtain optimum measuring results, the micrometer must be held on this sheathing.
Otherwise, the surfaces held in place may expand due to the body heat and falsify the measurement result.
The room temperature for the most accurate possible measurements is 20 °C.
There are analogue micrometers as well as those with an additional digital display of the measured values.
Different micrometers are required for internal, external and depth measurements or for measurements on gear flanks.
Depending on the requirements, micrometers can be tested internally by the quality assurance department or
by external certified calibration laboratories, as well as by the respective manufacturer using linear standards, gauge blocks or setting rings and plane-parallel test glasses.