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***Prior to reading, NOTE differing spellings of "Micrometer" and "micrometre".....the later being 1x10^-6, the former being the gauge***

A micrometer is used for measuring not only diameter, but width too, and is used where accuracy of measurement is needed (see vernier caliper comments).

The graduations on a standard micrometer are in 0.01mm graduations, however - subject to certain constraints - a good metrologist can subgraduate these to get a reasonably acurate reading of about 2 micrometres.

the certain constraints are numerous....temperature, anvil accuracy (and damage), calibration, softness of material being measured, speed of measurement.....

At these scales, temperature becomes critical.....I used to work in a temperature/humidity controlled environment for tolerances of less than 15 micrometres....that was both challenging and - occasionally quite boring.....after every pass, the workpiece had to be left with the coolant running to return to ambient temperature before measuring.

Measurement was also different. on tightening the screw on the micrometer, very very slow movements had to be undertaken in order to eliminate - as far as possible - the momentum of the gauge overtightening the workpiece against the anvil and giving a false reading....and then it was always measure 5 to 8 times to verify.

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1st of all we should calibrate this outside micrometer and check whether the reading is correct or not. the least count is zero if its range starts from zero since it depends sometimes the least count is 50mm. most often it is used to precise measurements such as journals of bearings...

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Least count of any instrument is the least value which can be measured with the instrument.

And least count of screw gauge (Micrometer screw gauage) is

=

value of one small division on the main scale / Number of divisions in circular scale.

Micrometer is used to take linear measurements like length, thickness, diameter.

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I was ALWAYS taught to use the ratchet at the end of the thimble to get consistent readings when checking multiple identical components, that's what it is for, also in our inspection department we had to have our micrometers checked against a standards block every week as we worked to very close tolerances (imperial measurements + / - .001- .0005 inch)

Things got easier when we got digital micrometers.

 

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