Question

what is the principle of micrometer screw

Answer 1

Micrometers use the principle of a screw to amplify small distances (that are too small to measure directly) into large rotations of thescrew that are big enough to read from a scale. The accuracy of a micrometer derives from the accuracy of the thread-forms that are central to the core of its design.

Answer 2

to amplify small distances into large rotations of the screw that are big enough to read from a small scale