The Micrometer Model shows the principle of operation and the physical parts of a real micrometer. 
Micrometers use a screw to amplify distances that are too small to measure directly into large rotations of the screw that are big enough to read from a scale.
The accuracy of a micrometer derives from the accuracy of the thread that is at its heart.
The basic operating principles of a micrometer is that the rotation of an accurately made screw can be directly and precisely correlated to a certain amount of axial movement (and vice-versa), through the constant known as the screw's lead.