Operating principles and mechnism of micromters

The essential element of measuring instruments operating on the micrometer principle is a screw with precisely controlled lead, having a pitch of usually, but not necessarily, 40 threads per inch. The screw of the micrometer is integral with the measuring spindle, whose face establishes the measuring contact with the object. The distance of that contact face from a fixed datum constitues the measuring length, which is then displayed by the scale graduations of the micrometer.
The micrometer has two basic scales. One is a linear scale to measure directly the axial advance of the spindle in increments large enough to provide distinct reading of the graduation lines. The graduations of this scale are usually identical with the pitch of the micrometer screw.The other is a circumferential scale around a hollow cyliner locked to the micrometer screw to indicate the amount of partial rotation when the last turn of the screw during the gaging process is less than a complete revolution. Micrometers designed for mesurement in finer increments also have a vernier scale to permit the evaluation of fractions of circumferential graduations.