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.