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micrometer caliper[mī′kräm·əd·ər ′kal·ə·pər]
a measuring instrument whose conversion mechanism consists of a screw-nut micropair. Micrometer calipers are used to measure linear dimensions by the absolute contact method.
A screw pair was used in an indicating device as early as the 16th century for the aiming mechanisms of cannon (1570); the screw was later used in various geodetic instruments. The first patent on a micrometer caliper as an independent measuring device was issued to J. L. Palmer in 1848 (France).
The operation of a micrometer caliper is based on the displacement of a screw along its axis when it is turned in a fixed nut. The displacement is proportional to the angle of rotation of the screw around its axis. Complete turns are read from a scale marked on the hub of the micrometer caliper, and fractions of a turn are read from a circular scale marked on the drum. A displacement of the screw of not more than 25 mm is optimum because of the difficulty of manufacturing a screw with an accurate pitch over a greater length. Therefore micrometer calipers are produced in several standard sizes, for measurements from 0 to 25 mm, from 25 to 50 mm, and so on. In a micrometer caliper with a measuring range from 0 to 25 mm, the zero mark of the scale on the hub should coincide precisely with the lengthwise mark on the drum, and the beveled edge of the drum should coincide with the zero mark of the scale on the hub when the measuring surfaces of the anvil and the micrometer screw are clamped together. Micrometer calipers with interchangeable anvils are used for measurements greater than 25 mm; such calipers are adjusted to zero by an adjustment gauge attached to them or by means of an end standard.
The object to be measured is clamped between the flat measuring surfaces of the micrometer caliper. The pitch of the screw is usually 0.5 or 1 mm, and the scale on the hub, correspondingly, has divisions of 0.5 or 1 mm; 50 or 100 divisions are marked on the drum, to give readings of every 0.01 mm. This is the most common value for a reading, but there are micrometers that read every 0.005, 0.002, and 0.001 mm. When the screw makes contact with an object, a constant axial force is provided by a ratchet friction device. Depending on the design (the shape of the body or the bracket into which a micrometer is built; the form of the measuring surfaces) or the purpose (measuring the thickness of plates and pipes and the depth of gear teeth), micrometer calipers are divided into flat, lever, sheet, and pipe calipers; screw-pitch calipers with inserts; and gear calipers. (See Table 1 for characteristics of Soviet micrometer calipers.)
|Table 1. Characteristics of some Soviet micrometer calipers|
|Flat ...............||0 to 600||±(2–10)|
|Lever ..............||0 to 2,000||±0–4)|
|Sheet ..............||0 to 5, 10, or 25||±4|
|Pipe ..............||0 to 10 or 25||±4|
|Gear ..............||0 to 1 00||±5|
|Bench ..............||0 to 10 or 20||±(2–3)|
Micrometer calipers are produced in manual and bench types, including those with a pointer indicator. Micrometer pairs are also used in depth gauges and internal micrometers. The most common are the flat types. Bench micrometer calipers (including those with pointer indicators), which are often called dial gauges, are designed for the measurement of small parts (up to 20 mm).
N. N. MARKOV