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Full text of "Modern Mechanical Engineering Vol-I"

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Measurement and Gauging.—The present system of measurement
is precise and positive, and is effected rapidly. The micrometer and vernier
tools are used for taking precise measurements, and the fixed gauges check
machined dimensions to predetermined limits.

Micrometer Calipers.—In micrometric measurement the pitch of
a fine screw thread is subdivided by means of graduations on the periphery
of a disk which revolves with it. In the English caliper (fig. 45) the screw
usually has 40 threads to
the inch, and the " thimble "
—the rotating element—
has 25 divisions. Since a
movement of the screw
through one revolution cor-
responds with a longitudinal
movement of •£& in., one
partial turn of the thimble
through one division moves the screw through -£§• of -^V in. = TUTTF in. To
enable the exact longitudinal movement of the screw to be read, the
barrel or " sleeve "—the cylindrical body—is divided in a line parallel with
the axis of the screw into 40 parts, but only every fourth division is stamped
i, 2, 3, &c., from zero, corresponding with o-i in., 0-2 in., 0-3 in., &c. Each
of these subdivisions thus represents 25 thousandths of an inch. To read
the caliper, therefore, multiply the number of divisions visible on the scale

Fig. 45.—Micrometer Caliper

Fig. 46. — Vernier Caliper

on the barrel by 25, and add the number of divisions on the scale of the
thimble reckoning from zero.
Vernier Calipers. — A vernier is fitted to instruments made for making
the finest measurements. An inch is usually divided (fig. 46) into tenths,
and a vernier, of length equal to nine of the divisions, is divided again into
ten parts. Each subdivision on the vernier is therefore jku in. shorter than
one division on the rule. When thousandths have to be read, each tenth
divisipn on the rule is subdivided into four, giving forty to the inch. Twenty-
four of these parts are taken on the vernier and subdivided into twenty-five
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