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End-Measuring Instrument
any of a certain class of instruments that are used to measure
or lay out inside and outside dimensions. Anend-measuring
instrument consists of two measuring surfaces, which are usually
called jaws. A dimensionis determined between the two measuring
surfaces. One of the surfaces constitutes the base of a singleunit
that includes a rule, or a graduated bar; the other surface is on a
head that slides along the rule. The
rule is divided into millimeters, and a vernier scale is
installed or engraved on the sliding head. To increasetheir
reliability, end-measuring instruments are made of materials that
have a high abrasion resistance andare not subject to corrosion,
such as hardened steels. In addition, the measuring surfaces are
chrome-plated or are reinforced by a hard alloy. Plastic
end-measuring instruments are also used.
In the USSR, several types of end-measuring instruments with
readings in 0.05 or 0.1 mm aremanufactured in several standard
sizes. The manufacture of end-measuring instruments with readings
in0.02 mm has been discontinued. Depending on their purpose and
design features, end-measuringinstruments are classified as vernier
calipers, height gauges, height-and-depth gauges (seeDEPTH
GAUGE),or gear-tooth calipers.
The vernier caliper (Figure 1) is the most widely used
end-measuring instrument. The first vernier calipersappeared in the
late 18th century in London, although wooden caliper rules without
verniers were used as
early as the 17th century. Three types of vernier calipers are
manufactured in eight standard sizes; thetypes differ in design and
in the number of measuring jaws. For measurements of up to 400 mm,
the jawsmay be extended from the zero reading. For larger
dimensions, the zero point of a measurement does notcoincide with
the zero mark.
Vernier calipers with an instrument range extending from 0 to
125 or 150 mm and with readings in 0.1 mmhave two sets of jaws
(Figure 1 ,a) as well as a rule depth gauge for measuring such
dimensions as thedepth to a shoulder or the depth of a groove or a
hole. The lower jaws are used to measure outsidedimensions; the
upper jaws, inside dimensions.
Vernier calipers with an instrument range extending up to 250 or
160 mm and with readings in 0.1 or 0.05mm (Figure l,b) also have
two sets of jaws. However, the lower jaws are used for both outside
and insidemeasurements, while the upper jaws are employed to lay
out or measure outside dimensions inside narrow
shallow grooves and holes. The upper jaws may be used to scribe
parallel lines, circles, and other elementsof the shape of a part
to be machined (seeLAYOUT TOOL). In such vernier calipers, the jaws
for measuringinside dimensions have cylindrical surfaces. The size
of the jaws when closed is usually 10 mm; the jawsize is stamped on
a surface that is not used for measurement and is added to the
reading when insidedimensions are measured.
Vernier calipers with an instrument range extending to an upper
limit of 400 to 2,000 mm have a single setof jaws that is similar
in design to the lower jaws of the caliper shown in Figure 1 ,b.
The last two types ofvernier calipers have a micrometer feed, which
is used mainly in layout work, for the more precisedetermination of
a dimension. The readings on such calipers are in 0.1 mm.
In contrast to a vernier caliper, a height gauge (Figure 2) has
a base rather than a fixed jaw. Measurementsare made from the
bottom of the base, which corresponds to the zero reading on the
scale. The slidinghead is equipped with a holder rather than a
movable jaw. For layout work, a stylus or a scriber is attachedto
the holder; for measurement, a special set of measuring jaws or a
bracket for fastening a metersuch asa dial indicatoris attached to
the holder. Height gauges are usually employed in layout work on
plates. Insuch work, the gauge is mounted on the plate together
with the workpiece to be laid out or measured. On aworkpiece to be
laid out, lines are scribed with a scriber when the height gauge is
moved over the surfaceof the plate. Height gauges are manufactured
in six standard sizes with readings in
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Figure 1. Vernier calipers: (1) graduated bar, (2) sliding head,
(3) vernier, (4) upper jaws, (5) lower jaws, (6) rule depth gauge,
(7)micrometer feed, (8) setscrews
Figure 2. A height gauge: (1) graduated bar, (2) base, (3)
scriber
0.05 mm for an instrument range extending to an upper limit of
400 mm or with readings in 0.1 mm forinstrument ranges extending to
an upper limit of 400 to 2,000 mm.
A gear-tooth caliper, which is a combination of a
height-and-depth gauge and a vernier caliper, is designedto measure
the thickness of gear teeth. The vertical rule of a gear-tooth
caliper is used to determine thedepth of a tooth from the tooths
tip, where the tooth thickness is measured. The horizontal caliper
is usedfor the direct measurement of the tooth thickness.
Gear-tooth calipers with readings in 0.05 mm aremanufactured in two
standard sizes for the measurement of the tooth thickness of gears
with a modulus ofup to 36 mm. Owing to the rapid abrasion of their
tips and to their relatively low accuracy, gear-toothcalipers are
being replaced to an increasing extent by displacement tooth gauges
and tangent toothgauges.
