A slide caliper is simple a rule with a fixed abutment, like a
hook rule, plus a sliding head. The head is arranged with a knife
edge that crosses the rule exactly in line with the movable jaw,
allowing direct reading of the dimension against the rule
markings. Usually there is also a means to lock the head, to avoid
disturbing the reading when the tool is withdrawn from the work.
Such tools still exist, just as described, but while they add
convenience ( and probably improve repeatability because of the
“cursor�?function of the sliding jaw), their precision is limited
by the resolution of the rule—never finer than 0.010 inch.
A very early modification to the slide caliper that greatly
improved its precision was the introduction of the vernier scale
invented by a French mathematician Pierre Vernier(1580-1637).
Although vernier calipers are now losing popularity to dial and
digital calipers , many machinists and mechanics still have a
fondness for them. This is partly because they are somewhat more
compact ( the dial on dial calipers occasionally gets in the way),
and partly because they may have concerns about the accuracy of
dial calipers over the long haul ( wear of the moving parts over
time can potentially introduce some “backlash�? . Partly, too ,
they may simply enjoy exercising an ancient skill that is
gradually becoming extinct.
On inch-measuring dial calipers , the main scale is usually
divided into 1/10-inch (0.1) units, while each dial marking
represents 0.001 inch ; one full revolution of the dial
corresponds to one division on the main scale. Beware, however,
that you should carefully scope any unfamiliar dial caliper the
first time you pick it up: There are some models that use 0.2-inch
units on the main scale. Thus, while a full revolution of the dial
still corresponds to one main scale division , those divisions are
twice as big. Likewise, each marking on the dial still represents
0.001 inch, but there are twice as many marks.
Metric dial calipers also come in two versions , perhaps more
common is the type with 0.02-millimeter divisions on the dial face
and in which one full revolution of the pointer corresponds to
2millimeters. Again , pay attention when handling unaccustomed
tools, because there are also versions that have 0.05-millimeter
dial divisions with a range of 5 millimeters per revolution , and
still others with 0.02-millimeter divisions but with a range of 1
millimeter per rev.
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