(6) Vernier calipers. Vernier calipers work like
slide calipers. Shown in figure 3-29, vernier calipers
Figure 3-29. Vernier Caliper
(7) Trammels. The trammel, shown in figure 3-
30, measures distances beyond the range of calipers.
The instrument consists of a rod or beam to which trams
are clamped. The trams carry chucks. The trammel
can also be used as a divider by changing the points.
Figure 3-30. Trammel
b. Use of Calipers. The operation of most calipers
is relatively straightforward. Vernier calipers, however,
can be finely adjusted to provide a very accurate
reading. The following paragraphs describe the
procedures used to make accurate measurements with
vernier calipers (see figure 3-31):
(1) Loosen the two locking screws (1) and (2).
(2) Move the movable jaw (5) along the rule
until the desired position is obtained.
(3) Retighten the locking screw (2) to secure
the movable jaw.
(4) Make fine adjustments to the vernier scale
(4) with the adjustment control (3).
(5) Tighten locking screw (1) and read caliper
in accordance with the following paragraphs.
c. Reading a Vernier Caliper. To read a vernier
caliper, the steel rule and the vernier scales must be
understood. These are shown in figure 3-32.
(1) Steel rule. The steel rule (1) is graduated
in 0.025 inch. Every fourth division (representing 1/10
inch) (3) is numbered.
(2) Vernier scale. The vernier scale (2) is
divided into 25 parts and numbered 0, 5, 10, 15, 20, and
25. These 25 parts are equal to 24 parts on the steel
rule (1). The difference between the width of one of the
25 spaces on the vernier scale (2) and one of the 24
spaces on the steel rule (1) is 1/1000 inch.
(3) Reading the measurement. Read the
measurement as outlined below:
Read the number of whole inches on the top
scale (5) to the left of the vernier zero index
(6) and record........................... 1.000 inch
Read the number of tenths (4) to the left of
the vernier zero index (6) and record ....
Read the number of twenty-fifths (7)
between the tenths mark (4) and the vernier
zero index (6) and record 3 x 0.025 = 0.075 inch
Read the highest line on the vernier scale
(2) which lines up with the lines on the top
scale (5) and record. (Remember that 1/25
= 0.001 inch) . ............... 11/25 = 0.011 inch
Total all preceding measurements1.486 inches
The measurement, therefore, is 1.486 inches.