vdoc

Chapter 7: A. Common Gages and Measurement Instruments 59
1. VARIABLE GAGES
Identify and use variable gages, including
micrometers, calipers, thread wires,
pitch micrometers, linear scales, etc.
(Application)
Body of Knowledge II.A.1
Most of the basic or general-purpose linear measuring instruments are typified by
the steel rule, the vernier caliper, or the micrometer caliper.
Steel Rules
Steel rules are used effectively as line measuring devices, which means that the
ends of a dimension being measured are aligned with the graduations of the scale,
from which the length is read directly. A depth rule for measuring the depth of
slots, holes, and so on, is a type of steel rule. Steel rules are also incorporated in
vernier calipers, where they are adapted to end-measuring operations. These are
often more accurate and easier to apply than in-line measuring devices.
Part II.A.1
Verniers
The vernier caliper shown in Figure 7.1 typifies instruments using the vernier principle
of measurement. The main or beam scale on a typical metric vernier caliper
is numbered in increments of 10 mm, with the smallest scale division being equivalent
to one mm. The vernier scale slides along the edge of the main scale and is
divided in to 50 divisions, and these 50 divisions are the same in total length as
49 divisions on the main scale. Each division on the vernier scale is then equal to
1/50 of (49 × 1) or 0.98 mm, which is 0.02 mm less than each division on the main
scale.
Aligning the zero lines on both scales would cause the first lines on each scale
to be 0.02 mm apart, the second lines 0.04 mm apart, and so on. A measurement on
a vernier is designated by the positions of its zero line and the line that coincides
with a line on the main scale. For example, the metric scale in Figure 7.1a shows
a reading of 12.42 mm. The zero index of the vernier is located just beyond the
line at 12 mm on the main scale, and line 21 (after zero) coincides with a line on
the main scale, indicating that the zero index is 0.42 mm beyond the line at 12 mm.
Thus 12.00 + 0.42 = 12.42 mm.
The vernier caliper illustrated in Figure 7.1 also has an inch scale so that it can
be used interchangeably for either inch or millimeter measurements. The smallest
division on the main scale represents 0.25 inches and the vernier is divided
into .001 inch increments. Thus the measurement illustrated is .475 away from the
main scale plus .014 from the vernier scale, for a total of .489 inches.