Chapter 4 Measurement
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number of threads per inch. Screw pitch gages
are available in US Conventional and SI Metric
thread sizes.
4.6.8 Fillet and Radius Gage
The thin steel blades of a fillet and radius gage,
Figure 4-52, are used to check concave and convex
radii on corners or against shoulders. The gage is
used for layout work and inspection, and as a tem-
plate when grinding form cutting tools. See Figure
4-53. The gages increase in radius in 1/64″ (0.5 mm)
increments.
4.6.9 Drill Rod
Drill rods are steel rods manufactured to close
tolerances to twist drill diameters. They are used to
inspect hole alignment, location, and diameter. Drill
rods are available in both US Conventional and SI
Metric sizes.
4.7 HELPER MEASURING TOOLS
Some measuring tools are not direct reading and
require the help of a rule, micrometer, or Vernier
caliper to determine the size of the measurement
taken. These are called helper measuring tools.
4.7.1 Calipers
External or internal measurements of 1/64″
(0.4 mm) can be made with calipers, Figure 4-54. A
caliper does not have a dial or scale that shows a
measurement; the distance between points must be
measured with a steel rule.
Round stock is measured by setting the caliper
square with the work and moving the caliper legs
down on the stock. Adjust the tool until the caliper
point bears lightly on the center line of the stock.
Caliper weight should cause the caliper to slip over
the diameter. Hold the caliper next to the rule to
make the reading, Figure 4-55.
An inside caliper is used to make internal mea-
surements where 1/64″ (0.4 mm) accuracy is accept-
able. Hole diameter can be measured by setting the
caliper to approximate size, and inserting the legs
into the opening. Hold one leg firmly against the
hole wall, and adjust the thumbscrew until the other
leg lightly touches the wall exactly opposite the first
Figure 4-50. A thickness gage is used to check part clearance.
Figure 4-51. Screw pitch gages are made for both inch-based
and metric threads. (L. S. Starrett Co.)
Figure 4-52. A set of radius and fillet gages. (L. S. Starrett Co.)