Chapter 4 Measurement
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• A dial indicator snap gage measures the
amount of variation in the part measurement.
The dial face has a double row of graduations
reading in opposite directions from zero.
Minus graduations are red and plus gradua-
tions are black. Both adjustable and nonad-
justable indicating snap gages are available.
See Figure 4-33.
On snap gages, the anvils should be narrower
than the work being measured. This will avoid
uneven wear on the measuring surfaces.
When working with gage blocks, keep the fol-
lowing tips in mind:
• Improper handling can cause temperature
changes in the block, resulting in measure-
ment errors. For the most accurate results,
blocks should be used in a temperature-
controlled room. Handle the blocks as little as
possible. When you must handle the blocks,
use the tips of your fingers, as shown in
Figure 4-37A.
• When wringing gage blocks together to build
up to desired size, wipe the blocks and then
carefully slide them together. They should
adhere to each other strongly. Separate the
blocks when you are finished. Leaving gage
blocks together for extended periods may
cause the contacting surfaces to corrode. See
Figure 4-37B.
Figure 4-33. A dial indicator snap gage. (L.S. Starrett Co.)
4.4.4 Thread Gages
Several types of gages are used to check screw
thread fits and tolerances. These gages are similar to
the gages already discussed:
• Thread plug gage.
• Thread ring gage.
• Thread roll snap gage.
These gages are illustrated in Figure 4-34.
4.4.5 Gage Blocks
Gage blocks, commonly known as Jo-blocks or
Johansson blocks, are precise steel measuring stan-
dards. Gage blocks can be purchased in various sets
ranging from a few commonly used block sizes to
more complete sets. See Figure 4-35.
Gage blocks are used to verify the accuracy of
master gages. They are also used as working gages
and for setting up machining work requiring great
accuracy. The Federal Accuracy Grades for gage
blocks are shown in Figure 4-36.
Figure 4-34. Thread gages. A—Thread plug gage. B—Thread
ring gage. C—Go/no-go thread snap gage. (Standard Tool Co.
and Taft-Pierce Co.)
C
B
A