Machinery Repairman

An explanation of the various methods normally
available to you is given in the following paragraphs.
Thread micrometers are used to measure the pitch
diameter of threads. They are graduated and read in
the same manner as ordinary micrometers. However,
the anvil and spindle are ground to the shape of a
thread, as shown in figure 6-88. Thread micrometers
Figure 6-89.—Thread gauges.
Figure 6-88.—Measuring threads with a thread micrometer.
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come in the same size ranges as ordinary micrometers:
0 to 1 inch, 1 to 2 inches, and so on. In
addition, they are available in various pitch ranges.
The number of threads per inch must be within the
pitch range of the thread.
Go and no-go gauges, such as those shown in
figure 6-89, are often used to check threaded parts.
The thread should fit the “go” portion of the gauge,
but should not screw into or onto the “no-go” portion.
Ring and plug gauges are available for the various
sizes and classes of fit of thread. They are probably
the most accurate method of checking threads because
they envelop the total thread form, and in effect,
check not only the pitch diameter and the major and
minor diameters, but also the lead of the thread.
The pitch diameter of a thread can be accurately
measured by an ordinary micrometer and three wires,
as shown in figure 6-90.
Figure 6-90.—Measuring threads using three wires.