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External Thread Inspection

Major Diameter
The major diameter is readily inspected with a standard micrometer.

Pitch
The pitch is most accurately measured utilizing an optical comparator. Pitch = 1/(number of threads per Inch)
Pitch Diameter
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The gold standard for measuring the pitch diameter of an external thread is
utilizing thread measuring wires as depicted in Figure 2. However, anyone who
has actually tried it, especially for small diameters with fi ne threads, will readily
admit that juggling the three wires into the correct thread position while trying to
mike over them can be a frustrating and time-consuming task.
However, a Pitch
Diameter Microme-
ter, shown in Figure
3, is a much better
measuring alterna-
tive. A pitch diameter
micrometer utilizes a
conical spindle and
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a specially ground,
grooved anvil to
provide a direct
measurement of the
Figure 2
thread pitch diam-
eter, as shown in Figure 4. It is important to note that,
due to the geometry restrictions of the anvil groove,
a given thread pitch micrometer can only be utilized
to measure a limited range of thread pitches. Thus, in
Figure 3
order to measure the pitch diameter of a large vari-
ety of threads, either an assortment of thread pitch
micrometers or a thread pitch micrometer with inter-
changeable anvils is required.
Thread Form
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Checking the thread form is a critical part of thread
inspection. Just because the O.D., Pitch and Pitch
Diameter are correct doesn’t mean that the thread is
acceptable. It is possible that either the 60 degree vee
angle or the thread root form might not be correct,
causing the thread to be out of specifi cation.
Figure 4
A Profi le Projector is widely used to check the thread
form quality, as it can magnify the key thread param-
eters enabling measurement of the fl ank angle, the
minor diameter, and the root radius of the thread.
Most commonly a horizontal Profi le projector is used
for this application. The correct staging of the thread
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The chart is prepared to represent the maximum EDM Today Magazine
is necessary so that the Profi le projector lens can
view it correctly as shown in Figure 5. This is achieved
through a combination of features like rotatable work
stage and accessories like a center holding device and
centers. The thread is on a helix angle, so that one has
to adjust the work’s stage angle so that the light path
is straight across the thread. This ensures that the
correct measurements are done.

An overlay chart is most commonly utilized in
conjunction with profi le thread form projection.

and minimum material conditions of the thread Figure 5
form as shown in Figure 6 and Figure 7.
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