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QUALITY CORNER | CONTINUED

Other Electrode Considerations

While not strictly related to the inspection of
threads, there are two considerations that need
to be accounted for when making threaded elec-
trodes:

Shrinkage
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For plastic or diecast molds, a shrinkage
allowance should be made to all dimensions.
While for most attributes of smaller threads that
might be negligible, the thread pitch shrinkage
allowance should always be applied. Otherwise,
the molded fastener could bind over a longer
thread length engagement when checked with a
functional gage even though the adjustment to
an individual pitch might seem unimportant.
Figure 6
Overcut
One detail in threaded electrode fabrication that
is often overlooked is the application of overcut
to the electrode design. A number of folks
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assume that overcut compensation is applied
equally to the O.D. and the Pitch Diameter of
the electrode. However, because overcut is per-
pendicular to the thread form and not the pitch
diameter, the overcut allowance per side for the
pitch diameter must be 2 times the actual over-
cut per side (or 4 times on the pitch diameter).
(See Figure 8) Therefore, if accurate internal
threads are required, the pitch diameter of the

than the shrinkage corrected pitch diameter of
the part.
Figure 7 electrode should be 4 times the overcut smaller
Once the mechanically measured thread parameters have
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been confi rmed, if the optical projection of the thread form
falls between the lines on the chart, the thread inspection is
complete.
Although not commonly used, thread functional gages can
also be utilized to check external threads. They basically
serve a function similar to “go” and “no-go” plug gages for
internal threads.
Roger Kern
Principal Consultant
www.itp4edm.com Figure 8
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EDM Today, Fall 2024 Issue