Page 2 - Fall 2024 EDM Today

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EDM’ing Tapped Holes

Typically, the only thread measurement necessary for EDM tapping holes is to assure that the thread pitch is correct
and to determine the undersize of the electrode. In the case of a non-orbiting EDM tapping electrode, this is typically
.001”-.002” per side overcut allowance. In the case of an orbit tapping electrode, the undersize is typically governed
by the tap drill size of the particular thread so that the electrode can move rapidly to the hole depth and then begin
the orbit burn. In either case, the electrode undersize can be determined by miking the O.D.
In most cases, the measurement of the resulting threads consists of making sure the mating fastener will thread
into the tapped hole.

In those cases where a thread class is specifi ed, the measuring procedures outlined in the next section will be utilized.
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EDM Mold Cavities or High Precision Tapped Holes

Before we get deep into the measurement of threads, we need to examine thread forms.
Screw Thread Systems

This discussion will be limited to Unifi ed
Screw Thread Forms. While there are
numerous other standards, this is the
most commonly used in the USA. Figure 1
depicts the basic dimensions of the Unifi ed
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Screw Thread Form. It is based upon a
60 degree vee thread form. Please note,
however, that the root and the crest of the
threads are truncated. The degree of the
truncation of root and crest are dictated
by the various thread class standards to
assure that the thread will always success-
fully mate with its counterpart. For a male
thread, the most commonly measured
attributes are:

• Pitch Diameter Figure 1
• Pitch (also known as the Lead)
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• Major Diameter
• Thread Form

Threads are classifi ed based on the degree of precision required. Each class is designated by a number and letter.
The number is related to the precision (or fi t) of the thread and the letter designates whether the class applies to a
male or female thread. There are three thread classes:

2. A, 2B
1. A, 1B

3. A, 3B
Each class has associated tolerance limits for the major and pitch diameters to assure that the external and
internal threads of a given class will assemble with a certain level of precision. The higher the class number, the
greater precision of the fi t, and the narrower the tolerance band for the male and female threads. Classes 2A and
2B are the recognized standard for normal production of screws and nuts. This class is also widely used for many
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other applications.

Internal Thread Inspection
The most common method of internal thread inspection is by means of “go” and “no-go” plug gages. The thread size
of these gages is based upon the given thread class and represents the maximum and minimum material conditions
for the threaded hole. If the “go” gage fi ts and the “no-go” does not, the hole is acceptable. However, if the “go” does
not enter the hole, you will have no idea why it does not enter as any one or more of the attributes listed in Figure 1
could be the culprit.

If the thread size is not a standard, as would be the case of a mold thread for which shrinkage has been applied
(more on that later), a cast of the cavity made with a material such as Facsimile needs to be made and inspected in
a manner similar to that of an external thread by the means described in the next section.
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(Continued on next page)

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EDM Today, Fall 2024 Issue