Figure 5-46.--Oversize drilled hole for tapping.
percent of the difference between the major and minor
diameters, subtracted from the major diameter.
When the tap hole is the right size, it is a little
larger than the root diameter of the tap, as shown in
figure 5-45. The tap will cut a thread in the work which
is only 75 percent as deep as the thread on the tap. The
Figure 5-44.--Tap drill size determination.
other 25 percent of the depth of thread on the tap
provides clearance between the tap hole and the root
DIAMETER. The diameter across the roots of the
diameter of the tap (see fig. 5-45). This makes tapping
thread is called the MINOR DIAMETER. The PITCH
is defined as the distance from any point on the thread
of a screw to the corresponding point on an adjacent
If the tap drill selected is oversize, the tap hole will
thread. It is usually measured from crest to crest and
be oversize, and the tap can cut only shallow threads
is expressed by a specific quantity of threads per inch.
in the work, as shown in figure 5-46. With less than a
full 75 percent depth of thread, stud or capscrew
Tap Drill Determination
threads usually strip.
If a threaded hole is to be made in a piece of metal,
If the tap drill selected is undersize, the tap hole
a hole of suitable size must first be drilled. The hole
will be undersize, being perhaps equal to the root
must be somewhat smaller than the size of the bolt to
diameter of the tap, as shown in figure 5-47. Then
be screwed into it.
there will be no clearance, and the tap will turn hard,
tear the threads, and probably break.
How do you determine how much smaller to drill
this hole? Figure 5-44 shows the system used for
The best method to determine the exact size of tap
figuring this. The resultant thread is known as a "75
drill to use is to refer to table 5-2. A chart similar to
percent thread" because the diameter of the hole is 75
this generally is included with a set of taps and dies.
Figure 5-47.--Undersize drilled hole for tapping.
Figure 5-45.--Proper size drilled hole for tapping.