Calculating tap drill the thread FORMING taps

[mklotz]

One way to minimize tap breakage is to use thread forming taps. However, they require a larger tap drill than that used for the equivalent thread cutting tap.

In my DRILL program, I use the following equation to find the tap drill for thread forming taps...

TD = MD - 0.0068*DOT/P

where:

TD = tap drill size
MD = major diameter of thread
DOT = desired depth of thread expressed as percentage
P = pitch of thread expressed as tpi

For example, a 1/4-20 thread with 75% DOT yields:

TD = 0.25 - 0.0068*75/20 = 0.2245"

Closest conventional drill is 5.7 mm but a #2 is close enough.