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.

[pennswoodsed]

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.

A little confusing, I am easily confused. Took me a moment or 2 to get 5.7mm from math shown.
Thank,Ed

[mklotz]

25.4 (mm/in) is an exact number, not an approximation. In fact, the American inch is defined as 25.4 mm. It's worth memorizing that number.

Note that the 5.7mm figure does not come directly from the math; it's merely the closest conventional drill size to the size dictated by the formula. It differs by a totally inconsequential ten-thousandth.