Threads and tap drills

TPI and pitch

Inch threads are named by diameter and threads per inch: 1/4-20 is a quarter inch with 20 threads per inch. Metric threads give the pitch, the distance from one thread to the next: M8×1.25 is 8 mm with a 1.25 mm pitch. They are two ways of saying the same thing: pitch = 1 ÷ TPI, so a 1/4-20 has a .050" pitch.

The tap drill

The tap drill sets how much thread the tap cuts, measured as percent of thread. A full 100% thread is hard to tap and barely stronger than about 75%, so 75% is the usual target in steel; in tough or hard materials, 60–65% makes tapping much easier for little loss of strength.

For a quick 75% drill: subtract the pitch from the major diameter. M8×1.25 → 8 − 1.25 = 6.8 mm. 1/4-20 → .250 − .050 = .200", and the nearest drill is a #7 (.201").

Form taps push the metal instead of cutting it, so there are no chips, but they need a bigger hole than a cut tap.

Rigid tapping feed

In rigid tapping the spindle and the Z axis are locked together, so the tap must advance exactly one pitch per turn. The feed rate is RPM ÷ TPI in inch, or RPM × pitch in metric. Get it wrong and the tap strips the thread or snaps.