Machining knowledge, made practical
Interactive TechTip
Cut taps and form taps need different holes. Pick the thread, pick the tap, set your engagement, and get the closest drill you actually have in the cabinet.
| Drill | Diameter | Engagement |
|---|
A cut tap carries the waste away as chip. A form tap has nowhere to put it: the lobes press into the wall and the metal has to flow up into the tooth profile. Drill it like a cut tap and there is simply no room for that metal to go.
Once engagement passes roughly 65%, forming torque rises far quicker than the extra strength you gain. That is the point where taps start seizing and snapping rather than forming.
Form taps: 60–65% in steel, stainless and ductile alloys. Cut taps: 65–75%. Above 75% on either type you are paying in torque and tap life for strength the joint will never use.
Before you tap
Most tap failures start in the pilot hole and end with a seized flute. Form tapping in particular lives or dies on lubrication — cold forming needs low friction far more than it needs cooling.
Tap-Paste Cutting Compound → Jobber Drills Full Tap ChartBoth systems use the standard 60° thread geometry. Hole diameter equals the major diameter minus the chosen percentage of thread height, where thread height per unit of pitch is 1.299 for cut taps and 0.68 for form taps — the same constants published as 0.01299 / 0.0068 per TPI in inch tables and as pitch/76.98 and pitch/147.06 in metric tables. Figures are a theoretical starting point: verify the drilled hole before tapping, since drill wear and runout shift the real diameter.