CNC / machinist calculator
Form (Roll) Tap Drill Size Calculator
Roll-form taps do not cut a chip; they push the metal up into the thread, so they need a larger starting hole than a cutting tap. Drill the cut-tap hole and a form tap will overfill and jam. Enter the thread and a target percent of thread, around sixty-five percent is typical, and this calculator gives the correct form-tap hole for inch or metric threads.
How it works
A form tap displaces material rather than removing it, raising the thread crest from the hole wall. Because that metal has to come from somewhere, the hole starts larger than for a cut tap and the tool forms the rest. The standard inch formula is the major diameter minus 0.0068 times the percent of thread times the pitch, with the percent as a whole number, and the metric form uses the pitch in millimetres.
For comparison, the cut-tap constant is about 0.013, roughly double, which is why a form tap always takes a noticeably bigger drill than a cut tap of the same size. Most form tapping targets about sixty-five percent of thread, a little less than the seventy-five percent common for cut taps, because a full form thread takes high torque and the rolled thread is strong.
Form tapping suits ductile materials like aluminum, mild steel, copper and brass, and it produces no chips and a stronger, burnished thread. Because spring-back varies with material, treat the result as a starting hole, round to the nearest available drill, and verify the produced thread with a gage.
Worked example
A 1/2-13 thread for a form tap at 65 percent: 0.5 - 0.0068 x 65 / 13 = 0.4660 in, noticeably larger than the ~0.435 in a cut tap would use.
Frequently asked questions
Why does a form tap need a bigger hole than a cut tap?
A form tap displaces metal to raise the thread instead of cutting it away, so the hole must start larger and the rolling action forms the crest. Using a cut-tap hole makes a form tap overfill and bind.
What hole size does a form tap need?
The hole is the major diameter minus 0.0068 times the percent of thread times the pitch. For a 1/2-13 thread at 65 percent that is about 0.466 inch, against roughly 0.435 inch for a cut tap of the same size.
What percent of thread should I target for form tapping?
Around 65 percent is typical, a little less than the 75 percent common for cut taps, because forming a full thread takes high torque and a rolled thread is already strong. A range of 55 to 75 percent is usual depending on material.
Which materials suit form tapping?
Ductile metals such as aluminum, mild steel, copper and brass form well, giving a chip-free, work-hardened, stronger thread. Hard or brittle materials like cast iron do not roll and should be cut tapped instead.
Are form-tap hole sizes exact?
They are a close starting point. Material spring-back changes the produced thread, and the size is sensitive to small drill changes, so round to the nearest available drill and confirm the first hole with a thread gage before running production.
Related calculators
Sources
- Form tap drill size formula and chart (Amesweb, cites Machinery's Handbook)
- Balax Thredfloer form tap drill data (PDF)
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.