Thread cutting · rigid tapping
Tapping Calculators
Size tapping operations correctly: recommended surface speed by material, exact feed rate for rigid tapping (feed = pitch × RPM), tap drill diameter for a target thread percentage, and reverse-calculation of thread engagement from an existing hole.
Recommended tap RPM
n = Vc·1000 / (π·D)Rigid tapping feed rate
F = pitch × RPMTap drill size
TD = D − 1.299 × % × pitchThread percentage from drilled hole
% = (D − drill) / (1.299·p)Formulas used
Rigid tapping: feed rate F = pitch × RPM. A M10×1.5 tap at 600 RPM needs F = 900 mm/min exactly.
Tap drill: TD = major diameter − 1.299038 × (%/100) × pitch. 75% engagement is the standard shop target — full strength with far lower tap stress than 100%.
Frequently asked questions
Why is 75% thread engagement the standard?
Thread strength increases little beyond ~75% engagement, but tapping torque rises steeply — 100% engagement roughly doubles torque and tap breakage risk. 75% gives full-strength threads with safe tapping.
What feed rate do I program for rigid tapping?
Exactly pitch × RPM. M10×1.5 at 500 RPM: F = 750 mm/min. In imperial, a 1/4-20 tap at 500 RPM: F = 25 IPM. Any mismatch between programmed feed and pitch forces the tap to stretch or compress.
What RPM should I tap at?
From the material's recommended surface speed: about 10 m/min (33 SFM) for mild steel, 5 m/min for stainless, 20 m/min for aluminum. Then n = Vc×1000/(π×D). Slower is always safer for form taps and blind holes.