CNC TRUECUT / MACHINING REFERENCE
Tap drill selection: cutting and forming taps
Choose tap drill information by thread size, pitch, tap type and material. Calculate an estimate, then confirm the hole specification.
Cutting taps and the diameter-minus-pitch estimate
For an ordinary cutting tap, subtracting pitch from nominal major diameter is a useful first estimate. For example, M10 × 1.5 gives 8.5 mm. The estimate does not include every thread engagement, material behaviour or hole tolerance. The tap drill calculator labels this result as an estimate so it is not confused with a controlled specification.
Inch threads and TPI
For an inch thread, first convert threads per inch to pitch. Pitch is 1/TPI inches, or 25.4/TPI millimetres. A 20 TPI thread has 0.05 inch pitch. Read the complete callout—including series and class—before selecting a drill. Equal nominal diameters can use different coarse or fine pitches.
Forming taps need their own drill table
Forming taps displace material instead of cutting chips. Their pre-hole size is especially sensitive to the material and desired thread fill, and it differs from a cutting-tap estimate. Use the exact forming-tap maker’s recommended range and hole tolerance. Do not apply the diameter-minus-pitch shortcut to a forming tap.
STI and pipe threads
An STI tap produces the receiving thread for a wire insert, so it needs its own drill and tap selection. Pipe threads add taper, engagement and gauging considerations. Use the dedicated references below rather than treating them as an ordinary bolt thread.
OSG manufacturer tap and STI drill charts · Wire-insert thread preparation · Pipe thread reference