Find the correct tap drill size for any inch or metric thread at your target percent of thread engagement.
The tap drill leaves the right amount of material for the tap to cut threads. Aiming for about 75% thread gives nearly full strength while keeping tapping torque manageable.
For inch threads, subtract roughly one pitch (1/TPI) from the major diameter; for metric, subtract the pitch.
Formula: Tap drill ≈ Major Ø − (1 / TPI) [inch]; Major Ø − Pitch [metric]
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