Tap Drill Size
Tap drill diameter for a target percent of full thread on a 60-degree (UN / ISO metric) thread: % = 76.98 x (D_major - D_drill) x TPI, so D_drill = D_major - % / (76.98 x TPI), with the nearest 1/64 in fraction (the named letter / number drill is a chart lookup) (first-principles thread geometry).
Formula and source
60-degree thread: percent of full thread = 76.98 x (D_major - D_drill) x TPI, so D_drill = D_major - % / (76.98 x TPI). The 76.98 constant is 1 / 0.012990; nearest 1/64 in fraction reported.
Tap drill for a target percent of full thread - first-principles 60-degree thread geometry as in Machinery's Handbook (Industrial Press), by name; public domain.
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