Tap Drill Size
Tap drill diameter for a target percent of full thread on a 60-degree (UN / ISO metric) thread.
Example
You enter
- Thread standard inch
- Major (nominal) diameter (in for inch, mm for metric) 0.25
- Threads per inch (TPI, inch) 20
- Target thread engagement (%) 75
You get
- Theoretical tap drill 0.201286
- Nearest 64th (in) 0.203125
- Nearest 64th (%) 72.1688
Details, formula, and sources
% = 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).
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.
Pure thread geometry, public; the major diameter, TPI / pitch, and target thread percent are user-supplied.
Estimate. AHJ and licensed professional govern.
Field names used by the API: thread_standard, major_dia_in, tpi, thread_percent, drill_dia_in, nearest_64th_in, nearest_64th_percent
- Thread form and target 60-degree included angle; 65-75% thread is the usual target, user-supplied (default 75%)tap-drill geometry