Tapping Drill Sizing

The correct size of tapping drill can be calculated using the following formula.
A thread depth between 63% and 83% will give satisfactory results. The use of 75% in the formula is suggested.

55 degree thread forms e.g BSW, BSF (inch calculation)

Drill Size = Nominal Tap Diameter - (1.28 / TPI  x  % of thread depth)

e.g. 5/16" BSF, 75% thread required

Drill Size = .3125" - (1.28 / 22  x  75 / 100) = .3125 - 0.0436 

Drill Size = 0.2689"

For metric drill size multiply answer by 25.4
therefore Metric drill size = 6.83mm
nearest Standard Drill = 6.8mm

60 degree thread forms e.g. BSCY, UNF, UNC (inch calculation)

Drill Size = Nominal Tap Diameter - (1.30 / TPI  x  % of thread depth)

Metric (ISO)

Drill Size = Nominal Tap diameter in mm - pitch

e.g. M8 (1.00mm pitch)

Drill size = 8 - 1 = 7.0mm

CLASSIC FASTENERS
PO Box 187
Welland SA 5007
Australia
Tel: +61 8 8346 9838
Fax: +61 8 8340 9898
E-mail: web@classicfasteners.com.au