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    • 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