Tap drill size calculator
The general form of every tap drill table: subtract the engagement fraction of the thread depth from the major diameter.
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Tap drill
6.985mm
- Tap drill in inches
- 0.2750
- Clearance hole
- 8.640
- Diameter minus pitch
- 6.750
- Pitch diameter
- 7.1881
- Minor diameter of the nut thread
- 6.6468
- Tensile stress area
- 36.609 mm2
- Threads per inch
- 20.320
Show the working
- Major diameter = 8
- Pitch = 1.25
- Thread engagement = 75
- Clearance hole allowance = 8
- Tap drill = 8 - 75/100 * 1.082532 * 1.25 = 6.985 13
- Diameter minus pitch = 8 - 1.25 = 6.75
- Clearance hole = 8 * (1 + 8/100) = 8.64
- Pitch diameter = 8 - 0.649519 * 1.25 = 7.1881
- Minor diameter of the nut thread = 8 - 1.082532 * 1.25 = 6.646 84
- Tensile stress area = pi/4 * (8 - 0.938194 * 1.25)^2 = 36.6085
- Threads per inch = 1.25 > 0 ? 25.4/1.25 : 0 = 20.32
- Tap drill in inches = 6.985 13 / 25.4 = 0.275 005
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- tap drill = D - (engagement/100) x 1.082532 x P
Always pick the next drill larger than the calculated size; a tight tapping hole snaps taps.