mklotz
Well-Known Member
Most of us are familiar with the simplistic formula for computing tap drill size.
TD = MD - 1/P
where:
TD = tap drill diameter (in)
MD = major diameter of tap/screw (in)
P = pitch of thread (tpi)
This formula is quick and handy but it doesn't take into account the depth of thread (DOT) which is normally expressed as a percentage. If you're tapping difficult material (e.g., naval bronze), using a smaller DOT than you get with this simple formula will save you from a lot of broken taps.
There is a formula that takes into account the DOT.
TD = MD - 0.013*DOT/P
We can see that the simplistic formula above has replaced 0.013*DOT with unity, "1", e.g.,
0.013*DOT = 1
Solving this equation for DOT yields DOT = 1/0.013 = 77%. In other words, the simple formula will always produce a tap drill diameter suitable for 77% DOT.
If you want something less than 77% DOT, use the formula above. For example, if I want to tap a 1/4-20 hole in managanese bronze with a 55% DOT, I would calculate:
TD = 0.25 - 0.013*55/20 = 0.25 - 0.03575 = 0.214 inches (#3 drill)
whereas the simple formula yields 0.25 - 0.05 = 0.2 (#7 drill).
Below is a chart from my DRILL program with some suggested DOTs for typical materials.
MILD AND UNTREATED STEELS 60-65
HIGH CARBON STEEL 50
HIGH SPEED STEEL 55
STAINLESS STEEL 50
FREE CUTTING STAINLESS STEEL 60
CAST IRON 70-75
WROUGHT ALUMINUM 65
CAST ALUMINUM 75
WROUGHT COPPER 60
FREE CUTTING YELLOW BRASS 70
DRAWN BRASS 65
MANGANESE BRONZE 55
MONEL METAL 55-60
NICKEL SILVER (GERMAN SILVER) 50-60
One further comment on this subject... I've encountered numerous newbies who, for some reason, can't decide what size hole to drill/bore before single-pointing internal threads on the lathe. The hole you bore is the same size you would use if you had a tap of the appropriate size. Use the formula above and you're good to go.
TD = MD - 1/P
where:
TD = tap drill diameter (in)
MD = major diameter of tap/screw (in)
P = pitch of thread (tpi)
This formula is quick and handy but it doesn't take into account the depth of thread (DOT) which is normally expressed as a percentage. If you're tapping difficult material (e.g., naval bronze), using a smaller DOT than you get with this simple formula will save you from a lot of broken taps.
There is a formula that takes into account the DOT.
TD = MD - 0.013*DOT/P
We can see that the simplistic formula above has replaced 0.013*DOT with unity, "1", e.g.,
0.013*DOT = 1
Solving this equation for DOT yields DOT = 1/0.013 = 77%. In other words, the simple formula will always produce a tap drill diameter suitable for 77% DOT.
If you want something less than 77% DOT, use the formula above. For example, if I want to tap a 1/4-20 hole in managanese bronze with a 55% DOT, I would calculate:
TD = 0.25 - 0.013*55/20 = 0.25 - 0.03575 = 0.214 inches (#3 drill)
whereas the simple formula yields 0.25 - 0.05 = 0.2 (#7 drill).
Below is a chart from my DRILL program with some suggested DOTs for typical materials.
MILD AND UNTREATED STEELS 60-65
HIGH CARBON STEEL 50
HIGH SPEED STEEL 55
STAINLESS STEEL 50
FREE CUTTING STAINLESS STEEL 60
CAST IRON 70-75
WROUGHT ALUMINUM 65
CAST ALUMINUM 75
WROUGHT COPPER 60
FREE CUTTING YELLOW BRASS 70
DRAWN BRASS 65
MANGANESE BRONZE 55
MONEL METAL 55-60
NICKEL SILVER (GERMAN SILVER) 50-60
One further comment on this subject... I've encountered numerous newbies who, for some reason, can't decide what size hole to drill/bore before single-pointing internal threads on the lathe. The hole you bore is the same size you would use if you had a tap of the appropriate size. Use the formula above and you're good to go.
