# Choosing the correct tap drill



## mklotz (Feb 3, 2008)

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.


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## AlanHaisley (Mar 13, 2008)

Marv,

This is really useful info. The missing link though is how large to make the screw shank before threading; unless MD is that number. Please explain that part too. ???

Alan (a true beginner)


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## mklotz (Mar 14, 2008)

Alan,

In a perfect world, running a die over a rod with diameter MD is fine and will leave you with threads with a sharp crest.

For casual threading this may be satisfactory or one can knock off a few thou if so inclined. The flat on the crest is seldom a critical dimension in the kind of work we amateurs do.

However, if you really want to make threads "according to the book", what you need is the attached diagram. In this diagram, I took the standard 60 degree thread form and worked out formulae for virtually every dimension of interest. The quantity labeled "diametrical undercut" is what you're looking for.


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## kellswaterri (Mar 14, 2008)

Hi Marv, ...'''attached diagram''...Where?    
                       John.


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## kellswaterri (Mar 15, 2008)

Thank you Marv, came through fine...
                         John.


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