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Old 06-08-2007, 07:28 AM
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Originally Posted by hemi43
Another way to find the tap drill size is to minus the pitch from the diameter.
i.e, 3/8-16 thread, .375 - .0625 = .3125 or 5/16
1/4-20 thread, .250-.050 = .200 or 13/64

to find the pitch divide 1 by the number of teeth per inch. Dan
Brilliant! I must admit my first reaction was entirely negative, as I've always used a chart. The connection between pitch and thread depth didn't hit me at first, but, of course, the pitch would be very, very close to double the thread depth.

This little tip is particularly handy when tapping for metric, where the pitch is given as part of the thread specification.

Here's another little tip an old timer gave me when I was starting at Chrysler back in the fifties:

The nominal diameter of a "numbered" screw thread, in thousandths, is equal to the number times 13 plus 60. For instance, for a number 10 screw, the nominal diameter is 10 time 13 plus 60 or 190 thousandths.

I wish there was a similar relationship for numbered drill bits.
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