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Old 06-11-2007, 02:18 PM
hemi43 hemi43 is offline

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Originally Posted by BillyShope
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.
Tap drill sizes are all based on a 75% contact area. The reason that the pitch works so well to figure out the tap drill is because national threads are based on a 60 degree tooth patern. BA threads are 55 degrees, so I don't think that this rule of thumb may work for those. Dan
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