More Tap Drill Size Tech

[Kojack]

Noodelz posted a link to a tap drill size chart which is a good thing to have around. Here's another way to determine the size if you don't have a chart available. Warning: you may need a calculator. The tap drill charts use this same math and list the closest drill for you. It's really simple though. Subtract one lead from the diameter of the tap you're using. One lead is the distance a thread will advance in one revolution, also equal to the fraction of 1 over the number of threads per inch.

For a simple example; a 3/8"x16 tap has a lead of 1/16". Subtract 1/16" from 3/8" and you get the tap drill size of 5/16".

For a 1/4"x20 tap you'd subtract 1/20". A calculator tells you that equals .050" so the drill size you need would be 1/4" or .250"-.050" or .200". The closest thing made is a "7" size drill at .201". Always round up on the size you get. If you go smaller you'll likely break the tap.

[noodelz]

Good post. It's funny when you're workin with that shit everyday it's like second nature. I single point whetever I can but when it comes to small shit a tap is good as long as it's straight.

[Kojack]

I've broken dozens but don't think I ever bent one. I used to get tap drill charts all the time but every time I really needed one they'd all run off to The Island of Lost Tools. That simple formula gives close to 75% thread depth which is most common.