Drill Size for Odd Taps

Do you have an odd size tap and don't know which drill to use? Here's one way suggested by Doc Nickel to calculate the drill size. For an example I'll use the tap for an AutoCocker 4-way which is 15/32-32.

To calculate the drill size you take the tap size and subtract the thread depth. To calculate the depth of the threads for an odd sized tap take the value from a known screw with the same thread pitch.

For a 15/32-32, try a 10-32 tap. The OD of a 10-32 thread is .1935 and the tap drill is .1590. Now subtract those two numbers and you get a thread depth of .0345". To get the drill size for a 15/32-32 tap subtract .0345" from .4688 (15/32") to get the drill size. That equals .4343 which is pretty close to 7/16" (.4375).

When I was taking machine tool classes I learned a much simpler method. Take the taps major diameter and subtract the pitch. Going with 15/32-32 again, it would be .469 - (1/32) = .438. Doing it this way will give you around a 75% thread. There are also a number of online thread calculators these days you can also use but I've found this works fine most of the time.