[TowerTalk] pipe tap drill size

[Charlie Gallo]

On 3/12/2010 jimlux wrote:

> And sometimes, it's stamped on the shank of the tap.

OK - The answer given is accurate enough, as it is (given the fact that you had to ask the question)

That said, the answer isn't as clear cut as #7 drill for 1/4-20

First - there are basically 2 types of tap for our purpose here - "cutting" taps (which are the taps you will see about 99% of the time - be they 2 flute, 4 flute, hand, spiral point, whatever) and Forming taps (which don't remove metal, but push the metal from the bottoms of the thread to form the crests - they only work on ductile materials, and the threads look weird, like they have a groove at the TOP of the thread)

Anyway, sticking with "cutting" taps, a #7 drill will give you a 70% thread, so it's actually a fairly good answer, but there are cases where you want a HIGHER percentage of thread (you never go to 100) or lower percentage (easier to tap, somewhat weaker threads)

IF you don't have a #7 drill around, you COULD, in theory go as LARGE as a #4 drill bit, and only get 57% of thread (I wouldn't ever GO that low except for threads that don't really hold anything), and go as small as a #9 drill bit(which will give you 77% of thread).  The danger there is breaking taps, and if you have a slightly large screw/bolt it won't go in real easy)

IN that range is a 13/64 inch drill bit, which will give you a slightly looser thread (66%) than the #7, BUT might be a LOT easier for you to come by
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