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
73 de KG2V
For the Children - RKBA!
My Website: http://www.thegallos.com
My Blog: http://kg2v.blogspot.com
A rose by any other name would be "deadly thorn-bearing assault vegetation."
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