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eeler1
05-18-2009, 11:43 PM
I've got a 9/32-32 tap, need to drill a hole the right size. I'm thinking its between a 1/4 and 5/16, so that would be between about .235 and .260+/-? Maybe a K or L drill bit? Any suggestions for figuring a drill size for an oddball thread?

LES A W HARRIS
05-19-2009, 12:08 AM
9/32-32, about .2473"- .2548", letter "D" .246" will probably cut a tad large and be on the low, .250" dia in the midrange, "F" .257" a bit oversize!

Cheers,

hornluv
05-19-2009, 12:36 AM
Subtracting the pitch from the nominal major diameter will give you approximately 75% thread depth, so in this case pitch is .03125 and your major diameter is .28125, so you end up with .250 for the tap drill.

Paul Alciatore
05-19-2009, 03:32 AM
Subtracting the pitch from the nominal major diameter will give you approximately 75% thread depth, so in this case pitch is .03125 and your major diameter is .28125, so you end up with .250 for the tap drill.

I get the same answer.

SGW
05-19-2009, 08:52 AM
0.2549" for a 65% thread

0.2508" for a 75% thread

0.2467" for an 85% thread

A quarter inch drill would probably be fine.

Lew Hartswick
05-19-2009, 09:24 AM
Yep. That "system" for metric tap drill works just fine for any
dimensioning system.
"Basic dimension minus pitch"
...lew...

eeler1
05-19-2009, 01:45 PM
Cool, and I already have a 1/4" drill bit to use. Thanks for the help guys.

lynnl
05-19-2009, 01:56 PM
Does that rule (nominal dia minus pitch) result in 75% for BOTH metric and std threads?

polepenhollow
05-19-2009, 05:25 PM
On a drill chart. find the proper size size tap drill dia for a 32 pitch thread. IE. (10-32- tap is a #21 (.157). <187- .157 = .020.
Take your major Dia of your thread and minus that Dia. (-.020)
This will work in all instances. All pitches and all Diameters. 1/2-13 or 5/16-24 and plus to infinity.
DOT is the same.
I'm just an old shop fart.

Lew Hartswick
05-19-2009, 05:47 PM
Does that rule (nominal dia minus pitch) result in 75% for BOTH metric and std threads?
Works for any 60 deg thread.

Whitworth and acme you have to look up. :-)
...lew...