View Full Version : Question on a tap.

Your Old Dog
02-02-2008, 11:21 PM
I'm making a tool for my shaper to allow me to slot inside a collar. I wanted to thread a 5/8s stud and in looking at my tap collection I find I have 11, 18 and 24 pitch taps. I searched through some of my books for the size of hole to drill to use it but came up blank. Anyone have this info or can tell me where to find it. This is a nice looking tap and I don't want to mess it up.

Thanks in advance.

02-02-2008, 11:29 PM
Ask and ye shall receive.


Bob Ford
02-02-2008, 11:38 PM

Bolt O D - (.975/thread pitch = Tap drill size.

.625 - .975/11 .53636


Paul Alciatore
02-03-2008, 01:58 AM
If you are asking about 5/8 - 11 and want real drills:

35/64" for 62 % of full thread

17/32" for 74 % or full thread

Or 5/8 - 18

37/64" for 61 % of full thread

9/16" for 81 % of full thread (you may not want to go there)

Or 5/8 - 24

19/32" for 54 % of full thread

37/64" for 81% of full thread (again you may not want to go there)

02-03-2008, 02:12 AM
I found a chart in the back of the Enco or J&L catalog. I put it in a clear sheet protector and taped it to the side of my tool box, next to the taps.
Got another one for feeds and speeds for different materials taped to wall, next to the mill...


02-03-2008, 02:26 AM
For any tap it's simply D- pitch

Your Old Dog
02-03-2008, 08:15 AM
Thanks everyone. I have several charts but could not find 24 tpi on any of them. I was starting to think it was some special automotive thing.

Thanks Brockley1, I had heard that once before many years ago but never needed it again and forgot it. You can bet I won't be forgetting it now!

Lew Hartswick
02-03-2008, 11:29 AM
For any tap it's simply D- pitch

Since that is the way all metric threading charts spec the hole size
I figure it's a good thing to use for any 60deg thread.

Question for Bob Ford: where did the .975 come from? Since
that makes the hole a bit smaller what percent thread does it
work out to be? Guess I could get out the calculator . :-)

Bob Ford
02-03-2008, 07:58 PM

Look in Machinery's Handbook 23third edition, page 1415.

For American National thread form.
Hole size = basic major diameter - 1.29904 x percent full thread / by number of threads per inch

1.29904 x 75% = .97428

The formula Bolt I D - .975 / threads per inch. Yields .57083 for a 5/8 x 18 for slightly more than a 75% thread

Machinery's Handbook yields .57087 for a 75% thread.

I think the shortened formula is easier to use.


02-03-2008, 09:01 PM
The easiest formula to remember for a tap drill is: 1 divided by the no. of threads per inch subtracted from the major diameter.

Example for 1"-8 thread tap is 1" minus 1/8" equals 7/8 " drilltap drill.

Bob C