Minor Bore Diameter

It has been many years since I have had the need to do internal threading, I seem to have forgotten the formula for finding the minor diameter for threading, I will be chasing 11 1/2 tpi.
Thanks in advance

My usual formula is: "Go to Machinery's Handbook and look it up."

If it is 11 1/2 TPI, is it (or should it be) a tapered pipe thread? If so, that is a different can of worms, but Machinery's Handbook still has the answers.

I use the Machinery Handbook or my Machinist Calc Pro 2 for these numbers. There are formula's that get you close, such as:

(.01299 x % of thread) / threads per inch. Subtract this from the major diameter.

Here's a link to a calculator you can use.
http://academic.evergreen.edu/projects/biophysics/technotes/fabric/js-tap.htm#tpi

Problem is, there is no perfect formula for depth of cut using a single point tool. It can get you close, but for final cut, you have to use the part it will fit to, a test bolt, or plug gage. With this said, a trick I learned on the PM forum:

Use the tap drill called out in MH or any other source for the bore, after that is done. I then cut a counter bore about .100" deep in the end of my thread area that is .002-.004 smaller than the major diameter. This is my final gage diameter. As you come up on your final cut, as soon as the tool marks the counterbore, you are at size, make a spring cut and it will be close fit, a second spring will loosen it up.

Edit: If you are cutting NPT than it's a different ballgame.

http://www.newmantools.com/tapdrill.htm
Scroll to the bottom of the page and you can put your own dimensions in the calculator.

Nominal diameter minus the lead.
In this case nominal OD - .087 for an 11 1/2 TPI thread, this will get you very close to the actual pitch diameter.

Also the ratio .866 will work as a starting point.

Thanks all, for the info. I think I found the right size from Newman tool web site. What I am trying to do is make an immersion heater for my anodizing set up
the heating element threaded area is 1.268"-1.270". I am not sure if it is a tapered thread as it is only about a 1/4" long. What I got from Newman's calculator is
1.179". I will be doing some trial and error this weekend.

Check out your small drill index, they almost always have charts for what drill to use for what thread. The tap drill size is usually almost always very close to the minor diameter for a 75% thread. The nominal diameter is the drill size listed. (only really works for small, less than critical holes)

there at straight pipe threads ant the trick was pointed out above example: 1/4 -20 major dia .250 minus pitch,1 over the number of threads 1 divided by 20 equals .050" .250- .050 equals .200. as there is no .200 drill round to the closest size in this case is #7 .201 this is the drill listed on most drill tap charts bill