1. Member
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Jan 2005
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## How to figure I.D. for threading

I need to thread a hole for 1.250 id by 12 how do i get the right size for the hole is it 1.160?

2. Senior Member
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1.160 is the correct minor diameter for a 1.250-12 thread

Machinery's Handbook 24th edition page 1554. If you do not have a M.H. you should consider getting one.

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OK Tim, I'm by far not an expert and this is a 'loaded' question, but I'll share what I use.

I use a 60 degree V thread rule of thumb value as my basis (.65). So the math goes like this:

.65/12= approximately .054" less in radius

Then double the thread depth you calculated to determine the change in diameter required for threading (.054 x 2= .108").

Finally, subtract the .108" value from your desired threaded hole size (1.250 - .108= 1.142").

Let the games begin!!!

Edit: I see Errol has posted. He has far more knowledge than I. Use his info...

Fred

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While you can look it up (and should take into account the percentage of thread you want) a good rule of thumb is that you'll get an approximate 75-80% thread by the formula: diameter = major - (1/tpi). It's a good check after running calculations, too.

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12/1=.0833 so 1.250-.08333= 1.1667" tighten it up if you want more bearing.

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## Math Trick

I am posting a link to a very easy formula to do this. This little trick got me with in .003 of the Machinery Handbook. It will get you closer on smaller sizes. There is also a formula for metric sizes.

http://www.machinist-guide.com/tap-drill-sizes.html

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\$2-\$5 for a poster sized drill chart for your wall, I've got 5 of 'em here. 1 7/64 is what she says.

OR, \$10-\$25 for a machinery's handbook on e-bay that will give you more info than you have ever cared to know. Any edition from the past 30 or 40 years is more than adequate.

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That little metal 'fishtail' that you use for aligning your threading tool with the part?

It has some numbers on it called 'Double depth of Amer. Nat Thd.' or something along those lines.

Those are the numbers you want to look at.

Pete

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Originally Posted by 10KPete
That little metal 'fishtail' that you use for aligning your threading tool with the part?
I think all those are for full "V" thread. Not accurate for our needs. I just grab the MHB, which I've done quite a lot lately.

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