cutting threads

[fourperf]

Good morning guys. I need to threadmill a 2.75 - 12 bolt and nut and for the life of me cannot figure out the maj and min diameters for the internal and external threads.

Can some one help me figure that out? Is there a formula? I have looked in the machinist handbook but that is all mambo jumbo to me.

thanks

mark

[DSL PWR]

OD threads start with the nominal size, in your case 2.75" less some clearance. This would be your major diameter. The book says 2.7481 to 2.7367. Now you need to know how deep to cut the threads. This is the pitch diameter. Your range is 2.694 to 2.6878. If I were doing this I'd start with 2.745 od, and cut the threads about .025 deep. This will give you a minor diameter of 2.695.

Now for the nut. I'd bore it to 2.697 (this gives .002 clearance to the minor diameter of the bolt) an cut the threads .0265 deep (this takes it out to 2.75, giving you .005 clearance on the od of the bolt).

This is how I would do it, but by no means is it gospel.

[fourperf]

thanks dsl power. Let me ask a stupid question because I am entering this into my cam software and I am a little confused about one thing. On the bolt obviously the 2.7481 is the maj and 2.715 is the min dia (those are the numbers I got when I subtracted .025 from 2.74) on the nut which number is the major and which is the minor (is the major always the larger number) or is it opposite to the bolt. Sorry for a stupid question.

Mark

[DSL PWR]

Major is always bigger. Just remember the major, and minor d on the nut needs to be bigger than the major, and minor d on the bolt. If it is the other way around you will create an interference fit.

[Geof]

Originally Posted by DSL PWR .... If I were doing this I'd start with 2.745 od, and cut the threads about .025 deep....

This seems a bit small for a 12 tpi thread. I am comparing it with 16 tpi where I cut 0.08 deep on the diameter giving 0.04 on the radius.