[bbulich]

I am getting into threading with my cnc lathe and can't seem to fully grasp a few concepts. I have been buried in the handbook but I think I just need someone to explain it to me. It seems most of the threads I will need to make are based on an existing part. If I have a threaded part with 10tip and an outside diameter of .622", how do I calculate the initial diameter to bore the diameter. I know I can find thread depth by 1/tip x .866 I am just having trouble understanding the initial OD and ID diameters for custom threads.

[chevydyl]

They make a micrometer for measuring threads minor diameter. Prolly get close with a set of calipers in some cases. But once you know the minor of an external thread not hard with the right math to figure out your major of the internal threads. Your on the right track. The major of the internal thread will be slightly under the minor of the external. Look at the tolerances of the closest size threads in the handbook and average out the min/max. Use that as a ball park and go for it. (Im saying if theres 5 thou clearance apply that same clearance to your custom measurement) Do some tests to see which diameters are gonna have the type of clearance your part may require. This is the only way I can think of without researching to do this type of job

[juergenwt]

bbulich - you have a 5/8-10 thread? Odd size. Not 5/8-11?
What you are looking for is the hole size? Nom. dia. minus pitch. In your case .625 - .100 = .525 hole size.
If it turns out to be 5/8-11 than divide 1" by 11 = .091. Subtract that from .625 and you have a hole size of .534

[Superman]

There are standards that threads must conform to....... but it depends on the end-user's application

link
see top left page as an example,

an external thread ie bolt, has more control on it's manufacture
an internal thread.... ie nut, is a little more generous in it's manufacture.

The nut's internal thread is "truncated" ( larger flats) to allow any grease, oil, dirt & other crap to move into the gap for the thread to be able to work without binding