Internal Threading question - what diameter to bore?

jgedde

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I'm making a 5C collet closer for my lathe. For the draw bar, I need to create an internal thread to grab the 1.238x20 external thread on the OD of a 5C collet.

I'm having a senior moment and can't seem to figure out what the ID of the workpiece should be bored to in order to cut these threads. Help!

I remember there's some way to compute it, but I can't remember how.

Any speedy help would be appreciated as I'm on hold until I figure it out.

Thanks!
John
 
1.238-20 UNS-2B

Minor diameter: 1.195/1.184
Major diameter: 1.238 min
Pitch diameter: 1.2115/1.2055
Thread height: 0.0271
Root width:0.0063
 
Hehe...Gagemaker software makes it all too easy.
 
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