Thread Clearance Question

bpimm

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I need to thread a piece of tubing and make a matching internal thread, I found formulas to calculate thread depth and came up with .047" for 13 TPI, I worked backwards from the ID of the tubing of 1.875 and added the wall thickness I want of .100" which gives me 2.075 for the minor diameter + .094 for the threads which gives me 2.169 for the major diameter. For the matching internal thread how do I calculate the initial bore size to start with?

I'm a threading novice, I just got my lathe set up to cut threads and cut my first one this morning.
 
When tapping holes, a tap drill which yields a a 75% thread is a good place to start. Since you have a non standard thread, tap drill tables won't help but the commonly use formula of major diameter - pitch = tap drill gives close to the 75% thread. For 13 tpi, the pitch is .077" so subtract that from your major diameter and you should be close.Since you are presumably single point threading, you could go slightly smaller but the gain in thread strength is not significant.
 
Use your male thread as a gage for fit as you are closing in on the final cut.
 
That's what I was thinking, tomorrow morning I'm going to try to cut some inside threads and get the feel before I try the 2" stuff.
 
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