Pipe Threading

08 Jun 2017 18:07 #94309 by 10K
Pipe Threading was created by 10K
Here's a routine for pipe threading. I'm kind of lazy, so I wrote it where ordinarily all you have to input is the nominal pipe size to use it. Everything else should be OK as is. You can also input the actual pipe size and TPI from Machinery's handbook if you want. For instance, 1/2" pipe is 0.840 OD and 14 TPI.

A few notes:
- the taper on a pipe is 3/4" per foot. For a 1/2" pipe, that's only about 0.030" in diameter change over about 1/2". So it's really important to be accurate when setting up your lathe, or the thread won't be right. There's an input parameter for Tip Radius. If you're having trouble with loose fits, increase this a bit. It makes a big difference, since it makes the cut 2 x Tip Radius larger.

- I had a lot of trouble with a hesitation between the effective thread and the vanish thread, which could be somewhat compensated for by threading at less than 100 RPM. There's a long thread about the cause of this at

if you're interested. In any case, include this code in your .INI file:

The code includes some notes. You may have to change initialization parameters, feeds and speeds starting at line 88 for your lathe. My lathe does not have speed control, but does have a spindle encoder that I use to cut threads. There's also a check to see that you're not using Tool 0 starting at line 21. This is mostly to remind me to set up the correct tool before I start.

File Attachment:

File Name: npt_pipe.ngc
File Size:5 KB

The following user(s) said Thank You: andypugh, snujcnc

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14 Jul 2017 02:31 #95783 by 10K
Replied by 10K on topic Pipe Threading
The pipe threading subroutine had a variable to decrement the cut taken that I didn't like that much. It worked OK, but took a lot of fiddling setting the initial cut, minimum cut, and decrement percentage to get a good result. Because of the taper, it was not readily apparent what depth of cut was being made, unless it was so large that it bogged down the lathe. I rewrote it to be automatic, and I think that it works much better now. It now scales the initial cut to the minimum cut based on the percentage of total thread depth that has been taken. Here's the latest version.

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