How to cut NPT thread on manual lathe?

[atr]

I want to cut a 1" npt thread on a pipe. from what I've seen the thread pitch is 11.5 tpi. I have an Italian lathe and the option of 11.5 tpi doesn't appear on the lathe's pitch's table in inches nor the possibility of 2.208mm in metric units. of course there is the possibility of 10,11 or 12tpi. it might be a silly question, but is there a way to overcome this problem?

[Charles T. McCullough]

The simple solution is to use a die.

[Bill Shields]

11.5 pitch was intentionally created to be not interchangeable with anything else.

Agree with die solution, which is what I do since I do not want to fool with the taper attachment

[atr]

The simple solution is to use a die.

I guess that's indeed the simple solution but in my case I need to cut only one thread and it's funny to buy a die for only a single use.

[Harold_V]

Without a taper attachment (or a hydraulic duplicator ---a "tracer") you would not be capable of creating that thread, assuming it's tapered. If you hope to create the thread, but straight, that's a different matter and may be addressed by altering the gears in the banjo. If you have the manual for your machine and the required change gears, it may provide the needed setup sequence. If not, you are likely restricted to the use of a die.

H

[liveaboard]

There is a program to find change gears; maybe it will help you find a combination that works.
do a search for "GEARSVB6".
It's free.

You input your leadsrew pitch (inch or metric) and list your available change gears. Enter desired pitch (inch or metric) and it presents you a list of possible gear combinations, with the error (if any).

Of course there's still the matter of cutting the taper...
I use the cross slide by hand while threading for that.

[Harold_V]

I use the cross slide by hand while threading for that.

Wow! That's no small feat, but certainly a solution when there are no other options. It may not be as bad as it sounds, as one usually has the left hand on the cross-slide handle, anyway. With a little practice, a functional thread most likely can be created.

Good comment!

H

[liveaboard]

Maybe I should elaborate a little...
I saw it done by a machinist in India, a man of some skill.

I started by cutting the blank cone to published dimension, and taking note of the total movement of the cross slide wheel required to follow the taper.
Then I practiced that a few times to get the feel of it.
During threading, I try to keep the cut depth even while rotating the cross slide wheel the predetermined amount during the cut.

The result was far from perfect of course, but quite usable; in this case I had a shoulder for the seal, but my Indian machinist's work held a seal on the threads, it was screwed into an aluminum engine block as an oil return line for a turbocharger.

[Harold_V]

One has a couple things going for them with tapered pipe threads. For one, they are a full form (100% thread), plus they are generally created on a relatively thin wall, so the base material (the tubing on which the thread is created) can conform to the tapped thread by deforming. So long as the thread form is reasonable and full (not truncated), a seal can be expected by simply tightening adequately.

Pipe threads have 3/4"/foot taper, so the amount of cross-slide movement is easily deducted. This system would certainly not pass in a rigid controlled environment (it most likely would NOT pass any inspection procedures) but so long as the end result is functional and suits the needs of the maker, it's a great way to solve an otherwise difficult problem. It wouldn't have been acceptable where I was trained, where all features received full inspection. That's not what we're talking about her, though!

H

[Bill Shields]

Find a friend with a die?