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+--- Thread: NPT Internal Threads (/thread-3545.html)
NPT Internal Threads - Wongster - 04-28-2017
Hi folks,
Someone asked me to help cut some internal threads for some fittings without the taper. I've no experience with NPT and so I though if you folks can help me out.
For NPT without the taper, would it be like those usual threads on bolts? The pitch I gathered is to be 11.5 tpi (which I'm also not sure if my lathe has the gear combination for).
TIA.
Wongster
RE: NPT Internal Threads - TomG - 04-28-2017
The taper is indeed the only thing that makes NPT threads unique, other than the odd pitches and diameters. A 60º tool will work just fine.
Tom
RE: NPT Internal Threads - Wongster - 04-28-2017
Thanks, Tom. Been waiting for new videos... lol.
I look up the gear combination for threading but only see whole number pitch. Any trick to cut the 11.5 tpi?
RE: NPT Internal Threads - Vinny - 04-28-2017
On my Logan it's one of the gearbox selections. Guessing you don't have a gearbox, you'll need to calculate what combos will do it and you may have to buy a gear or two.
RE: NPT Internal Threads - Wongster - 04-28-2017
I only have this chart from the manufacturer of the lathe to help me with:
![[Image: 54976DE3-07B0-4C3C-8B9B-A95CE3F8B155_zpskws5c5wf.jpg]](http://i1207.photobucket.com/albums/bb469/wongstersproduction/Mobile%20Uploads/54976DE3-07B0-4C3C-8B9B-A95CE3F8B155_zpskws5c5wf.jpg)
How do I do the calculation?
RE: NPT Internal Threads - Vinny - 04-28-2017
There's a calculator here: http://littlemachineshop.com/reference/change_gears.php If I can remember how to do it manually I'll let you know.
RE: NPT Internal Threads - Wongster - 04-28-2017
Thanks Vinny. Can I assume that the calculator works regardless of the lathe i use?
RE: NPT Internal Threads - Vinny - 04-28-2017
The bottom one is for any lathe. The formula is DRIVEN/DRIVER X DRIVEN/DRIVER X Leadscrew TPI but I'm having a problem with the formula with your chart.
RE: NPT Internal Threads - Vinny - 04-28-2017
Here's another one: http://bilar.co.uk/cgi-bin/change-gear-calculator.pl And here's some more info on calculating it: http://www.lathes.co.uk/latheparts/page14.html
RE: NPT Internal Threads - Wongster - 04-29-2017
Thank you, Vinny.
I'll study through and see if I can get the project going. If I can make it, it will be a paid job.