NPT Internal Threads

[Wongster]

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

[TomG]

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

[Wongster]

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?

[Vinny]

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.

[Wongster]

I only have this chart from the manufacturer of the lathe to help me with:


How do I do the calculation?

[Vinny]

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.

[Wongster]

Thanks Vinny. Can I assume that the calculator works regardless of the lathe i use?

[Vinny]

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.

[Vinny]

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

[Wongster]

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.