MetalworkingFun Forum

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

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

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?

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.

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


How do I do the calculation?

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.

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

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.

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

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.