NPT Internal Threads

[Vinny]

Cool! Good Luck!!!

[Wongster]

Does it matter if the lead screw on my lathe is metric?

[Vinny]

Slightly different formula. (DRIVER/DRIVEN) * (DRIVER/DRIVEN) * LS PITCH = PITCH. 25.4/PITCH will give TPI. You'll probably also have to use +/- 5% in your calculations.

The app at littlemachineshop.com should accept a metric lead screw. Also example 4 here: http://www.lathes.co.uk/latheparts/page14.html covers it.

[Dr Stan]

One thing many/most forget is the 60 degree cutting tool should be aligned with the taper, not the axis of the lathe.

BTW, the only thing that crosses/interchanges between Imperial & Metric threads are the taper pipe threads.  Learned this on a trip to Germany.

[Wongster]

I proceeded with test cut when the requestor asked me to try with 12TPI.  But I was so scared of crashing into the chuck that I disengaged the leadscrew when I'm supposed to reverse or stop the spindle....





My lathe doesn't have a threading dial like some I saw on YouTube. The cross slide has to be engaged at all time during threading. 

Any tips on how to see if I'm at the end of the cut to stop and reverse the spindle with the threading tool backed off the part?

[Mayhem]

1. Engage the half nuts to take the first pass.
2. Turn the motor off and at the same time, advance the cross slide to move the tool out of the thread (this prevents turning a concentric ring that will induce a stress point. If you can turn a groove for the tool to enter, or the piece is threaded all the way through it makes it easier, as you know the tool has cleared the thread. Otherwise, set a stop or mark the tool and you can always stop a fraction early and then turn the chuck by hand.
3. Advance the cross slide so that the tool completely clears the thread (I usually go one revolution on the dial, so I don't get confused).
4. Put the lathe into reverse and back the tool out until you are clear, and stop the lathe.
5. Set your next depth of cut, put the lathe back into forward and turn the lathe back on.
6. Repeat steps 2 through 6 until your thread is complete.

I do a spring pass every third pass because internal tools will flex a little. You simply take another pass without advancing the tool (i.e. take the same pass as the last cut).

Of course, if your chuck isn't a screw on one, you could turn the tool upside down and cut the thread on the back of the piece. To do this, you place the lathe in reverse to take the cut, as the carriage will then run toward the tailstock. No risk of crashing the tool into the chuck. Same process as described above but the directions are reverse.

Hope this helps and that I haven't missed something but that is how I do it.

[stevec]

Darren, doesn't NPT mean national pipe TAPER as opposed to NPS or national pipe STRAIGHT?
How do you achieve the taper in your procedure?

[Mayhem]

I cannot thread a taper at present Steve, so I was simply describing how it do an internal thread. I don't think that Wongster can do it either unless his lathe is CNC (I think he has a CNC mill) or he has a taper attachment (I assume you can thread using a taper attachment).

I've screwed external NPT threads into female NPS threads that I've cut before. Some thread sealant and they hold OK. Of course, I'm not using them in high pressure applications just the oiler on my mill.

[Vinny]

Earlier Wongster said it'd be NPT without the taper. So probably just a standard thread.

Wongster, when cutting a thread that's really close to the chuck, and especially for internal threads no matter where they are, I put the tool on the opposite side of the work and run the machine in reverse to cut the threads. That way the end of the threading is the end of the workpiece and if you can't disengage the the lead screw, just let it keep going until it's far enough to out of the way to check the thread. Adding a threading dial would be helpful!

[Wongster]

Wow!!! Thanks guys!!!

I should call it NPS. That guy just told me NPT but ignore the taper... lol. I'm clueless.

Anyway, I do have a tiny cnc lathe. Not full cnc, only 2 axis, unlike those machining center I watched on YouTube. It's a Sherline lathe with long bed. It's currently beneath the workbench to make space in the otherwise super crowded "shop".

Cutting in reverse direction is a good idea. And about threading dial, is it something I can just add on to my kind of lathe? It's a Proxxon PD400 with a 12mm x 1.5mm leadscrew. With a threading dial, can I disengage the leadscrew when I reached the end of a cut? I feel safer that way; to use the lever to engage and disengage the saddle from the leadscrew. Too nervous to do this job.