Cutting NPT threads on a mini-lathe?

According to SomeBody :

Yes -- you need a taper attachment for cutting the tapered (NPT) thread mentioned in the "Subjet: " header -- as well as needing the ability to cut the correct number of threads per inch.

Which Mini-Lathe is this, and what thread pitches can it cut?

What diameter pipe are you planning to thread? For any reasonable length, you need to have a large enough lathe so the pipe will pass through the spindle, so you can cut the threads close to the chuck. This sounds out of the reach of the typical Mini-Lathe.

Also -- normal straight threads terminate in a groove to give you time to disengage the half nuts (if any). Tapered pipe threads can go on to terminate in the air.

Most people cut pipe threads with dies, which are a lot more portable than a lathe -- even a Mini-Lathe.

Good Luck, DoN.

[ ... ]

You can if the lathe has a taper attachment -- but I don't think that any of the mini-lathes have such an attachment.

Enjoy, DoN.

Actually, I think it is possible to do so if one uses one of the NPT thread chasers that are typically used in die heads. Die heads typically have 4 or

6 chasers and the operator would have to rig up a way to mount one chaser to present to the work. That might be done by holding it in a QC-type tool holder or turret tool post.

Mike

You could always make one, though:

If you don't have a taper attachment for your mini lathe then the tail stock has to be used. I don't know if your mini lathe tailstock has the ability to be set off center. If it does, then moving it toward you 3/8" per foot will give you the proper taper for NPT threads. The taper is 3/4" inch per foot on the diameter. Or 1 degree, 47 minutes. So you could take a piece of pipe and hold it between centers with the tailstock set over and single point the threads. The tough part about this is driving the pipe. Using a faceplate, a lathe dog, and a center in the spindle is one way to drive the part. The lathe dog has a screw which tightens against the part and a bent over tang that fits into one of the slots in the faceplate. The tang will fit loose in the faceplate. So use some binding wire to hold it against the side of the slot that does the driving. This will keep it from bouncing around. Remember that the taper per foot is 3/8" per side per foot. This means that if you thread a piece 4" long the tail stock only needs to be set over 1/8" If you don't know some of the terms I used then google for them or go to the library and check out the book: How To Run A Lathe. This book is old, and will assume you are using a different type of cutting tool and tool post. But the lathe itself is little changed and you will see good pictures of faceplates and driving dogs. I just got the book off the shelf and checked for tapered thread cutting. And there is a drawing of the setup needed to cut the tapered thread. It shows an important part of the setup. This is making sure the threading tool is square to the straight part of pipe. There is a tool called a "fish tail" that is used for grinding threading tools and for setting the tool square to the work. They are cheap. And if you are gonna grind your own tools (and you should learn how) then get one. They are very handy. Cheers, Eric

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Great write up! I learned something today... thanks.

Well, you're welcome! Eric

Agreed! Well said, Eric.

Harold

According to Eric R Snow :

With the proviso that the workpiece must be short enough to turn between centers.

And if it is longer, then my earlier requirement that the spindle through hole must be large enough to accept the pipe through the spindle still applies, along with the need for a taper attachment.

Granted, on a Unimat SL-1000, the headstock could be swiveled to produce the taper -- but cutting threads on it is too much of an exercise even with straight threads. The taper would make for other problems with the threading attachment for that machine.

Enjoy, DoN.

You could always make one, though:

Thanks!

Great write-up, it looks as though I have my work cut out for me. I will have to attempt some trial pieces before embarking on the project.

Thanks.

Nice write up Eric, Thanks.