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Npt thread cutting

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John_Dennis

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#1
I am making a 3/4 NPT pipe nipple out of a piece of 316 stainless pipe. I have a 3/4 pipe die but am afraid that stainless will be a little tough to cut. I do not have a taper attachment on my lathe, should I try to cut straight threads and then use the die to chase them to a taper or cut a smooth taper and thread with the die?

Thanks

-John
 

benmychree

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#2
That would work and save your die from destruction (hopefully) Use the best sort of tap lube for the job, I like TapMagic.
 

markba633csi

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#3
I like your first idea, cut straight threads and then chase them to a taper- much less brute force required especially with stainless
mark
 

benmychree

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#4
316 SS is nasty stuff; ideally, the chasers should be HSS
 

P. Waller

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#5
Process plumbers use dies to thread SS pipe every day without problems using common Rigid pipe threading tools.

Once in a while I thread 2 1/2 SS pipe using this method for the local sewerage department. The biggest problem is often holding the work if you do not have dedicated pipe threading equipment, a small 3 Jaw scroll chuck in an engine lathe will not likely hold it.

This is not threading but I use this machine for it, I place a 1 1/2" square steel bar in the tool post to keep the die stock handle from rotating, this is a 20" chuck but 2 1/2" pipe will spin in it if not VERY tight.
Have at it, the worst that can happen is that you break a die or spin the pipe.
Failed to add the picture.
Serrated jaws will not always hold the part as mentioned by benmy, these are W&S jaws with heavy serrations and if you spin the part you will have made 24 grooves in it.
 
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benmychree

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#6
Even a 10" 3 jaw with smooth jaws can hardly be tightened up tight enough to cut pipe threads with dies in a lathe, even on ordinary steel pipe.
 

Technical Ted

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#8
Is it possible to put your piece of pipe on a homemade (or store bought) mandrel and use tail stock offset to turn then thread? I think that's what I would try if I really wanted to thread it on my lathe without a taper attachment.

Good luck,
Ted
 

Video_man

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#9
A 3/4 inch pipe is not a long taper, you could cut it with your compound fairly easily, set to NPT taper (3/4 inch per foot, or about 1 3/4 degrees (1 deg 47 min 24 sec, actually). I've done this with such tapers, makes the die cutting much easier. Obviously, you need to chuck the workpiece fairly close to the jaws or provide a steady rest otherwise.
 
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