Can you cut pipe threads on a lathe?
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  1. #1
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    This may be a dumb question, but I've never seen it addressed --

    I know that pipe threads are tapered. Could a person cut those threads on a lathe equipped with a taper attachment set to the appropriate taper?

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    YES, They can. It's sometimes easier to rough chase straight & run a die on though.

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    Here's a boiler wash out plug being made without a taper attachment between offset centers:


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    Yes. I tried it today on a SBL with taper attachment.

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    I've done tapered threads by the ways listed above. The fastest way I have ever found on a manual lathe to chase them is with a tracer. That only works if you need to do a couple of dozen or more.

    The usual way I do them is to turn the tool upside down, and thread away from the chuck. Then I just turn the compound in as I feed off the part after I get the whole thread to the big end size. Do the math as to how much to feed in each revolution. With a little practice as to how fast to feed in, and you can pass a thread gage test.

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    The way I was shown, although dont know how many do it this way becuase there are faster ways, i guess, is to machine the taper(I believe its 1 degree and 47'?)then set your tool bit square with the diameter,NOT the taper you just cut...then thread away..

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    On a lathe that doesn't have too much backlash and with some practice its possible to freehand the taper if the lathe doesn't have a taper attachment. Just cut the ouside taper with the compound first to have something to follow. It doesn't make a thread as accurate as if cut with a taper attachment. But sometimes it does the job if the pipe isn't running much pressure and that theres no chance for it to kill someone if it leaks. You can always chase it with a die after to fine tune it.

    At first I thought it was a crazy idea to freehand a pipe thread but after doing a couple I was quite pleased with the results. Although I wouldn't do it on anything critical or anything that was being sold to a customer. I'm just mentioning it because some guys just can't spend $1000 on a taper attachment to cut a thread for some widget they're making for the backyard.

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    does anyone do it the way I mentioned or am i way out in left field?

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    I do it just like gbent posted , its fast and works fine for pipe.

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    CatHead, always set your tool square with the part. It's the way its supposed to be done. Never set the tool to the taper. So you're doing it right.

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    That deal of setting square to the part is also how I was taught, and makes sense.

    It is also a really good argument for using a taper attachment.... You are setting square to the CENTERLINE of the part, which is actually at an angle if you are taper turning by offsetting T/S. Makes it hard to set to.

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    That's correct, don't set to the taper.

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    Also I read somehwere that for may pipe sizes it it acceptible to use a tapered female with a straight male thread, machineries handbook maybe ??

    Parts would not be to print so it would only be for parts you are making for your own use. I have used 1/8 NPS nipples before in a pinch and they worked fine with some teflon tape on them..that was before I read the information.

    Bill

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    I've got the taper attachment, assuming I ever get my lathe put back together! [img]smile.gif[/img]

    Do you taper the piece before threading, or just taper the threads themselves? And how much taper? Somewhere I think I read 1/4" per foot -- is that right, or did I dream that somewhere?

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    With regard to mixing straight and tapered threads, the following may be helpful:
    "Pressure tight joints for low pressure servce are sometimes made with straight internal threads and the Americian Standard taper external threads. The ductility of the coupling enables the straight threads to conform to the taper of the pipe thread. In commercial practice straight-tapped couplings are furnished for standard-weight (Schedule 40) pipe sizes 2" and smaller."

    pg 7-150, Piping Handbook, 5 ed, McGraw Hill. Chapter 7 was written by Helmult Theilsch, manager of the Research and Development Division, Grinell Corp, and one of the recognized authorities on piping. In many ways the 5th edition of this handbook is better than the 6th edition.

    Oh, Yes, the taper is 3/4 inch per foot.

    Thermo1

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    You dont have to taper the work piece before threading. The threads will be tapered

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    Is it possible to do this properly by off-setting the tailstock? I am worried about that boiler washout plug now ... Not something to get "roughly right".


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