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Thread: Cutting NPT on a lathe

  1. #1
    IronSkull is offline Plastic
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    Default Cutting NPT on a lathe

    Is is possible to cut a 2" pipe thread with a manual lathe with taper attachment? This particular thread it on a casting and the thread needs to end tight into a shoulder. Cant get close enough with a die.

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    johnoder's Avatar
    johnoder is offline Diamond
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    Yes - set the T/A to 3/4" per foot on dia.

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    IronSkull is offline Plastic
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    60deg single point tool still set up as if cutting a straight thread? 90deg to axis of work and 30deg on compound?

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    S_W_Bausch is offline Diamond
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    Those NPT threads aren't on all lathes, so good luck.

    Seems to be 11 1/2 tpi.

    National Standard Taper Pipe Threads Size NPT Chart - Engineers Edge

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    vettepicking is offline Hot Rolled
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    A taper attachment is the only easy way on a lathe that i can think of. moving the tailstock over might work but the threading tool needs to be square to the shalf of pipe, not the taper where the threads are cut . if that makes any sense.

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    thermite is offline Diamond
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    Quote Originally Posted by IronSkull View Post
    Is is possible to cut a 2" pipe thread with a manual lathe with taper attachment? This particular thread it on a casting and the thread needs to end tight into a shoulder. Cant get close enough with a die.
    Have a care you don't get TOO close to the shoulder, else the NPT won't be able to reliably seal with the less-than perfectly repeatable fittings that may be offered to mate to it.

    You cannot/dare not turn an NPT die bass-ackwards as you might a straight thread to a shoulder, but NPT dies are common enough that you might also give some thought to grinding one 'just enough' thinner to serve to chase threads made by an unmodified one and pick up a bit extra towards the shoulder, as it won't need the long-lead-in when tracking a precut thread.

    Bill
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    IronSkull is offline Plastic
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    I have a big axleson lathe that will do 11 1/2 per in. And I understand the set up. Just like cutting straight thread, set the tool 90 to the work, and the cut follows 3/4 over a foot taper. Thats all I needed to know. Thanks

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    johnoder's Avatar
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    Actually if you are just talking one side, the cut follows 3/8" in a foot, because the 3/4" per foot is on diameter.

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    Limy Sami is online now Diamond
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    Quote Originally Posted by IronSkull View Post
    60deg single point tool still set up as if cutting a straight thread? 90deg to axis of work and 30deg on compound?
    Yes - only set the compound to 29deg, IME it gives a better finish on threads.

    If I was working up close to a shoulder or other obstruction I'd reverse the spindle, invert the tool and cut away from danger

    It works for me on a TA.

    FWIW - If it needs to seal ''right'' as in not overun and foul the shoulder etc etc, either get a gauge (warning big $$) or a sample of what has to screw on to it.

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    oldbrock is offline Hot Rolled
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    No prob. do it all the time. Presume you have 11 1/2 tpi on your qcgb. Hope you have a telescoping type taper attachment otherwise you will be feeding thread depth with the compound. Make sure you snug up the gibs, the backlash in the cross slide screw will allow the pressure on the tool to push the cross slide back that amount. Other than that, a piece of cake. Peter

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