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  • I'm having problems cutting thread in pvc

    I am not a plumber or work with pipes normally, but am an engineer by trade.

    I'm trying to cut threads on a 1 1/4 pvc sch40 pipe. This will be used to house electronic equipment and not used for any pressure purpose. I have a special 12R rigid die for pvc, using a handle.

    While cutting the threads, I get about 3/4 of the way through, when the handle stops suddenly. It requires an enormous amount of pressure to continue. However I noticed when then happens, I back it off just slightly and notice that it looks like it's taking a rather large bite of material, spanning about 4 thread widths. It usually cuts very small strips that exit the die separately, but this is a large chunk of plastic spanning about 4 threads that is all connected together.

    If I back off even further, causing one of the cutter blades to pass backwards over that spot, it seems to remove that 'chunk' of material and I can continue using only light force. When it gets to where the pipe is just barely covering all the cutters, the handle stops again. This time, it's permanent. I can't make it cut beyond that, no matter what I try. The pipe turns in the vise if I use more force.

    It was my understanding that the end of the pvc pipe should extend to the end of the die and not just barely covering all the cutters.

    I also noticed that when I try to screw an end cap on, on some threads, the cap goes on many turns before it tightens while on other threads, it only goes on a couple of threads before stopping.

    Is this normal? If not, what am I doing wrong or what do I need to do right. I am not using any cutting fluid or oil. I'm applying as even a force from start to finish. The die is brand new from Rigid. I'm using a wood plug inside the pvc pipe so it doesn't deform. This wood dowel does not extend to the threading end, but only where the vise holds the pipe. The force required to turn the handle is so light (until it stops), that I felt the wood dowel inside wouldn't make a difference. I may be wrong.

    Thank you for any advice.

  • #2
    Re: I'm having problems cutting thread in pvc

    First you probably need to 'set' the dies. Do this with a new factory metal nipple. Loosen the 4 screws on the diehead about 2 turns. Thread the metal nipple all the way in. Then tighten the screws in a clockwise order.

    The wooden dowel needs to extend all the way to the end of the pipe. Any deflexion in the plastic will give you a poor thread. Try a little liquid soap instead of oil.
    Last edited by plumberscrack; 10-13-2010, 06:41 PM.

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    • #3
      Re: I'm having problems cutting thread in pvc

      Wow! Quick response. I'm impressed.

      To make sure I understand, I loosen the 4 screws holding the back plate of the die. I use a factory metal nipple (not sure what this is, but assuming that I use a normal metal pipe end?) and put it all the way into the die until it just passes the end of the cutters or to the end of the die (flush to the back plate?). Then re tighten the screws. I'm assuming that this procedure lines up the cutters with a 'normal' thread.

      Thanks in advance for helping to clarify my misunderstandings.

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      • #4
        Re: I'm having problems cutting thread in pvc

        Yes, normal pipe thread but NOT one that was cut by hand. Use a store bought 1-1/4" nipple. Thread all the way to the INSIDE of the backplate or just past the cutters (I find going all the way to the end gives you too deep of a thread. Others may tell you differently) Where ever the nipple thread ends up, don't force it. Only go hand tight then maybe a 1/4 turn more. Then tighten screws with nipple in place. You should be good to go then.

        FYI: schedule 40 is always a PITA to thread. Sch. 80 is easier

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        • #5
          Re: I'm having problems cutting thread in pvc

          Thank you so much for taking the time to help me. I really appreciate it.

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          • #6
            Re: I'm having problems cutting thread in pvc

            i have a 1'' pvc die and it works good.

            are you using fresh pipe and is it the the grey pvc?

            older pipe tends to get brittle.

            open the dies up like bill said and you should be fine with new clean pipe.

            rick.
            phoebe it is

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            • #7
              Re: I'm having problems cutting thread in pvc

              Why can't you use a male adapter???

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