Cutting threads on the inside of a 3" socket end of PVC pipe

I am in the process of making an item for my saltwater setups, and have to thread the inside diam of a 3" socket joint (happens to be
correct bore needed for my application and only needs threads) which are 13 tpi. My biggest problem is if I tighten the piece up in the chuck it distorts it. I ruined a bunch of pieces trying to find a sweet spot n chucking prressure with out distortin oor without having it slip. Even with light cuts its gone wrong. Perhaps I need to put it all away and give it a try on anaother day, unless someone can suggest something that may work to hold this piece in the chuck without distortion or slipping. I have played with super glueiing it, double faced tapes, but one thing I have not tried net is to glue two or three lugs on the outside of the piece I am turning and allow them to hold against the chuck jaws sides.........hmmmmmmmthat idea just hit me, is there anything else that may work?
PVC is neat to machine, but it can get awfull flimsey and such when chucked up.....
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Suggestions, I haven't tried 'em:
Cut a short length of Sched 40 or thicker 3" pipe to go in the opposite end of the joint. Now the wall thickness is almost doubled, and should not distort quite so much.
Drill holes into the opposite end of the joint and drive small screws into them. With those screw heads sticking up, the piece shouldn't rotate in the chuck.
or....do both of the above.
Best -- Terry
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On 22 Jan 2006 14:45:33 -0800, snipped-for-privacy@hotmail.com wrote:

Not a possibility as threads need to go through entire length of the piece except for about 3/32" of it at most.

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Turn a wood plug that will drive into the pipe. It will stiffen the pipe enough to hold in the chuck. Bugs
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On Sun, 22 Jan 2006 22:34:49 GMT, snipped-for-privacy@hotmail.com (Roy) wrote:

What about mounting the PVC joint in the center of metal (or wood) block, with some countersunk screws (in the sides of the block) to keep the PVC joint from spinning loose.
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BTDT: PVC is pretty flimsy stuff. I'd solvent glue it to a 1/2" pvc plate, chuck up the plate, turn the threads, cut it off. Of course, I have some PVC sheet stock available so that is the first choice.
Roy wrote:

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I have a limited uspply of sheet pvc (1/4") and had considered doing this, but was a bit leary of cutting up the sheetstock and then screw up cutting the threads..... What I am making is a retaining ring that has a flange on it so it can clamp a funnel shapped cup to a flange with an O-ring seal between the two.
I happened on a few items called protein skimmers that leaked at the adjustment knob shaft, and figured out a way to cut that apart and remachine the shaft and housing to include a o-ring seal like later models have, unfortunately when the company threw out these leaking skimmers on warranty replacement they retained the colleciton cups and pumps. The going price for a replacement cup complete with retaining ring is $48.90 each.......and that does not include the cap for the cup, which would be a piece of cake to make once I get the cup and retaining ring made.....so is life.....Pumps I have , so I can save a lot of $$$$ if I can get the retaining rings made. I also need to find the name of a plastic, which is described in another post. Regards

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Roy wrote:

Make a ring of steel, aluminum or any other material that the pipe will just slide into the bored hole. cut a slot in the ring so that when the chuck jaws close on the ring there is enough gap that the ring will grab the pipe tightly. I have used this method on thin wall parts many times. It works.
John
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Roy I am not sure I understand exactly what you want to do but get a larger piece of PVC that will not distort as easily. The bore it to fit your flimsy stock. Glue it in if needed. Then thread away and cut away the bad from the good.
Bob AZ
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Roy, This is an excellent place for soft jaws. If you make some jaws that are broad enough, they'll enclose the piece almost totally, eliminating distortion. It's worth the effort, for then you'll have the jaws for other applications, and you'll be surprised at how often you'll use them once you have them. Nice part is you'll be able to take the piece out and put it back and have it run nearly perfectly. The advantages of soft jaws are almost endless.
Harold
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On Sun, 22 Jan 2006 22:34:49 GMT, snipped-for-privacy@hotmail.com (Roy) wrote:

You can make "pie jaws" for your chuck. These are shaped like sections of a pie. Use sections that are 120 degrees. Close them until the sides almost touch and then bore to fit your part. These will hold your parts with small enough distortion, and with enough grip, to single point your threads. Another method would be to turn and bore a piece of good wood (not pine, but maybe poplar?) so that your part just slips into the wood piece. Make one saw cut so that the wood can grip the plastic when the chuck jaws tighten. A 3/4" wall on the wood piece should be adequate. If slippage does occur then glue some fine sandpaper to the wood in the bore. 320 or 240 grit would work fine. ERS
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Roy wrote:

Make a custom "collet" for it. Hopefully, these parts are quite cylindrical (probably so if you were chucking them). So, you get a piece of pipe, aluminum or steel, that has a slightly smaller ID than the PVC part's OD. Turn in the lathe until the PVC part is a very tight press fit in the ring. Then, slit the ring at one point and clean up the edges. When you put the PVC part in the ring, and then clamp the ring in the 3-jaw, it will close up the slit and grip the part. If the ring is stronger than the PVC it will distort much less, and put the PVC part in circumferential stress, so as to minimize distortion. The PVC part would bulge out between the chuck jaws without this ring. I've made this sort of fixture a number of times when turning flimsy parts, and it has never failed to fix the problem.
Jon
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I'd suggest you measure the ID of your socket and the OD of any PVC pipe you decide to use in a few places first. If it's anything like the 4" piece I used to make a housing a while back, it ain't round to start out with. I ended up turning a wood plug as a pilot.
You may want to get a cleanout adapter to slip inside or over the piece you're threading and work from there...
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Well I took a different approach.I first started with PVC pipe fittings as they were close to being what I needed, and would work if I pursued it with a proper method of retaiing the piece in the chuck.
What I did was laminate two pieces of 1/2" pvc sheet stock together. I made them 4 1/2" square. Mounted it in a 4 jaw, cut the center out roughly with a hole saw mounted in the tailstock, cleaned up the hole to required ID, cut the threads and shoulder lip it needed, removed the piece and remounted it in a 3 jaw with the jaws holding the work internally and machined off the outer portion to obtain the round shape and ran a light knurl on it as well..... Looks great......I guess using an already produced form or shape is not always the easiest route to go. It took far less and lots less waste overall doing it this way than it did my old way and getting nowhere...
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Roy wrote:

Oh, that's familiar. There's an approach that looks very promising, but after adjusting and/or fixing one complication after another, you realize that a whole-other approach is really called for. The new way usually turns out to be as good or better. Bob
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