anybody have experience turning teflon????



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Thread: anybody have experience turning teflon????

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    Default anybody have experience turning teflon????

    we are turning a .125 diameter by .400 long teflon part on a citizen B12(.250 diameter barstock). the problems we are encountering is that the oal is not repeating. we are running dry with a constant air blast @ cutting tool. basically we are doing this. we have to hold +/-.001 on OAL but we are absorbing the entire tolerance and them some.

    collet is set just tight enough to grip mat'l without slippage and GB is set with very minimal drag.

    ANY HELP WOULD BE HUGE!! We always struggle with Teflon

    Thanks


    1-face
    2-spot
    3-drill thru(.050 dia)
    4-re-face & turn(one cut to Z.450)
    5-cutoff

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    Registered MikeMc's Avatar
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    I am wondering why air? I always run this stuff with oil.

    www.atmswiss.com


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    I was cutting "virgin" teflon and found it to strech into the tool as I was cutting which caused over cutting. Even .005" seemed too much to face off in one pass, material is too soft. Also the cutoff could be doing the same. Try lighter cuts to be sure this is not your problem. The bar stock also whips between the collet and guidebushing which pulls the stock back into the guidebushing.



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    Red face No oil

    Mike Mc.....We generally run air for TFE for chip control, it helps to keep chips blowing away from part & cutting tool. I think this is a pretty standard process. On the other hand oil has shown to have negative effects on many types of plastics.



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    Default over cutting

    cogsman....i have experienced the same condition in the past. from the best of my knowledge we have our process' down so that we take a "light" cut on finish faces.

    i am wonder if something is happening (compressing....stretching) between the guide bushing and the main collet that is causing the variations despite our efforts to avoid that very thing.



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    I've run into the same problem. I had the operator set the GB with no drag and programmed the cutoff to cutoff .003 over, and then move in and create a finish cutoff pass to size.

    I did have to remember to move the main spindle back in .003 afterwards though. After both of those remedies the parts repeated.

    However, it's a fine line between tight and loose when setting the GB. I've noticed as the material machines it will change the diameter enough to affect the turn. I have the operators run 5 parts after the machine sits for a while before considering something to be a repeating dimension.

    Oh, and we do use oil. If you are dry machining, I'd still try wiping the bar down with some oil and see if that helps the GB not grab the part.



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    I hope ur not using an insert as a cutoff. ..u need to use a carbide "knife" tool....guaranteed to repeat oal!



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    Hey, I run bunches of teflon.

    What elsanch said. I made a holder for teflon that has a slot in it for a peice of razor blade to fit in, and some tiny set screws to hold the blade in place. this helps loads with holding size and burr control( +/- .001 OAL, and .030" +/-.001 hole thru. )

    and yeah,run it slow to keep heat down. Usually the material we get is pretty consistent, but trying to turn it like delrin is asking for trouble. i think max is 3000RPM on those parts i mentioned above. Takes awhile, but shorter than fighting the damn thing.
    Oh, also, Take out all the G0 moves in z and replace them with G01Z___F.005, and don't be shy to use a bunch of g04s to make sure the stock isnt springing.

    Hope this helps!!



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    I appreciate all the suggestions. The problems seem to have stabilized to a managable level. I don't use a cutoff insert, I use a .008 wide micrograin carbide tool. The razor blade idea is interesting though, I may have to give that a try.
    Thanks



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    Default Re: anybody have experience turning teflon????

    Quote Originally Posted by chet470 View Post
    I appreciate all the suggestions. The problems seem to have stabilized to a managable level. I don't use a cutoff insert, I use a .008 wide micrograin carbide tool. The razor blade idea is interesting though, I may have to give that a try.
    Thanks
    I know this is an ancient thread, but I rarely see this mentioned. Teflon is a funny material, it undergoes a phase change at around 68 degrees F I believe. 1.7% change in volume because of this. If your shop were below 68F, and then when you machined it, it warmed up, then it would grow 1.7% while machining. My knowledge kind of peters out at this point, but in theory, if you took the cube root of the 1.7% you get about 1.19 which would be the linear growth in each direction. A 0.400" long part would grow almost .005" in length. Similarly (proportionally) in diameter as well.

    Your part is pretty small, so it isn't a big deal, but on a 5" diameter part, it might grow many thousandths.

    We are building parts for cryogenic service, so we have to deal with this phase change shrinkage, as well as normal shrinkage due to change in temperature. I'm going by memory, but a roughly 3" diameter teflon ball valve seat shrinks about 0.040" after cooling to -260F, on top of about 0.035" growth from phase change.

    Here is a link to a study on this. If you are machining lots of teflon, you may want to talk to someone more knowledgeable.
    https://pdfs.semanticscholar.org/5c8...e457fc4abe.pdf



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    Default Re: anybody have experience turning teflon????

    I run Teflon with Super duper double dog sharp tools! It really needs water in my opinion to keep that temp constant.
    For the tools, I take some diamond paste and load up some brass then make them super shiny. 500, 1500 and 4K grit I think... I forget exactly, but if you make the cutting edge super sharp you wont get any hairs.

    I also have one of those lab refrigerator thermometers (50 bucks on amazon) and stick the probe in the coolant. Then you make a part, note the coolant temp, then put it in the inspection room, note the air temp. Wait 30 min, measure the part.
    Then you can adjust and run them. They you have to do some kind of black magic ritual to make sure the rest come out the same. Those tolerances sound easy but are kind of tough with dealing with long skinny parts in plastic on a machine that heats up...



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    Default Re: anybody have experience turning teflon????

    Great information. Super specific! I have found that the Teflon (PTFE) is super soft, and loves to move around a lot. I think your insight into the 68 degree is invaluable. Thanks.



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anybody have experience turning teflon????

anybody have experience turning teflon????

anybody have experience turning teflon????