Looking for a Good Plastic for Some Fixturing
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  1. #1
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    Default Looking for a Good Plastic for Some Fixturing

    We're looking for some hard plastic for some fixturing that will support some thin wall parts during machining.

    Essentially the plastic will act as plugs, but it needs to be really precise in overall length and diameter.

    We want something that handles threaded and/or tapped holes nicely.

    Our first thoughts are nylon.

    It seems to have the desired hardness, it is decently priced, and it seems to handle temperature fluctuations comparatively well.

    Any suggestions?

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    Nylon is a real pain in the ass to machine, especially if you are trying to hold a few tenths. Delrin is pretty much the defacto plastic for what you are looking for. Next would be PVC to save a tiny amount of money, or if you want to glue pieces together.

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    gotta agree delrin, finishing a vacuum fixture made out of delrin tomorrow. made lots of fixtures with delrin that are thread as well.
    abs works but sucks and does grow with coolant I only did it cause I had 3'x6'x 2" sheets here

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    Not trying to hijack your thread but when you say thin walled parts are you talking about a part with thin interior walls or profile walls? When I do thin wall stuff I use that 2 part spray foam insulation to keep the chatter and flex out. After the part is finished I just soak it in a solvent and done.

    But to through in another cheap fixture plastic that holds size is ABS.

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    Delrin would get my vote

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    Acetal resin (Delrin) gets my vote, but as long as impact isn't an issue, acrylic can work pretty well too and may be cheaper. Just keep the sections pretty thick where possible, and avoid sharp inside corners.

    Nylon would be far down my list due to hygroscopic behavior, lower stiffness, and "ehh" machinability.

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    Quote Originally Posted by Milland View Post
    Acetal resin (Delrin) gets my vote, but as long as impact isn't an issue, acrylic can work pretty well too and may be cheaper. Just keep the sections pretty thick where possible, and avoid sharp inside corners.

    Nylon would be far down my list due to hygroscopic behavior, lower stiffness, and "ehh" machinability.
    +1.
    Nylon is rubbish. It can absorb 3% by volume.
    The best material is natural acetal.
    Delrin is okay, but natural acetal is the best material and is what is used for spindle liners (as told to me by Hydrafeed).

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    Quote Originally Posted by DavidScott View Post
    Nylon is a real pain in the ass to machine, especially if you are trying to hold a few tenths. Delrin is pretty much the defacto plastic for what you are looking for. Next would be PVC to save a tiny amount of money, or if you want to glue pieces together.
    Thanks - I've never machined it and didn't realize it was that difficult to machine.

    Based on all of this great feedback I will definitely look into delrin, and the other materials mentioned too.

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    Quote Originally Posted by barbter View Post
    +1.
    Nylon is rubbish. It can absorb 3% by volume.
    The best material is natural acetal.
    Thank you - I didn't know that nylon was so difficult to work with. I will definitely look into acetal too.

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    Quote Originally Posted by g-coder05 View Post
    When I do thin wall stuff I use that 2 part spray foam insulation to keep the chatter and flex out. After the part is finished I just soak it in a solvent and done. But to through in another cheap fixture plastic that holds size is ABS.
    Awesome - thanks. I will consider these two options as well.

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    Delrin = Acetal Homopolymer

    I have cut a bunch of parts out of Acetal Copolymer and we had serious issues with dimensional changes due to humidity and temperature in the shop. Parts were good coming off the machine, and then some would fit their mating part and some wouldn't as the seasons changed.

    Actual Delrin is supposed to be much better but if I recall was a good bit more expensive.

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    Delrin, we use it for a lot of different operations here...cuts easy, stays accurate, and is pretty wear resistant.

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    McMaster has some surprisingly well priced delrin - all the way up to 10.0" diameter.

    We'll probably give that a shot.

    Thank you all.

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    Quote Originally Posted by Delw View Post
    gotta agree delrin, finishing a vacuum fixture made out of delrin tomorrow. made lots of fixtures with delrin that are thread as well.
    abs works but sucks and does grow with coolant I only did it cause I had 3'x6'x 2" sheets here
    Those are some very $ sheets. I have some small pieces of 1" and 1.5" Acetal plate, but nothing like that. Acetal rod, I have plenty of that. Ran it in bar feed mode in the lathe all day yesterday, and will run all day today too. And all next week.

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    Quote Originally Posted by Scruffy887 View Post
    Those are some very $ sheets. I have some small pieces of 1" and 1.5" Acetal plate, but nothing like that. Acetal rod, I have plenty of that. Ran it in bar feed mode in the lathe all day yesterday, and will run all day today too. And all next week.
    Yes they are, a buddy of mines work shut down about 8-10 years ago. I got a hd2500 filled to the top of the bed with ABS , Delrin,nylon and a bunch of other types of plastic from 1/16 thick to 6" thick. mostly full sheets and big blocks. cost me like 250 bucks. been making fixtures out of it for 10+ years, cause I didnt have any certs for it. 2 years ago I threw and gave away about 500lbs, didnt really want the non certed suff in shop.
    we used to run lots of sheet/block delrin and 1" round teflon. The certed stuff I still have a back office filled with it.


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