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Thread: Cutting UHMWV - bearings from flat

  1. #1
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    Default Cutting UHMWV - bearings from flat

    What is the best way to cut some plain bearings from a UHMWV flat. I have a piece maybe 4" x 6" x 1/2" thick. I need to 6 pieces 1.000 OD x 0.800 ID x 1/2" long. These will function as plain bearings in a part that I am making. Basically just stabilizing and allowing smooth/easy low speed (by hand) rotation in a lazy-susan type deal. How well will a hole saw work? Better to trepan? Or is there some better way?

  2. #2
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    Do you mean Ultra High Molecular Weight Polyethylene which I've usally seen as UHMW?

    I've machined UHMW with endmills but it's majorly gummy. Super sharp HSS works better than carbide. It's so gummy that I'd say that the likelihood of holding accuracy in the thousands with the stuff is pretty low. A hole saw will probably do better than other options but be prepared for it to load up. The stuff wants to melt if cut too fast and it wants to flow generally.

    Also, don't put it in an oven at 300F: That will lead to the part warping and becoming scrap. Won't say how I know this UHMW is more fun to machine than A2 tool steel but not quite as durable.

  3. #3
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    I just made several 2" diameter inserts on the lathe and had no problem. I used a sever rake with moderately radiusd cutting edge and it gave me a smooth shiney factory look surface.

    I cut mine out in squares, drew an X across them, drilled them out. Then stuck a bolt in the 3jaw slid the square over it and tightened a nut down over it and turned it.
    Last edited by Your Old Dog; 12-08-2008 at 07:04 AM.
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  4. #4
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    I frequently cut UHMW on my lathe using a DCGX insert designed for cutting aluminum and get that same smooth shiney factory looking finish that YOD gets. No problem with gumming and I spin it at around 900 RPM.
    Ernie (VE7ERN)

    May the wind be always at your back

  5. #5
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    The cost of your time to post this is more than the cost of a 3ft stick of 1" UHMW. Its not a rare or expensive material, buy a stick and make it a quick lathe job, life is too short.
    James Kilroy

  6. #6
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    ckellog: Yes, that's it. Just got my acronym scrambled. And I'll remember not to cook it...

    YOD: Thanks, cutting into squares was on my list, just thought maybe there was a better way. This chunk I have has one hole that looks like maybe a hole saw, but it just didn't sound like a good idea given what I think I know about this stuff.

    Doc: I've got some nice "scooped" high rake inserts. But I would probably just use HSS for this.

    JKilroy: I'm an HSM, therefore my time is by definition worthless. But I get your point. However, I have this chunk that's been in the way for a while, waiting for "just such an occasion". If I bypass it now, it will be in the way for another eternity. And it's against my religion to throw it out. Anyway, I did make an inquiry about local sources. That seems like a good thing to have on hand given the number of "hinge/bearing like things" I make...

  7. #7
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    BadDog:
    If UHMW cuts anything like HDPE, then a holesaw is fine. Just go slow and drill a through hole tangent to the waste side of the cut for chips to fall into. Going fast causes the plastic to weld to the saw.

    I cut 2" dia x 0.5" thick circles out of HDPE sheet, then clamped to a screw through the center hole and held the screw head in my mill's spindle. I used sandpaper between the washers and the plastic since HDPE is so slippery. Using an HSS lathe bit I was able to turn it within .005" or so of the diameter I needed. That was holding the screw head in the crappy drill chuck that came with my mill with all its runout. With a good lathe chuck you should be able to do much better with no extra effort.

  8. #8
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    lwalker: Thanks.

    But that brings up another (perhaps better?) question. In spite of my earlier joke, I'm not averse to buying stuff to save trouble and time. But, if I'm buying, then the question becomes, "What is the best material from which to make simple low friction light duty bearings?" UHMW, HDPE, ???

    This specific task is making a hub for a tooling holder. It sits on an upright pole of 0.800 diameter. The pocket for the "bearing" will be 1.000. And the hub sets on a roller ball thrust bearing (had several in stock). So all the plastic needs to do is hold the hub co-axial with the shaft to handle any imbalance caused by tool loading. It also should turn smoothly and easily, though not "too easily" (don't want it auto rotating due to vibration etc). But I seem to do a lot of stuff like this. No terribly high loads, no high speed, just needs to move freely and not wear into the shaft or corrode. It is also often in this size range since my "axle shaft" is often a bolt shank or the like.

    So, if I order a 3' stick of 1" rod, what material will best suit, be readily available, work freely/easily, and not cost much? I recall posts by Evan and others about various special plastics having all sorts of interesting properties, but I really don't think I want or need anything exotic...

  9. #9
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    I'm not a mechanical engineer (ask me electronics and software questions :-) but IIRC, it depends on the PV relationship (PSI of the weight bearing on the contact area * velocity of shaft) of the shaft you're bearing on. That number will determine if the material can handle it. Different materials will have different PV ratings.

  10. #10
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    Nylon makes good bushings if you think you might need more.

    Personally I'd use Delrin. Plenty good for your application as a bushing. And the remainder is a good all-around plastic. Easiest machining of the plastics I've used. As in it breaks a chip well. (I hate UHMW and HDPE). I use aluminum speeds and up the feed until it chips or the finish suffers and back off a hair. Pretty good strength.

    Lets see if I can get a link to McMaster to work this time.

    PLASTICS
    Jon Bohlander
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