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Turning UHMW

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  • Turning UHMW

    Hi, Iv recently gotten a large chunk (12" x 6" x 1.5", about $35) of UHMW from the plastics store and started turning it, but getting a decent finish with a couple of the tools I usally use on steel/aluminum has been problematic.

    It either chatters, produceing nice small swaff but a bad finish, or closer to the headstock (no tailstock support used, 6" long 1.5" diamiter peice, 4 jaw chuck holding 'square' unfinished section) produces long curly swaff that seems to mainly enjoy turning into a rats nest around the work, being recut and damageing the finish when it does

    I was turning it at 300rpms, tryed a sharp HSS tool with chipbraker and also tryed some random carbide insert with chipbraker tooling.

    diffrent tool angle did seem to rather affect chip generation but I was wondering if anyone knew of some HSS grind profiles that did really well for finishing UHMW, or tips in cutting it without a 'ratsnest' forming of fine swaff.

    Cutting it in my porta-bandsaw sure was intresting. produced a giant ratsnest in the lower guide roller making a 6" long cut 1.5" cut height, did'nt seem to stall it out/damage it or melt into a goo, god bless UMHW slipperyness
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

  • #2
    Well, I've found the best way to cut pieces to size is with a table saw. One of my blades has been resharpened to leave the tops of the teeth flat, though I did that for a woodworking use. On uhmw it takes out little square divets, leaving the edges clean. Pretty much any sharp blade will work, though.

    On the lathe I have the same problems you do. I did resort to using a saw blade in the TP grinder, and that pretty much chips out the uhmw like the table saw, in other words no long stringy shoelaces or gummy mess. For a nice smooth finish after this, you would have to experiment a bit- if you leave several thou to take off with a sharp tool, you can come to the finished size in one pass. Trying to take off a thou or two can be tricky. It likes to be sheared rather than chipped, unless the chipping is very fast, as it is with the table saw.

    The cutting tool is going to either want to push away from the uhmw or get drawn into it, so some experimenting will tell you how many thou you have to take per pass to actually take that much off. Dialling in too little and it won't remove that much- dial in too much and more than that will come off. The right amount to dial in will depend on how your cutting tool is ground.

    Don't take this as gospel, but in my experience that's how it works. Some other plastics behave the same way, and many don't.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #3
      Accept the fact that it's going to be a rats nest until you get to near size then just do a finishing cut.
      Play with speeds and feeds, at some point it's usually possible to get it the throw the cuttings off in a big string well clear of the machine so they don't ravel.

      Use HSS for the finishing cut with massive top rake, I use about 30 degrees and hone all the edges, you need a small corner radius honed on.

      I normally finish at about 1700 rpm and 20 thou cut no idea of feeds it's no 3 on both levers but it's the same feed I use for roughing.

      .
      .

      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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      • #4
        I machine a fair amount of plastic and much of it is UHMW
        I found a parting tool the best.
        see link:
        http://www.kennametal.com/e-catalog/...y%3A4073&logo=

        e2die
        please visit my webpage:
        http://motorworks88.webs.com/

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        • #5
          I'm turning mainly polypropylene at the moment,dunno how similar to what you're using it is.
          I'm turning 5" diameter wheels and for facing and turning I'm using a HSS bit in a diamond style tool holder and turning about 250rpm with a 0.50" depth of cut and 0.004 feed,but I'm only a hobbyist so not in a production environment.Like John says,a good bit of top rake seems to do the trick.
          For boring I'm mostly using a positive rake insert tool,same speeds and feeds.

          The stuff comes off in long strings and is quite warm which surprised me and soon fills a scrap bin.
          One thing to bear in mind is that although the job doesn't feel particularly warm,plastic expands quite a bit,so I let it cool for a while before doing a finishing cut to get the finished size,especially when turning the inside of the wheel,the rings formed tend to rub on the tool and the tool gets pretty warm from the friction.

          I cut the initial parts with a coarse blade in a jigsaw and slowest speed and push the saw into the cut. I mount it on a piece of MDF fastened on a backplate securing it with a good bead of hot melt glue while I knock the corners off until I get it to a size I can mount in a three jaw chuck.

          I've found that for turning the inside of the wheels that the stuff starts to come off in an ever increasing ring so I score the surface with a stanley knife several times on different diameters as the lathe's turning and this makes the ring formed break off as it hits the scoring.

          Allan
          Last edited by Allan Waterfall; 10-30-2009, 06:02 AM.

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          • #6
            When you're down to finishing cuts, try using a vacuum cleaner to remove the swarf as you cut.
            Paul Compton
            www.morini-mania.co.uk
            http://www.youtube.com/user/EVguru

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            • #7
              For parting off larger diameter rounds I use the saw blade idea that I believe John was the original poster of a couple years back. The mount I made for my QCTP lets me adjust the rake quickly and I can part off about a 5 1/2" diameter piece in one shot with a 7 1/4" blade.

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              • #8
                I have done the same as EVguru except that I use the shop vac for the roughing cut. As the long curly stuff comes off the bit, it goes right down the hose. I just fasten the hose behind the compound.

                Patrick

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                • #9
                  Originally posted by John Stevenson

                  Use HSS for the finishing cut with massive top rake, I use about 30 degrees and hone all the edges, you need a small corner radius honed on.


                  .


                  UHMW is a PITA, Johns emphasis on very sharp HSS is key to a good starting point, Black moon a chipbreaker is useless --- your not going to get it to happen so go for nice long continues and find a way of getting rid of it (like the shop vac suggestions)
                  This has to be the same material they use in pop can holders --- it will stretch and stretch and not break,
                  When cutting large diameter pieces in a saw you should start pounding a wedge after the blade gives enough clearance or it will bind and then derail your blade.

                  On the flip side if you have multiple pieces you can actually get it dialed to where the stuff turns off the lathe and goes single file right into your garbage can.

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                  • #10
                    Originally posted by John Stevenson

                    Use HSS for the finishing cut with massive top rake, I use about 30 degrees and hone all the edges, you need a small corner radius honed on.

                    .
                    John, How do you grind the bit with that top rake ? ie. how far back
                    along the bit does the rake go. With that rake and a bit of front
                    clearance the edge is pretty "delicate" yes? :-)
                    ...lew...

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                    • #11
                      Nylon can be bad for that too.

                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        I use a tangential tool which naturally has fairly high rake. A chip brush held against the rear side of the work keeps the chip from wrapping - this can be hand held for small work or a holder mounted on the CS for larger items. I normally hand feed for roughing so pausing the feed occasionally breaks the chip.

                        John

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                        • #13
                          Evan: lol, thanks for making me feel better/not alone in this :P

                          Evguru: Thanks for the idea.. I was kinda thinking of it but when I tryed the braker blew (Too much other stuff on that circuit) and it kinda distracted me I bet I could make a nice split collar to clamp the hose onto a magbase or something.

                          I kinda suspected the chipbraker was usless. the deep interrupted cuts made really nice 9 shavings, but I suspect thats because it was interrupted, not sure what a deep continual cut would do, likey long curlys.

                          So I guess the large front rake basicly requires a 'chipbraker' style grind except its not really braking anything so I guess the smoothness of it doesnt matter.
                          Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                          • #14
                            Would a set of wood turning tools be useful in turning UHMW plastic 'freehand'? (ie setup a 1/2" bar into my tool holder to act as a wood lathe tool rest for the chisel)
                            Like this set: http://www.kmstools.com/maxwood-6pc-...-tool-kit-3748 ?
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                            • #15
                              I have discovered that the best insert I have found for machining plastics is a Sandvik dcgx 3(2.5)1 Al H10 designed for cutting Aluminium. It cuts clean, leaves a surface smooth as glass, and I love how it throws the string AWAY from the chuck so as not to get all wound up.



                              I also use my compound mitre saw with a carbide blade for cutting it to size.
                              Ernie (VE7ERN)

                              May the wind be always at your back

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