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Is cast urethane machineable?

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  • Is cast urethane machineable?

    And is there a danged "e" in the middle of "machineable"? Haahaa. Man I got a horrible education. Working on that .... anyhow ..... onward here.

    Most of you have seen those red shock absorber bushings made of (usually) red urethane. I need to come up with a replacement drive cushion for these older motorcycles. They would replace these old rubber "cartriges" that are hot-pressed into the stock wheel. Those cushions are not available (and never were) from Honda, but many of us that ride these 1980's machines are having to deal with this issue.

    This "drive spider" (aka "driven member") simply presses into the cushions easily by hand.

    It looks like this when it's all assembled. The splines on the driven member mate up with the splines on the drive member. Away we go! The drive spider is located l-r by an o-ring on the splines. The o-ring slips into a groove in the drive member to locate the driven member in the proper spot within the wheel, so that it stays centered in track. The drive cushions do just that. Cushion forward power drive. The wheel itself stays centered by the use of several spacers fitted on the axle on each side of the wheel.

    My thoughts were to replace them with cast urethane cushions Without getting into the engineering deets, the harder durometer of urethane is of zero consequence in this case. They don't align the wheel, the bike's weight is not supported by them. All they do is cushion the drivetrain from harsh engine pulses. That's it. Other bikes that use this exact same wheel made by the same bike manufacturer and the exact same engine use no drive cushions at all. So that implies that the actual durometer of these cushions is of little importance. They are only on some bikes to make them more "delux" with smoother riding/less perceived engine vibrations etc ...

    Never the less thay ARE on there.

    Anyhow, I either must make precise molds and cast new properly sized cushions for the drive section, or use some existing urethane bushings and machine them down to spec. That is, if this stuff is workable at all. Biggest problem seems to be fixturing. How to chuck-up something that is a bit squishy and not have it whacked into distortion during material removal. Mandrel of sorts perhaps?

    Anyhow .... just thought I'd ask. You never know what kind of experience people have until you ask.

    Thanks tons.
    Last edited by Farndurk; 09-10-2011, 12:41 AM.
    I make messes.

  • #2
    Also, those metal sleeves in and out of each cushion are also unecessary. Those are part of the prefabbed cartrige that some vendor supplied to Honda like that. The inner and outter sleeves are bonded to the rubber cushion. This allows the entire thing to be treated as a cartrige. When pressed in correctly they cannot distort the rubber itself during installation. So it's not like those metal sleeves are actually part of the engineering of the wheel. They are part of the available part that Honda's contracted vendor dsupplied to them back then.

    Just need some kind of cushioned insert that will pretty much stay in place. I've even considered that windshield installation goo, I've seen that stuff used as engine mounts for those that want to "cast" their own urethane engine mounts. Make a mold out of duct tape and get with it!

    Anyhow ......
    I make messes.


    • #3
      You'd have better luck grinding urethane rubber parts instead of trying to turn them- pretty much like any rubber parts. Toolpost grinder would be the way to do it. Holding is a trick- if you can press fit a bushing onto a mandrel, then grind it, that works well. If you can press the bushing into a recess of appropriate size to grind the ID, that works too.

      If you don't have a toolpost grinder, you can do the job on a table saw. You'd have to have a hole in the bushing first, then you'd make a shaft to press the bushing onto, then make a jig to hold the shaft horizontally and above the blade. You arrange to turn the shaft, and manipulate the jig side to side- you raise the blade up to the bushing, turn the shaft, and move the jig to remove the total circumference of the bushing according to your settings.

      I've done similar many times and I know it works. It's basically milling- but at 200 ft/second cutting speed or faster, and 20 to 120 teeth involved. The teeth go by so fast that the rubber doesn't push out of the way while being 'machined'. It's messy, and can be dangerous.

      An alternative in this case would be to cast a liquid rubber in place. You'd have to mount the part well enough centered and adjusted for axial position, then pour each cavity full of some liquid solution. They do make epoxy rubber compounds and two-part urethane compounds, but better top up your wallet before shopping.

      If you can handle going through several separate 'pours', you could use shoo goo or any of the other-named similar compounds- goop is another name. Give each hole about 1/8 inch of goop at a time and let it cure- keep adding layers until the cavities are full. Goop doesn't pour very well, but it can be done. The part will never come loose-

      Too handle coating will also work, but it too is not all that pourable. It can be thinned with naptha, but it will also shrink quite a bit during cure, and it would have to be applied in layers, same as goop.

      There may not be much said about durometer for these bushings, but because the application is as a damper, it's going to have a resonant frequency where it works the best. You could have a situation where at some speeds it makes vibrations worse. I'm thinking that a medium durometer urethane would be about right, and that could be about what those bushings you have are. If I was doing that job, I'd have the table saw jig half made by now-

      If they are to be a press-fit in the holes, use some dish detergent to lube them so they install fully. That won't harm anything, and will eventually get washed away.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


      • #4
        Yes, grinding is the way to go....
        Precision takes time.


        • #5
          Great stuff guys! I can't hang around .. we have a huge Haboob coming down on us right now (Yuma, Arizona) I'm actually on generator power at this moment. Tied the awnings down to the Toyota, got everything put away so no-one gets killed by an 80mph lawn chair.

          AACK! Damned lightning too! It's no different than beadblasting, blowing sand creates static charges that build up and release when they find a ground.

          WOW! What was THAT? Ok ... Thanks .. gotta jet.
          I make messes.


          • #6
            Contact these folks bet they can help you. Paul Oliver has been a big help to me in the past. As him about glue for urethane also.



            • #7

              This is the place to go for urethane stock. They can also mold your bushing for you if you can make a sketch.

              I use them all the time when I build custom dies at work. They do very good work never had a problem with them.
              "The Administration does not support blowing up planets." --- Finally some SENSIBLE policy from the Gov!


              • #8
                Check McMaster Carr. They have a lot of different types of rubber rods, both solid and tubular in different materials and hardness (durometer). You might just luck out and find some tubular stuff that you can get to fit with out grinding.



                • #9
                  Blah, you don't want to machine cast rubber. You want billet rubber! Just kidden.

                  But seriously, an existing bushing + metal ID and/or OD bushing sounds like a good idea. Ie, all you need to do is find one with same or smaller OD and same or larger ID
                  Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


                  • #10
                    Originally posted by Black_Moons
                    Blah, you don't want to machine cast rubber. You want billet rubber! Just kidden.

                    But seriously, an existing bushing + metal ID and/or OD bushing sounds like a good idea. Ie, all you need to do is find one with same or smaller OD and same or larger ID
                    Like I said McMaster has a good selection of materials. Since rubber is so forgiving he may find something close enough to just press in with a little dish washing soap for lube. Onle work would be cut to length.



                    • #11
                      I'm sure the original bushings are vulcanized to the rubber in which case you can burn the rubber out and reuse the sleeves.



                      • #12
                        Rubber is usually frozen for machining. Possibly dry ice would be good. I suppose you'd have to work fast,or have the work mounted in a collet so you could remove it,re freeze,and re mount.

                        I got a 2 part kit for casting urethane some years ago. Not too cheap. I was working as toolmaker then,and can't recall what reason I bought it. Didn't use it,and gave it to someone who needed it since its shelf life was being reached.

                        It is NOT something you want to breathe AT ALL. We had a strong spray booth there to mix it in. I would not recommend mixing it under a kitchen stove hood,just not powerful enough. Be careful.


                        • #13
                          I use urethane as die pads for "bump dies" and as die cushions for forming dies.

                          One part if you can picture it looked like a * with three inch long legs. The legs needed to be bent 90 degrees at a certain point.

                          What I did was build a forming die with a urethane pad in the center. The force of the press pushed the pad down as the mandril forced the part down. Retract the mandril and the urethane popped back up ejecting the part.

                          Doing it this way allowed us to use a smaller simpler press and increase output... The smaller press cycles faster than the big one and it's less complex to set up which is a boon for the operator.

                          Great stuff. The supplier I mentioned above can help you find the right material for the job... in fact I would be surprised if they haven't supplied material for these bushings before as there are a metric f*ck-ton of motorcycle hobbyist and restoration fanatics around here! (I am not one of them... I simply provide the occasional machined doo-dad or technical advice. )
                          "The Administration does not support blowing up planets." --- Finally some SENSIBLE policy from the Gov!


                          • #14
                            Probably be easier to mould them from some urethane that "Smooth-On" sell.

                            A mould made from polypropylene would be virtually self releasing because the urethane won't stick to it.
                            I made a mould for the rubber insert on a model aircraft engine starter extension I made.



                            • #15
                     has come castable stuff About $1.25 a cubic inch. And some tube that you might be able to cut to length.
                              Last edited by 914Wilhelm; 09-10-2011, 05:34 PM.