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Do I cut the inside or outside first?

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  • Do I cut the inside or outside first?

    I have to make a bushing from aluminum with thin walls. Somewhere around six to ten thou.
    Any preference to do first and last?

  • #2
    How long? 1 diameter or 5diameters?

    If short, I would do OD, then ID then part.

    Long. ID, Length, Mount on mandrel and turn od.


    • #3
      Originally posted by 1-800miner View Post
      I have to make a bushing from aluminum with thin walls. Somewhere around six to ten thou.
      Any preference to do first and last?
      I have started from ID, plug the hole with wooden dovel, play doh or rubber bung and turn OD
      Location: Helsinki, Finland, Europe


      • #4
        Do the inside hole first. If you machine the outer diameter first, the walls will collapse when you try and hold it to drill the hole.
        Brian Rupnow
        Design engineer
        Barrie, Ontario, Canada


        • #5
          ID first, turning peels material off so has an advantage to boring as boring takes more pressure to cut due to the geometry of the cut,,,

          still - aluminum and that thin is going to most likely take some kind of internal reinforcement after you get the center bored,

          like others have stated more info would help


          • #6
            6 to 10 thou? Wrap a couple layers of aluminum foil around the bolt

            I would do ID first assuming your going to have a piece of full thickness stock in the chuck for the OD. Then again if this is a 3' bushing, that's a whole other level of difficult.

            Is drilling out the hole for the next size of bolt possible? Or drilling it a little larger to take a thicker bushing?
            My recommendation?

            No matter what I tell you, get a second opinion.


            • #7
              I have to make four. only about an inch long but four different diameters.
              So I need to think this through and use the same mandrel for each. Just turn it down to do the next smaller size.

              Well I just thought about it and the i.d. stays the same. One size mandrel for everyone.
              Life just got better.

              The big camel back that went through the fire is slowly coming to life.
              The bottom drive shaft sat in Babbitt on the end of a large cast iron " wishbone"
              I have never done Babbitt on that small of scale and decided to replace Babbitt with bronze bushing.
              Well I screwed up and bored the cast oversize of the bronze bushing and now I gat to pay twice for my mistake.
              As in, aluminum bushing to center the bronze ones.

              Then comes the hard part of getting the wishbone affair registered straight and true on the line boring machine to i.d. the bronze.
              the wishbone is nearly a hundred pounds with no two surfaces parallel.
              Going to involve two tombstones, two large angle plates and lots of shimming and jacks.
              Last edited by 1-800miner; 09-21-2018, 11:21 AM.


              • #8
                rough the id rough od finish id finish od


                • #9
                  Turn the ID and fill with Cerrobend . Turn the outside and the melt out the Cerrobend. My temptation if possible is to leave the Cerrobend in place and push the bushing in place and then melt out the filling. That option depends upon where the bushing is being used.



                  • #10
                    Joe Pi on YT did some thin wall plastic bushings and used the "trick" where the amount removed on the OD was done all in one go. The idea is that you only get support from the material that is next to be removed. So ID first so the tube or whatever has the support needed for drilling or boring then remove the entire outside amount in one pass.

                    I used the same idea on a test trial for turning a very small pin from solid. For giggles I cut an .025" pin from .25 rod. Worked really well. Clearly no way to go back and take a finish cut but by using a very light pressure so the shaving was quite fine but continuous it worked out to be a decent cut.

                    There IS another method you could use if you only need that much of a bushing. And that would be to set up and knurl a raised collar on the smooth upper shank of the bolt just under the head. Or if the bolt is short enough that it is threaded all the way then to wind a couple of turns of a slightly smaller spring of the right size wire so it crowns up above the OD of the thread by the amount needed. And since it would grab quite well you might even be able to dress it down slightly to fit perfectly.

                    In fact it was easy to find the link with "thin wall bushings" in the YT search line.


                    I'd say "good luck" but from the test I did for the small pin I'm sure that this technique will serve your needs without the requirement of filling the center or making a mandrel.
                    Chilliwack BC, Canada


                    • #11
                      How big in diameter?

                      Thin and aluminum is gonna be a problem no matter what.

                      OD turning puts (with most tool grinds) a good deal of radial "push" to get the tool to cut. And then there is the force of the cutting. Even with a mandrel that fills up the part, the tool may just "catch" the material and tear it with a part that is literally only a few sheets of foil thick.

                      I assume there is no way to just use a piece of shim stock wrapped around whatever it is?

                      I have done several fairly thin wall parts in steel, making electronic plug pin removers. I actually drilled out the ID as the final operation for those. A quick check says one of them has about 7 thou walls. The OD is around 0.125 for that one, the other is slightly larger. (looks like one is a little burred-up from use)

                      CNC machines only go through the motions


                      • #12
                        Finish the bore, then mount on an expanding mandrel and cut the OD. You don't need to use much pressure with the mandrel expansion and any heat-treated aluminum alloy will cut to that thinness without 'tearing'. If you will be removing large amounts of material from the OD, you must keep the part cool. Thermal expansion on thin aluminum can cause the part to slip on the mandrel due to the light grip required, and then it may bunch or tear.
                        Southwest Utah


                        • #13
                          Originally posted by J Tiers View Post
                          OD turning puts (with most tool grinds) a good deal of radial "push" to get the tool to cut. And then there is the force of the cutting. Even with a mandrel that fills up the part, the tool may just "catch" the material and tear it with a part that is literally only a few sheets of foil thick.
                          turning actually puts less pressure on the part again due to the geometry and being able to "peel" the material off,

                          but there's more to it than just that - it's more structurally rigid in comparison to pushing from the inside, kinda like how an egg is strong for outside defense yet how a weak little chick can peck it's way out of it,,,

                          you would be surprised at what can be achieved with high RPM's and a threading insert (or the like) and slow feed rate with skin cuts,,, the trick is to keep the unit pressure high enough to bust through the surface and yet the surface area it's contacting very low, this equates to being able to skin cut without much overall pressure - threading inserts (or equivalent are ideal for this situation)


                          • #14
                            Sharp tools and tiny "nibble" cuts may be best..... unless you are taking off a LOT of material, in which case a tool that cuts only on the left side may allow a cut direct to final size, so that the part is never touched when at final size. I do this for thin long parts,like an 0.093 stem on a 3/8" part, but they are not hollow.

                            A LOT of material means an amount much larger than the final thickness, so taking off 0.075" depth of cut might be a "lot". Even 0.040 DOC may be a lot when final size is 0.006 thick.

                            I thought about the idea of leaving the part on a solid stub of the original stock.... and then parting it. I think you would need to part with most of the cut in the solid part, just for support and depth control. Might still tear the material.

                            If you can keep the expanding mandrel tight, I agree, the chance for tearing is less. If you glued it on the mandrel, and both were the same material, that would possibly be best way for glue. Heat parts to remove (loctite, etc).
                            CNC machines only go through the motions


                            • #15
                              Like JTiers I also wonder why you can't just wrap a bit of shim stock around it. Form them neatly around something round so it stays tubular. Would certainly be a LOT easier.

                              Not many of us have a drawer full of various size expanding mandrels handy. And for all but the shortest of ring style bushings small nibbles puts too much stress into the remaining material. Again I strongly suggest watching and trying the Joe Pi "all at one go" method. It really does work.
                              Chilliwack BC, Canada