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Lathe - centering a long part after flipping

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  • Lathe - centering a long part after flipping

    I am going to lathe a part that is about 3.5" long. I will work one end, then flip it and do a final boring & outside operation.

    When I flip it, I will only be able to hold about 0.5" of material.

    I will be using a 4 jaw chuck with copper plates for soft jaws. No problem centering the 4 jaw, but how do I ensure the part is parallel to the jaws/ways?

    thanks for all the help!

  • #2
    If your part is only 3-1/2" long start with a piece of material that is an inch or more longer than your finished piece,
    grab it in the chuck and machine as much as possible in one setup...
    Keith
    __________________________
    Just one project too many--that's what finally got him...

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    • #3
      Originally posted by JW1942 View Post
      I am going to lathe a part that is about 3.5" long. I will work one end, then flip it and do a final boring & outside operation..........
      If you are boring it after you flip it, I would suggest you use a steady rest to hold the shaft true while you bore it and then use a revolving center in the bored end while you clean up the outside. If you can't use the revolving center, can you use a previously made center drilled plug in the freshly bored hole to ensure the shaft doesn't get pushed sideways in the jaws?
      Location: Saskatoon, Saskatchewan, Canada

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      • #4
        To ensure that it is mounted such that the sides are parallel to the axis of the lathe; first make sure that the piece is centered. Then fix an indicator with a fat, flat tip to the cross slide and sweep it along the length of the work by moving the saddle left and right. If everything is perfect, the needle will barely move. Rotate the work 90 degrees and repeat. If both tests are good you are ready to go.

        Dan
        At the end of the project, there is a profound difference between spare parts and extra parts.

        Location: SF East Bay.

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        • #5
          Give us a better description of the part or even a drawing what you are trying to attempt.
          Boring 3.5" long part hold only 1/2" in chuck is a quick way to many disasters if you are inexperienced.

          recipe could be steady rest, different order of operations, turning between centers or just plain old whack it with inertial adjustment tool until dial indicator looks good.

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          • #6
            Excellent ideas - thanks team!

            FWIW - here is the part I am making. https://bbs.homeshopmachinist.net/fo...ension-adapter

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            • #7
              Originally posted by JW1942 View Post
              I
              I will be using a 4 jaw chuck with copper plates for soft jaws. No problem centering the 4 jaw, but how do I ensure the part is parallel to the jaws/ways?
              !
              btw, this should always be a concern, i.e. dialing work in in the four jaw. Most seem to get dialing it in at the chuck, and if the goal is something really accurate you get it to a tenth or two.......but the chuck isn't prefect and I guarantee a few inches out from the there it will not be to a tenth. What do you do? tap tap tap....its an royal PITA iterative process of indicating in two spots and using light taps get them both concentric.

              where possible, Instead work between centres as it is the easiest way to maintain concentricity when swapping a part around
              Last edited by Mcgyver; 03-23-2020, 07:54 AM.
              .

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              • #8
                I would probably start by turning a center drilled shouldered plug that fits in the 28,75mm bore. (if you have enough large diameter live center this wouldn't be necessary)

                After that turn the 44.50mm lower section external features, grip from the 44,50 diameter section in 4-jaw, bore the hole to size, plug it with the previously made plug and turn the OD to 32,40mm.
                That way the wall thickness is still plentifull when boring the hole and plug is supporting the thin walled section during OD turning. Thin walled parts are cha-cha-chattery.

                And since this is suspension adapter and not a machine spindle bearing housing tolerances are probably somewhat more loose.

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                • #9
                  I would do the long end first, turning the OD, then boring the ID while it is still on the bar stock, Then part off, a little bit long. Do all the parts like that first. Then make a mandrel that's about .001 slip fit in the bore, and put that in the chuck or 5C e collet. Then you can do the short end while holding the long end without damage to the thin wall of the tube. Do all the parts on the second end like that, and you're done.

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                  • #10
                    If the part is already cut to approx. length, drill an undersized through hole and then do the 32.4 dia. end work first, to include finishing the 28.75 bore and the 44.5 dia. od. Then flip the part, holding by the 32.4 dia. Pushing the part up against the shoulder of the 44.5 dia should hold the piece straight in the chuck. Then it's only a matter of indicating the 44.5 dia to get the part running concentric before finishing the detail of the 34.6 dia end along with the 20.4 dia bore.

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                    • #11
                      Thanks a lot for the ideas guys - very much appreciated. I need to do some measurements and then will start in on the process. I will keep you all posted.

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                      • #12
                        I don't see this as too big to do in one go. But which you do first and second after the flip depend on which surfaces are critical to proper operation.

                        Joe Pie on YT did a good video on cutting thin wall tubing like features and very thin diameters. You may want to look that up thoughts for the thin section.

                        Another option is to do the drilling and boring all in one setup. Including the thin wall portion. Then with the two (or more) bushings done turn an expanding mandrel from scrap stock to let you slip the thin wall portion on and hold by the ID. You could turn the mandrel from aluminium then drill and partly tap for a large set screw and then make a couple of cross cuts along the axis with the arbor still in the lathe. It wouldn't take all THAT much time by hand. But if you have a hand held band saw it would be easy. This mandrel would be 28.75mm D x 58L and if the splitting cuts are about 20mm long that should be enough to let pressure from the set screw in the center push out by the .01 to .02mm needed to firmly grip the bushings for the rest of the work and hold them centered really well.
                        Chilliwack BC, Canada

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                        • #13
                          Very good video. Outside last. Lot of support - do not bore all through. Cut the OD in a single pass.

                          I need to do a few large diameter, thin wall bushings - this is a great technique for them!

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                          • #14
                            Chuck it as originally planned. Set a dial indicator against the side of the part at the outboard end. Rotate the part and tap against the side until it runs true. Centerdrill. Support the part with a center and then set a steady rest at the outboard end.

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                            • #15
                              Originally posted by JW1942 View Post
                              Very good video. Outside last. Lot of support - do not bore all through. Cut the OD in a single pass.

                              I need to do a few large diameter, thin wall bushings - this is a great technique for them!
                              Yep, that's the one. Quite the eye brow raiser, eh?

                              I've done the pretty small diameter and LONG pins for a couple of my own jobs of and the same trick of taking the whole cut at one time did the trick. Due to the depth of the cut and not wanting to induce any push away I hand fed the cuts with just enough pressure to keep a nice fine ribbon coming off the nice freshly sharpened cutter.

                              He's got another about order of cuts for a part. Another good one. And it's that inspiration that makes me think you might be able to do all your critical cuts in one setting then flip and mount onto that expanding mandrel I suggested and do the rest of the stuff with a low tolerance even if it's not exactly spot on like you get with a one grab only.
                              Chilliwack BC, Canada

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