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  • Seeking help setting up a workpiece

    Hello all,

    I am a hobbiest and relatively new to machining. I have been working on my 7x10 desktop lathe learning the basics (turning/facing/threading operations) all on pretty small aluminum stock. I have turned larger pieces on an engine lathe while I was a student but now i am looking to turn a 2" diameter by 4" length piece of aluminum on my lathe. I currently have a 3" auto centering 3 jaw chuck and I am having trouble getting the workpiece trued in the chuck. What is a good procedure for making sure the piece is straight and centered, or do I really need to get a 4 jaw chuck OR is this much more of an advanced level project than I am thinking.

    Thank you in advance

  • #2
    A 3-jaw can only go so far to center work unless you try to shim each jaw. You really need a 4-jaw independent chuck to center or re-center pieces. If you continue to use the 3-jaw do not remove the workpiece any sooner than you have to unless you mark where a particular jaw was, otherwise it will no longer be centered.

    Lots of other posts regarding this matter on this forum a quick search should turn them up for you.

    Make sure you support the tail end of that larger piece ...
    "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

    -- Harold "Doc" Edgerton

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    • #3
      cool, thank you for the reply thats exactly what i was looking to learn
      I'll give that search a try and see whats already been said too

      Comment


      • #4
        Even a nice true chuck can hold a piece a bit off kilter so it can wobble a bit.
        Here's a little tool I made to help in centering workpieces in the three-jaw chuck. It's a simple roller that I can move inward to touch the work and nudge it to center while turning the chuck by hand or very slowly under power with the jaws tightened lightly:



        Once the piece runs true, I tighten the jaws and I'm ready to go.


        And, here's the little video that inspired me:

        http://www.youtube.com/watch?v=tn7A9PqNftY
        Cheers,

        Frank Ford
        HomeShopTech

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        • #5
          Thanks for mentioning that Frank. I also made one of those centering tools, very handy! It certainly helps take the wobble out of things that are longer and are only just barely held in the chuck. I really like it for taking the wobble out by using the face of a piece, like a thick washer that needs to be thinned.
          "Work hard. Tell everyone everything you know. Close a deal with a handshake. Have fun!"

          -- Harold "Doc" Edgerton

          Comment


          • #6
            I use the tool often for long pieces, getting them to run straight even if they hang way out. Makes it really easy to center drill the end for the tailstock center.

            Before I made the roller tool, I just ran up the tool post to nudge the work. Roller makes the job that much better!
            Cheers,

            Frank Ford
            HomeShopTech

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            • #7
              thr --- since you seem to be new here, you may not be familiar with
              Frank Ford and his web site ... believe me, it'll be well worth your time to put that site in your favorites for an treasure of tips/tricks like the one he mentions here.

              Frank, I had forgotten about this little alignment aid gadget and will again put it in the 'to do' pile! (By the way, from the post a few days ago on cutting a taper, you told of your gadget for making the tool post mounted indicator, reading off the back side on a bar -- well thats been my project for the last couple days and it every bit as neat as I thought it would be! -- thanks!)
              If everything seems to be going well, you have obviously overlooked something........

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              • #8
                Its not difficult

                Originally posted by thr
                Hello all,

                I am a hobbiest and relatively new to machining. I have been working on my 7x10 desktop lathe learning the basics (turning/facing/threading operations) all on pretty small aluminum stock. I have turned larger pieces on an engine lathe while I was a student but now i am looking to turn a 2" diameter by 4" length piece of aluminum on my lathe. I currently have a 3" auto centering 3 jaw chuck and I am having trouble getting the workpiece trued in the chuck. What is a good procedure for making sure the piece is straight and centered, or do I really need to get a 4 jaw chuck OR is this much more of an advanced level project than I am thinking.

                Thank you in advance
                Hi thr.

                First of all, just what degree of accuracy do you require in your set-up - and why?

                If you are going to machine stuff off that gets below any "off-centre" in the set-up, it doesn't matter - most times.

                "Adequate support" for the job is probably more important than the centreing of that job.

                The lathe spindle is trying to push the job down onto and past the cutting tool which is intended to remove metal from the job in the process.

                The reaction - due to resistance - of the tool is to stop the work passing it. If that force is strong enough, the work will tend to "climb over" the tool as the lathe spindle and the job keep turning/"twisting". If it gets beyond the capacity of the chuck to hold onto the job, the job will or may get torn out of the chuck. That is why "end support" at the tail-stock - by way of a "fixed/dead" or "live/rotating" centre in the tail-stock quill is so important in resisting these sorts of loads.

                No chuck - 3 or 4 jaw - is perfect as there is some "run-out" even when new and even before it is mounted on the back-plate or the lathe - each of which can or may have contributing additive errors as well.

                Setting aside wear on the chuck ("bell-mouthing" and worn/distorted "ways" for the jaws), the errors in a chuck will be either or both:
                - "cylindrical": where the test piece is "out" by the same amount along a specified distance; and
                - "conical" where the "run-out" varies over the length of the test piece.

                Here are the specifications for two new 3 jawed chucks:




                For what its worth, all mill and lathe spindles have similar errors on their tapers as do collet chucks and collets - even though they are much less - ie "closer" - than that allowed for a lathe chuck.

                For your job, I'd have clamped it firmly but not tightly in the 3-jawed chuck and then spun the spindle by hand and "eyed" the error and keep on tapping the high spot/s at the outer (tail-stock) end to remove about half the error (run-out) until it "looked good" (its surprising how good your eye and a bit of practice can get you in this regard). Then I'd clamp the chuck and re-check. I'd have used a lathe tool with a bit of white paper on the lathe bed to "sight it in". That is often good enough. Use a dial indicator only when you need to.

                Then drill the tail-stock end with a centre-drill, fit the tail-stock centre and engage it into/with the job (to support the job) and you will be safer and get a better result from the "stiffer" (more rigid) set-up.

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