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Help with screwy four jaw chuck

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  • Help with screwy four jaw chuck

    Brand new four jaw chuck - if I put a straight test bar in it and indicate on the middle of the bar till it's concentric then the ends of the bar go round like a baton twirlers baton. Same deal if I indicate on one end of the bar - the other end is miles out.

    If I indicate on the outer diameter of the chuck body then it's concentric to 0.02mm - it's difficult to indicate on the front face because of all the markings / bolt holes etc. Does this all suggest that the chuck is concentric but not mounted square or the jaws aren't square to the chuck axis? I tried reversing all the jaws but got the same result.

    Assuming the body is on square do I need to correct this by somehow machining the jaws? Chuck is a front mount din 6350 style if that makes any difference.

    Cheers for any advice,

    Simon.

  • #2
    I suggest as a first step taking all the jaws out and giving it a good clean, maybe there is something in there. Next step, put the jaws in reversed and recheck.

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    • #3
      First thing I checked was for swarf, and as I already said reversing the jaws gives the same effect.

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      • #4
        Ooops, sorry about not reading carefully enough.

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        • #5
          Jaws not perpendicular

          The axis of a straight test bar when mounted in the chuck should be reasonably parallel with the axis of rotation of the headstock of the lathe. You should be able to traverse a test indicator along the mounted test bar and not have the reading change a great deal. If it does, that suggests that the gripping surfaces of the jaws are not parallel with the headstock axis of rotation and the jaws need to be re-ground.

          gordon

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          • #6
            Originally posted by ironnut
            the jaws are not parallel with the headstock axis of rotation and the jaws need to be re-ground.
            Your correct in the diagnosis, but since this is a brand new chuck it needs to go back to the distributor for a replacement or refund.

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            • #7
              The chuck is new in that it's not been used but I bought it over six months ago - don't think I'm going to have much success returning it.

              The chuck is mounted via an adapter plate - I'm wondering if the plate is at fault rather than the jaws - think I need to check it carefully before condeming the jaws or chuck.

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              • #8
                Originally posted by sco
                The chuck is new in that it's not been used but I bought it over six months ago - don't think I'm going to have much success returning it.

                The chuck is mounted via an adapter plate - I'm wondering if the plate is at fault rather than the jaws - think I need to check it carefully before condeming the jaws or chuck.
                That's a good idea as it could just be some dirt or a burr. Did you machine the backing plate while it was mounted on your lathe, or was it already on the chuck? If the latter I'd determine if there is enough material to face off the plate and machine it true to your lathe.

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                • #9
                  Chuck

                  Check the spindle nose for damage or run-out, then install the mounting plate and take a light cut on the mounting surfaces. mount the chuck and check the face and OD without the jaws in place. Then you will be ready to install the jaws and a test piece.

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                  • #10
                    Simon,

                    Take the chuck off the backing plate and run an indicator on the face of the plate to check for wobble. If it runs true then your problem lies with the chuck. To machine the back of the chuck true to the jaws, chuck a piece of round stock in your three jaw and take a clean-up cut on it. Grab the freshly turned round stock with the jaws of your four jaw and indicate it so it runs true radially. Then you can machine the mounting surface of the chuck relative to the jaws to make it run true when re-mounted on the backing plate.

                    That should cure your wobble issues.

                    Tom
                    Tom's Techniques

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                    • #11
                      Since the chuck is new, it may not have been deburred well enough. I would check each piece and remove any build-ups, etc. You mentioned that the body seems to run pretty close (.02mm), which is good since you don't need any extra vibration from imbalance. If you have to throw that out to get the jaws to align, that would be a bummer.

                      It would seem that if you can change jaws around and the problem persists, you are largely taking the blame off the jaws. Perhaps one or more of the slots have not been machined parallel to the face of the chuck, or perhaps the jaws are loose in the slots until tightened, in which case they would skew. If you can rule out sloppiness in the jaw fits, then I would start with one jaw and run it in each slot, using a dial indicator to see whether the inside face of the jaw is parallel to the spindle axis. You will probably have to keep some outward pressure on the jaw to somewhat simulate it being tight while you roll the carriage back and forth with the indicator. Be careful as you don't want the indicator tip to catch in the gaps on the gripping edge of the jaw.

                      Do the same test with each jaw in every slot. From this you'll know if the jaws are off, or if the slots are off.

                      To be a good test, you'd have to check with the jaw in different positions along the slot. It won't be any good if they grip accurately at say 1/2 inch diameter, but skew the workpiece at another diameter.

                      It would almost be safe to say that if you do any machining to a jaw, you would then have to dedicate that jaw to that slot.

                      If you can check the concentricity of the gripping edge to the spindle axis at a few different locations of the jaw along the slot, and find that the error is consistent, then you could theoretically improve things by grinding the gripping edge. And of course this will all get messy right away- you'll have to clean, clean, clean the slots, jaws, etc as you go, otherwise the dust will not only throw the process off- it will end up being lapping compound. If you do end up grinding the jaws, you will have to find a way of simulating that they are tightened against a workpiece, so any tilt or rocking motion possible is 'locked out' before you grind.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                      • #12
                        Thanks for all replies. The adapter plate was machined on the lathe but I'll check it first. Might also ask the inspection department at work to check the chuck on it's own before machining it's back face to double check.

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                        • #13
                          Simon, can I ask a question, are you experienced at dialing work in with a 4 jaw and this one is throwing you for a loop, or is this your first go round at dialing in?
                          James

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                          • #14
                            I would say I'm fairly experienced - I'm not very slick at doing it but I can get the required result. The confusing thing was that I could get one part of the bar running concentric but then the ends would be miles off - so it's not like a brick in a washing machine but as I said before like a baton twirler's baton. Reading all the replies and thinking about it a bit more I think for whatever reason the jaws aren't parallel with the lathe axis - just need to work out where the problem lies and the easiest way of correcting it.

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                            • #15
                              Update on this - spent all evening making measurements;

                              Adapter plate on it's own across the mounting face: 0.02mm
                              Chuck mounted on the plate across the front face: 0.05mm
                              Across the chuck jaw running faces: 0.08mm
                              Chucked up my eight inch test bar and got it running concentric up close to the chuck, at the end of the bar the run out is then just over a 1mm - tightening the jaws an extra amount opposite the high point it comes down to ~0.8mm.

                              Somebody suggested putting a bar in the 4 jaw chuck and then holding the bar in the three jaw and skimming the chuck mounting face - I've set this up but not machined it - the mounting face moves axially 0.35mm as the spindle rotates. I'm worried that this approach may only reduce some of the problem and might only work for one bar diameter.

                              Grinding the jaws seems like it might be a better option but how to go about this exactly?

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