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Chuck Scroll Woes

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  • Chuck Scroll Woes

    Don't recall if this has been up on the board recently...

    Any known cure for an unevenly worn three-jaw lathe chuck scroll?

    (I imagine this is the case. Chuck is ancient. Cleaned, relubed, scraped, shimmed, remounted, jaws ground internally to niiice TIR at one particular diameter. Any other diameter is totally all over the place. By a LOT- like +/- a millimeter or two or more.)

    Wait a sec- Just remembered, wasn't there an article in HSM about this? Forrest, was that you?

  • #2
    The scroll may have collected regions with bumps/dents in them.

    When you ground the jaws, did you have them "loaded" in the proper clamping direction using a special fixture or centrifugal force (dangerous but effective in my own case)?

    Without loading, results can be random and with loading in the wrong direction, the jaw "teeth" are pressing the wrong side of the scroll groove.



    • #3
      Oh, yeah. Loaded, centered, best keyhole. As one does, as is known in the art.


      • #4
        Is it loose on its pivot?
        CNC machines only go through the motions


        • #5
          Don't be so sure it's the scroll. Check at different diameters and record the readings referenced to each jaw. If the results always show one particular jaw is high or low, then it is likely the teeth on the jaws. A new set of jaws might help. Or some carefull work on the jaw teeth.

          Paul A.
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.


          • #6
            How much vertical play do the jaws have in their "guides"? If they have excess play, the result can be axial misalignment causing gyration of the part about the spindle axis.

            Another thing to check out is uniformity of the jaw's grip from front to rear. Have the chuck grab a precision rod wrapped in one layer of letter paper and then examine the jaw pattern. If the jaws grip at the rear or the front only, the gyration mentioned above can also occur.

            I had to re-tru a 6" Buck set-tru chuck some months back and it took several tries due to wear in the guides. After loading and cutting did not give good results (similar to yours actually), I put a protective band of metal around the chuck body secured to prevent the jaws from flying out. I then opened them to about 1.4" and ran a 1" indexable carbide boring bar down thru it as it spun at around 500 rpm for centrifugal loading only. In my case (perhaps by luck), the jaws did not move during cutting and the carbide left a uniform jeweled/rainbow like appearance the length of the cut.

            The end result is that the centering is pretty good (and can be zeroed), grip is very uniform, very little "gyration". What must have happened is that the jaws "found" a good combination of loading, placement in the slides, etc.

            [This message has been edited by nheng (edited 02-04-2005).]


            • #7
              Frankly, I do not think that there is a cure.
              I recall the articles and comments in Model Engineer of say 30 years back, and trying to improve a Union 3 Jaw chuck.
              Simply putting and machining a retaining ring on the jaws fails because the improved jaws are not in compression when in use.
              One improvement was to insert three pieces of identical pieces of steel between the jaws and tightened up. At this point, the jaws are in compression as they would be use and can be machined.
              After this, the jaws have to ground to narrow the new widths which result.

              At least, the lathe can hold accuracy at one diameter. The suggested methods given earlier cannot.

              Once this is done, the register of the backplate is removed and the chuck can be set to tenths by knocking the chuck into position.

              I bought a new chuck!!!!!!!!



              • #8
                I have two chucks which I ground the jaws of, and BOTH are now much more accurate over the whole range. A third has had the pivot shimmed and is now good.

                If you think you need to grind.....

                Check several diameters....if each is out a different amount, check the scroll pivot...

                If scroll pivot is not loose, bag the chuck.

                If scroll pivot is loose, shim or bush it (I like shims, less invasive and harder to do wrong)

                If diameters are consistently off, and chuck is otherwise concentric, grinding will work.

                But the best method IMO is Rich Carlstedt's method...
                Drill a small (1/8) hole in the "top" end of each jaw. Location is not very critical, but it would be good to get the hole centered. Insert a pin that fits closely. Close the pins on a ring (slice of pipe works).
                Now grind. The jaws are both loaded correctly, and clear for grinding on full length.

                The only other sensible method IMO for one-piece jaws is the one posted on the Logan site, which involves a plate cut to hold the jaws by their tapered sides. The plate dimensions are super-critical, and I found it hard to make right. it's easy to drill holes.

                CNC machines only go through the motions