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3 Jaw Chuck Jaws Not Evenly Contacting Workpiece

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  • #16
    Originally posted by kuksul08 View Post
    Here are some photos of a jaw. I can see the grind marks are continuous, ...
    With the continuous grind marks, I have wonder how the upper & lower pieces are not aligned? How could they be ground together & not be aligned? They were probably ground separately, but on the same grinder, with the same orientation. So the uppers are original, but mismatched by the manufacturer - a QC failure.

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    • #17
      Looks clear to me, the grind marks are not the point.

      Look instead at the grip faces. Notice that they look a little wider on the top jaw. If true, that indicates that they are farther back than the narrower marks on the bottom portion of the composite jaw. The two were ground together at least on the tapered sides, but the top part either has more wear, was not ground on the grip face at the same time as the bottom one, or the manufacturer did not spark out on the whole composite jaw when grinding the grip faces.

      The grip faces on the bottom also do not even look consistent along that jaw, but that may be a trick of the reflections.
      CNC machines only go through the motions

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      • #18
        I would regrind those jaws in situ,after throughly cleaning the chuck parts. Then afterwards, go through the same cleaning to make sure there was no grit left to cause wear. The chucks that I have ground can be trusted to run at 0.001" tir at any diameter, I must be very lucky to get 3 chucks that good.
        With those reversable jaw tops, I would only grind the full length of the inside but not the centre when grinding the jaws when they are set outside. It would be very unlikely to be able to match the master jaws with the top parts both ways around.
        Last edited by old mart; 11-08-2021, 11:13 AM.

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        • #19
          The first picture with the grinding shown on the angled faces sure does look like it's a match.

          Then given that the masters are a little "lower" it might be some other issue. If the chuck is new to use but previously used perhaps someone before you loaded the jaws with a small puck of metal held back by the master jaws then lightly ground the slave jaws true? Otherwise it would appear to be a QC issue?

          You might get away with simply grinding the masters so the parts are only held in the slave jaws. You might not need to grind the whole length. But then if you're going to set up to grind the jaws anyway.... might as well do it all so you get full length support.
          Chilliwack BC, Canada

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          • #20
            The top jaw has a key that nests into the bottom jaw. Is there any play in that area? Perhaps the jaws were pushed toward center before they would ground to 'spec'. Then when the pressure is put on a workpiece, the play moved the other way, leaving you with the gap.

            One cure could have been to use a shim on the appropriate side of the key- but then that shim would always be required when remounting the top jaws. It would be better to take up that play in the outward direction before tightening the jaw down- then go through the process of grinding the jaws true.

            Your solution sounds like what I would have done. I can set up to grind jaws, but I have never done that- yet. My one-piece jaws are out by less than two thou in concentricity, and yes I'd like to get that down to a couple tenths ( is that laughter I hear?) but it will never be that close over the whole range- and likely not repeatable either. Apparently for a three jaw, being out by 2 thou is pretty good. In that vein, your method of sanding back the bottom jaws is perfectly acceptable. The testing is pretty easy too- and if your final sanding takes a little off the top jaws as the surfaces even out, that's ok. Ground in place would give you a slight hollow in each jaw, which I think is to be preferred, but the grip surface changes anyway as you clamp different diameters, so it's kind of just academic at this point. It's better to achieve full jaw length contact with the workpiece, and it looks like you have done that.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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            • #21
              It seems to me that.... (and I could be wrong) The surface that was shown to be ground in situ is the tapered side of the jaw, not the gripping surface. Grinding on the side like that allows more clearance as you tighten it down on a small rod. One situation where a person would do that is to allow the chuck to close down on a smaller work piece.

              I'm surprised to see the teeth on the master jaw too. The only value that I see in that is the ability to use it without top jaws when you need all the length that you can get. It would allow you to cut all the way back to nearly the face of the chuck.

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

              Location: SF East Bay.

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              • #22
                It is incorrect to say that workholding can not be done with the master jaws.
                Just take off the top jaws and chuck up your work. Much more positive holding force
                from the master jaws. Done all the time.

                --Doozer
                DZER

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                • #23
                  Originally posted by Doozer View Post
                  It is incorrect to say that workholding can not be done with the master jaws.
                  Just take off the top jaws and chuck up your work. Much more positive holding force
                  from the master jaws. Done all the time.

                  --Doozer
                  This could actually be a great advantage if you need to hold something oddball
                  just make a sacrificial top jaw for the odd job
                  25 miles north of Buffalo NY, USA

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                  • #24
                    Originally posted by nickel-city-fab View Post

                    This could actually be a great advantage if you need to hold something oddball
                    just make a sacrificial top jaw for the odd job
                    That's what soft jaws are for.

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                    • #25
                      Originally posted by kuksul08 View Post
                      In my lathe, the jaws are two-pieces and bolt together. One part rubs against the scroll and has some contact area with the workpiece, and the outer part touches the workpiece only.


                      Can someone shed some light on this?
                      Not sure I can but will try.

                      I turn some tubing and no matter what I always had a flat spot" with my three jaw set. I would crush the tube just to get a good bite on it to turn it. Frustrating. I was using over sized tube just to keep from deforming it.

                      Then the six jaw, solved my issues. It is still not my prefered method, which is a four jaw.

                      I like to dial in the part with a four jaw most of the time.JR

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