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3 Jaw Chuck Jaws Not Evenly Contacting Workpiece

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  • 3 Jaw Chuck Jaws Not Evenly Contacting Workpiece

    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.

    I found that there is a gap between the workpiece and bolt-on jaws. The gap is perfectly parallel with the work, telling me it's not wear or damage, but actually some kind of incorrect offset.

    I would say the gap is .001-.002", enough to see light through, and enough to induce chatter on heavier cuts.

    In aluminum, it's small enough of a gap that tightening the jaws deforms the stock and it holds fine, but in steels it does not.

    Can someone shed some light on this? Is this a common thing, an if so how might I address it?

  • #2
    Somehow I doubt that this is a common thing, would it be acceptable to simply put copper shims over the jaws? Since most people do that anyway, to prevent marring the work.
    25 miles north of Buffalo NY, USA


    • #3
      Have you swapped the upper and lower parts of the jaws? Depending on how accurately they were manufactured in the first place, it possible that they are not 100% interchangeable. Is this a new problem, or just noticed?
      "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979


      • #4
        Shouldn't be doing that, could be those are not the proper or original outside jaws for the chuck


        • #5
          Put a ring, something pretty like a bearing race on the od jaws and turn or grind the offenders.


          • #6
            Pull top jaws, measure from grip face to some feature like the ledge that sits in movable jaw with scroll. Are they all the same dim?


            • #7
              It looks like the top-jaws might not be original. My chuck has original top and master jaws, and the master jaw is recessed just a little on all 3 jaws.

              In general, you don't use the master jaw (bottom) for holding. It's just a mount (or carrier ) for the top jaw. The top jaw is what should hold the work. I'd recommend removing the master jaws and carefully grinding that edge down just enough to provide clearance. No need to go overboard. Just enough that a straight edge shows that there is clearance.

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

              Location: SF East Bay.


              • #8
                Ok so I took the jaws off the chuck. Each jaw is numbered 1-3 and matches with the associated half, as well as the slot in the chuck. There are grind marks on the flat angled sections indicating they were bolted together and ground as one piece. The variation comes from the narrow workpiece contact portion.


                • #9
                  You need to grind the master jaws. Ideally you would grind the top jaws in the same setup. That woud assure full contact of the work on both the master jaws and the top jaws. You do need to preload the jaws before grinding.


                  • #10
                    Hmm, I wonder how I can do that with tools I have and also preloading the jaws towards the center.


                    • #11
                      Ditto on Danlb, they are not meant to grip the work, just remove them and take a little off the tips. They don't need to match, just need to be smaller than the top jaws


                      • #12
                        Look at how the slave jaws are shorter than the masters. If we look around at online images I see that some, not most but some chucks with two piece jaws have slave jaws that are wider than the master jaw. But ALL of the slave jaws are the same or a whisker longer than the master jaws. Yours are quite a bit shorter on the outside and apparently a few thou shorter on the inside too. Makes me think that these are not the proper slave jaws for that chuck.

                        In another thread about replaceable jaw chucks someone alluded to a standard for the jaws as far as fitting to the spine and cross bar goes. If that is so it would explain why the slave jaws fit. But if there's no standard for the outer shape of the slave jaws it might explain why your jaws in are not a good fit and might not be the proper ones for this chuck in the first place.

                        You might want to look into that idea of a common fitting format and if so shop for a new set which starts out longer than the master jaws.
                        Chilliwack BC, Canada


                        • #13
                          Here are some photos of a jaw. I can see the grind marks are continuous, so I have no reason to believe these top jaws are for a different set of master jaws.

                          What I did was put some fine sandpaper on a 90 degree angle plate and put that on a surface plate, then carefully removed about .002" from the master jaw only. Now there is about a .001" gap, which allows the top jaws to make full contact. Amazing how much more rigid everything is!!!

                          Thanks for the recommendation guys.


                          • #14
                            Nice fix. Needs some pretty accurate sanding to get that right.. But if you turn the jaws around for external work wont they also be wrong. Looks like they can be slightly adjusted . Isnt the problem the key that holds the movable jaw to the main jaw.


                            • #15
                              Originally posted by plunger View Post
                              Nice fix. Needs some pretty accurate sanding to get that right.. But if you turn the jaws around for external work wont they also be wrong. Looks like they can be slightly adjusted . Isnt the problem the key that holds the movable jaw to the main jaw.
                              The key is a precision tight fit. They can be removed by hand but just barely, it's very snug. What I did should have no effect on the alignment if I flip the jaws around.

                              I think the ultimate solution here would be to grind the master jaws even lower, then place a rod in there to preload against, then grind the top jaws as was recommended.

                              I think what I have will work for my purposes.