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Making a 3 jaw standard chuck into a set-tru chuck

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  • #16
    I have both 6" and 8" 6 jaw Bison set-tru chucks. I use one or the other just about every day, have been for many years. I keep them pretty well dialed in. Once in a while I will need to make a minor adjustment. When that happens, I don't loosen the mounting bolts any, as the setscrews have plenty of mechanical advantage to slide the chuck a tiny bid sideways under tension. It only takes a couple of minutes to nip it up, and I don't have to fight with something moving when re tightening the mounting bolts. These chucks have almost no wear after all this use. I do take out the jaws and clean the jaws and scroll every week or two, or whenever the chuck key gets slightly hard to turn. I think many people never clean the chips out.
    Kansas City area


    • #17
      I agree that bell mouth is a valid reason for grinding the jaws. That can be improved and bell mouth jaws are a real PITA.

      I thought of including that in my post above, but I know that I have a tendency to be verbose and I really do try to keep things as short as possible. Thanks for mentioning it.

      Originally posted by mikey553 View Post

      Excellent review Paul! I can only add that grinding jaws in place can correct the bell mouth and the wear of the jaws. That's about it. My 6" 3-jaw still gives me about .001" TIR positioning, but bell mouth is becoming a problem.
      Paul A.
      SE Texas

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


      • #18
        Yes, runout in the scroll's bearing can be a problem. This is most likely due to the original manufacture and not from wear as the bearing surfaces on a typical scroll are fairly large and the rotation is at extremely low speeds for very short intervals.

        Checking this runout should be fairly easy once the chuck is disassembled.

        Originally posted by J Tiers View Post
        Grinding jaws should NOT be relied upon for concentricity. It's really ONLY for fixing bell-mouthed chuck jaws, which it does very well.

        Check the scroll bearing if the chuck does not repeat well, you may be able to shim it to be a better fit.

        Yes, second ops is why I have a set-tru chuck.
        Paul A.
        SE Texas

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


        • #19
          Originally posted by Paul Alciatore View Post
          Yes, runout in the scroll's bearing can be a problem. This is most likely due to the original manufacture and not from wear as the bearing surfaces on a typical scroll are fairly large and the rotation is at extremely low speeds for very short intervals.

          That's true theoretically . But in the real world, "theory" gives way to reality. I've found that wear situation on 2 chucks out of the 5 I have had. And I never even looked for that on one of them, I just replaced it.

          BTW, at least one of the chucks had only one pinion, so the problem was not cured by using just one. That can "help" make the chuck repeat the same error, but won't "fix" the error.

          I don't think you can so easily and flippantly discount it, given my experience. After all, many here have chucks that came with the lathe. That makes them "used" if the lathe was.

          Think about it.... the jaws wear when parts are put in and taken out (most do not "spin" parts in the jaws on purpose). And every time that is done, the scroll is turned. A lot of turning if you have to adjust the jaws for a different size workpiece.

          'Nuff said.

          4357 2773 5150 9120 9135 8645 1007 1190 2133 9120 5942

          Keep eye on ball.
          Hashim Khan

          Everything not impossible is compulsory

          "There's no pleasing these serpents"......Lewis Carroll


          • #20
            Originally posted by Doozer View Post

            Unless you are doing a second operation,
            all features turned in the same chucking
            will be concentric. But you knew that already.

            It is difficult to machine both ends of a part in one setup, But you knew that already (-:

            For very close work I use a tenth reading indicator, actually this one.


            • #21
              Check out this video by Tigermoth Racing.



              • #22
                These days most people seem to forget about turning between centres. Its a useful technique, you can swap from end to end and its got to be concentric, can't be anything else.
                'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger


                • #23
                  Sure, there's lots of options. But getting back to the question......

                  I've thought a few times about the idea of doing the set-tru mod. And this thread has me thinking about it again. A few things to note in relation to doing the mod to the chuck and backing plate.... This assumes there is a backing plate since a direct mount chuck is one piece and can't be adapted to any sort of set-tru style mod......

                  The easy and quick way is to remove the chuck and reduce the diameter of the register shoulder on the back plate so there's a little float And depending on how close a fit there is for the mounting bolts some modification might be needed there too. Tight fitting bolt shanks or socket cap heads in close fitting counterbores would need to be relieved in some manner to provide the plus and minus .005 to .010 that will be wanted.

                  For adjustments I guess I'd try the bump method with no adjustment screws first. It's easy to do and needs no modification to the chuck itself. Just the mods to the backing plate to permit the adjustment play. But I wonder about how it would be for adjusting? I tend to use a side mounted dial gauge in my tool post holder and do my adjustments on the four jaw from the "waist" of the chuck. But if I had a "hit to move" setup I'd wonder about the weight of the chuck causing some downward fall as it bumped to the side. This could be easily cured with a dial gauge in a holder that reads off the top of the item and just rotate around to the high reading and give it a slight bump down towards the target reading. Rinse and repeat until running within spec. Might be a pretty easy peasy mod.

                  The next step up the food chain for making a cheap machinist's Set-Tru would be drilling the chuck body to allow for four setting screws that push against the register of the backing plate. Now we'd simply set the zero just like we'd do with a four jaw but using a hex key instead of a chuck key. And with this idea it should be pretty quick and easy to use two hex keys to zero one axis and the other after it. And I could use the gauge in the QCTP holder I have already instead of setting up the mag base and indicator for the mallet method.

                  Brian, does your lathe have the integrated flange mount similar to the mini lathes or does it have a "proper" mount? I seem to recall that this model is set up with the integrated flange mount. If so then in order to keep any other chucks or equipment that mounts to the flange in spec I'd be looking doing the machining to permit the set tru action in the ID of the back of the chuck body. That way only the one item intended to be made adjustable does in fact end up adjustable.

                  Often the register shoulder is pretty short. And that is why I was thinking that holes which are slightly slanted would be wise. And a brass floating plug goes in first for the set screws to push against so the tips don't dig in. Or perhaps smoothen out and slightly dome or do a matching conical nose to the set screws? Whatever it takes so the tips don't dig in.
                  Chilliwack BC, Canada


                  • #24
                    When I machined the 5C adapter plate for the 5" chuck for my T&C grinder I ground the register surface about .003 under so if I want to dial a part in to zero runout I loosen the bolts and lightly tap the chuck around with a small brass hammer. I dialed it in using a 1" gage pin and left it there. For most things it's good enough.
                    There wasn't enough room for to use the normal 4 set screws. The register surface of the back plate fits into the chuck by about .188.

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                    • #25
                      At the museum, we have just finished fitting the Burnerd 100mm to a matching size backplate with 6 fixings rather than the common three, and of course the register has about 0.006" play for fine tuning. With a test bar in the chuck yesterday, I got 0.001" tir next to the chuck and 0.004" tir at 9" out, not really bad enough for any further work, like grinding. I could have dialled in zero next to the jaws, but only bothered to get the od of the body within 0.0005" tir and left it where it was. One thing to remember about the set tru style of chucks is the extra length which might be annoying if your lathe has a short bed.
                      The 100mm, 125mm, 5" and 160mm are all modified, the only chuck which won't be getting modded is the 6 3/4" Pratt with the serrated jaws. I do have two sets of hard jaws for it, but they have been ground both ways round and they don't get much use, the soft jaws are the preferred ones for that one.
                      I use a nylon headed hammer on the od of the body after slackening the mounting screws/bolts slightly. It takes a bit of practice to get the chuck to move with the minimum tapping, but not moving by itself. Then tightening bit by bit and checking if the lot is staying where you want it. I have been very lucky with these chucks, 2 are British, and the others are Chinese and Indian, they all hold within 0.003" tir over their entire holding range with both sets of jaws. I have another Burnerd 5", but it lives on the rt most of the time.
                      Last edited by old mart; 01-07-2022, 04:38 PM.