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

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

    I just watched this Youtube video while prowling around looking at youtube posts. This guy has turned his standard 3 jaw chuck into a set tru style chuck by addition of an adaptor plate. This looks really neat, and he seems to be satisfied that he can dial any run out down to zero. I'm not sure I would do that, but my lathe chuck has about 0.003" total indicated run-out, and it would be nice to get it to zero.---Brian
    https://www.youtube.com/watch?v=5Wltvmcubdw
    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

  • #2
    I have done it. It works well but is a real pain to do for every part so I only do it when absolutely necessary. That said, I have 5C collet setups on both my lathes which is my preferred way to use the lathe if at all possible. I also have a 4" 3 jaw chuck that has a 5C mount which makes mounting it a snap, no need to remove the collet closer, 4" works for a lot of the parts I make, if not then I mount the bigger chucks.

    For the kind of work you do, a 5C collet setup would be ideal. There is nothing like it. For one thing, it grips the part evenly around its full circumference so it does not mark the part. You can also remove the part and put it back in and it stays running true. Once you have a 5C lathe setup, you can then get square and hex 5C blocks and a spindexer to use those same collets on the mill. They are extremely handy on the mill also.

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    • #3
      Perhaps a small thing? But he's now increased his overhang by a good 3/4 of an inch. I've always found that anything I can do to decrease the overhang in all places leads to less chatter and flex.

      A lot of folks have made their 3 jaw chucks into fake Set-Tru chucks simply by drilling and threading for four set screws around the back rim of the main chuck body and those screws bear against the chuck to back plate registration step on the back plate. The register step being reduced in size by around 5 to 10 thou so there's from 10 to 20 thou of total side to side range of movement. This also requires access to the tightening bolt heads so the chuck can be semi released to allow the set screws to slide the chuck. Then tighten for actual turning. This direct modification to the chuck body method having the advantage that no increase in the overhang occurs and one can do the work pretty easily.

      Of course none of this is possible if the chuck is a direct mount style.

      For bigger stuff that I need to true up I've got the big 4 jaw independent. And more recently (3? years now?) the 5C collet adapter I made up. And for smaller parts needing secondary operations or other times smaller parts need to be zeroed up on a feature I've got a handy little 4 jaw on a shank that I can use in my 3 jaw.

      If you're simply hoping to reduce the .003 runout then I might suggest making up a jaw tensioning jig and grind the jaws. I know that I need to do this with mine since it's got a pretty significant amount of bell mouthing wear. And it certainly does affect the consistency of the hold.

      Another trick that I wonder about is to index the jaws around the mounting slots. Keep them in order of course. But if the jaws all steps one way or the other there may be an order where they reduce the runout? Costs nothing to try....
      Chilliwack BC, Canada

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      • #4
        How often do your customers specify a lathe part with a concentricity deviation of 0.001" between features?
        Last edited by Bented; 01-05-2022, 09:47 PM.

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        • #5
          It's not necessarily a precision specification. Often, when making small parts for display the mismatch between parts is visible to the eye or by feel.

          I have a set-true style chuck on one of my lathes and I've been spoiled by the way that anything from 1/4 inch to 2 inches is centered quite well without changing anything. It makes me lazy about remounting parts for second ops.

          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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          • #6
            What's wrong with sliding a paper or brass shim between the work and the offending jaw? I keep a small flat box of them on the headstock. New work never needs them. repairs always do.

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            • #7
              Originally posted by Bented View Post
              How often do your customers specify a lathe part with a concentricity deviation of 0.001" between features?
              Most of us don't have customers bented. However, for a lot of shop made tooling, like I make, the more concentric it is, the better quality your parts come out. So the better it is, the better.
              21" Royersford Excelsior CamelBack Drillpress Restoration
              1943 Sidney 16x54 Lathe Restoration

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              • #8
                Many just make the "spigot" undersize by a little. Then they loosen the fixing crews slightly, and give the chuck a tap with a soft hammer to get the part running true, then tighten them again. "Poor man's Adjust-Tru".

                (Or just use the 4 jaw to begin with.)

                I have an actual Buck adjust-tru, and it is quite handy for making multiple parts when there is an already-finished diameter. Always grabs the same diameter part on-center once adjusted. Bented should like that, no fussy fiddling for each part. Almost as good as soft jaws. Fast enough for production.

                Hobby people don't need more than one part? And have all day to work on it? Nah.... I've needed to make up to 50 of the same part before, and probably will need to again. Fast and good works for me.
                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

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                • #9
                  I admit to being a purely hobby machinist these days. I have no customers, except myself, making one part is pretty normal, making 2 parts counts as a production run.
                  I've got a 1936 4" (UK speak, 8" US speak) lathe with a 3 jaw which gives me 0.003" TIR. Thats good enough for most of my work, coupled to careful work planning so 'concentric' parts are machined in the same set up, and if that doesn't work, then its the 4 jaw. Thats good enough for me.
                  '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

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                  • #10
                    Years ago I got a nice little 100mm German made chuck really cheap from an MSC clearance flyer. Careful as I was making a back plate it didn't run true. Even after grinding the jaw faces with a TPG it still wasn't as true as I wanted. I too recently watched a couple of YouTube videos on set-tru mods. I did it and I can dial it right in now. I haven't used it much yet, and I don't know how much of a range over which it will stay true, but I'm hopeful. I have a set of 4C collets, but only by 16ths, so I plan to use the chuck for small work that's in-between sizes. I also made an adapter to fit it to a tailstock center, so I can use it for tube, or long pieces without a center hole. Maybe I'll add photos tomorrow.

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                    • #11
                      I made a conversion when I first made the back plate for may three jaw. It works just fine. Some points to keep in mind:

                      This is not a set and forget thing. The source of the greatest inaccuracies in a scroll chuck is the scroll. Even new scroll chucks will have different runout when different diameter parts are gripped. Those with abused scrolls can have significant differences even when the "same" diameter parts are gripped. So, to get the best accuracy, you need to dial in each part.

                      Since my chuck was purchased new and I haven't abused it, it will run fairly true if I dial it in only on the first part of a given diameter. And even if I don't dial it in at all, it is OK for most parts. I only need to dial it in for those parts that need the best accuracy.

                      If you make a lot of parts with close to the same diameter, you will probably create a problem spot on the scroll. This is due to the wear on the scroll at those points where the individual jaws contact it. With small differences in diameter, you can get significant differences in concentricity. This is normal with any chuck.

                      The contact areas between the jaws and the scroll of a three jaw or the adjusting screws of a four jaw are significantly different. The four jaw chucks have a half thread on the outside of the screws and a fairly large and flat bearing surface between those screws and the jaws while the three jaw jaws have curved fingers which only contact the scroll along a single line. The three jaw chucks will wear more than the four jaw ones will. This is in the nature of the beasts.

                      These differences in the nature of the contact between jaws and scroll vs. jaws and screws and the uneven wear in the three jaw is the reason why grinding the jaws often will not improve the runout of a worn three jaw chuck. And yes, I have tried that and it didn't make much difference. But that chuck was very worn.
                      Paul A.
                      SE Texas

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

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                      • #12
                        Originally posted by Bented View Post
                        How often do your customers specify a lathe part with a concentricity deviation of 0.001" between features?
                        Unless you are doing a second operation,
                        all features turned in the same chucking
                        will be concentric. But you knew that already.

                        -D
                        Last edited by Doozer; 01-06-2022, 09:32 AM.
                        DZER

                        Comment


                        • #13
                          Originally posted by Paul Alciatore View Post
                          I made a conversion when I first made the back plate for may three jaw. It works just fine. Some points to keep in mind:

                          This is not a set and forget thing. The source of the greatest inaccuracies in a scroll chuck is the scroll. Even new scroll chucks will have different runout when different diameter parts are gripped. Those with abused scrolls can have significant differences even when the "same" diameter parts are gripped. So, to get the best accuracy, you need to dial in each part.

                          Since my chuck was purchased new and I haven't abused it, it will run fairly true if I dial it in only on the first part of a given diameter. And even if I don't dial it in at all, it is OK for most parts. I only need to dial it in for those parts that need the best accuracy.

                          If you make a lot of parts with close to the same diameter, you will probably create a problem spot on the scroll. This is due to the wear on the scroll at those points where the individual jaws contact it. With small differences in diameter, you can get significant differences in concentricity. This is normal with any chuck.

                          The contact areas between the jaws and the scroll of a three jaw or the adjusting screws of a four jaw are significantly different. The four jaw chucks have a half thread on the outside of the screws and a fairly large and flat bearing surface between those screws and the jaws while the three jaw jaws have curved fingers which only contact the scroll along a single line. The three jaw chucks will wear more than the four jaw ones will. This is in the nature of the beasts.

                          These differences in the nature of the contact between jaws and scroll vs. jaws and screws and the uneven wear in the three jaw is the reason why grinding the jaws often will not improve the runout of a worn three jaw chuck. And yes, I have tried that and it didn't make much difference. But that chuck was very worn.
                          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.

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                          • #14
                            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.

                            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.

                            -D
                            Yes, second ops is why I have a set-tru chuck.
                            Last edited by J Tiers; 01-06-2022, 03:26 PM.
                            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

                            Comment


                            • #15
                              Paul makes the key points above about the scroll error. I would add 'what do you think keeps the scroll itself centred?' This is why it is normally recommended that you use the same socket for tightening if the chuck has more than one to keep the scroll biased in the same direction each time, but of course over time it also makes it worse as the stress/wear is at the same point every time too. You can't win.

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