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Expected runout for a 3 jaw chuck?

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  • Expected runout for a 3 jaw chuck?

    I just replaced my old 3 jaw chuck with a TOS 3 jaw, and it seems to be a nice chuck. It's a plain backplate version. Fiddling around with the backplate, changing the orientation etc I can get the runout down to around 0.01 mm on a piece of ground 12 mm rod measured just out from the jaws. Normally for really precise work I'd use collets, and indeed do mostly use them, so I'm not sure just how accurate I should be able to get a brand new chuck like this. Is it worth continuing to try to refine it more or does that sound about what I should expect?

    Pete

  • #2
    0.01mm is way better than you have a right to expect from a low cost 3 jaw. However before you get to excited recheck the run-out with a 1" rod without readjusting the chuck position on the backplate.

    Phil

    Originally posted by PeteF
    I just replaced my old 3 jaw chuck with a TOS 3 jaw, and it seems to be a nice chuck. It's a plain backplate version. Fiddling around with the backplate, changing the orientation etc I can get the runout down to around 0.01 mm on a piece of ground 12 mm rod measured just out from the jaws. Normally for really precise work I'd use collets, and indeed do mostly use them, so I'm not sure just how accurate I should be able to get a brand new chuck like this. Is it worth continuing to try to refine it more or does that sound about what I should expect?

    Pete

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    • #3
      Thanks Phil, I wouldn't have any precision ground rod that size here that I can recall. I just measured that before I stopped working for the night, but tomorrow I'll also do the usual tricks for more accurately locating the rod in the chuck etc. However I just don't want to be spending hours fiddling around with something if at the end of the day it's already about as good as I should expect. Having endured clapped out chucks for so long now, it's a bit of a treat having some brand new chucks!

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      • #4
        Probably within 0.05mm would be good.... The TOS 3 jaw I bought years ago (little 100mm one) had horrendous runout, but was probably within spec...
        Precision takes time.

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        • #5
          Concentricity on a 3 jaw self centering chuck is not a high priority. If you need concentricity use a 4 jaw independant or a collet or centers. A 3 jaw is about convenience, once you skim the workpiece it will be concentric to your spindle precision anyway.

          Phil

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          • #6
            i believe a straight rod is not needed. if you make multiple measuments rotating the rod in the chuck and find the largest runnout, then rotate the rod 90 ° and measure again, you are measuring the runnout of the chuck (assuming the rod is round). is this correct?

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            • #7
              Originally posted by philbur
              Concentricity on a 3 jaw self centering chuck is not a high priority. If you need concentricity use a 4 jaw independant or a collet or centers. A 3 jaw is about convenience, once you skim the workpiece it will be concentric to your spindle precision anyway.

              Phil
              With due respect Phil, it certainly is when you mount it! Otherwise one would just buy the cheapest Chinese POS chuck available, not bother to even turn a register, and simply bolt it to a piece of rough cast slag beaten into submission! Instead, clearly when mounting a 3 jaw, one tries instead to get it as concentric as is possible, so the only concentric errors when it comes time to actually use it, are those that occur due to the very nature of the design ie the scroll and the errors that design naturally introduces at different diameters, as you implied earlier.

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              • #8
                Originally posted by philbur
                Concentricity on a 3 jaw self centering chuck is not a high priority. If you need concentricity use a 4 jaw independant or a collet or centers. A 3 jaw is about convenience, once you skim the workpiece it will be concentric to your spindle precision anyway.

                Phil
                Actually many job shops use 3 jaw chucks even for work that requires concentricity. Most of them do this by using "soft jaws", typically aluminum jaws that you bolt in in place of the normal jaws (on 2 part chuck jaws) and then bore to size right at the size of your workpiece.

                The other way to acheive this in a home shop is to use a "Tru-Adjust" style chuck that has set screws that let you dial in the concentricity at your workpiece diameter.

                But if even you don't have a Tru-Adjust chuck there's a "poor man's" way to make yours into one. Take the chuck off the back plate, and cafefully machine the shoulder on the plate so instead of being a close slip fit its about 0.005" less in diameter than the mounting recess on the chuck.

                Now when you need high accuracy, loosen the mounting bolts on the chuck, put an indicator on your workpiece and use a rubber mallet to tap the chuck in for zero runout and then tighten.

                Ok, here comes the truth. I made my first home made tru-adjust chuck like this by accident when I blew it and over cut the shoulder on the mounting plate for a new chuck by about 0.005". But I had some work I needed to do and I decided to go ahead and use it that way and tapped it in for zero run out on that workpiece before tightening the bolts, and then realized how nice that was, its worked great since then.

                Paul T.
                www.power-t.com

                Comment


                • #9
                  Paul, that is the correct way to mount a chuck anyway ... well except machining the register undersized bit

                  Even if the resister is quite a tight fit, the final step when mounting should be to loosen the bolts, chuck a bar in the jaws and indicate the high point. Then use a mallet to shift the chuck on the backplate. Of course the tighter you've machined the register, the less it will move when you do this. I think a lot of people don't know about/or do this final step, but even how you torque the bolts up will influence the runout ever so slowly. It's splitting hairs, but why not, it's probably the chuck most of us use the most so why not get it as close as the design allows!

                  When I first turned my register I took it down to precisely 100.00 mm and it needed to be hammered onto the chuck. However the chuck ran horribly. When I indicated the register it turned out that it wasn't perfectly concentric. I put this down to the carbide insert tools I was using as they weren't machining this CI too well at all. So I swapped out the carbide for a sharp HSS tool and took a very slight skim off the register to correct it. However it is now of course half a poofteenth undersize, but I'm happy to say not enough to consider machining it off and starting again, but I was very lucky as it wouldn't want to be any smaller.

                  Comment


                  • #10
                    You can actually take the register right off with no consequences. With four bolts gripping the chuck onto the backplate it's going nowhere. The register on a 5" chuck I have is 1mm undersize and has given no problems in 25 years. As Paul points out it makes it a poor mans Adjust True.

                    Phil


                    Originally posted by PeteF
                    ........ but I'm happy to say not enough to consider machining it off and starting again, but I was very lucky as it wouldn't want to be any smaller.

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                    • #11
                      Pete, chasing 0.01mm concentricity on a 3 jaw scroll chuck is a pointless exercise. My 25 year old 5” has a concentricity error upto 0.1mm and I’ve yet to have a situation where I need to do anything about it.

                      Phil

                      Originally posted by PeteF
                      With due respect Phil, it certainly is when you mount it!

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                      • #12
                        From what I've been able to ascertain the 0.01 mm runout is about as good as I'm likely to get for a 3 jaw and I'll just leave it as is. I'll just confirm more carefully that's what it is today, counterbore the socket head bolts and call the job done. I hadn't realised until today that the chuck's master pinion is marked so I'll check my measurements more carefully using that pinion to initially tighten the jaws.

                        Phil, you're right, the register isn't needed, but if it's turned accurately on the lathe the chuck belongs to, and it's a firm fit into the back of the chuck, the chuck should automatically turn to the limits of its tolerance. The "bolt trick" I mentioned above can then be used to help take out some of those tolerances. In theory all the bolts should be doing are stopping the chuck falling, rather embarrassingly, onto the lathe bed and spoiling your day. They shouldn't be part of the chuck registration per se. What I'd be concerned with is in the event of a crash ... not that I'd ever do that of course if you're relying on 3 pissy 8 mm bolts for accurate registration, the chuck could be shifted off centre in the crash. Even done up tight I reckon a crash could move a chuck against it's backplate if that's all that was keeping it in position! I don't know, but that's just my gut feeling. The chucks are certainly designed to sit against a register, so I'm guessing the manufacturers do that for a good reason.

                        Pete

                        Comment


                        • #13
                          Phil, just to let you know I got the TOS chuck properly mounted and finished. I indicated a Compac 215 dti onto a ground shaft, tightened with the master pinion, and carefully set up the DTI to be correctly reading. The 215 is 0.002 mm per division and the needle barely moved! I'll be conservative and say 1/2 a division, so 0.001 mm runout. Obviously as you pointed out there's no guarantee that figure will repeat at any other diameter, indeed it almost certainly won't. However the object of the exercise was to get the chuck mounted as concentrically as possible and it's fair to say I'm extremely happy that it's that close, at least at this diameter of work. The spec sheet calls for a maximum of 0.025 (IIRC) runout throughout its range, which is still quite good in my book. If I need any better and I'll use the 4 jaw or collets as you mentioned. I'm certainly very impressed with this chuck and only wished I'd thrown my old one in the bin (ie sold it on eBay) sooner!

                          Pete

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                          • #14
                            As long as you are happy Pete, that's the main thing.

                            Phil

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                            • #15
                              Who wouldn't be happy with that amount of runout from a non-precision 3 jaw!

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