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Gettint a "set-tru" chuck to stay true?

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  • Gettint a "set-tru" chuck to stay true?

    I have a 10" buck set true scroll chuck for my lathe. It is currently my only chuck, and is in good condition. The problem is that the chuck walks back of 0, and it is driving me nuts!

    I tighten the opposing grub screw as tight as I can with a 18" ratchet, I tighten the face screw that lock it to the back plat as tight as I can in a star pattern. Zero is good as long as I take care with my tightening sequence. The issue is, take one even slightly interrupted cut and it starts to run out. Am I doing something wrong? Is there a way to make these things stay reasonably zeroed? I realize it is only going to be "perfect" at the diameter it was set with, but this can happen even during the same part w/o disturbing setup.

    Thanks,
    Jason

  • #2
    I've a 6" Pratt-Burnerd adjust-true 3-jaw chuck that I use when doing runs of things too big for collets. I frequently work with 4 1/2" PVC rod in this chuck, running low speeds & heavy feeds, sometimes hard enough to explode the parts or throw them out of the chuck. I have never had the chuck's adjustment disturbed by these, ah, issues.
    I don't tighten it nearly so tight as you indicate, either. Were I you, I'd have the chuck & backplate apart to check for burrs, chips, and such and also to find out if the chuck is drawing up tightly against the backplate when the front screws are tightened.

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    • #3
      I've got a non-genuine set tru (Taiwan maybe) that works fine for me. The bolts holding the chuck to the backplate are snugged but not seriously tightened. I never touch them when adjusting centering. I don't put nearly as much torque on the adjustment screws either - just a little more on one if I need a thousandth or so, loosen the opposite if I'm out several thou. I'm wondering if you've got a lot of stress set up with tightening that gets re-distributed under load. No support for this suggestion, just hypothesis.
      .
      "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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      • #4
        The grub screws are only intended to push on the "hub" for centering purposes. I suspect that by tightening them, the screws at 90 degrees from the others are fighting to prevent movement and then with any cutting forces, the chuck jumps to a more favorable position. Gently snug the grubs only for centering and then tighten the face screws to maintain that position. Den

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        • #5
          One of the first things I did when I got the lathe (and chuck with it) was to completely disassemble the chuck clean and lube the scroll, and cleaned the chuck and back plate. I'm not saying it might not be full of crap again, it has seen about two years of intermittent hobby use, but it has done this from the first time I ever zeroed it.

          Originally I did not tighten anything to the extent mentioned above, I have just gradually cinched it down more and more every time it drifts off...

          I'll pull it apart and check its condition. The biggest issue arises when the sucker does not make it through a whole job.

          Thanks,
          Jason

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          • #6
            Originally posted by nheng
            The grub screws are only intended to push on the "hub" for centering purposes. I suspect that by tightening them, the screws at 90 degrees from the others are fighting to prevent movement and then with any cutting forces, the chuck jumps to a more favorable position. Gently snug the grubs only for centering and then tighten the face screws to maintain that position. Den
            How loose do you take the face screws? I typically just lightly break them loose (I know this is subjective, I should throw a torque wrench on it) and then adjust and tighten them back up. Any time I have tried to just lightly adjust the grubs just to center it, then when I tighten down the face screws, I find I am out again. I am assuming this is due to the chuck tilting a bit perpendicular to the spindle and then when I pull the chuck tight to the back plate it gets pulled parallel to the ways and the run out is back.

            I need to get this thing virtually perfect to finish up my high speed spindle head. I need to turn an arbor for the router drive gear, the last one was out a few thou and it nearly destroyed itself when I bumped the router on...

            Thanks for the advise guys. Keep it coming if you have any more thoughts!

            Thanks,
            Jason

            Comment


            • #7
              Consider this: three jaw chucks have limitations that have to be factored into the productivity equation, even adjustable three jaws chucks. The chuck is adjustable so you can tweak it when you need extra close concentricity Normal operation imnclidees having to adjust the work concentricity if drifts a little under heavy cuts or after changing work diameter.

              The chuck should ordinarily grip reliably under all normal work conditions but there are exceptions related to material stability, thin walls, short grips, large overhangs, heavy cutting forces, and so on. If the work requires refined concentricity there hould be a point before the first finish cuts there the operator hould make a concentricity check.

              Also hard jaws are slippery. If the surface available for gripping is of short aspect ratio perhaps soft jaws affording an enveloping grip should be considered.

              I'm not poo-pooing your problem statement. I'm pointing the hitches and considerations in the way of simple progress from raw stock to finished part.
              Last edited by Forrest Addy; 12-26-2009, 01:52 AM.

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              • #8
                Hard to say whether the problem is your chuck or not. But, I can say for sure the Buck set true's are crap compared to the P-B equivalents.

                Even a light crash on a Buck and it's toast.

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                • #9
                  Did you add any oil/grease after cleaning the chuck?

                  Purists say it must be 'clean and oil free!' to hold firm.

                  I hate the idea of 'clean' steel myself. Spells rust in my book.

                  maybe clean it and recoat with a more protective but less lubracating formual. like LPS3.
                  Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                  • #10
                    Originally posted by Forrest Addy
                    If the work requires refined concentricity there hould be a point before the first finish cuts there the operator hould make a concentricity check.
                    Can you expound a bit on how to do this? Keep in mind that this bulletin board and books are my only source of education on machining in general. I don't even have the slimmest clue as to how I would measure for concetricity.

                    Originally posted by DR
                    Hard to say whether the problem is your chuck or not.
                    What else might you think it could be?

                    Originally posted by Black_Moons
                    Did you add any oil/grease after cleaning the chuck?

                    Purists say it must be 'clean and oil free!' to hold firm.

                    I hate the idea of 'clean' steel myself. Spells rust in my book.

                    maybe clean it and recoat with a more protective but less lubracating formual. like LPS3.
                    Actually, it had about 10 lbs of grease in it which had all flung to the outer edges. If I remember correctly, I sprayed the scroll w/ thin film lubericant. The jaws do not loosen, are you thinking that stuff in the scroll causes the zero to drift?

                    Thanks,
                    Jason
                    Last edited by jacampb2; 12-26-2009, 02:21 AM.

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                    • #11
                      No I think the stuff thats leaked onto the surface beween the chuck and its backplate is what causes the zero to drift, the rest of it can probley stay in there
                      the scroll afaik is not designed to hold the chuck in zero, just move it while the front bolts are slightly loose.
                      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                      • #12
                        I'm with Tgtool here. You are wayyyy overdoing it. Take apart and clean and deburr. The clamping screws in the face should be tight enough to hold it flat and still allow adjustment with the adjusting screws on the OD. You might check and make sure that the plunger like pieces under the adjusting screws are there. I used to do aircraft turbine parts on these and had to be with in .0005. No problem at all. The way you are tightening things here tells me you may have sprung jaws. damaged scroll, etc. These are precision tools and should be treated as such.

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                        • #13
                          Originally posted by jacampb2

                          .................................................. ......................

                          What else might you think it could be?

                          .................................................. .....................
                          The chuck mount to the spindle.

                          The jaws (out of square).

                          The operator (you don't need exceptional tightening of any of the screws).

                          As I said, with Buck's even a light crash and they're toast. I have a six and eight that can be trued, but will only stay for a part or two. Both were mildly crashed and still appear like new.

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                          • #14
                            set tru

                            i have many times had set screws self descruct ,first thing i would check

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                            • #15
                              Originally posted by jacampb2
                              The issue is, take one even slightly interrupted cut and it starts to run out. Am I doing something wrong? Is there a way to make these things stay reasonably zeroed? I realize it is only going to be "perfect" at the diameter it was set with, but this can happen even during the same part w/o disturbing setup.

                              Thanks,
                              Jason
                              Im not gonna say the buck chucks arent nice. But I have had issues with the jaws not locking down consistently. And I dont expect much from my scroll chucks. But I do expect a certain amount of consistency. Or actually holding ability.

                              I have two small lathes and had a nice Buck chuck. Did all I could to keep the jaws from pulling the work as I gripped it. I may have had a bad scroll, but I tore it down and cleaned it up, still issues..

                              Then I bought a Pratt Burnerd chuck for one of the lathes. WOW!! What a difference. And it wasnt a PBA chuck but an import, used stuff. The jaws locked down square to the work at all diameters. I was impressed. Got another one for the other lathe. Same thing. Solid chuck. Then got a couple 6 jaw chucks and now thats all I use for scroll chucks on the two lathes. The PB chucks are both steel body chucks and the three jaw is a "semi" steel body I think, what ever semi steel is Im sticking with the PB chucks for scroll chucks. Although. Got a nice brand new SCA 3 jaw chuck that is really nice but I keep the PB 6 jaw on the lathe so the SCA is outta the loop. JR
                              My old yahoo group. Bridgeport Mill Group

                              https://groups.yahoo.com/neo/groups/...port_mill/info

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