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4-indep-jaw chuck on rotary table: Centering not critical, right?

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  • 4-indep-jaw chuck on rotary table: Centering not critical, right?

    If you attach a 4-independent-jaw chuck on a rotary table, the centering of the chuck to the table is not critical, right?

    I mean, you want to get close, but no need to use a DTI to get the chuck concentric on the rotab is there? Because the four jaws are independent?

  • #2
    Not until you rotate the table..

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    • #3
      Hm. OK, how about this as an example.

      The rotab is centered (exactly) under the quill, with DTI used on the rotab's center hole to get it exactly concentric. Then put on the 4-jaw-independent chuck. Now assume that chuck goes on sloppy, .1 out of round, let's say. Next I mount the part in the chuck and use the quill-mounted DTI on the part until it's concentric (or, alternately, that I'm picking up a punch mark on the work so that it's centered to the quill...whatever). The PART and the ROTAB are concentric, but the chuck is not...isn't it somewhat irrelevant if the chuck is not concentric, aren't the differential jaws compensating and making a "correct" center, if I am aligning the part concentric with the quill?

      I guess what I am saying is, if I am going to "sweat" (1) getting the rotab exactly centered under the quill and (2) getting the part exactly centered under the quill, is it necessary that I also "sweat" getting the 4-jaw independent chuck exactly centered on the rotab? (Obviously if it were a scroll chuck of any sort, it WOULD matter a great deal.)
      Last edited by Bolster; 12-19-2010, 01:36 PM.

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      • #4
        Logic says that you will adjust the jaws till the part is centered to the rotab's rotation. So no, the chuck body should not need to be perfectly centered.

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

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        • #5
          Well that's where I'm netting out. But I get a lot of people responding like Moe above, who say that even a 4-jaw independent chuck must also be DTI'd into concentricity.

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          • #6
            Originally posted by Bolster
            But I get a lot of people responding like Moe above, who say that even a 4-jaw independent chuck must also be DTI'd into concentricity.
            Don't listen to 'em. Better yet---set it up; make your part. Prove them wrong.
            Last edited by Arthur.Marks; 12-19-2010, 02:50 PM.

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            • #7
              I guess another way to think about it is to look at mounting a 4-jaw chuck to a backplate for a lathe. If you attach a backplate, it doesn't have to be dead on concentric with the chuck, but you probably don't want to have it off by 1/2" and have your lathe walking and dancing around the shop. If the 4 jaws on the lathe are concentric with the part held there, you should be good to go, even if the chuck is doing the whorley-gurley. So, no the centering on the rotab on the mill should not be critical, only the 4 jaws holding the part.
              Cheers,
              Gary

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              • #8
                Originally posted by Arthur.Marks
                Don't listen to 'em. Better yet---set it up; make your part. Prove them wrong.


                That's what I would do.

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                • #9
                  The reason this is an issue, is because I expect to move the chuck on and off the rotab frequently. So it would save a lot of fussing if I knew that concentricity of chuck-on-rotab wasn't critical.

                  And of course, since it's a rotab, I don't have any issues of a heavy part wobbling at high speed. My RPM on a rotab is what, one rotation every three minutes or so.

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                  • #10
                    I missed #3 post. That will work but an accurately located three jaw really speeds things up.
                    Last edited by moe1942; 12-19-2010, 04:55 PM.

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                    • #11
                      Bolster, the thing is to get your workpiece to run concentrically with the rotab's _true_ axis.

                      A lot of people have an idea of indicating the hole in the rotab to center it, not knowing whether the hole is actually running true. Bad idea.

                      Instead, first, mount your workpiece on the rotab/in the chuck and use a dial indicator to get it true by turning the rotab (dial indicator fixed to the mill table) and adjusting the workpiece position in the chuck ("bumping it in").

                      When that's done, get your workpiece to turn concentrically with the spindle by placing a dial indicator in the spindle, and turning the rotab while moving the mill table until your dial indicator is stable.

                      Did I mention centering the chuck? No, because it's not necessary!

                      Benta.

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                      • #12
                        I recognize that sequence from Harvey's book (Machine Shop Trade Secrets)! Sounds good to me, thanks.

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                        • #13
                          Originally posted by Benta
                          Bolster, the thing is to get your workpiece to run concentrically with the rotab's _true_ axis.

                          A lot of people have an idea of indicating the hole in the rotab to center it, not knowing whether the hole is actually running true. Bad idea.

                          Instead, first, mount your workpiece on the rotab/in the chuck and use a dial indicator to get it true by turning the rotab (dial indicator fixed to the mill table) and adjusting the workpiece position in the chuck ("bumping it in").

                          When that's done, get your workpiece to turn concentrically with the spindle by placing a dial indicator in the spindle, and turning the rotab while moving the mill table until your dial indicator is stable.

                          Did I mention centering the chuck? No, because it's not necessary!

                          Benta.

                          Perfect reply. For sloppy work, you can center the rotab by sweeping the center hole but if you need to hold a close tolerance, Benta's got the procedure to do it.

                          I had a few scrap parts before I finally started doing things the way Benta described.

                          Comment


                          • #14
                            Concentricity on a rotab

                            Originally posted by Bolster
                            If you attach a 4-independent-jaw chuck on a rotary table, the centering of the chuck to the table is not critical, right?

                            I mean, you want to get close, but no need to use a DTI to get the chuck concentric on the rotab is there? Because the four jaws are independent?
                            There is rarely if ever a need to get a chuck concentric to a rotary table with the (probably) single exception that a job is concentric with the chuck and the job was centred in which case the concentricity of the chuck was a chance by-product of centreing the job on/to the rotary table.

                            I never use a 4-jaw chuck on a rotary table unless I cannot effectively grip the job in a 3-jaw chuck or a collet.

                            Not all "centres" (ie morse tapers) are precisely or at all accurately either concentric with the table axis or have no "conical" misalignment either.

                            Here are the test piece figures for the concentricity at the table and 4" out from it:

                            6" Vertex rotary table: 0.0008" (0.02mm) at the table and 0.0012" (0.03mm) 4" from the table.

                            8" Vertex rotary table: 0.0008"

                            Here are the inspection sheets for my pretty good "Vertex" 6" and 8" rotary tables:





                            There are really mainly (only) two "centreing" situations that pertain to rotary tables:

                            1). getting the axis of a job and the table to be co-incident/aligned; and

                            2). getting the co-incident axis of the table and the job to be co-incident with a mill quill axis.

                            Here is my 3-jaw chuck and my ER-32 collets on my rotary table:





                            I can centre the job to my rotary table and then centre both the job and the rotary table to the mill quill axis to within a Total Indicated Run-out (TIR) of 0.01mm (~0.0004") - or better if required - within a manner of a couple of minutes - and all with just a Test Dial Indicator (TDI) and neither need nor use a "centreing" device/adaptor.

                            I will cover the methods later.

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                            • #15
                              Thanks for the replies...please note I will be inserting non-circular work into my chuck. Seems most replies so far are assuming circular work.

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