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Chuck backstop

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  • Chuck backstop


    I have a lot of disk shaped workpieces and I struggle always to align them properly in the chuck. Normaly I use parallels against the chuck body, but thats a crutch...and better not forget to take the parallels out before you turn on the machine. Don't ask how I know...

    Then there are chuck spiders like the Posistop:

    They are expensive, only made from aluminum, prone to get chips under them (They have magnets that hold them in place..and attract chips) - Long story short, I dont like them.

    Then I came across a design that Robin Renzetti has shown in one of his videos (Around 23:50):

    I was pretty impressed with that design and tried to build something along that lines. But my chuck is to small (125mm) to have the heightadjustable screws as a stop - Thats the reason I went with standoffs that are ground to a fixed height.

    And thats what I ended up with:

    I have some sliding baseparts that are clamped in place. The sliding baseparts have three threads for standoffs and are fully adjustable.
    The standoffs them self are machined from drill rod, have their head hardened and then they are ground to the same height.

    Here in use to clamp something with the normal jaws:

    With a small change in setup they work also with the revered jaws:

    Full text with a lot more photos here:

    I am using that thing now for quite some time and I realy like it. Easy to setup, no fiddling around with parallels and very precise.
    Personal website

  • #2
    Very nice setup, I like it.

    Do you have more standoffs of different heights? The shown set appears to leave an extremely thin gripping area- unless you had a very thin disc to work on, that strikes me as dangerously little holding power. A couple additional standoffs to allow more depth if the job allows it, would be very useful.

    Doc's Machine. (Probably not what you expect.)


    • #3
      Soft jaws would work well if there are not too many different diameters to chuck.


      • #4
        Very nice, but Renzetti's device does not depend on the chuck face's having zero runout.


        • #5
          I made a bunch of spiders like these: http://www.homemodelenginemachinist....ead.php?t=5849

          for different thicknesses and one that has 3 adjusting set screws.

          I rarely use the adjustable one since I have enough fixed sizes for most work.

          Hardwood or even MDF work well, quick to make and disposable.


          • #6
            Royal make these........

            I had looked at them years ago but they are a bit pricey for what the are and I don't think they had any for 6 jaw chucks. They are held in place by magnets but that means they rely on the chuck face being square and they will attract chips and fine filings that could cause error.

            For similar setup's like yours I have a 1" dia. tappet from an old Briggs engine that I use in the lathes TS.
            It's only needed if your work has to be mounted on the very end of the jaws or if the part is like a thin disc where there isn't enough part to jaw contact to ensure the part is mounted square.

            Last edited by JoeLee; 10-28-2016, 09:33 AM.


            • #7
              Originally posted by stefang View Post
              Full text with a lot more photos here:
              Very nicely done. I like that "lab book" look of your web page. Black text on a white background with pictures sprinkled in to document what's
              going on, and it stays on message. One small observation. You used the word "trough" a couple of times. It's spelled "through". Once again, nice job.
              Location: Long Island, N.Y.


              • #8
                Stefang, brilliant idea nicely executed. You guys offering up other spacers (they are not spiders, a spider goes on the outboard end of spindle) are missing the coolness of this. Grinding sets of three pins in different lengths is only part of it, its being able to alter the diameter that puts this idea way out in front. So many time a spacer is the wrong shape for the diameter of what you need to hold or the bore you want to put in it
                Last edited by Mcgyver; 10-28-2016, 07:00 PM.
                in Toronto Ontario - where are you?