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  • Backplate Design

    I am designing my own backplate for a 6" chuck and a SB9 lathe (screw mount). It will have a fairly simple cross section but I am wondering about how thick it should be. I intend to turn the area where the perimeter of the chuck seats down somewhat, leaving a boss in the center that will fit loosely into the hollow area in the back of the chuck. This will reduce the thickness of the metal of the backplate at that point. I am wondering is 5/8" is thick enough or should it be more? The central area where the boss and mounting threads are will be about 3/4" +/-.

    Paul A.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  • #2
    Paul - The pre-threaded (1.5"-8) back plate I bought with my independent 4-jaw Bison chuck is 1.4"thick - almost exactly divided between the hub and the plate proper. I made the boss a just fit to the chuck as recommended by Bison, and bored out the hub to a close fit on the registration portion of my (JET 9x20) lathe spindle.

    I also bored 60 holes parallel with the axis of the back plate near the periphery so I could use them for indexing. Laid the coordinates out in a table, and drilled and reamed them on my mill/drill. Very tedious!

    ------------------

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    • #3
      if im to read you correctly are you going to make to backplate loosly fitting in the chuck body,if so,before you do , please get further advice, im sure the backplate MUST have a very snug fit even a pressed fit if its a self centreing chuck. theres a lot of experienced guys on here to give you better advice on making a backplate than i can give,


      bill

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      • #4
        The center of the body will have to be thick enough to accomodate the thread mount. It should be the total length of the threaded and plain portion of the spindle.

        I have made two piece back plates when there was not enough material for a suitably thick center.

        The thickness of the OD of the mounting flange can depend on the method of mounting. If the capscrews thread into the backplate, enough meat should be left to acommodate a good thread engagement, typically from one to two diameters. ie a 3/8"-16 should have 3/8"-3/4" thickness. If the capscrews thread into the chuck, enough meat should be left to countersink a socket head screw plus sufficient material to ensure a strong mount that will not pull out. Probably 1/4"-1/2" would suffice.

        The capscrews should be countersunk as a safety precaution to prevent tools and body parts from becoming snagged.

        Some people undercut the spigot on the back-plate and drill the mounting holes oversize to permit tapping the chuck around as a kind of "poor man's" adjust-tru. This works pretty well with a smaller machine when cutting forces are not great, but it must be kept in mind that a jam or wreck can knock it out of whack.
        Jim H.

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        • #5
          Thanks for the replys.

          Actually I am doing two of these, one for a 3 and one for a 4 jaw. I am trying to design an adjustable mount like the Tru Set types. I have a couple of 6.75" d x 1" thick pieces of 1018 steel for the project. The chucks are about 6.25" OD so I will have an extra 1/4" to play with. I want to recess the chuck into the backplate about 3/8" to allow room for four, radial mounted dog tip set screws. This should provide the centering adjustment. That's why I plan on a loose fit in the hollow back. I want to leave about 1/32" all around. That should provide enough range to center even a badly worn chuck. Mine are brand new now but I hope to live a few more years... Six 10mm SHCS should provide enough holding pressure to resist all but the largest crashes. I am quite conservative in the feeds I use.

          I love the 60 hole idea. I'm going to try to find a way to incorporate it.

          So if I have a total thickness of 5/8" at the point where the mounting screws are and I try to counter bore them, how deep should I go? Half of that or 5/16". That would leave about 1/16" still exposed. That's not too much. But is the 5/16" under the screw head OK? Or should it be more? Perhaps 3/8", leaving 1/8" of the screw head exposed? That still doesn't sound like a lot and I don't want it cracking after some use.

          Paul A.
          Paul A.

          Make it fit.
          You can't win and there is a penalty for trying!

          Comment


          • #6
            I have an 8" Pratt-Burnerd on which the back plate, out at the edge, is only 3/8" thick. So I'd think you'll have plenty of 'meat' left.

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            • #7
              JC said

              <font face="Verdana, Arial" size="2">Some people undercut the spigot on the back-plate and drill the mounting holes oversize to permit tapping the chuck around as a kind of "poor man's" adjust-tru. This works pretty well with a smaller machine when cutting forces are not great, but it must be kept in mind that a jam or wreck can knock it out of whack.</font>
              Exactly what I did with my early model 5" 3jaw Bison over 20 years ago. I have never readjusted it since and it has been used well but never wrecked or abused. Just out of curiosity a few nights ago I decided to check it. I used two test pieces, a short piece of precision ground 1 1/4" steel bar and a 3/4" piece of hard anodized round rail from an optical system. Both measured about .002 runout at the chuck. The optical rail measured about .0035 at six inches from the chuck and was very repeatable with rechucking. I didn't measure with the 1 1/4 inch material because it is too large to chuck up balanced in the spindle bore.
              Free software for calculating bolt circles and similar: Click Here

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