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Lathe chuck question

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  • Lathe chuck question

    I have a G4003G lathe with D1-5 cam lock chucks. The lathe came with a 6" 3-jaw and an 8" 4 jaw. I tend to prefer the 4 - jaw.

    I also have an old Cushman 6" 3-jaw 234B chuck. I don't think it has ever been used. I'm wondering if it's worth trying to get a backing plate for it for using on the Grizzly lathe.

    I'm not familiar with various backing plate designs or various chuck mounts. The Cushman chuck has 3 - 3/8" cap screws on about a 5-1/2" bolt circle that I assume are used for mounting the chuck to the backing plate. I would assume a backing plate would have to have some alignment feature to center up the chuck? What do I need for a backing plate? and is there any advantage to the Cushman chuck over the chinese grizzly chuck?


  • #2

    Cushman chucks are excellent. It would be very worth while to mount it.

    Cast iron or cast steel is generally the preferred materials for back plates.

    It is a fairly easy operation to mount a new plate to your lathe and turn a tight fitting bolster to fit. Then you just need to drill a matching bolt hole circle to bolt the chuck on.
    If you think you understand what is going on, you haven't been paying attention.


    • #3
      So what I need then is a blank D1-5 backing plate - then machine the face myself for fitting the cushman chuck? Sounds like a decent little project.


      • #4
        So what I need then is a blank D1-5 backing plate
        Not necessarily - on a lot of those old chucks the back plate was integral to the scroll operation, if so I dont know of a way to adapt another back plate to them. Take that plate off to make sure ...
        If everything seems to be going well, you have obviously overlooked something........


        • #5
          It looks like the main housing of the chuck includes the bolt circle. The back of the chuck is recessed about .125 to a diameter of 4.50 The recessed area appears to be how the guts of the chuck would be removed - There is a 4.50 stamped into the back face on the bolt circle/main body. The main body is machined to an ID of 4.70. Between the 4.50 x 0.125 recess and the 4.70 machined id of the main body it is recessed a little deeper. The guts seem to be completely contained within the body of the chuck.

          The backside of the chuck from the 4.70 id to the 6-1/8" OD appears to be the bearing surface and it contains the mounting bolts. I think a backing plate machined to align with the 4.70 diameter and relieved enough to not bear on the 4.50 ID should do the trick (?)


          • #6
            A Cushman chuck is a very desirable piece of gear, especially an unused one. Get a D1-5 backplate for a 6 or 6-1/4 chuck. Make sure every mating surface is surgically clean with no burrs. Mount the backplate on the lathe and carefully machine it to a .001 press fit on the appropriate ID of the chuck. Drill & tap for the mounting holes, press and bolt them together.
            Taking the time to get this right can give you a 3 jaw chuck you will enjoy using for years to come.
            Kansas City area


            • #7
              Cushman or Chinese? DUH! A old worn out Cushman is not necessarily good though. Bob.


              • #8
                Thanks - I'll start looking for a backplate.


                • #9
                  I know nothing about the Cushman chuck, but it seems these days when I come across something older I pay attention. Old good beats new crap most of the time.
                  I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                  • #10
                    Ok, Found a blank D1-5 back plate, machined the bolster, drilled and tapped the mount holes, Warmed the chuck in a 200deg oven to get it to expand enough to fit over the bolster. Turned the back plate to eliminate the slight step between the backplate and the chuck. Everything seems nice and snug.

                    I have a 1/2" (chinese) center drill. I chucked it into the chuck and set my dial indicator up. I get a max - min variation of about 0.003. I haven't checked the Griz chuck to see how it compares. For 3 jaw work is that normal or not so good? I do use my 4 jaw when I'm trying to be a bit more precise - it downs't seem to be all that difficult to get under 0.001 with the 4 jaw - which is about the limit of my dial indicator anyway.

                    I presume one could tune the chuck. I've read about using a dremel tool or a die grinder mounted to the carriage to tune jaws.


                    • #11
                      A good 3 jaw will run around .003 or less. You can often tune them a little by having an indicator on the workpiece and tapping the jaws with a soft mallet or rubber (deadblow) hammer to get it a little better. If it's out .003 TIR (total indicator reading) it would only have to move .0015 to be on center.
                      Kansas City area


                      • #12
                        Thanks toolguy! - you've been a big help. I did chuck up a stub of 1-1/2" aluminum bar stock and ran it over the dial indicator. At that diameter it's better than the .003 I was getting using the 1/2" center drill.

                        First time I've turned cast iron. nice to machine - but a PITA to clean up!