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Making a chuck plate "101"

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  • Making a chuck plate "101"

    I read the previous responses regarding back plates but still need help. I would like to make a back plate for a 6" 3 jaw chuck for my 9" SB lathe. I have a 6" 4 jaw chuck on the lathe now. Can I use this chuck to make the plate? I saw a reference to "register" and what does this refer to? I was going to use a 6" x 5/8" bar bell weight - will this be adequate? What are the critical machining operations needed to get a functional plate?
    I would appreciate any help or references you can give me. Thanks Paul

  • #2

    The register is the area on the spindle nose that has no threads and includes the flange that the chuck's backplate will fit up next to. In my opinion the tighter the fit the better the chucks repeatability will be. The register is supposed to line everything up.

    About using a barbell for the backplate: I tried that but found the material to be hard in some places and very soft in others, I also found holes in the casting. I would think that barbells are made from the cheapest of metal so I don't know if I would trust it. The possibility of a chuck flying across the room will not be something you would enjoy! Also, the 5/8" thickness is way to thin for a safe plate.

    I too have a 9" SB lathe so here's how I made mine: (remember I'm not an expert)

    First machine an exact duplicate of the spindle nose (threads and register) of the Southbend. This will be used for a "gage" for the new backplate.

    Then get a piece of flat steel (4140 annealed would be nice) that is at least 6" X 6" and 1.25" thick, and put it in the 4 jaw chuck. Bore and thread this steel such that your spindle "gage" has a nice tight fit. Remember, the register will also be bored at this setup (you're making this piece backwards from fitting on the lathe). Also face the entire piece of metal. I repeat, do not remove or adjust this piece of material while it is in the chuck until the spindle fit is acceptable and material faced. This is needed to maintain accuracy (that's why the "gage" of your spindle was made).

    Now remove the material from chuck and take a band saw and remove corners. Replace the 4 jaw chuck with this "new" backplate on the spindle and finish the side and face to suit your chuck.

    I think using this method will ensure that you will have the maximum accuracy that your spindle will allow.

    By the way, you don't have to use the square piece of steel- a round would be better- but I did not have a round that big in my supply.

    This was really a fun project for me - I think you will enjoy it too.....Mike

    [This message has been edited by Mike Burdick (edited 02-05-2003).]


    • #3

      For a threaded spindle there are three key features -
      a) the thread
      b) the area immediately behind the thread - "register"
      c) the radial flange or shoulder that the backplate runs up against

      The register centers the backplate and is normally the same as the major diameter of the threads themselves. The shoulder has two functions - keeping the chuck perindicular to the spindle and driving the chuck.

      Therefore for maximum accuracy the threads must be very close fit to the backplate. The inner diameter of the backplate and the rear surface can be carefully scraped for a precision fit. When this is done, the chuck will always return to the same relative position on the spindle giving maximum consistancy and accuracy. It is important that inside and outside corners (where appropriate) be chamfered to properly clear mating part corner radius' (or lack thereof). DO NOT SCRAPE THE SPINDLE - only the backplate! Sometimes the register and flange must be turned and re-faced to facilitate maximum accuracy, but it should only be done if absolutely no other choice exisits (last resort). Threaded holes should also be counterbored a couple threads to prevent pull out of the threads above the surface - this can cause accuracy problems if a bur is raised by tightening a bolt.

      Paying attention to the details here will gve superior results. I also make withness marks to line up backplates with the chucks and number bolts to their respective holes incase it is disassembled by others.

      [This message has been edited by Thrud (edited 02-05-2003).]


      • #4
        Does it make sense to produce the dummy spindle nose that Mike Burdick describes by making a rubber mold and casting - like the dentist does?



        • #5
          Mike Burdick:

          "Then get a piece of flat steel (4140 annealed would be nice) that is at least 6" X 6" and 1.25" thick,..."

          What do you pay for a piece of 4140 6" x 6" 1.25" thick? I puchased a piece of 1018 1 inch thick cut off of a 6.25" diameter round for about $15. Is the 4140 less expensive? Do you really need 4140 for a backplate?


          • #6

            The 4140 annealed would not be required for a backplate and it is more expensive than 1018. I would imagine that any steel would work, but some are easier to work with than others. I used 4140 for mine because it's nice to work with and seems to be very stable (lack of internal stresses due to working) and more importantly, I had some!

            I'm definitely not an expert, I just expressed how I made my backplates with my meager equipment.


            [This message has been edited by Mike Burdick (edited 02-05-2003).]


            • #7

              No, but it makes sense to make a metal copy and a female for making copies of your spindle for things like dividing heads. The rubber repro is ok for metrology, but not fitting and scraping backplates to your spindle.

              When you make the copy of the spindle make sure you include the rear shoulder and the spindle register. It is easy to make errors on accessories that attach to the spindle if you do not - trust me, been there, done that.


              • #8
                We are lazy down here in Texas because it is HOT for most of the year. I am too lazy to make a spindle adaptor, so I bought one on Ebay!!! Called a "GO gage" - it is machined much more accurately than I could ever do, and it fits the factory backplate of my 2-1/4" - 8 Sheldon like butter. Cost $15 + postage. MSC has them new for about $200. These are taken good care of usually (in someone's metrology dept), so it is easy to find one in good condition. It does not have the shoulder register, so you would have to add that if you need one for fitting purposes. I still need the plug gage for the bore, so this is an admission against my own interest!! Don't y'all go bidding on that GO bore gage when one comes up for bid, y'hear!! A.T.


                • #9
                  Here's my 2 cents worth. Go ahead and make an exact duplicate of your lathe's spindle nose. It's great practice, and will come in usefull in the future. Buy a raw cast iron blank and machine it to fit. Pay particular care to the register portion and scape in the mating surface to the spindle.

                  In searching the web I found widely varying prices for chuck backing plates. Victor Machinery ( has a raw cast iron back plate that'll fit a SB 9" for $12.50. One already machined starting at $41.00. Not bad either way.



                  • #10
                    Thanks, Thrud, for the advice about casting a copy of lathe spindle nose.

                    Spinrow asked a question I've been thinking of, too. And I didn't notice the answer. Could I use the body of the crummy 4-jaw chuck that JET supplied with my 9 x 20 lathe to make a back plate for a good Bison chuck?



                    • #11
                      Using the mounting of another chuck might be a good shortcut, especially if the mounting elements (threads, register or taper, etc.) are in good condition and there is sufficient material so that you can machine it to fit.

                      I recently picked up some worn out L0 chucks for a few bucks each. I have separated the backs from them and they appear to have enough meat to them that I can face them and use them to mount flat back chucks.

                      Don Kinzer
                      Portland, OR
                      Don Kinzer
                      Portland, OR


                      • #12
                        Well, here goes !!!
                        There is no such thing as a register !
                        The counterbore relief area is just that , a relief area.
                        The "critical areas are the threads, ie pitch diameter and distance (TPI), AND the face at the rear of the back, that stops against the spindle. This surface MUST be square to the threads ( Read: made at same time as threads)

                        The reason is that a thread is a conical helix and registers itself, and the "stop" faec must be square to prevent sideload.
                        Green Bay, WI


                        • #13
                          That is one of the theories of chuck accuracy in relation to screw mounts - it has never been proven however. All three in unison produce more accurate and consistant mounting of the chuck. In theory it is posible to acheive the same accuracy with just the thread and shoulder but I have never seen it done.

                          You can if you like, but a high quality bison cast iron backplate is only about $40 - so why bother with the junk?


                          • #14
                            Thrud -
                            I couldn't see such a thing in my MSC catalog. Where to buy?



                            • #15
                              Since every lathe and chuck manufacturer that I am aware of provides a register finished to very close dimensions, I must believe there is such a thing.
                              There must be some play in threads to allow them to tighten up. The register prevents the back plate from squirming on the threads when final tightening is done.
                              All three items, threads, register and squareness of shoulder are important in proper fitting of chuck, and the closer attention is payed to each, the better the job will result.
                              Jim H.