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  • Southbend backplate register

    I am trying to help a mate with his southbend nine lathe..He needs me to machine a backplate to hold his 3 and 4 jaw chuck. I see its a 1 1/2 inch spindle with 8 tpi. Would this be 60 degrees or 55 degrees.

    Also how does this backplate register to the spindle. Does it use the threads to align it when the backplate bottoms out on the shoulder. Doesnt sound right. I am thinking of making a dummy spindle to do the testing. Any hints and tips on doing this will be appreciated.

  • #2
    Standard US thread with 60 degree thread form. On the spindle the threads are concentric to a locating diameter at the back of the threads. This diameter will have a squared off face perpendicular to the threads so the chuck will sit square to the threads when seated against it. Making a dummy spindle is a good idea. Gives you some practice at threading and will help with this project. Make that dummy spindle in such a way so that you can use it to mount a chuck on some other machinne tool, like maybe a mill table, in the future.

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    • #3
      That threaded spindle subject has been hashed a lot on the forum.
      I made a ER32 chuck awhile back, I think there is info searchable on that.

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      • #4
        It uses 60 degrees, and yes it does bottom out on the shoulder.
        The thread itself provides a centering effect.
        I think it is best to face the backplate while it is mounted on the spindle that it will be used on.
        On both sides.
        I usually use 1 and 3/8 hole size before threading.
        But gewinde-normen.de calls for 35.1mm, a small bit tighter.
        I don't think the difference matters enough to worry over.
        25 miles north of Buffalo NY, USA

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        • #5
          I have made both a back plate for a new chuck and tooling that uses the 1.5 x 8 thread for my SB9.

          The important areas for mating the thread to the spindle are the rear face and the thread. The rear face determines the angle at which the back plate or other device will sit on the spindle and the THREAD determines the centering. The area called the "register" is not an actual register and should be used for NOTHING. It should be a clearance fit to allow the thread to do it's job.

          On my first attempt to fit an accessory to the spindle thread I did make the mistake of trying to use the "register" area for centering. It seems to be obvious but that is deceptive. What I wound up with was a mount where the two centering features, the thread and the "register", were fighting each other. I could not change the thread so I would up enlarging the diameter at the "register" area and then, it worked perfectly. And it has retained it's center accuracy for many years of use.

          Another important point is to be sure to cut the thread long enough (in the axial direction) so that it does not bottom out. I made that mistake too and had to cut a second part because synchronizing with the internal thread in order to extend it was just too difficult. Do make that internal thread at least one thread longer than the external thread on the spindle.
          Last edited by Paul Alciatore; 08-10-2021, 06:54 PM.
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.

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          • #6
            I had the same experience; once I opened the hole up to about .020 clearance (on the diameter), it worked perfectly, and still does.

            Originally posted by Paul Alciatore View Post
            On my first attempt to fit an accessory to the spindle thread I did make the mistake of trying to use the "register" area for centering. It seems to be obvious but that is deceptive. What I wound up with was a mount where the two centering features, the thread and the "register", were fighting each other. I could not change the thread so I would up enlarging the diameter at the "register" area and then, it worked perfectly. And it has retained it's center accuracy for many years of use.
            25 miles north of Buffalo NY, USA

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            • #7
              There are differences of opinion on register vs threads centering the backplate. It's been fought out many times and each position seems satisfied with their own arguments.

              Here is one instruction for making a backplate at lathes.co.uk. I find the argument for threads being sufficient fairly persuasive but manufacturers have been making close fitting registers successfully for many years too.

              I think it's true that both features aren't required together, but I have fitted backplates with a close fit and it does work fine that way. If there's any conflict between which feature will control, it probably means that the two are not concentric. They should NOT conflict. IF there's a conflict, put some clearance on the register section because the threads are never going to give up their own effort.
              .
              "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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              • #8
                Originally posted by TGTool View Post
                They should NOT conflict. IF there's a conflict, put some clearance on the register section because the threads are never going to give up their own effort.
                Way I learned it, the straight section isn't even a register, it's just clearance. The actual register is the square shoulder of the spindle which is at perpendicular to the threads -- right up against the head stock. The threads will always pull the plate in square and true to that.
                Last edited by nickel-city-fab; 08-10-2021, 08:52 PM.
                25 miles north of Buffalo NY, USA

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                • #9
                  The straight part is for pre-aligning the thread as you put the chuck on the spindle. Kind of like a "dog-point" screw, but in reverse. It lets you get the thread started without damage. There is a secondary fi=unction of allowing the spindle to not have threads down to the shoulder.

                  If made perfectly concentric, there would be no issue with it being a closer fit, but that is not needed.
                  2730

                  Keep eye on ball.
                  Hashim Khan

                  Everything not impossible is compulsory

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                  • #10
                    The thread is 60ยบ. The vertical face on the spindle is the register, so make sure the back of the backplate is flat. The backplate will slide up the threads on contact with the spindle register and center itself. The recess in the back of the plate has nothing to do with registering the chuck. This recess is usually cut 0.010" or more above the diameter of the spindle. This area is just for clearance.

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                    • #11
                      The sizes are in the Southbend Run a Lathe book..

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                      • #12
                        Yes, the back of the register is vital to ensure alignment, but Burnerd, one of the best chuck manufacturers in the UK, back in the day when I was but a lad, were quite explicit that the backplate should be a close fit on the register for concentricity. Thats just the advice of a high class chuck manufacturer, but sure, what do they know compared to you guys?

                        I've always followed their advice and regarded the thread as the thing that tightens the backplate against the back face of the register, and indeed it doesn't matter too much if its a free fit.
                        Raglan, a good class UK lathe manufacturer used a spindle nose with one register at the back, another smaller one at the front, and a square thread, which obviously could not contribute anything to concentricity
                        'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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                        • #13
                          Originally posted by Richard P Wilson View Post
                          Yes, the back of the register is vital to ensure alignment, but Burnerd, one of the best chuck manufacturers in the UK, back in the day when I was but a lad, were quite explicit that the backplate should be a close fit on the register for concentricity. Thats just the advice of a high class chuck manufacturer, but sure, what do they know compared to you guys?
                          That just goes to show that not everyone at a manufacturer knows what the hell they are talking about.

                          Originally posted by Richard P Wilson View Post
                          ...and a square thread, which obviously could not contribute anything to concentricity
                          Sure it can... It's a rotational device (ie, inclined plane), isn't it? While a vee thread will have forces on the face in both X and Y axis, a square thread will only have forces in the Y axis. Apparently, having only the Y axis force is enough to force the backplate up flat against the register.

                          To illustrate, these two images can help. In the first, we have a simplified cut-a-way of a threaded spindle and a backplate. In the second, we see these two elements fully engaged (or screwed on). The forces pushing back from the spindle register must equal the forces pushing forward from the backplate. As the backplate and the spindle engage, the two forces will probably not be equal, which means that once forces will push more than the other. When the backplate is against the register, the only direction the chuck can move is UP, meaning that it climbs the threads. It will continue to climb until the top force and the bottom force are equal. At this point, the chuck is as centered as it can get. Also, the spindle "flat area with no threads" and the backplate recess are no where near touching - a condition that would be required IF those two areas were the register for the chuck.

                          Ask any lathe owner with a threaded spindle what their spindle "flat area with no threads" OD is compared to their plate recess ID. You will be hard pressed to find any that say "my OD and ID are the same", because if they were, threading on a chuck would be a nightmare (try assembling a plug into a hole when the two diameters are the same). For instance, my South Bend H10 three jaw has a difference between these two diameters of 0.020", yet when installed, it has a TIR of less than 0.002", which is not possible to the "flat area with no threads and the backplate recess is the register" crowd.


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                          • #14
                            I do not have a lathe with a threaded spindle end, mine has a D1-4 end. I have never made a threaded chuck backplate, so what do I know?

                            In my other life I design big centrifugal compressors, some of them are driven by a drive shaft with the same threaded arrangement as the lathes with a threaded spindle. No cylindrical register, just threads and a shoulder. It is sufficient for most applications, but for the fastest compressors we provide a cylindrical register to improve centering of the drive shaft. When you spin at 10-20 thousands RPM and transfer thousands of horsepower, you want the minimal runout of the drive shaft, a few tens - no more. This is not an ideal design since it is difficult to make and difficult to assemble. We never open the cylindrical register - it defeats the purpose. Forgot to mention - each compressor is about a million USD.

                            In the recent years we started to deviate from the threaded shaft design just because there are better solutions available. This is similar to what happened with lathe threaded noses. They work, but there are better designs around.

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                            • #15
                              Originally posted by SLK001 View Post
                              For instance, my South Bend H10 three jaw has a difference between these two diameters of 0.020", yet when installed, it has a TIR of less than 0.002", which is not possible to the "flat area with no threads and the backplate recess is the register" crowd.

                              ]
                              Quite similar to mine.
                              When I made the backplate for the 4-jaw, and made the ER32 chuck, and made the faceplate....
                              I left around .020 clearance above the major diameter. In each case,
                              they thread on and off with zero to half a thou runout every single time.
                              And sometimes it takes quite a bit of force to unscrew.

                              IMHO facing the part flat and square and true to the spindle shoulder is the most important thing.
                              Because that will determine the truth and repeatability of eveything else.
                              25 miles north of Buffalo NY, USA

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