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Fitting chuck back plate?

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  • Fitting chuck back plate?

    I know this is a newbie question, but what can I say, I am a newbie. I am new to machining and recently got my first lathe. It is an Atlas 10F. It is in really good shape except the chuck jaws are bad. Sense the chuck looked original means it is probably 50+ years old so I purchased a new plain back chuck and back plate for it but I have a question before I fit it. The backplate is threaded for my spindle but the threads don't start until .5" into the plate. This means that when the backplate bottoms onto the shoulder of the spindle only about .5" engaged in the thread. My question is should I have the shoulder of the backplate turned down or is that enough engagement on the spindle threads?

  • #2
    I would leave it as it is. With a 8tpi nose thread half an inch of engagement still gives you 4 threads. It's about what my Atlas 10 was engaging on too.

    When you fit the plate to turn the register for the chuck, don't cut the step deeper than necessary to locate the back of the chuck, that way if you over-do it you'll have enough on the thickness for a second go.
    Peter - novice home machinist, modern motorcycle enthusiast.

    Denford Viceroy 280 Synchro (11 x 24)
    Herbert 0V adapted to R8 by 'Sir John'.
    Monarch 10EE 1942


    • #3
      I had the same issue with a chuck backplate I bought for my South Bend 10K. Those threaded plates have to fit every spindle nose on every lathe ever made with that size spindle, so on any given lathe they may leave something to be desired. I turned off the excess on mine, and consider it an improvement. (I also totally bored out the thread, fitted a plug, and re-threaded it to a closer tolerance too, but that may have been a bit much.)

      If you do take off the excess, spend some time getting really good contact between the back surface of the chuck and the back shoulder of the spindle that the chuck butts up against. I used some hi-spot blue and scraped it in until it was bearing all around.
      Try to make a living, not a killing. -- Utah Phillips
      Don't believe everything you know. -- Bumper sticker
      Everybody is ignorant, only on different subjects. -- Will Rogers
      There are lots of people who mistake their imagination for their memory. - Josh Billings
      Law of Logical Argument - Anything is possible if you don't know what you are talking about.
      Don't own anything you have to feed or paint. - Hood River Blackie


      • #4
        I'm with Peter on this. At 8 TPI, four threads is quite enough. And, if you try to turn it off, you will need to get the setup perfect to avoid turning the rear face of the backplate at an angle. This face determines the orientation of the chuck when it is mounted and any error in it's angle vs the chuck's axis will show up in anything you turn with it later.

        When I make a back plate, I do the back side first, including the threading, IN ONE MOUNTING on the lathe. This ensures that all the surfaces that are used in mounting the chuch are absolutely concentric. Then I remove it from the chuck or other device that held it for this "rear" work and mount it directly on the spindle for finishing the front and outside surfaces. This ensures that these surfaces, that the chuck mounts on, are dead concentric with the LATHE AXIS as determined by the way that actual back plate mounts on it.

        If you do cut the rear down, then mount the back plate by itself and check all front surfaces for run out. You may need to touch them up.

        Personally, I would leave it alone unless a specific problem shows up.
        Paul A.

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


        • #5
          I mounted the back plate by itself and check all front surfaces for run out and there is .003" that need to be trued up. I think I will leave the back as it is and true the face and fit the chuck. Like Peter said, I'll turn the register only enough to locate the chuck so if I have problems I can redue it after I get more experience.


          • #6
            Fitting chuck backplate

            I did this exact same thing last fall when I put a new chuck on my 10" Atlas lathe. My backplate (as purchased from CDCO) fit just about as you describe. Right or wrong, I used a roller bearing cup that would fit over the spindle as a spacer and mounted the backplate backwards on the lathe. Then I turned off about 3/8" of the unthreaded portion of the backplate. I figured 3/8" equaled 3 more threads of spindle engaged. Maybe I was just lucky, but by using the lathe to cut the backplate down, the plate still spun true when I turned it around.


            • #7
              Originally posted by 50BMGBOB
              I mounted the back plate by itself and check all front surfaces for run out and there is .003" that need to be trued up. I think I will leave the back as it is and true the face and fit the chuck. Like Peter said, I'll turn the register only enough to locate the chuck so if I have problems I can redue it after I get more experience.
              Before You turn the front of the plate ''true'' check both the spindle and back plate for any dings , and remove with a stone or file if they are really bad. Then as pointed out previously check the ''fit'' of the back plate to the lathe spindle shoulder for full contact.

              In fact if You can screw the faceplate on the spindle backwards and check to see if the run out stays the same and check the run out on the faceplate shoulder area. You could consider machining off some of that counter bore to get more thread engagement also.



              • #8
                Conventional wisdom is to turn the register for a snug fit for the chuck.

                If you make it a few thou undersize, then you can loosen the chuck mounting bolts and tap the chuck to centre work more accurately (on those occasions when you need to). This is a frugal mans' grip-tru.
                Paul Compton


                • #9
                  More great things to check. I only get a few hours a week to work on it so it may be a few more days until I get back to it. Better to take my time than rush and screw it up.

                  I did get the back plate and chuck from CDCO. I will have to double check for dings but I didn't see any last night when I got the parts. I did try threading it on backwards but I need to find a spacer (like the roller bearing Racerx used) to see how that compares to the front.

                  Mine did come with an old 4 jaw as well, it is worn, but still spins and grips true. I have been using it after cleaning it up but I wanted to get a servicable self centering chuck. As I said, I am learning as I go and having a lot of fun along the way!


                  • #10
                    Well any job worth doing, is worth redoing until you get it right!

                    I found a bearing to use to turn down the back like Racerx suggested. That worked great.

                    I then turned the front and cut the register and screwed up and took to much off. Wasn't that measure once, cut twice???

                    Being cheap (and broke) I carefully welded up the register cut I had made and re-squared the front and cut the register. Came out good this time.

                    I marked and drilled the mounting holes and one was slightly off. A little file work and they line up now. I did damage the threads on one of the mounting bolts and sense they are metric while I had some hex headed bolts, just not the socketed (hardened) ones that came with the chuck I have to go to the store tomorrow. I did temporarily mount it with the bolts I have and now my run out with a piece of cold roll is .002-.004" (range of run out from several times chucking, measuring and removing it). I will check it again after I get the right bolts but I think I will be vary happy with it.

                    Thanks to all of you. I am learning!