1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Fitting a lathe chuck backplate adapter

Discussion in 'QUESTIONS & ANSWERS (Get Help Fast Here!)' started by architard, Oct 3, 2012.

  1. architard

    architard United States Active User Active Member

    Likes Received:
    Trophy Points:
    District Of Columbia
    I'm making steady progress on my Clausing 4914 rehab and in anticipation of finally being able to make some chips I'm starting to plan out my first few projects.

    Absolutely no tooling came with this lathe, so I have to start from zero. I bought a backplate adapter from AMTOOLS that fits the rare 1-3/4 x 8 spindle nose of this lathe. I plan on using this backplate on a new Shars 6" 4 jaw independent chuck.

    The problem is that the diameter of the spindle register is slightly undersized so that the backplate can not be screwed completely on to the point that it abuts the spindle face. I judge the discrepancy as less than a 64th of an inch (sorry I'm an architect I'm still getting used to using decimal dimensions). As I understand backplates and chucks, the accuracy of the surface of the spindle abutment face is what really dictates the alignment of the backplate, NOT the accuracy of the threads.

    My plan is to mount the backplate on the spindle backwards, and bore out the spindle register diameter so that the it can fully thread onto the spindle. However I will be relying solely on the threads at this point to machine that diameter. My question is how accurate does the clearance need to be for that spindle register diameter? Can it be slightly over sized since the real workhorse in accuracy is the spindle abutment face?

    Which brings me to my next question. Can I rely on the machined surface of the spindle abutment face from that factory? Or while I have the backplate mounted backwards should I also machine that face so that it aligns with my lathe? Again, same question as before, if it's only being held on by the threads how accurate can I expect that face to be machined? Am I worrying too much at this point?

    Finally, I have one more concern. Since I have absolutely no way of holding anything in the lathe until I get this backplate fitted to a chuck I have no way of practicing any machining. So this boring and facing of the backplate could possibly be my first operation ever on a lathe. Obviously I don't want to screw up my brand new backplate. What are my options for mounting something in the lathe to practice on without a chuck? I was thinking of mounting some practice stock between centers, however I don't have a faceplate to drive a lathe dog, as well as wouldn't that also require me to center drill the practice stock? How do you center drill the stock without first being able to put it in a chuck? I could just center drill in a drill press but I don't know how accurately on center I can get that way. Although I'd assume for practice it won't matter all that much.

    I guess I have a sort of chicken and egg problem. Or maybe more that I can't get there from here. :dunno:

    Here is a sketch just to illustrate the terms I am using in this post.

  2. 8ntsane

    8ntsane Active User Active Member

    Likes Received:
    Trophy Points:
    Not knowing if you have a mill handy, but if you do happen to have one, just clamp it to a trammed in table. Indicate the part in, and use a boring head to open to size. If not, you can turn it around backwards as you mention. Some claim the threads keep it centred up with in a few thou, but I personally would check the runout once you have it screwed on to the spindle. Do you have a face plate you could mount it to? Atleast you could bump it around to zero it out.

    I would shoot for the min clearance between the two, and though Ive seen many that are over size, I would perfer it not. If you can get it 0.002 or better would be great. They sell the plates like this so the end user has to fit it to your machine. Though not the end of the world if its a bit over size, you should shoot for a very close fit.

    The face that butts up to the spindle should certainly be checked with a indicater, but should allready be properly machined. I would not machine that part , unless the indicater showed a problem after the back plate was bored, and mounted the proper way.

    First things first, you need to get that bore opened up to get the back plate fully seated. after getting that done, run your indicater over the back plate and check for problems. The register to the back of the chuck will, or should allso over sized, and will need turning to size as well.

    Once your plate is on the spindle, you can do a skim cut to the OD to true that. Next would be the step that goes into the back of the chuck. That should be oversized also, and need to be sized too.
    That will be a snug fit, not press fit just snug. You will want to get the fit here so you have no play between the two parts, again just a snug fit, not press fit.

    The face of the back plate should also have a light skim cut, to true it as well. Once trued, you should be ready to mount your chuck, and mark the bolt pattern for drilling and tapping.
  3. Newmetalmark

    Newmetalmark United States Active User Active Member

    Likes Received:
    Trophy Points:

Share This Page