Advice on machining a backplate
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  1. #1
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    Default Advice on machining a backplate

    I'm pretty new to lathes and I need to get a new chuck for my south bend 9a. I'm looking at Bison's offerings for a 6" 3 jaw chuck. I have no issue turning it down to the right size. I'm just wondering what's the best way to mark the holes to drill for the screws. Thanks.

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    I cut a punch on my lathe to fit snug in the holes in my 4 jaw a put some witness marks, then drilled and taped them on the mill.

    You got a mill or drill press?

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    Before you buy a new chuck, check your other thread for some more posts that may help.

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    Good use for a transfer punch, or like Bob said, make one.

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    Transfer punch is the answer. Make sure your holes are big enough to allow a little movement. It is not the bolts that assure alignment but the recess in the chuck itself. The bolts should just hold it all together with a little play before you tighten everything down.

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    It's your funeral but ;-

    A 6'' 3 jaw is over size for a 9'' SB, ......6'' 4J lightweight yes, 3 J no, 5'' is more than large enough

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    I have a SB-9 and was given the same advise about a six inch chuck. I bought it with two six inch chucks, three jaw and four jaw. I had no problems with those original six inch chucks and I did some math that showed me that there were no limitations due to the size of a six inch. I could fit any sized (diameter) work that a five jaw would hold and some that it would not.

    Another argument that I heard against the six inch was the weight of the chuck. A SB-9 has a journal bearing which has 100 times more support area than a ball/roller bearing would have. The chuck weight may be a factor on a lathe with ball or roller bearings, but for a journal bearing it is a piece of cake.

    I purchased a new, six inch, three jaw with reversible jaw tops. It has worked just fine. I love it.

    As for machining the back plate I did this:

    1. Mounted the blank in my four jaw and centered.
    2. Took a shallow cut to make it round and a facing cut.
    3. Reversed it in the four jaw gripping it on the freshly cut OD.
    4. Turned the rear part of the back plate as needed (I had made a drawing but that is optional.)
    5. Drilled and bored the hole for the threads.
    6. Bored what is called the "register". This should be somewhat larger than the diameter of your spindle that it will sit above. It does not need to contact the spindle as the thread will do the centering.
    7. Cut the internal thread. I had made a replica of my spindle threads to test it with, BUT
    8. Without taking it out of the four jaw chuck I took the chuck with the new back plate on it off the spindle, reversed it and tested the thread I had just cut. I had to take off a bit more and test it at least twice more. This ensured a good fit on my spindle and kept my threading tool in sync with the already cut thread so I could just go right back to making the thread a bit deeper. This worked like a charm.
    9. Only after I was satisfied with the fit on the spindle, I removed it from the four jaw and put it on the spindle.
    10. Then I finished it to fit my new three jaw. This ensured almost perfect concentricity of the mounting thread with the surfaces that located the new three jaw.

    As for the holes, I did not use a through-the-hole transfer punch. With tapped, blind holes in the new three jaw, that would have been difficult, no IMPOSSIBLE. You could use those short, threaded punches that fit in threaded holes, but I did not have any. What I did was to install the six mounting bolts in the holes in the chuck and take three outside and three inside dimensions. I added all six of them together and divided by six. That, hopefully gave me the average, center-to-center distance between opposite holes.

    I then put the back plate on my rotary table on my mill and centered it. I had noted the center position of the RT before mounting the back plate so I returned to that position. Then I moved the X axis for half the center-to-center distance I had found above. I did drill the holes somewhat oversized so there was some slop to allow for errors. It worked just fine.

    As an alternate if you do not have a RT, you could use a pointed lathe bit to mark the needed diameter on the back plate while it is still in the lathe (step 4-5 above). Then you could set a pair of dividers to the radius needed and step off six divisions on that circle. Go around once or twice to test your setting and adjust the dividers as needed before making the final marks.



    Quote Originally Posted by Limy Sami View Post
    It's your funeral but ;-

    A 6'' 3 jaw is over size for a 9'' SB, ......6'' 4J lightweight yes, 3 J no, 5'' is more than large enough

  9. Likes Limy Sami, vicfi, crossthread liked this post
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    Fair comment EPA,

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    I was over complicating machining a backplate. I'll buy a semi-finished back plate that already has the threads.

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    EPA..good advice but I have one question. You stated:

    "Bored what is called the "register". This should be somewhat larger than the diameter of your spindle that it will sit above. It does not need to contact the spindle as the thread will do the centering. "

    I was taught that the "register" is actually what assures the chuck is square to the spindle. The threads can't do that. I just wanted to clarify in my mind what you meant by the "register". To me that would be the bevel that turns up against the opposing bevel of the spindle. If so, then I believe this is the most important feature when turning the backplate and should not be somewhat larger. I totally agree with you that a 6" chuck is fine on an SB 9. That was my first lathe and I had a 6" 3 jaw and a 6" 4 jaw. I gave the lathe to my son and it is still working perfectly to this day with those chucks. I had to move up to a Monarch due to the spindle bore limitations on the SB.

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    Indeed - and folks like P&W carried this to extremes - like their hardened and ground conical nose on the early lathe - including Model B

    conical-nose-p-w-lathe.jpg



    Quote Originally Posted by crossthread View Post
    EPA..good advice but I have one question. You stated:

    "Bored what is called the "register". This should be somewhat larger than the diameter of your spindle that it will sit above. It does not need to contact the spindle as the thread will do the centering. "

    I was taught that the "register" is actually what assures the chuck is square to the spindle. The threads can't do that. I just wanted to clarify in my mind what you meant by the "register". To me that would be the bevel that turns up against the opposing bevel of the spindle. If so, then I believe this is the most important feature when turning the backplate and should not be somewhat larger. I totally agree with you that a 6" chuck is fine on an SB 9. That was my first lathe and I had a 6" 3 jaw and a 6" 4 jaw. I gave the lathe to my son and it is still working perfectly to this day with those chucks. I had to move up to a Monarch due to the spindle bore limitations on the SB.

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    No biggie, but the ball bearing in my Logan is rated for 8000 lb. It would surprise me if the bearing of the SB is rated that high, but it clearly is rated for the typical work of the lathe.

    A journal bearing (other than for power turbines and the like) starts up with zero oil pressure, and as it starts, it is rubbing the metal together. As soon as the "oil wedge" builds up, the spindle no longer touches.

    However, the mass of a chuck (and work) does cause, and if excessive, extends, the time during which there is metal-to-metal rubbing.

    Personally, the chuck mass is not an issue IMO, simply because the machine is designed to have actual work held on the spindle. Being a 9", one could reasonably expect to machine 3.5" steel bars which are the length of the bed. The marketing information commonly shows such operations.

    Such a bar will put at least 50lb on the headstock bearing, plus the weight of any other devices, such as a chuck, or a center and dog, etc. The chuck would weigh only 30 to 40 lb if it were solid, and it is not solid.

    Larger work would likely not be turned often, but if it were, the forces on the bearings would be higher. One assumes the designers specified the bearings to handle that possibility, however unlikely.


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