Lathe chuck back plate - finished vs semi-finished
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    Default Lathe chuck back plate - finished vs semi-finished

    Hi all, sorry for the rookie lathe question but I can't find a good explanation for the life of me. What's the reason for places selling a lathe chuck with a semi-finished back plate, specifically for a 4-jaw independent chuck? I can appreciate an on-the-machine finished back plate for a self centering chuck. It seems for an independent jaw chuck, a small amount of runout on the chuck body wouldn't be a big deal as long as the chuck doesn't move on the back plate. Is it so the clamping interface between the back plate and chuck is perfectly planar and perpendicular to the spindle axis?

    P.S. My question is in regards to a manual lathe with L00 nose, but should apply to other nose types as well.

    Thanks in advance!

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    All back plates should be trued to the lathe for best accuracy .

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    Fully finished adapter plates are finish machined on the chuck side, so that they can only accept one specific chuck, or exact copies by other chuck makers. For example, 6" Buck Ajust-tru, 6" Pratt Burnerd Setrite and 6" BTC Zero Set chucks all fit the same adapter plate. The major chuck makers usually stock adapters for their own current products. But there are so many different chucks, new and used, that it is more efficient for tool dealers to stock semi-finished adapter plates and let the customer do the machining of the spigot and holes to fit their own chuck.

    On my lathe with L00 spindle, I used finished adapters for my Setrite chucks, but I had to machine semi-finished adapters for a Sjogren No.2, PBI KC25, and a Swiss combination 4-jaw.

    And yes, all adapters need to be finish faced on the lathe on which they will be used.

    Larry

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    Thanks, that's good info! To test my understanding, if you move an L00 mounted chuck to a different lathe you would technically break it down and re-face the back plate each time? If it stays on the same lathe it's fine to install and remove chuck from that lathe without re-facing as long as it sits on the nose with acceptable repeatability?

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    Don't underestimate vibration. Chucks are heavy, and if not running true will have an influence on vibration and thus surface finish.
    Last edited by Benta; 05-31-2019 at 01:52 PM.

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    Quote Originally Posted by dingo16123 View Post
    Thanks, that's good info! To test my understanding, if you move an L00 mounted chuck to a different lathe you would technically break it down and re-face the back plate each time? If it stays on the same lathe it's fine to install and remove chuck from that lathe without re-facing as long as it sits on the nose with acceptable repeatability?
    No-It should be a one time deal.

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    Quote Originally Posted by dingo16123 View Post
    Hi all, sorry for the rookie lathe question but I can't find a good explanation for the life of me. What's the reason for places selling a lathe chuck with a semi-finished back plate, specifically for a 4-jaw independent chuck? I can appreciate an on-the-machine finished back plate for a self centering chuck. It seems for an independent jaw chuck, a small amount of runout on the chuck body wouldn't be a big deal as long as the chuck doesn't move on the back plate. Is it so the clamping interface between the back plate and chuck is perfectly planar and perpendicular to the spindle axis?

    P.S. My question is in regards to a manual lathe with L00 nose, but should apply to other nose types as well.

    Thanks in advance!
    .
    a semi finished one you final machine or true it so it will have no runout.
    .
    a finish machined one almost always have some runout due to the machine it is mounting on have some runout errors. you literally can mount a finished machine chuck on 10 different lathes and get 10 different runout amounts.
    .
    i have made chuck mount before and most of the labor is getting it to mount on the lathe. once that is done its often less than 1 hour to true it up, transfer punch the holes and drill the holes on a drill press or whatever you have available.


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