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firbikrhd1
07-19-2003, 11:49 PM
Tonight I was in the shop and decided to machine a back plate for a lathe chuck I obtained a while back. The back plate I had was prethreaded but the register needed to be bored to fit my lathe. Previously I have read the various posts about backplate machining and gathered that usually the register bore is machined at the time the threads are cut, otherwise the register may not be concentric with the threads. That was impossible in this situation. Since no one has addressed this particular circumstance I want to tell what I did and see if anyone has a better solution.
I got a race for a tapered roller bearing that fit over my lathe spindle, placed it on the spindle and threaded the backplate on backwards with the side that will eventually be touching the chuck toward the headstock. These races, in my experience tend to be very accurate in width. This spaced the back plate out past the lathe register and allowed it to center on the threads so I could bore the backplate to the proper diameter to receive the register. I had turned a gauge the exact diameter of the register to check the bore as I went. When it fit nicely, I stopped, took the back plate off and reversed it to the position it will run with the chuck attached. The fit was excellent and the back plate runs true. Now I'm ready to turn the face to fit the chuck recess.
This may come up again and I don't know if I was just lucky or my method was sound. I would appreciate any input you pros out there have.
Thanks!

SGW
07-20-2003, 12:01 PM
Certainly close enough, since you're turning the plate to fit the chuck recess with the plate mounted on the spindle nose. Even if there was a slight error as a result of your setup, it won't matter.

Thrud
07-21-2003, 02:57 AM
Your chuck should just press on the back plate with firm hand pressure. Make sure that you bevel the corners and undercut the inside corners of your backplate so that there is no interferrence with the chuck when fully seated. Better chucks are undercut in the inside corners already, but it is always wise to check for it.

If your chuck bolts from the front counter bore the the first thread or two out of the backplate chuck bolt holes so that when the chuck is bolted down it will not pull threads proud of the backplates surface.

Paul Alciatore
07-21-2003, 03:39 PM
I think your method is absolutely sound. The threads should be self centering to a great degree so the register bore's axis should be within a few tenths of the thread axis. One thing I would have done at the time would be to take a light facing cut on the back surface that will also contact the spindle. This surface will do more to hold the backplate perpendicular to the axis than the registry bore which mainly centers it. By machinning both of these at once you would have assured the best possible mating. But don't go back and try to do it now, you'll never get the exact same position on the spindle and if it's working, don't fix it.

I'll have to remember the idea of using bearing races for spacers. Good one! Not only are they very parallel, but I bet several bearings from the same source will be very close to the same size.

Thanks for sharing.

Thrud
07-22-2003, 05:57 AM
Paul
Freshly removed hard disk platter spacers are also a good alternative throw-away (they work at least once). I keep stacks of them for disposable service on the lathe and mill. They are handy because you can cut into them and not chew up a good endmill or insert like you would with the hardened & ground bearing spacers. I have yet to use one of the 4" diamerter rings from 14" platters, but the 5 1/4" and 8" are handy sized...