# 8" backplate for a Bison 6 jaw Set-Tru chuck



## Badabinski (Apr 4, 2021)

Another backplate post! This one is going to be more detailed.

A couple of months ago, I bought an 8" Bison 6 jaw set-tru chuck. It came without a backplate, so I needed to fit one up.

I started by buying a backplate off of Shars. My lathe has a 1¾"-8 threaded spindle nose, so it's a bit of a pain to find pre-machined backplates. Shars was about the only place I could find an 8" backplate for a reasonable price. The backplate came in, I mounted it on the lathe, and then realized that it was too thin. Set-tru chucks need a pretty deep boss. In my case, the boss was .710" tall, so the 1" thick backplate from Shars would be too thin by the time I took off all the metal. I had already machined the thing so I can't return it. I guess I just have an unfinished face plate now.

So after pouting for a bit, I bought this chungus piece (8.25" OD, 2.5" thick) of cast iron.




This was almost too big. The jaws were just barely clearing the ways. Any larger, and I would have been bolting this thing to a faceplate.

After getting the stock centered up, I skimmed the outside a bit so my facing tool could make full passes.




Then I faced, drilled, and bored out the center for a 1¾"-8 thread. I cut the thread in, and then mounted the backplate on the spindle.




Then I started facing it. This was so, so slow. At this diameter, my lathe can face about .025" before it starts getting too bogged down. I had to remove approximately 0.800" from the face. That's 32 passes, with each pass taking roughly 3 minutes.






After that, I started making the boss by bringing the rest of the body down to 1" thick. Rather than face to a shoulder, I took passes along the OD. I started out at .030" passes, slowly working my way up as the diameter got smaller until I was removing .110" from the diameter with each pass. This took a while, but it was a heck of a lot faster than the facing!






After a bunch of finishing passes, the chuck was the right shape! I made the boss about .010" undersized to ensure I had enough wiggle room to center up the chuck.




(This was a pain to get off of the spindle. I ended up clamping a C clamp on the boss with some copper to protect the surface, and then I beat on the C clamp until the backplate came off. Not ideal, but it worked.)

I didn't get any pictures of the mill work because I was tired and in a rush, but I just clamped the backplate to the table, did some edge finding, and then drilled and tapped a 3 hole bolt circle for M10-1.5.

Here's the chuck on the lathe!




I need to skim the backplate down so it matches the chuck, but I'm way too lazy to do that right now. Here's a video of it spinning:






And here's the runout I'm getting on a ½" tap follower after zeroing it, taking it out, and then putting it back in the chuck:






.0005" is good enough for me! That's what I usually shoot for in my 4 jaw chuck, so I'm totally pleased with that.

Overall, the backplate turned out pretty well. I absolutely hate machining these though. Single pointing 8 TPI threads is annoying, the stock is usually quite large, and cast iron (even ductile nodular stuff) just sucks. There's abrasive grime EVERYWHERE, so I'm going to need to spend a few hours cleaning it all up. This is partially my fault, because my starting stock was a bit too big. If I had tried harder I might have been able to get some slightly thinner stock. As it is, I took a 33 pound piece of stock and machined off 13-14 pounds.

Thankfully, I won't need to make another backplate for a while. I can't wait to put this 6 jaw chuck to work!

EDIT: Apologies for all of the large photos. My internet connection is really speedy so I didn't realize that my pictures were like 3 megs a pop. I'll try to make sure my images are smaller in the future.


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## Janderso (Apr 5, 2021)

That's a nice chuck.
Great job on the backing plate. Not a job for the timid for sure.


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