Seized jaws in 4 jaw chuck

[rwm]

The back plate made wrong. The boss on the back plate is too shallow and is making contact with the edge of the chuck. When you tighten the mounting screws it pulls the chuck into a concave dish. You just need to machine the back plate so contact with the chuck is made in the central area, leaving a slight air gap around the edge. You need to be very careful not to go undersize on the diameter. It should be a snug fit. This is less important on a 4 jaw. I find it difficult to add material back to a part when working on the lathe...
I had this exact same issue and someone smart on this website clued me in! I think it was @pontiac428

 

[Fixitagain]

So contact is being made on the outer flange of the backing plate currently. I’ll machine down the outer ring so it makes contact with the center of the chuck instead.
Just have to wait for fedex to deliver some tooling. I’ll report back once it’s done

 

[Fixitagain]

As for the diameter of the mounting plate. It’s TIGHT. Ie have to use bolts to pull it on and then hammer it off while working on it. But it does run true to 1/2 thou. I won’t touch that edge

 

[rwm]

I'm confident this will fix it.

 

[Fixitagain]

I turned about 1/8" off the face of the backing plate this evening. now the chuck sits supported where the bolt holes pass through the chuck and the jaws run smoothly now. (well almost.. one is a bit sticky but not seized up anymore). huge improvement!
I also learned about turning speeds for cast iron and hss. taking hints from someone talking about seeing sparks and squealing and spinning too fast, use bull low gear. worked great! onto my next project which is turning down the flat bar to a tee nut to mount the QTCP.


thanks everyone for the help and ideas

 

[rwm]

Nice job! Show us a pic of your lathe?

 

[Fixitagain]

Freshly unloaded into my carport while I get around to renting a skid steer to take it into my shop in the back yard