I have a lathe with a 2 1/4" threaded spindle with 6 tpi. It came with a3-jaw but no 4-jaw chuck. I got an 8-inch 4-jaw off ebay that didn't fit and then a cast iron backplate which had the proper thread. Saturday, I cut down the backplate to fit the chuck and drilled and tapped it to attach.
First time I used it, it got no heavy loading, but I though I was never going to get the chuck back off. Eventually a bunch of hammering on a lever bar broke it free. The 3-jaw has never needed more than a couple hits with a dead-blow hammer, even after I jammed a tool into the work.
Is there anything obvious I should think about to keep it easier to remove? The main interface between the backplate and the spindle seems to be the flat surface on the backplate that is about 3 3/8" in diameter. I never turned that part of the backplate, but I have since polished it with some fine emory and greased it.
Anything obvious I should do beyond what I have already done?