xray wrote:

I have a lathe with a 2 1/4" threaded spindle with 6 tpi. It came with a
3-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?

Clean the threads meticulously, then put a drop of oil on them each time you
mount a chuck. The book "How To Run A Lathe" shows how to make a little tool to
clean out the internal threads on a chuck backplate. I haven't ever done that,
but I use an old toothbrush and occasionally I take my chucks apart and clean
everything with solvent, brushes, and compressed air.

If it goes on easily and jams coming off, it may be due to a chip stuck in there
is what I'm thinking. The flat area at the back of the backplate is called the
"register" and this by rights should be hand-scraped to closely fit. If it's a
bad fit and spun on tightly, this could have the effect of jamming the threads
and making removal difficult.

Normal chuck removal procedu chuck a 1' long piece of 2x4 so it sticks
sideways out the jaws. Put the lathe in backgear and strike the 2x4 once hard,
in the direction to unscrew the lathe. It should free it.

Grant