Use a parting tool to cut a grove either side of the ring you want to form. Then round over the outside of the ring and use the tool for the inner and to part off.
Longworth jaws are awkward to tighten, need a third hand. Try making a set of reversing jaws out of 12mm rigid plastic, like the Nova ones, as I did. And buy the buttons from Charnwood.
I also use an Axminster Precision Chuck, my chuck was made in Poland.
My backplate is 1/1/4", I agree with CHJ, 3/4" is rather small diameter for a very heavy chuck.
My chuck insert came loose with a grub screw. Remove and clean insert and chuck, use a drop of thread lock and tighten, cured mine.
Hope this helps, Tony.