I also use an Atlas 12x36 with a 3 jaw chuck. I think it is a 6inch. Anyway a friend gave me an 8 inch Chinese chuck with 3 hole tightening holes. But I needed to buy an adapter plate. I decided to use a bronze rod and weld in a new tooth. I did and used files to dress the weld and it worked very well, not perfect but well enough. I did buy and fit a back plate for the 8 inch chuck and I could not asked for a better fit. I cleaned, oiled, and saved my old chuck for better usage if necessary. The cheapest way is to weld and file it to fit and it will do you good service for years.