For forming a circle is used only mandrel and fingers. I've tried to use comercially made tools but in this case there is no need for them. If you have strong fingers it's easy, if not than you need to play with hammer. First thing you bend ring for soldering, it is not bent round but more like half moon. The reason is to allign both ends more accurate in all dimensions. And after soldering just mandrel and hammer. For me this is faster (I'm very lasy to get my ass out of chair and go to the table with this circle former, and even after that to get perfect roundness you need mandrel and hammer). So in my opinion it's uselees.
Sorry for my poor english, hope it's understandable.
