Ok you lot I have a confession to make . The circle was done with a cutters compass. You put the plasma head or a gas cutting head in there and do perfect circles.
Freehand on 1/2" plate just using a straight edge to follow produces this. makes virtually no slag on the underside.
A gem , I was talked into it by my boc supplier when the gas was on a shortage. I have only just started to play with it , it seems happy up to about 20 ish mm and will sever 27mm bar so good enough for anything I need it for.
Bob