Dave Staeheli
I've been using an "arch punch". Good steel and perfectly round, which hole saws are not. Initially you can use it without modification, but then I just rub the cutting edge against a diamond hone. Yes, it flattens the edge, but creates a new edge with the flattened edge and the inside bevel. Remember, it is the inside edge that does the cutting on a sphere. A bit pricy though, especially in the larger sizes.