I used a 4" grinder with a cut off disc and really took my time. Youtube has lots of people showing how they do it (some will make a jig, etc.). I used a plasma cutter on the first keggle (my brew pot), and that went through it like butter
. I don't have a really steady hand, so the plasma cutter makes me a bit nervous. I have more room for error with the grinder & cut-off wheel combo.