I find ball nose cutters quite slow at removing material as the periferal speed near the centre is very slow. You would be better of with a "radius" cutter these are like a 12mm Dia endmill but the corners are all radiused to 3mm.
As this is only for weld prep you may just as well take an old blunt 1/2" milling cutter and round off the corners on the hand grinder, you don't need them to be exact. I use this method quite a bit when fabricating parts from solid to get the look of a cast fillet in the corners
Can also be done with a flycutter.
Edited By JasonB on 17/12/2012 12:02:16