i picked a pair of these up at Nothern Tool, and ummm... do you have to drill a pilot hole so these things dont "walk" all over the place?
http://www2.northerntool.com/welding/spot-welders/item-154830.htm
You shouldn't have to
i beleive the point of the spot weld cutter is designed to cut thru the weld on one peice of metal and save the other peice. for instance a damaged rocker attached to the floorpan... use the bit to drill the spot welds out (which should be a little dimple which should also help locate the bit) and once it has cut thru all spot welds the rocker would come off and it would still have the floor panel metal to weld on the new spot welds for the new rocker. I hope that made sense