[Bigdav160]

i am going to agree with Robert. My favorite are the Spot eze. They do leave an 1/8" hole in the bottom layer/ The drill bits do break off occasionally but are replaceable. They will cut through many 100's of spot welds before they wear out.

I also have the roto broach style, and as mentioned, the spring loaded center needs a decent divot to stay centered. The benefit to these cutters is they generally leave the lower panel undisturbed and the cutting bits are replaceable and fairly inexpensive. They also don't stay sharp as long as the Spot eze.

I don't do well with the grinder or burrs. Once I start I tend to lose the position of the spot weld.