For the sake of your fingers and unless you enjoy the sight of your own blood, if you're drilling or hole cutting in sheet metal, put the workpiece in a vise or clamp it securely . As the drill bit breaks though its likely to snag the work piece and twirl it around, nicely butchering your hand. The chassis punches mentioned earlier are great things, but the ones I've used require a hole drilled in the center of the work piece, so still be careful.