Rob is exactly right. Step drills are the perfect tool to use to drill holes in sheet metal. I'll bet the big drill grabbed and spun the drill motor as it broke through. Step drills don't do that. Much safer for your wrists.
And if there are ragged edges on the back side of the hole, just run the step drill in from that side and it will take them off easily.
You can get a cheap set at Harbor Freight, usually for about 14 bucks or just an 1/8" to 3/4" bit for $6.
