As you grind off the rivet head, you may lose sight of the rivet's center when you grind a bit of the frame. Wait a minute and let it cool, then you should see the outline of the rivet. Use a center punch to make astart for the drill bit. I use a 1/8" hardened bit to go all the way thru, then step up to a 1/4" or 5/16". When you step up, don't have to go all the way thru, just to the other side of the frame. Then take the big punch and BFH and give it a few good whacks. If there's any piece of the rivet head left, it will be much harder to break it loose. Once you get a rivet or two out, you can whack on the bracket and it will often take the rest of the rivets out that you've ground off flush with the frame, JSM84