The grit is what does the cutting. The metal just pushes it about and into the glass.
The important thing here is to keep lifting the cutter up about every few seconds (you'll hear the cutting sound change) to get fresh grit into the cut. This operation will probably take you about 1/2 hour or so, depending upon how fast the cutter is (you can't go too fast or the grit will be thrown from the cut by the speed of the cutter).
Actually, you dnn't really need to put teeth in but they do act as resovoirs for the grit. You actually want to have the teeth backwards relative to a wood saw as you are pushing the grit into the working surface rather than trying to lift glass off of the working surface.
Most experienced trepanners use flashing (18ga. sheetmetal about 6" to 12" wide used for roofinng purposes) for the cutter with a plywood disc to shape the flashing into the proper round shape. The sheet metal works just fine and the cut is fairly thin so you don't have to remove too much glass as you go and it also makes a ring big enough to take a maximum cut out of the piece of glass although more rigid stuff like a piece of pipe will allow for overflowing the sides of a piece of glass thus getting a 6" mirror out of a 6" square piece of glass.

• Thanks again Bob! - gregted - August 19, 2002 - 22:33 UTC