The idea of pendulum grinders for roughing out is, of course not new, but this may have a new twist. The blade from my tile saw was mounted on an arbor and attached directly to the shaft of an electric motor. The motor was then mounted on the end of the pendulum with a shroud made from plywood and some inner tube. The wooden part of the pendulum was laminated from two pieces of 1 X 2 to make a 2 X 2. Before laminating, the inner faces of the 1 X 2s were routed out for six feet at the top end so that a hollow, ½ X ½ was formed when the parts were glued up. Also a ½" nut was imbedded in the top end of the pendulum. A six foot, ½" threaded rod could now be threaded into the pendulum to vary the over all length. A ball and socket joint was attached to the top end of the rod and the pendulum suspended on a moveable bracket on a vertical rail in the garage attic.
The mirror blank is mounted on a three legged table with adjustable feet. To rough out the mirror, the pendulum is adjusted with the threaded rod to give the desired length between the bottom of the blade and the ball joint. The bracket in the attic is raised or lowered till the blade is just about to touch the mirror. Next, turn on a water supply and the motor and rotate the pendulum clockwise about an 1/8th of a turn to lengthen the pendulum and a small area of the mirror can now be ground out by moving the pendulum about over the mirror. When no more glass is being removed, draw the grinder to one side, move clockwise again another 1/8th of a turn and repeat grinding. If the area being ground is not centered on the mirror the table legs are adjusted. About three full turns around the table and the mirror was roughed out to within a ½" of the edge. It took about 15 minutes of actual grinding to rough out the 18" mirror, but probably another two hours of adjusting and cleaning up the ground glass slurry. The usual precautions of wearing a mask, working in a well ventilated area and washing away any slurry apply.Some of the advantages of this setup are; you know definitely what the radius is that you are making; a wide range of radii can be ground, mirror does not have to be on a turn table.
Some ideas for next time: I would do most of the adjusting, grinding out a dummy mirror from a disc of MDF to be used in forming the fine grinding tool and lap; mount a router in the table and the backing discs for the fine grinding tool and the lap on the pendulum, in place of the motor, to route out a convex surface to match the mirror.