I’m in the picture frame business and came up with this simple idea to ensure perfect miters every time. Cut a block of 3/4″ plywood so one corner is exactly 90°. Clamp this block to the sander table as close to 45° to the disc as you can manage. Now cut a pair of miters and trim them on the disc sander, one on the right hand side of the block, the other on the left. You’ll find that even if the block is a fraction of a degree out, the miters will still fit perfectly.
– Sam Laughlin
Santa Rosa, California