Sand perfect circles
Try a disc sander to achieve true roundness. First, from 3⁄4 " plywood, build an auxiliary table (about twice the size of the table on your disc sander) with a 3⁄8 "-deep slot 3⁄4 " wide. Now, fit a hardwood bar a few inches longer than the auxiliary table so it slides freely in the slot. About 1" from one end of the slide, fit a 3⁄32 " pin that protrudes 1⁄4 ". Bore a 3⁄32 " hole at the center of the circular workpiece, place it over the pin on the slide, and slowly advance it into the sanding disc. When you reach the scribed circumference, clamp the slide into the auxiliary table and spin the workpiece for a perfect circle.
—Edward Hanselman, Hoosick Falls, N.Y.