While trying to find the center of a circle, it occurred to me that pairing a combination square and a framing square should help do the trick. I clamped the squares together with a spring clamp as shown, and then marked a center line. Next, I rotated the disc about 90°—it's not critical—and marked another line that intersects with the first. That intersection is the center of the circle.—Jim Skrocki, Painesville, Ohio