Here's a magical way to trace arcs and circles. Tap two finish nails at the ends of a desired diameter, then rotate a framing square against the nails while holding a pencil in the square's corner. Spray the underside of the square with silicone or rub on some paraffin so it'll glide smoother. Then practice a little to get the feel of the three-point contact technique.
Woodpeckers One-Time Tool Pocket Compass. The unique design of the Pocket Compass gives you an accurate, convenient tool for drawing arcs and circles from one inch up to 6-1/2” in diameter. It also eliminates the need for a ruler and even has a self-contained stainless steel pivot point. All that and it’s about the same size as a pocket knife. Pre-Order before July 1st, 2013, not available after 7/1/2013.