The Family Handyman
All you need to trace an exact arc is a thin strip of metal, a threaded rod and a couple of wing nuts and washers. Drill holes in the ends of the metal bar or strip, then bend the ends in a vise. Slide in a threaded rod and tighten the wing nuts to create the desired curve for tracing. Because of its density, the metal maintains a nearly uniform radius when pressed into a curve.
We experimented with various thicknesses, lengths and types of metal and got the best results for large-radius curves with a 1/8 x 1-in. x 4-ft. aluminum bar ($8.99 at Ace Hardware). For smaller radius arcs, a 12-in. x 1-in. .032 brass strip ($2 at Ace Hardware) works great. Use 1/4-in. threaded rod ($3) for the aluminum bar and 6-32 threaded rod ($1) for the brass strip. This shapely tip is from Bryan Morse.