Tape measure trammel helps you mark big arcs
When you need to scribe a large, precise arc or circle on a workpiece, just reach for a tape measure. First, drill a 1⁄16 " hole centered on the tape's 10" mark: This keeps the tape's metal hook out of the way, and 10 is an easy number to work with. (Don't worry about damaging the tape measure—a few tiny holes won't break it.)
Then, determine the radius of your circle. Remember to add 10" to it to account for the position of the first hole; then drill another centered 1⁄16 " hole in the tape measure at that location. Use a push pin to anchor the 10" point at the center of the circle, place a pencil in the other hole, and scribe the arc.
—Bill Wells, Olympia, Wash.