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In my work I often need to draw large-radii curves, for which I use this long beam compass. Built from scrap wood and a dowel center, it’s cheap and easy to make in any length. It can be quickly adjusted and doesn’t need locking clamps or screws to hold its setting.
The compass consists of a beam with an inverted friction-fit saddle. A dowel center inserted into the underside of the saddle serves as a pivot point. Make the beam from 3⁄4” x 1″ stock, and glue a small block onto the end to help hold the pencil. Drill through the block and beam to create a hole that will grip a pencil with a press fit. (I use a round rather than faceted pencil.) Make the 6″-long saddle from a piece of solid wood glued between two pieces of 1⁄4“-thick plywood. Cut the center piece about 1⁄32” narrower than the beam to ensure a friction fit that allows you to adjust the saddle position but that holds its setting without clamping. Drill a hole in the underside of the saddle to accept a dowel center, which is sold for aligning mating dowel holes.
To use the compass, adjust the distance between the pencil’s point and the dowel-center tip to match the desired radius. Use an awl to poke the center point of your circle or arc into your workpiece, register the dowel-center point tip in the divot, and swing the compass to make your mark. — Larry LaBeau
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