Using a regular compass for small circles is OK, but for projects involving large circles or arcs, beam compass heads are ideal.
The aluminum heads clamp onto a piece of wood from 15.9 – 19.0mm thick, creating a compass whose radius is limited only by the length of stock you have at hand. Each brass clamping knob holds the head securely to the beam using a trapped pin that prevents pivoting and shifting. The pins withdraw as the clamping knob is backed off. The heads need only be clamped approximately as one of the points has an eccentric tip that can be used to achieve 3.2mm of fine adjustment.
Both points have been hardened and can be used to mark hard surfaces such as steel plate, sheet metal or aluminum. Also included is a small pencil for marking wood and gyprock. The heads, points and pencil all pack into the slim tubular hard-shell case for storage in a toolbox.