This beam compass (trammel) is used to scribe a circle or arc, by using a sharpened point at one end and a lead pencil at the other. The radius can be adjusted by sliding the pencil holder across the tool’s beam, and locking it in place by turning a knob at the desired location. The beam compass can draw and divide circles that are larger than those made by a regular pair of compasses. This version allows the precise drawing of circles up to ~160 mm in diameter. 

This model uses a sharpened finish nail as its fixed point. I used a M4 socket head screw and brass insert to secure the pencil holder. A carriage bolt might be better. A M2 socket head screw secures the pencil.