This is an updated version of the trammel based on suggestions I received from Bassposauist. It adds a scale and is a little more compact. Another suggestion was to use the knob designed by  ItsAzuurfor_ for a M4 hex head bolt [https://www.printables.com/de/model/650554-leveling-knob-m4-bolt/files](https://www.printables.com/de/model/650554-leveling-knob-m4-bolt/files) 

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 hex head bolt and brass insert to secure the pencil holder. A M2 socket head screw secures the pencil.