Rusty Compass

The rusty compass, or fixed compass, is generally more of a mathematical abstraction than an actual drawing tool. But sometimes it's useful to have a tool that will not budge from its setting, say for the quick and accurate bisection of a line.

Four simple fixed beam compasses — of radius 30, 50, 75 and 100 mm — are included here. Source for the 50 mm compass is included, as all the others are derived from this.

In addition, you will need:

• plastic-headed poster pushpins

• M3 - 12 mm length machine screws, M3 nut. Washer optional

• short lengths of 2 mm drawing graphite, preferably sanded to an angled point.

Building:

1. Glue the pushpin onto the end of the beam, pushing the point through the small hole

2. Loosely assemble the screw and nut into the beam

3. Push the graphite through the split hole in the beam from the top. Make sure it's about the same length through the beam as the pushpin. Gently tighten the screw to stop the graphite slipping.

The compasses are shown on top of a construction of Hofstetter's A Simple Ruler and Rusty Compass Construction of the Regular Pentagon