Blackboard compass

This compass is intended for use on a blackboard. I generally use only a string as a compass on the blackboard, but this becomes impractical when the circle arc is too big (or for full circles).

This model has a hole in the middle (either to centre the compass correctly or to put something through and hold it in place). A cross-hair is also there to make sure the centre is right and there is a niche to glue a magnet (more practical for full circles or arcs with a very big radius). I used a magnet with a hole in the middle so that I still see very well where I put the compass. A clip is used to adjust the length of the string.

The Openscad file is attached so you can change the dimensions (depending on the size of your string, magnet and/or the hole in the middle).