HOW TO – Make circles Freddi C. style
Here’s a post on painting perfect circles using old school drafting tools.
http://www.makezine.com/blog/archive/2006/11/how_to_make_cir.html
I used to use one of those pens that you drop the ink into when I was in drafting class. Never thought of using it for painting. A compass set at Staples is about $15. You can also use the pen with a ruler to make straight lines or one of those bendy curve things to make compex curves.
– Dave