Yep, go buy a good quality compass/divider. Not the el chaepo ones you give kids, but good draftsmans tools. Most have enough "points" that you can use as a scribe to get a good line going, and enough passes if your styrene isn't crazy thick should get you through. My current stable is a couple of "Weems and Plath" ones from the old sailing days, with the screw type fitting.
Just like this:
http://www.mdnautical.com/weems_plath/cp_176.jpg
You get what you pay for, yeah $20 isn't exactly cheap, but this is such a multipurpose tool (think measuring symmetrically, scribing arcs, making styrene circles, etc etc etc) that its worth having a nice one for.
Some searching should even reveal a compass with a attachment for a No. 11 blade. I use to have one but took mercy on a poor fellow modeler who was having the same issues you were having and gave it to him. Hard to find, but they ARE out there. Pick you center, carefully spin it around and around making progressively deeper passes and before you know it you have a instant circle out of styrene.
Hope this helps
Just like this:
http://www.mdnautical.com/weems_plath/cp_176.jpg
You get what you pay for, yeah $20 isn't exactly cheap, but this is such a multipurpose tool (think measuring symmetrically, scribing arcs, making styrene circles, etc etc etc) that its worth having a nice one for.
Some searching should even reveal a compass with a attachment for a No. 11 blade. I use to have one but took mercy on a poor fellow modeler who was having the same issues you were having and gave it to him. Hard to find, but they ARE out there. Pick you center, carefully spin it around and around making progressively deeper passes and before you know it you have a instant circle out of styrene.
Hope this helps