Awhile ago someone mentioned using an Olfa Compass Cutter for cutting
centering rings from soft materials, such as cardboard and foamboard.
I bought one, and have used it, and it's useful, but htere are
problems as well. First, a description of the tool: it looks a
little like a drill-press circile cutter, only it's made of plastic.
It has a little metal spike in the middle, like a compass for drawing
circles, and use small X-acto-like blades.
The compass cuttert has the same problem that a regular compass has
when drawing a circle, namely, that it's practically impossible to
hold it straight upwards while cutting, meaning that it's very hard to
draw a perfect circle. I've found that what you have to do is to use
it to cut a pattern (for example, a template printed with RockSim or
VCP)--the advantage of using the compass cutter over a regular knife
is that is makes a nice, straight, clean cut. The blade is just
barely long enough to cut 3/16" (5mm) foamboard, though at times I've
had to cut the bottom layer of cardboard from the other side (slightly
annoying, but the main thing is to get a straight, clean cut through
the foam). It will also cut _very_ small holes---it did a pretty good
job cutting out foamboard centering rings to mount a 10.5mm engine
tube in a BT-56 body.
Another poster mentioned using the compass cutter with a drill press.
I've found that will this is the only way to produce a perfect circle,
it's very hard to center the circle or control its size: the problem
is that the top of the cutter (the handle), which has to be inserted
into the drill chuck, is tapered rather than straight, and that makes
it almost impossible to mount it straight into the drill. I've given
up on using this way, and only use it by hand.