I’m pretty certain that the Sharpie isn’t causing the weird ripple. It’s in an adapter, inside the normal tool holder, and tightened down pretty well. It’s also just barely touching the surface of the paper, so I don’t think resistance is causing it to move. I’ve been testing this with a sheet of MDF under a roll of thin paper on which the Sharpie is drawing. I’d say it’s very flat (not contributing to error). Yeah, there’s definitely no spring involved, but honestly, it really seems like this should work with no problems.
I’ve been testing this way for two reasons: 1. it’s easy with minimal waste, and 2. I’m still waiting for an adapter so the 1/8 in bits I have will work in my router (which normally accepts 1/4th inch bits). I’ll definitely try cutting some foam as soon as I get that adapter collet in the mail.