Remix of Marble Caster by HipsterLogic (http://www.thingiverse.com/thing:8959/)
Designed for a 5/8" ball bearing (http://www.mcmaster.com/#96455k58/=z8321g), which operates smoother than the marble.
Full drawing robot project documented at http://www.instructables.com/id/Low-Cost-Arduino-Compatible-Drawing-Robot/
If the bearing does not fit, use a hair-drier or hot air gun to soften the lip.
Remixed From Marble Caster by onebytegone