This is a simple caster designed for a robot I'm making, hopeful one of these days it will be finished. The caster uses a 16mm marble for the ball. It may be possible to print a ball for it, but I have not yet tried it. A few things to note are: Most marbles are glass, so there is a chance of them breaking and you may need to increase the "ballDiameter" variable, in the scad file, by 0.5mm if you are using a different marble size so it can rotate freely.