This thing was made with Tinkercad. 8-ball bearings. Noodling around. A well-tuned and precise printer might just make this work. Can't test until I get my Printrbot, but might benefit from more bearings, larger diameter, scaling up. The individual balls just kiss the races inside and out. This design might also benefit from some kind of bearing holder.You can order one here: http://www.shapeways.com/shops/bitforgeCheck out the "Improved Ball Bearings" for greater precision and maybe even functionality.I am not sure how round the bearings in this Thing will come out. When you print it, let me know. Thanks! I had a couple of these printed up by Shapeways, and as a bearing, they are terrible. The precision of the printing process used is such that the bearings are fused to the inner and outer races. If you could break them free (and I haven't), they would be a lumpy, dumpy bearing that would soon self-destruct. I think a better plan of action is to print the inner and outer races, adjusting for the precision of your printer, and use off the shelf round things for bearings (BBs, pellets, airsoft, etc). Keep me posted, Thingmakers.