I was interested in the idea to 3D print ball bearings. So I designed a parametric model with Fusion 360. The model can generate 3D printable ball bearing shells according to the number of 6mm airsoft BB it will hold.  More BBs in the bearing, larger the bearing will be.  Press the BBs into the shells to make the bearing. This process can be mastered with a few attempts. 

The Fusion file and a few example .stl files are enclosed.