This thing was made with Tinkercad.
Improved Ball Bearings with 9 ball bearings
6mm ID / 24mm OD
Based on the first 8-ball Ball Bearings cartridge I designed, I packed another bearing in the races to better support the structure. Who knows how this gizmacci will print, but the design has bearings precisely dimensioned (5mm) to fit the races - see image. I don't know yet how round these balls will print, but they will likely need to be broken from races before they will rotate. This is not a precision bearing, this is proof of concept, but a high precision 3D printer might actually produce a usable low-speed, low-load bearing.
+++ I have further improved this design by using a shape script for the spherical bearings, that creates smaller facets and then laboriously and precisely positioning the bearings in the race, so that there is absolutely minimal contact between bearings and races. +++