For Sawppy V1 I tried to create a design that would be tolerant of variations between 3D printers, but I failed to overcome two problem areas. Both are where 3D-printed plastic mated up against unyielding metal. One area were holes for 608 bearings used in each axis of rotation, a feature I plan to keep and scale down by using 623 bearings. The other area are holes for heat-set inserts, which has proved problematic in multiple ways and thus something I am now trying to avoid.
To deal with the bearings, I reminded myself exactly why they are a part of my rover designs: smooth rotation. The bearings are to ensure loads are carried through axes of rotation without introducing friction that could bind up. Friction is especially bad in the rocker-bogie suspension system, because it is a completely passive design for distributing weight of the rover across all six wheels. If any of the joints seize up, then the rover’s weight would not be distributed properly.
So smoothness of ball race bearing are the critical feature here, and the fact they have the strength of steel is less important. Weight of a rover is a tiny fraction of the maximum weight capacity of these ball bearings, which means I have the option to mount ball bearings using crush ribs. This is a concept I read about on Hackaday and I think it is appropriate to use for a small 3D-printed rover design. Previously I would allocate a cylindrical cavity to hold a bearing, but that meant the precise diameter of the cylinder became critical for a proper fit. Too large, and the bearing would move about loosely. Too small, and the bearing could not be inserted or perhaps damage the surrounding plastic during installation.
So instead of trying to hit the perfect diameter in a smooth cylindrical cavity, I could design a slightly too-large hole but with a few small ribs inside. When a bearing is inserted, the steel outer race will push little extraneous bits of plastic out of the way and dig itself a cozy home in between these ribs. If a 3D printer prints a little more or less than ideal amount of plastic, the size of those ribs would change but it is easier for a bearing to crush small ribs than to reshape the entire cylinder.
This approach has two problems:
- The bearing load is transmitted only through these small contact patches. But again the forces here are small and could be handled by the crushed ribs.
- The center of the bearing will end up in an unpredictable location. It will be within a small range but the actual position will depend on which ribs give way more than others. Fortunately, precise location is not critical for a little rover.
These problems make crush ribs unsuitable for most precision metal machining, which demands high precision for metal parts to work together. But we are in the world of 3D printing, where tolerances are quite loose relative to those seen in machining. Crush ribs allow us to turn the problem of loose tolerance into a feature and I can proceed with mechanical design for a cute little rover.