Imagine someone wants to press the bearing into the hole. If the size of the hole is incorrect, the bearing will not be tightly fixed. If the hole is too large, the bearing is loose. Too small, the bearing cannot be installed at all. Since the difference of 0.1 mm can significantly affect the looseness of the assembly, correct installation is very important.
For 3D printed objects, holes designed to have a diameter of 20 mm (for example) will be slightly different when printed. The usual way to solve this problem is to adjust the printer settings or modify the object until the magic combination that produces the correct result is found. This is also called the Goldilocks method. However, this means that 3D models can only appear on specific printers, which is a problem for designs to be shared. Since [Dan] works on a robot with 3D printing elements, it is especially important to find a solution to this problem.
The solution he borrowed from the world of injection molding was to use squeezing ribs, which can be thought of as a set of very small supports that deform as the parts are pressed in. Instead of bringing one piece of hardware into contact with the entire inner surface of the hole, it only makes contact with the extrusion rib. It is much easier (and more forgiving) to press the part into the compression rib than to try to get the entire mating surface completely correct.
Using extruded ribs in this way is a bit cumbersome, because their original purpose in injection molding is somewhat different. Walls in injection molded parts are rarely really flat because this makes them more difficult to eject from the mold. Therefore, the surface is slightly inclined, which is called the draft. This slight angle means that press-fit parts are a problem because the sides of any injection holes will be inclined. The solution is to use squeezing ribs. Unlike walls, the squeezing ribs are straight. The ribs are small enough that they will not stick to the mold and provide the mating surface required for press-fit hardware. [Dan] has embedded a short video below on how to apply this technique to 3D printed objects.
Designing around the limitations of manufacturing methods is the key to good results, and technologies in one field can pollinate each other like another. Whether you are curious or trying something like hot glue injection molding, it is worth taking a moment to learn more about injection molding and its design considerations, because this is one of the technologies that makes the world work.