[Dan Royer] shared a tip about how to get a reliably tight fit between 3D printed parts and other hardware (like bearings, for example.) He suggests using crush ribs, a tried-and-true solution borrowed from the world of injection molding and repurposed with 3D printing in mind. Before we explain the solution, let’s first look at the problem a little more closely.
Imagine one wishes to press-fit a bearing into a hole. If that hole isn’t just the right size, the bearing won’t be held snugly. If the hole is a little too big, the bearing is loose. Too small, and the bearing won’t fit at all. Since a 0.1 mm difference can have a noticeable effect on how loose or snug a fit is, it’s important to get it right.
For a 3D printed object, a hole designed with a diameter of 20 mm (for example) will come out slightly different when printed. The usual way around this is to adjust printer settings or modify the object until the magic combination that yields exactly the right outcome is found, also known as the Goldilocks approach. However, this means the 3D model only comes out right on a specific printer, which is a problem for a design that is meant to be shared. Since [Dan] works on robots with 3D printed elements, finding a solution to this problem was particularly important.
The solution he borrowed from the world of injection molding is to use crush ribs, which can be thought of as a set of very small standoffs that deform as a part is press-fit into them. Instead of a piece of hardware making contact with the entire inside surface of a hole, it makes contact only with the crush ribs. Press fitting a part into crush ribs is far easier (and more forgiving) than trying to get the entire mating surface exactly right.
Using crush ribs in this way is a bit of a hack since their original purpose in injection molding is somewhat different. Walls in injection-molded parts are rarely truly flat, because that makes them harder to eject from a mold. Surfaces therefore have a slight cant to them, which is called a draft. This slight angle means that press fitting parts becomes a problem, because any injection-molded hole will have slanted sides. The solution is crush ribs, which — unlike the walls — are modeled straight. The ribs are small enough that they don’t have an issue with sticking in the mold, and provide the mating surface that a press-fit piece of hardware requires. [Dan] has a short video about applying this technique to 3D printed objects, embedded below.
Designing around a manufacturing method’s limitations is key to getting good results, and techniques from one area can cross-pollinate with another, as they did here. Whether you’re just curious, or experimenting with something like hot-glue injection molding, it’s worth spending a little time to learn more about injection molding and its design considerations because it’s one of those technologies that makes the world go round.