How Tightly To Bolt Onto Extrusion Beam? A Second Example.

A recurring design concern when using this construction technique of aluminum extrusion beams and 3D-printed connectors is how tightly the plastic parts should be grabbing on to their beams. In the theoretical world, we can precisely measure the forces these parts need to handle and based on that information, derive the number and location of fasteners. The real Mars rover Curiosity underwent this type of rigorous engineering study. But for our hobbyist-level Sawppy the Rover construction project, it’s an iterative process of trial and error. The previous post discussed Sawppy’s rocker-bogie suspension system components and how they were slimmed down for the second draft. This post talks about where the suspension assembly bolts onto the chassis.

This body mount point will take almost half the weight of the rover, so it definitely needs to be one of the strongest parts we print. Exactly how strong we need, though, is an unknown which meant the first draft is likely to be overbuilt out of caution. So when it came to extrusion mount, I saw the print orientation allowed the mount to completely surround the extrusion on all four sides and bolt onto three of them, and that’s what the first draft had.

This was proven to be overkill pretty much immediately with two important traits:

Making the shape completely surround the beam meant the only way to install/remove this mount is to slide the whole thing off the end of a beam. Which also means everything else bolted to that beam has to be removed beforehand. This makes it a huge hassle to experiment with different designs.

The second downside of a complete surround is lack of flexibility to adjust for the imprecise nature of 3D printed plastic. If the mount is in an open U shape, the opening can flex a little bit to help installation. When the mount is a closed O, there’s nothing to open and flex and help out. Which means if the extrusion beam slot is printed just a little small… the open end U-shape can make things work but the enclosed O-shape would get stuck

Too Much Extrusion Support

Which is exactly what happened to the test piece on the left: A scrap piece of extrusion was installed to test its fit inside the enclosed mount, and it was tight. After apply a little force in an attempt to help, the extrusion scrap is now completely stuck and will need to be cut free sometime down the line.

On the right, the second draft for suspension body mount with an open U-shaped beam channel that was successfully installed without trouble thanks to its little bit of flexibility. And while we gave up some strength by holding on to three sides (and bolting on to just two) in practice this piece seems strong enough for the task printed in PLA. In fact we might have room to slim it down a little further and reduce the number of fasteners as a future optimization.

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