In school (AE) I gained a fair understanding of monocoque structures and how they behave and can be optimized. The thing with thin shells is they are efficient wrt weight, but they buckle easy under compression. The loads on a hollow tube are concentrated at the top and bottom (vertical load only), and the buckling mode on the compressed side will take many forms… it will end up looking like a wave… like what happens when you crush a soda can. This wave shape would require not just a filling that withstands compression (like concrete), it would also need adhesion to the walls to prevent outward deflection. This alone will make most fillers (short of welded fillers) not very effective.
Otoh, assuming the accelerations are reasonable wrt stepper torque, going to a solid bar would be much stiffer than hollow bar. Someone who has access to cheap solid bar could try this out… see if a primo or lr can handle the weight.
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Excellent. “Buckling” in my mind is a plastic process but, of course, it must happen elastically first. So, gasp, there’s a trade off. Larger diameter tubes are stiffer from the moment of inertia but thinner tubes buckle more.
In any case, the combined effects of the moment of inertia and buckling must be visible under static loads, so it should be say enough to test.
Tricky stuff, this engineering…
Edit: I wonder what is happening to the pipe under the concentrated load of the bearings resting on top of the pipe of the X-axis (router platform); this is exactly where the buckling loads are highest.
Looking at my DOM tubes and bearings, there is definitely a squeeze acting on the tubes as indicated by the 3 flat marks left by the bearings. Also the hardened bearing surfaces are completely chowdered at this point (cheap bearings). At the end of the day, those core z clamps, trucks, and core clamps are probably well under what it would take to induce DOM tubes to buckle. Even with a heavy router installed the pla will yield first.
That is exactly why I decided to print my parts with 4 walls. That makes the weak points a lot stronger I think.
Ryan ran flat spots into 1" stainless tubes. The point loading on the bearings is huge.