After repeatedly running the base plate through the sander, the mirror cell now slides into the tube. It is a tight fit--but not so tight that I can't loosen the bolts in the flanges and slide the entire mirror cell back and forth.
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I took off only as much of the base plate as I needed to barely fit into the tube. For that reason, the tube, which was delivered somewhat out of round, is now close enough to round for my purposes.
Unfortunately, I can't use the same trick for the upper tube assembly. The spider legs were designed to be light, and to hold the diagonal mirror holder in position under tension. But because the upper tube assembly is so far out of round (even more so than the lower tube assembly), I don't have any easy way to get the diagonal mirror holder exactly centered using the tensioned legs approach.
I am thinking of replacing the .050" carbon steel legs with something a bit thicker and stiffer--perhaps .100" carbon steel. These might be stiff enough to actually force the tube walls closer to circular. In this case, they aren't held in tension, but add rigidity to the upper tube assembly.
I wish that there was some way to force the upper tube to be round, but the aluminum is just stiff enough that while I can bend it to round, it won't stay there once I remove the clamp. I can't imagine a technique for applying that force that won't obstruct the diameter. I thought about applying an epoxy coat to the outside of the tube, while I have it clamped to round--but once the clamp comes off, I expect the tube's desire to return to its natural state will crack the epoxy.
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