Here it is, printed, with just the dog screw installed:
Now with the fixing screws installed:
Installed on the Z torsion rod:
My son helped me tighten it as evenly as possible, line everything up, and then crank the bed up and down.
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The bed moves smoothly up and down, even if the corners are out of tram by over a centimeter.
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Trammed as well as we could, the corners were out of tram by over a centimeter. This means that getting the tension even on all four ends in each bundle wasn’t feasible with this design, and I don’t have ideas for how to make it secure and adjustable that make sense to me.
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The cord wrapped around the tension rod nicely and tracked along the rod in a spiral without wrapping over itself. So if we had been able to do fine adjustments, it would have worked nicely.
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The suspension blocks were great. The line stayed put. They are clearly very strong, but also very easy to put the cord onto and the cord stays put. I really like this design.
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This last version of the tension rod cord fixture didn’t slip on the rod. Might finally have that solved, just in time to abandon the project.
Oh well, this was a fun idea to play with!
I’ll switch to trapezoidal rod, but I haven’t worked out the belt path(s). I might try making my own closed-loop belt. A thread on reprap.org recommends Black Witch neoprene glue. I have some IC-2000 rubberized CA that is also recommended for joining belts. I don’t have any experience forming a scarf joint in timing belts but why should I let that stop me?
Also, while I use KFL08 pillow blocks on my 3D printer where the thrust loading is low (the bed doesn’t weigh very much) I hesitate to use them here where the loading will be much higher. My experience with their KP08 cousins doesn’t make me expect some random KFL08 to be high quality anyway. So now I’m pondering making something with needle or taper bearings for support, and 608 bearings to just hold them in place.