The quest for Strandbeest monster robot continues. Since my previous update I have been working on to improve the joints. The previous prototype used use M3 nuts and bolts to hold the pieces together. After some use the bolts would loosen and the whole thing would fall apart.
My goal was to make a design which would require only one screw per joint to fasten the joints. After some doodling on whiteboard and a week of simmering, I finally found a solution that seems to work pretty well and allows nice compact structures.
You can see the design below. I can't wait the when I'm able to share these ideas straight from Tinker.
The basic idea is that the nub is fasted to the bottom most leg part with a screw. The narrow part of the nub is slightly longer than the topmost leg piece, which allows me to fasten the nub tightly to the other leg part. I noticed that 1mm tolerance is good when the pieces are printed with Thing-O-Matic.
The reason for that much extra space with holes is arc shrinkage. HydraRaptor blogs goes into great detail why it actually happens. In a nutshell, the arcs are represented as tiny line segments and the printers smoothes these out a bit. These two things together makes the holes and round shapes a bit hard to reproduce. The height tolerance is explained by the roughness of the printed surface.
<This time around I also designed both legs to see any additional problems it might bubble up. One thing that came up was how to transmit the power to all the legs. Initially I was thinking about some kind of gear, but it looks like it might be tricky to attach the motor. I'm probably going to try to build the legs so that when they are connected to gather the center rotating pieces will become a crankshaft.
Next I will try to see if I can shrink the design a bit. Currently it takes almost the whole day to print all the parts as one side of the leg needs 2 prints. The current version uses 16mm circles around the joints, and I was able to make it work really nicely with just 8mm circles.