This project continues to deliver DUH! moments. The first three joints of the arm--you can think of them as the waist, shoulder, and elbow in human terms--came together pretty quickly and easily. They're large, and there's plenty of room to locate motors, bearings, and such. But the wrist proved to be a whole other matter entirely.
This should have been obvious, because mechanically it's the most complicated part of the arm. There's three rotary axes, and because it's all hanging out on the end of the arm (where you're trading against payload ounce-for-ounce), there's pressure to keep it relatively small and light. Oh, and remember, I'm trying to keep this as cheap as possible, too, so that means no fancy internal gears or other goodies that cost $100 a pop.
The design I ended up with (which you can see mostly finished in one of the pictures) is functional but one that I think will bear a lot of refinement. The motor for one of the axes is hanging off the side, and I suspect the wiring will be a bit of a rat's nest. I haven't even tried working out limit/home switches yet, but think I'll be able to find a place for them. There's also a lot of bearings involved, which add a lot of weight. On the plus side, they're all super-cheap. When the dust settles, I think it's going to eat up a good deal of payload, so I will probably be tempted to go back and see if I can reduce the weight enough to matter. The real solution may be to add more torque at the elbow and shoulder, which have the hardest job.