I hadn’t really thought the motion platform through yet, but I knew that it was going to have to pitch and yaw, but NOT PIVOT. The best way to do this is probably with a universal joint with a flange welded to both ends. Unfortunately, I don’t have enough time for that, so here’s another solution: Two ball-bearing hinges cut in the middle and welded together. Turns out they still had a little too much ability to twist, so I bolted on some angle iron to prevent that.
This isn’t ideal, since you’re not pitching and yawing on the same plane, you get a little translation in the other axis. But it’s good enough.
Instant U joint!