I'm trying to make an elbow joint for a robotic arm that is actuated with a stepper motor and two gears. Here's a quick sketch of what I'm trying to do: http://imgur.com/mBlg7 The two bottom plates will be attached to the 'bicep' and the two upper plates will be attached to the 'forearm.' The forearm pivots about the elbow shaft when the stepper motor rotates. In order to work, the elbow shaft has to be fixed to the upper two plates but free to rotate within the bottom plates. Allowing it to rotate within the bottom plates is easy enough...could probably just provide large enough holes in the plates and let them act as bushings. What I can't figure out is how to fix the shaft to the upper plates. Anyone have any ideas? Or maybe a better way to go about this entirely?