These are my knees. They are made of wood with a routed edge. I purchased the plans for the lower section of the robot from Bermuda Triangle Engineering. I deviated from the plans and went with the "stepped" look, where the center rib is the widest and they step smaller as they go up/down. I believe this is the way the real robot was.
The plans say to coat them by hand with rubberized auto undercoating. I found it very difficult to spread the coating on the knees. Even after sanding I could not get a smooth surface. I decided to spray the coating directly on the knees and go with the slightly textured natural look. I really like how they turned out as they have more of a rubbery look and feel.
Here are the different parts of the hinge. The main part of the hinge is styrene plastic. The center is cut from a 1 inch PVC rod. The two washers are actually machine shims. In order to get a washer with such a large opening and thickness, I had to order machine "Arbor Shims" from McMaster-Carr.
Here it is mounted to one of my knees. They are only painted with primer at this point and I only made one so far. Call it a prototype. I wanted to see how it would turn out before doing the rest. I only hope the rest come out this good.
The inner, inside edge of the knee has a single bracket. I placed it dead center. Besides provided a solid platform for the legs and everything above, the brackets align the knees evenly so the legs will set down on the threaded rod at each end of the knee.