The goal of this project was to build a rig and animate an inorganic, non-bipedal, character.  The main challenges being constraining the movement of the joints to rotate only on the axes that the mesh would visually be able to rotate on. For instance, an axle joint does not have a great a range of movement as a ball-and-socket joint. I succeeded in constraining the motion of each joint to their logical axes (see rig showcase below).  

The nature of the constrained joints led to the difficulty of "sliding feet". All creatures that walk on legs usually have some rotating mechanism at their ankles that allow their feet to stay put as they shift their center of gravity.  Since this robot's feet were constrained to rotate only on a single axis, I had to be extra careful when animating the torso in a walk cycle, to ensure the feet wouldn't be sliding over the ground mid-step. Luckily this turned out to be a kind of blessing as it forced me to animate the torso to move more realistically, i.e. in ways that would allow the feet to stay planted.  

The second challenge I encountered was making the robot move in a machine-like way.  My initial animation tests mimicked much more closely the walk cycle of a dog which just did not look right for a big hulking machine. So after careful keyframe manipulation I managed to convey a sense of weight and "machine-ness" to the robot. 

I assembled the robot, and the surrounding scene, mostly from photo-scanned assets of a B52 Bomber, a tugboat, and various other hardware "greebles" obtained from Ian Hubert here: https://www.patreon.com/IanHubert/posts