Bobcat

Proof of Concept

The Bobcat came about as a fluke. I was gathering supplies for the Men at Work diorama, and came across the GHQ model I'd built for the White River & Northern IV. I'd planned to use it as a static background prop on the diorama, but the more I stared at it, the more I thought it might be fun if it was animated. I was quite divided for a time—it seemed much too small to attempt. But the more I stared at it, the more I thought, what's the worst that could happen?

Well, the worst is I could fail. But I could succeed, which somehow I did. The biggest technical challenge was the size of the pivot points: there's so little material in the main arm assembly that even a #86 drill can split a part in half if the hole is not perfectly dead-on. So, I created more robust pivots using eyelets made from bits of GHQ etched bass industrial ladders, cut as shown below. The pivots were completed using 0.008" phosphor-bronze wire. It took more than a day to make four pivots.

From there the challenge I set for myself was to produce all of the other motions with a single mechanism, in the tradition of old mechanical toys. This was quite a fun challenge that resulted in one of the most complex little mechanisms I've ever devised, which took over two days to develop. Using only a reciprocating crank, the Bobcat backs away from area A while simultaneously raising the bucket; it then rotates about 100 degrees and approaches area B, tilting the still-raised bucket. Finally, it reverses the sequence to start over at area A.

Originally I was just going to rotate the Bobcat in place, which would not be outside the realm of possibility, since the machine is capable of zero-radius turns—that is, it can turn in place. But I felt it would look a bit hokey, so I created a double-jointed mechanism that moves the Bobcat in a complex pattern big enough to look plausible but compact enough to avoid having a large, visible slot in the base. From there it was a matter of assembling a bunch of levers and cams to move the bucket up and down and tilt it.

The bucket is raised and lowered with a piece of fine wire, much like a rod puppet, while it's tilted with monofilament thread, like the excavator. However, the mechanism had some issues: the biggest one was relying on gravity for the bucket movements. Given the amount of friction present in such tiny devices, it's an unreliable method. So, I treated it as a proof-of-concept exercise: it showed what was possible; now I needed to start over and refine the design.

The New and Improved Bobcat

The prior iteration was an excellent exercise that gave rise to a new Bobcat, one with greatly improved mechanical design, function and cosmetics. I took my time with this one, spending a week developing the mechanism one tiny part at a time.

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