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Reinventing the Wheel

A tale of adventure and self-discovery at Barobo Inc.

By Brian Broderick

“Brian, I need you to design a new wheel for the Mobot.” Graham, founder and head engineer of Barobo, went on to explain that the new wheel model would have to interface well with both carpeted and smooth surfaces, in addition to being robust enough to survive the frequent drops and bumps that occur in the classroom environment. These were my first real design problems of my first real project at my first real job and the intensity of the moment was palpable. I knew nothing about wheels, even less about designing them, but I also knew great engineering begins with fearless grit. So, with nothing but an internet full of resources and a freshly installed copy of SolidWorks, I quelled my inhibitions and embarked on the journey of a lifetime.

Engineers and laymen alike know the intimidation of beginning a new project. Not knowing where to begin and in a slight panic, I racked my memory for any experience I had with wheels. Inspiration came, surprisingly enough, from my childhood: the LEGO tire, a regular component of my juvenile creations. Both functional and simple, the lego tire has tread sections alternating on either side providing good traction while maintaining a constant radius for smooth rolling. It was as good a start as any, so the question became: how could I redesign the features of this timeless classic to work with the mobot? The most prudent modification seemed to be a rounded rolling surface to maintain good contact even when the Mobot wasn’t fully extended and, after some rough sketches and a design check with Graham, I began to CAD model my first part.

The geometry of the wheel was fairly simple, little more than a flat disk with a few cuts and extrusions. But, as I progressed in the modeling process, I began to feel something primal stir within me. Seeing the growingly complex features appear beneath my mouse cursor, the sensation that Michelangelo must have had while carving David became very clear to me. This was the manifestation of thought in reality, a chasmic leap from the neurons of my mind to the pixels of my computer screen. Unlike Michelangelo though, I was unbound by the the limits of stone and chizzle. All I could conceive was possible and as I delved deeper into the design, the boundary between imagination and reality became as nebulous as a thin fog. In short, I was Ellen Page in Inception, though much less attractive.

Once I had tasted the sweet nectar of design, I knew I could never return to the tranquil docility of my earlier life and design became the fixation of my every waking minute. Fortunately, though, Barobo was an oasis of design possibilities and the open ended wheel project gave me the opportunity to satiate my creative cravings. I began to create models of growing complexity and absurdity, monstrosities better suited to science fiction films that children’s robots. As I felt the fingers of insanity clawing at my mind, Graham descended to my rescue with a revelation that wrenched me from my mania and refocused me on the mission like a starving cheetah on a limping gazelle. The original design derived from the Lego tire was adequate, he thought, but now what was needed was to convert it to a two-half, shell design that could snap together for simple and rapid assembly and save on material costs by allowing dead space in the interior. He went on to informed me, in a tone the betrayed nothing of the immense gravity of his words, that the part I was designing might ultimately be plastic injection molded.

I spent the next few days frantically researching snap connection joints. With our circular geometry, an annular snap joint like those found on over-the-counter tylenol bottles seemed ideal. It would allow for quick, secure, and irreversible assembly and would integrate well into the interior space of the model. Snap connectors, however, generally require overhang. This complicated the project somewhat because plastic molds that can form overhang are typically very expensive. My only chance at avoiding this added expense would be to design my part so that it could be molded using what’s called a “bump-off”. This is where the freshly molded plastic stretches around overhanging features of the mold when being ejected from the mold, allowing an overhang on the part to be formed without expensive extra techniques. After designing a prototype of the part and submitting it to our plastic injection service I found that while one half of the design would, indeed, be compatible with the bump-off process, the other half that required overhang on the exterior rim would be impossible without using the expensive methods.