At Dartmouth, ENGS 76 (Machine Engineering) is a class that provides hands on experience with mechanical design and analysis through a semester long project to design a machine to complete a challenge. When I took the class, the challenge was to create a machine that could assemble a bridge to span a 12 inch “canyon.” The bridge had to be constructed from a fixed set of materials, including plastic or foam blocks for support pylons and aluminum plates for the deck. The bridges were scored based on the amount of material used and the time required to complete the bridge, among other factors. The machines were controlled by human operators, using a RC transmitter. We were supplied with a kit of basic mechanical parts, motors and other electronics. All other parts had to be fabricated using the variety of machines available in the machine shop.

I worked with a team of four other students to design and fabricate our machine. We started with brainstorming several high level approaches and creating mockups using foam-core and other materials. We assessed our designs based on factors such as fabrication difficulty, speed and required operator skill. We ended up choosing a design that I originally proposed. This design involves a track that allows the pylons to be inserted horizontally and automatically fall and turn vertical at the correct location. The deck plates are wider, which allows them to ride on a separate track and just be pushed into place. Once all the pieces are in place, the two rails of the track retract, leaving the bridge self-supported. The pieces are placed onto the track using a small wheeled vehicle that carries an electromagnet on the end of an arm.

We primarily constructed our design from laser cut plywood and 3D-printed plastic, as these materials had sufficient strength and fast turn around times for prototyping. Once we had decided on our basic approach, our team divided up the design between ourselves. I primarily worked on the track geometry and retraction mechanism. I also developed the final iterations of the wheeled vehicle.

At the end of the term, we had a competition between the different groups in the class. Pairs of teams ran their machines at the same time, but they were not directly competing (in fact, there were extra points awarded if both teams were successful). One of our two runs can be seen in the video below:

Our machine ended up earning the second highest score, with one team beating our time by a small margin. Our design was by far the most reliable and produced a stable bridge every time.

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