Considering I spent countless hours of my child hood constructing remote control aircraft with foam board, this project was a lot of fun. Given some dimensional constraints and 10 pieces of 1/4” foam board, our assignment was to construct a chair weighing under 500 g that could hold the weight of our 80 kg professor. The chair could only be made of foam board with no tape or glue, so only friction fit assemblies were allowed. This project mainly explored the process of light weighting and the techniques that modern structures use to reduce material and cost. Some modern structures used for study included bike frames, bridges, and truss structures. My favorite example from class was the evolution of high performance bike frames over the past few decades, where the professor went over the changes in material and dimensions implemented to bring the weight to a bare minimum. Although a sub 500 g chair resulted in full credit for the assignment, I decided I would push the limits of how light a chair could be and still hold the weight of an adult man. Weighing 352 g, this chair exceeded my expectations for how basic design elements can minimize a structure’s mass while remaining functional.
I knew based on the weigh distribution and spacing of legs that a diagonal main support configuration would work the best. I also knew that a single layer of board would buckle at the outer corners, corner supports were later added in the CAD design. Below is the concept drawing, which ended up being very similar to the final product.
Basic stress analysis was done in Inventor to identify points of concentrated stress. Material was added to the supports near the seat while it was taken out of the back rest and seat to compensate for the added weight.
Design of this chair was simple as it was just a few simple 2D drawings. Fillets and light weighting spaces were implemented to reduce material to an absolute minimum without sacrificing structural integrity. The chair’s component faces were exported as DXF files and laser cut for assembly.
After laser cutting, I nervously assembled this chair in a couple minutes expecting it to fail miserably. Instead I was pleasantly surprised as it held my full weight with only a few sketchy cracking noises.