For my midterm project, I decided to make a round combustion engine from one of the woodworking books.

I began with a low-fidelty prototype made from cardboard. The goal was simply to understand what the device would look like and visualize how each component would function. I used the Boss laser cutter to cut out the pieces, which I took from the template in the book. I used the saw downstairs to cut all the dowels in this project.

I learned a lot from from this prototype. The most important lesson was that the template in the book was not very accurate. I had to bend the cardboard in order to make the pieces fit together:

I had also forgotten to size the holes to fit the dowels, so I came up with some… creative ways of adapting some parts. I spent a lot of time forcing oversized dowels through the holes.

I also learned that the bearings in particular would need a complete redesign for the next prototype. They feature holes oriented perpendicular to each other (for the vertical dowels, and for the crank shaft). Since the laser cutter can only create two-dimensional parts, I would need more than just the pattern from the template. Similarly, I discovered that I would need a way to make a somewhat-cylindrical piston from laser-cut parts.

For my medium fidelity prototype, I took all these changes into account.

I redesigned every component in my Illustrator file using precise shapes instead of simply tracing the outlines from the book. I changed the design for my bearings so that they consisted of several stacked circular pieces glued to two outriggers, which were in turn attached to the vertical dowels.

Some additional drawbacks: I found out that I needed to make the holes slightly larger than the diameter of the dowel (instead of exactly the same size). I spent a lot of extra time sanding down the dowels and holes, attempting to force the pieces to fit together. I eventually resorted to using a pair of pliers to twist the dowels into and out of the holes, which left ugly marks on the dowels.

I also realized that the top piston channel needed to be longer – every time I tried to operate the mechanism, the top and bottom of the piston would move a small amount horizontally in opposite directions. This motion caused the piston to get stuck inside the shaft as opposed to moving fluidly up and down.

My final prototype is nearly the same as my medium-fidelity prototype with slightly larger holes and a longer piston channel.

To make the piston slide better in its shaft, I decided to modify its design. I cut two pieces with slots in them and fit them together so they formed an “x” shape. I found that the design moves up and down nicely in the longer channel.

I liked the look of some of previous midterm project devices with darkly stained wood, so I decided to apply staining to my project as well. I used light staining for the dowels and dark staining for everything else.

This project turned out to be a lot of work, but I’m definitely satisfied with the result.