Names: David Doucet, Harrison Lin, Claire O’Malley, Avinash Shivakumar
For our final project for ENGI 210, we were required to build a gear train toy or appliance. At first, we had too many ideas to narrow down. Eventually, Dr.Wettergreen, seeing our indecision, suggested that we create a spirograph toy. We liked the idea, so we set about to do exactly that.
The Spirograph
First we did some research on the spirograph to understand the theory behind it. The spirograph is a type of toy based on gears, where an outer-toothed gear (with an included hole for a writing utensil) meshes with an inner-toothed gear. The displacement of the hole in the outer-toothed gear from its center governs how much the resulting pattern sketched by the writing utensil will deviate from pure circular behavior. In allowing the writing utensil to continue sketching for the full mesh along the entire length of the inner-toothed gear, a unique pattern will result.
Sketching & Planning
We began our process with a few sketches of what we thought our spirograph should look like, based on designs we had seen online. Mainly, we realized that a planetary gear system would enable us to build a spirograph that met the design specifications for this project. For this, all the planet gears could either all be the same size, which would yield circles and ovals as the drawings, or they could all be different sizes, which would yield circles and hypotrochoidal figures. Both ideas would require holes within the planet gears in order to insert a writing utensil or drawing medium.
Printing & Fabrication, Failure & Reiteration
Looking online, we found some designs for both of our planetary system ideas. First, we started with the different-sized planet gear option. As per Dr. Wettergreen’s preference, so that we might later cast our 3D printed parts as opposed to casting something that was laser cut, we modified some CAD files that we found online and sent them to the 3D printer. Most of the 5 gears we were printing had overhangs, which would have been impossible to mold, so we sliced them horizontally through the middle so that we could make molds (either 1 part or 2 part) based on these simpler shapes. Our parts ended up taking 13 hours to print completely. Once out of the 3D printer, we immediately began the process of casting the gear set in the Oomoo material. We encountered quite a number of hiccups in this process as we soon realized that our 3D printed parts, while significantly thick for their purpose within the planetary gear system, were a little too thin to effectively cast and then replicate from the 300 material. Although our original intent was to make two of each gear from the molds so that we could combine them into a 3-layered mesh, this became less of a possibility if casting was to be done. Because we already had the 3D printed parts, we did not want them to go to waste, so we assembled this idea using the 3D printed gears and attached the gear mechanism to a laser cut, circular acrylic sheet, which we then spray-painted blue for the aesthetic.
Redesigning & Prototyping Project
Meanwhile, we modified some designs that we had found online in order to make a small high-fidelity prototype of the equal-sized planet gear system. We laser cut our designs in plywood and acrylic and used some wood glue and wooden dowels to fasten the prototype together. One issue that we encountered in designing the prototype was that the gears were slightly too large to mesh together properly and easily. Our remedy for the prototype was to sand down the teeth and the inner radii of the gears in order to get them to a size where they would mesh properly. This helped to tell us that for the final product, we would need to cut the gears slightly smaller than we had originally thought.
When we were prepared to make the full sized version of the equal-sized planet gear system, we also decided to cut the entire assembly in acrylic, both for facilitated casting and for a sleeker look. We also designed holes in each of the gears, in case we wanted to use the assembly as a drawing tool, albeit one that draws circles and ovals. We decided to cut our parts at approximately 2 times the sizes of those for the prototype, remembering to size down our planet gears. When we scaled up, the ratio of diametral pitches (number of teeth per inch, diametral pitch = N/d, where N = # of teeth d = diameter of gear) between the gears (which should be the same for the outside and inside gears) was off: the outside gears had a greater diametral pitch than the inner gears. Therefore, we re-cut and re-cast our gears to appropriately match diametral pitches; this took a few iterations of cuts and then a few more of molds/casts to get just right especially considering the tedium involved in the molding and casting process. A video of our prototype is attached here as well.
Finishing Process & Final Details
Our main challenges with finishing were figuring out how to connect the different layers of our device, figuring out what kind of handle to best use for the spirograph, and how to incorporate a self-drawing mechanism that holds the pencil and applies pressure to the tip.
Layering: The first thing with layering was that we cut holes that were the perfect size to hold quarter inch nuts and bolts. We wanted nuts and bolts because we wanted the device to be easily assembled and disassembled. This was because we wanted to adjust how tightly the layers were held together.
Handles: In order to turn the gears, an appropriate amount of torque must be applied. Thus, we fabricated wooden handles that provide the lever arm to apply the torque to our turning mechanism, while maintaining all other aspects of the functionality of the mechanism. Using the drill press, we drilled a ¼” hole radially through a dowel that was thicker than our central dowel that held the sun gear in place. Then, sanding down the sun gear’s dowel, we managed to fit the larger dowel onto the sun gear’s dowel to create an effective handle. We noticed that the devices from last year contained no lever arms or handles to turn the gears, which made it very difficult to turn the planetary gear mechanism.
Self-drawing mechanism: We found it challenging to both hold the pencil and turn the gears at the same time. In order to draw a clean line, pressure must be applied to the tip, and the pencil and handle must not collide as the mechanism is turned. We created a tube holder that kept the pencil in an upright position that, while applying pressure to the tip of the pencil, also help the pencil in place as the mechanism was turned. To create a compressive force, we used small rubber bands that went across the top. This allowed us to create much higher fidelity drawings.
Colors and Finishing: We also sanded and spray-painted the planet gears to make it appear more child-friendly and more like a toy. As a toy, this project could illustrate the workings of a planetary gear system to a young child, and it could illustrate that a planetary gear system can be operated by the rotation of the sun gear, the inner-toothed gear, or a combination of the two. Lastly, we removed the wax paper from the acrylic in order to showcase it sleek, glossy surface. A video demonstration of our final product is included below.
