Class project for ME328 (Design for Strength and Stiffness) at Cal Poly

Overview

The final design project for my Design for Strength and Stiffness class was to design, analyze, manufacture, and test a racing kart. The initial design requirements included a maximum size and load cases to design for deflection. Respectively, the kart had to fit through a door and be ridden at least long enough to test function and deflection results. 

Approaching this project, I first wanted to push the boundaries beyond what has been done before and prove a concept I have been interested in for a long time. Thus, I decided to design and 3D print all the structural joints within a reasonable cost and design life. Other high-level design choices included using PVC pipes for the structural members and a simple friction-held steering system. 

Design & Manufacturing Process

I designed each type of joint specifically with its load cases, manufacturability, and assembly in mind (largely guided by intuition, experience, and a few rapid prototypes). For the material, I ultimately decided to print almost all of the joints, except for the PETG steering joint, from PLA because the project had a short design life and I reasoned any part failure would be more likely from a design oversight rather than material properties. As you can see (bottom right), I designed each joint with curved webbing and fillets to reinforce them against bending, preventing shearing at the corners, which would be a common problem for this application. 

The Solidworks CAD was done by modeling each pipe and joint. Each part was then constrained within the assembly to ensure parts would fit and had realistic tolerances (top right). After, the kart was simply modeled as sketch lines for beam FEA using a Weldments feature within Solidworks. Stress and deflection were both modeled for a 180lbf weight on the seat (middle right) and a 60lbf load on the push bar. 

For assembly, each joint was 3D printed with a custom profile to maximize speed and strength on my printer. Otherwise, PVC pipes were cut, wheels were assembled, and the seat platform was cut and sanded. Assembly was remarkably straightforward as the joints were all tight press fits that required some torque to install but an even greater force to pull back out due to friction. The kart performed well and had a comfortable amount of flex, ultimately meeting the design requirements.  

Conclusions

To further improve the model, I would reprint all of the joints with stronger material and fasten a couple of them with screws or bolts with embedded nuts. I was initially surprised that the racing kart performed so well and was relatively similar to the initial design. As this design was completely original, I had to rely significantly on my intuition for DFM and part strength. Through this project, I further developed my skills within the design process and learned a lot about what goes into making informed design and manufacturing decisions within a formal engineering process. 

Using an API for San Francisco's transit system (MUNI), this piece displays the locations of an inbound/outbound light-rail train on a 3D printed topographic map. The code utilizes real-time data from MUNI's website to update a strip of NeoPixel LEDs, which shine through the layers of the elevation-accurate map of the San Francisco County. Made for both form and function, the map can be hung on the wall, where it can help people know when to leave for work/school. 

Forming these knives required careful cutting and sanding to emphasize the natural aesthetic of the wood in such a small format. Building off my previous woodworking experience, I kept track of each angled piece and worked each step in preparation for the next step. After aligning each segment, I spent hours power-sanding and hand-sanding to create a comfortable, sleek grip. This food-safe finish of tung oil and mineral oil protects the raw wood while emphasizing the natural tone and grain.