So, welcome to the long, exhausting but fruitful journey that was our midterm project. We approached the project by first brainstorming ideas for movements we wanted to depict. We thought about motions we see in daily life, from helicopters and birds flying to people biking or waving. Ultimately, a previous year’s project inspired our eventual design. Their mechanism portrayed a cat sliding back and forth by converting rotational motion to translational, but we felt like there was something missing after watching their demonstration video. Thus, our goal became to incorporate leg motion along with linear movement. As for the animal, a lion seemed like a fitting step up from the cat.
Afterward, we drew a diagram for the system. Our conceptual design involves the same rotational to a translational mechanism to push a wooden box with gear attached. Gears on opposite sides of the box would both attach to an axle, with the entire arrangement resembling a cart or wagon with gears for wheels. These gears are then attached to the legs of the lion, achieving a circular motion movement for the legs. From watching videos of lions walking, we noticed that they moved the legs on the same side of the body together at the same time. Thus, we decided to offset the gears such that the gears on one side are “in phase” and the opposing facing gears are out of phase.
We created a low-fidelity prototype to test individual mechanisms. Our device has three mechanisms excluding the walking motion of the legs. Linear to rotational (movement 24) and (2) rotational to rotational (movement 92) were relatively easy for us to envision and prototype because these were the mechanisms that the previous cat team had used on their project and we made only partial changes to suit our case. Mechanism (3) linear to rotational (movement 124) was what proved to be the most challenging to implement. Initially, we thought of having a system of gears turning each other or some sort of pulley system. However, we quickly realized that the mechanism we were trying to produce was very similar to a car and the axles that join the wheels. Hence having the axles attaching to the leg gears (acting as the wheels) and then having them move along a gear rack made the most sense. With the major design decisions made we confirmed that each mechanism was achievable using cardboard.
To test the walking motion, I first found a lion cutout pattern online and modified it so that it had a rotational joint at the hip and knee. However, when I attempted to move the feet in a circular motion, as would occur with the gear, I realized the joint at the hip prevented a complete rotation. From this, I realized that the hips required a degree of linear motion, replacing the hole for an axle with a slit.
When it came to the high-fidelity prototype, we laser cut a 15 cm diameter gear for the rotational to linear motion and a smaller gear that actuates it. Additionally, four strips of gear tracks were combined into two wide strips and four small gears that interface with the teeth in the gear track were laser cut. Two finger-joint boxes, one to house the entire mechanism and the other as the “cart”, were cut too. Finally, we plasma cut two pieces of metal, one that acts as a rod to achieve the rotational to translational motion and the other placed inside the cart to act as a weight.
After gathering the parts, we post-processed them before assembly. The wood for the lion and the box encasing the mechanisms were sanded and coated with a finishing oil that provided a slightly orange shade. The pieces of the lion representing the nose and the mane were spray-painted black. We removed the slag from the metal pieces with an angle grinder and sanded with 120 grit with an orbital sander before applying a clear coat and sanding with 400 grit after.
When assembling the entire mechanism, ample use of woodglue helped put the pieces together. Glue stuck together the outer box, the various parts of the lion, the washers at the axles for the joints, and the axles themselves. We first put together the lion and then the cart it went on. Next was gluing the gear racks and axles to the floor of the box encasing the mechanism. Finally, the box itself was put together.
Here is a video demonstration of the final device:
The link to the slides for our project can also be found here.
Through the project, we had a few takeaways:
- Some things are easier said than done. We glossed over details, such as how the lion would attach to the cart, that, in hindsight, careful deliberation beforehand could have benefited.
- Testing separate parts is important but integration also cannot be overlooked. Some parts of the mechanism for our lion are slightly unstable, and a low-fidelity prototype of the entire model could have helped us avoid some mistakes like understanding which tolerances were important.
- Attachment points between different components in an assembly are very big potential failure points so a lot of thought needs to be put into how to design them and if possible minimize the number of such points in your assembly.
- Good time management requires more than just planning the most “important” parts. Tasks like post-processing, testing, and troubleshooting were overlooked and took more time than expected.
Signing off from the project: Andrew Sun & Fred Gachoka, good luck to anyone who is inspired by our design and would like to improve on it in future!
Cost Analysis:
Cost of Materials:
Plywood (Hardwood Plywood):
Unit Cost: $25.78 for 4 x 8 ft plywood = $0.81 / square foot
Cost: $0.81 / square foot x 16 square foot used = $12.89
Steel (MetalsDepot® )
Unit Cost: $14.00 for 1×2 ft piece = $7.00 / square foot
Cost: $7.00 / square foot x ¼ square foot used = $1.75
Wood oil (Wood Polish & Conditioner, 16 oz, Orange):
Unit Cost: $14.98 / bottle
Cost: $14.98 / bottle * ½ bottle used = $7.49
Cost of Equipment:
The Ion Maker Space Rental Rate: $99 / month
Cost: $99 / month x ¾ month = $74.25
Cost of Labor:
Wage: $7.25 / h
Cost: $7.25/h/person * 25 hrs * 2 people = $362.5
