2) Design Process - Onion Cutter


The general design process we followed was: 1)  Define the Problem 2) Ideate 3) Iterate 4) Prototype 5) Test. In the beginning of the project, we made a list of problems we needed to solve. For example, a knee slider (For knee surgery), a bubble blower, or an umbrella stabilizer. After a bit of consideration, we narrowed down the project to the onion cutter. We ultimately decided on the onion cutter for a couple of reasons. First, given the capabilities of our servo (5 V), the torque should be enough to slice through an onion. Second, the cutting motion would be natural to mimic with links and joints, and it would allow for an interesting study of the motion a knife makes for optimal cutting. Finally, the coupling of the knife mechanism with the crank would apply concepts from class (Transmission Ratio, bevel gears, and coupled motion) that would elevate the complexity of our project.

The early drawings of the onion cutter were very different from the final product. We ran into a couple problems early on, the first was the coupled motion. We had to figure out how to transform the knife's perpendicular cutting with the servo to the cranks parallel sliding with the servo. Andy came up with the brilliant idea of using a 90 degree bevel gear. The early drawings had the crank on the side of the cutting board. We found that this took up a lot of room and made clearance very finicky for the knife links and cutting board links. 

There were several key iterations of the project. Often these iterations were prompted by a problem we ran into. Below is a list of issues we ram into, we will go into more detail on the critical thinking behind solving the issues.

  • Coupled motion 
  • Link Clearance
  • Input torque required for dual-mechanisms

Our proudest problem resolution was the coupled motion. Transforming the input clockwise motion of the servo on the knife to perpendicular counterclockwise motion proved to be a challenge. Some of our initial strategies to compress the onion included using gravitational effects, spring loaded slider, or a wound up string. After a bit of consideration we decided to make our mechanism more complex with the use of a 2:1 90 degree bevel gear transmission ratio. This ingenious idea was coined by Andy. The second issue we ran into during our initial prototyping phase was the link clearance, for the knife and corresponding links. The initial sweep of the knife went 360 degrees, but after some consideration we decided to limit it to about 180 degrees (hence the 2:1 bevel gear). This adjustment made sure the links were not crossing whenever the input angle reached its critical point. We also adjusted the CAD to allow for some wiggle room between the rotational links.