10.3 - Design Process

Iteration 1: Slider-Crank Mechanism

As stated in the proposal, our end goal is for the matcha whisk to move in a zigzag pattern across the bowl. The first stage of our design process was to determine what type of mechanism we should incorporate to achieve this shape. We brainstormed a couple different ideas including one with a cam follower that is outlined in the proposal. Ultimately, we decided against that idea because of the complexity of the cam follower system. We went with a system of two slider-cranks that work together to produce two-axis motion. Each converts rotary input into linear motion: one controls the whisk’s left-to-right movement, while the other controls its up-and-down motion within the first slider. One crank rotates at three times the angular speed of the other, allowing for the zig-zag path needed for effective whisking.

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An animation was created using MotionGen to visualize the motion of the dual slider-crank system. This simulation shows both cranks operating simultaneously at the 1:3 frequency ratio. It's important to note that we decided to have the vertical slider moving at three times the speed of its horizontal counterpart, whereas the animation shows the opposite. Regardless, the simulation indicates that our system will output the zigzag motion we desire.

Iteration 2: Ideal Link Lengths and Gear Size

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The next iteration of our prototype involved deciding on ideal link lengths and how we would power our system. Initially we thought about having two motors to power each of the cranks separately. We eventually decided to make it more complex by having a motor power one crank that was connected to the other by a gear train. This allowed us to power the whole system with only one motor and control the 1:3 angular speed ratio through the size of the gears. We calculated maximums and minimums to determine the link lengths, gear dimensions, and number of teeth we would use in the physical prototype. We then used SolidWorks to CAD the system of slider-cranks, the gear train, and the body on which it would rest, and created an assembly to visualize all of those parts together.

Iteration 3: Experimenting with Laser Cut Links and Gears

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Our next step was to laser cut the links and gears to make sure they worked with each other and could press fit onto the shafts and bearings we were provided. This is where we had to take a lot of measurements and reiterate many versions of our links and gears. We were working with ball bearings of many different outer diameters, so we had to account for which bearing we were using for which joint. We also took into account the materials we were working with. It made sense for us to use 3mm plywood for the links, because of the thickness of the bearings, and 6mm plywood for the gears, to maximize the contact between the pinion and the gear.

Iteration 4: Physical Prototype

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After we finalized the links and gears we were going to use in the physical prototype, we began to build. We ran into challenges with cutting the shafts down to the size we needed. Initially we were sawing the stainless-steel shafts by hand which was taking a lot of time and effort. Then we moved to having the machine shop cut them for us which was much more efficient. Overall, the gear train and dual slider-cranks work to generate the motion we need to whisk the matcha. However, there are facets of the design we can improve to make the motion smoother before we add the motor.

Key Takeaways and Design Updates:

• The horizontal slider needs to be a little longer to accommodate the full range of motion of the whisk.

• Need to figure out how to screw the horizontal slider to the linear bearing housing. Right now, the screws are too short to fit a nut on the end. Previously we tried longer screws, but they hit the side of the bowl and interfered with the motion.

• The 3D printed elements should be made out of PETG instead of PLA so they will be stronger.

• The platform that the gear train is attached to needs to be 12 mm taller.

• The shaft that the vertical crank is grounded to needs to be 1 inch upwards in the y-direction.

• The links and gears should be laser cut out of acrylic instead of plywood so that they are smoother.

• The press fit of the gears and the links needs to be more precise so that the motor doesn’t pull anything loose.

• Need to identify exact placement of links, joints, and gears in the z-axis so as to eliminate any motion in that direction and leave only planar motion.

• Overall, ensuring that the end motion is incredibly smooth before we add the motor.

Iteration 5: Final CAD Model

Once we had a clear list of design changes that needed to be made, we went back to the drawing board and updated the CAD model to fix these issues. The horizontal slider was made longer and the top platform was made taller. We ended up reconfiguring the electronics so that the motor sat on top of the mechanism while in the original design it was underneath. This allowed the crank to make a full rotation without the coupler running into any other components and provided more space for us to house the electronics. We also added a cover over the electronics that made our overall design look sleeker while still allowing visibility and access to the motor and battery. Most importantly, we shifted the placement of the gear shafts to allow for both cranks to make a full rotation. This slight change allowed us to observe the full range of motion that we weren’t getting with the prototype.

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Iteration 6: Final Build

After 3D printing and laser cutting all the components, we moved on to assembling the final build. We ended up printing extra spacers to raise the platform even more to accommodate the length of the actual chasen (bamboo whisk). Gear alignment was our biggest issue during this time. Any deviation in gear placement would throw the whole mechanism off balance and we wouldn’t get that smooth motion that we were looking for. It took a lot of minute changes in z-axis placement to get it exactly right. Another issue we had was that we were running a 12V motor with a 9V battery. This caused the battery to drain quickly, and we constantly had to have an extra battery charging and ready to act as a replacement. Finally, we added a big red button to start and stop the motion of our mechanism.

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