Final Product: The Call Me Maybe YoYo

Team Call Me Maybe:

YoYo Overview:

Team Call Me Maybe was inspired by old fashion rotary phones for the design of the Yo-Yo.  Our YoYo is made up of two symmetrical halves that are made up of five main components: the base, dial, fingerstop, retaining ring, and number pad. Our YoYo also includes a spring and bearing to help mimic the spring-back action of rotary phones. During the ideation and design of our YoYo we thought that creating a finished product with an interesting mechanical property (ie. the rotating front) would make the design process more difficult and demanding, but the final product much more interesting. 

The major design considerations we took into account when designing our YoYo included: critical press-fit dimensions, YoYo mass and moment of inertia, and the YoYo string gap. We accounted for each of these in the designs for our molds. For example, the core mold for our Base included critical dimensions for the Finger Stop press fit, bearing press fit, and a ledge for steel shims we over-molded into our YoYos. 

Our YoYo was also designed to be assembled in a specific way. Each of our components fits into the YoYo assembly process in a specific order. Each half of the YoYo is made from a Base, Dial, Number Pad, Retaining Ring, Finger Stop, Bearing, Spring, and 10-32 nut. The steel shim and 10-32 were injected into our base when manufacturing parts. The bearing was later press fit into the Dial of our YoYo.

The final design of the YoYo can be seen in the following figures. The first image is an exploded view of of YoYo. This shows a side view of each of the components from our YoYo. Below the initial exploded view you’ll see and isometric view of both a fully assembled and exploded YoYo – we included these to help one better understand how the YoYo fits together. Finally we have a nice product shot of our YoYo in the last figure.

Figure 1.1: Exploded View of YoYo

Figure 1.2: Exploded View Iso

Figure 1.3: Isometric View

Figure 1.4: Fully Assembled YoYo

Comparison to 3D Printed Prototype:

During the early stages of our design work we 3D printed prototypes of all the components of of our YoYo.  The SLA printed prototypes had significantly better surface finished and better overall quality than the FDM printed YoYos.  Although this was the case, both were crucial in verifying our designs and ensuring our critical features and fits worked.

When compared to the injection molded parts, the 3D printed parts have poorer overall surface quality.  However, the 3D printed parts have better dimensional accuracy than the injection molded parts since they are not susceptible to shrinkage.  This resulted in the 3D printed parts having better and more consistent fits than the injection molded parts.
    

Figure 2.1: SLA Printed Prototype Full YoYo

Figure 2.2: SLA Printed Fingerstop Tests

Figure 2.3: FDM Printed Dial

Figure 2.4: FDM Printed Full YoYo

 Comparison of Designed and Measured Specifications:

Figure 3.1: Designed Specifications

Figure 3.2: Measured Specifications

The critical dimension that was the most accurate was the finger stop width. This is because the mold for the finger stop was created first since it had the longest machining run time. The design specification for the finger stop hole width was designed around this. Ideally, there would be an interference fit between the finger stop width and finger stop hole of 0.001” as this allowed for the best fit between the base and finger stop while testing. For this reason, the measured spec of the finger stop hole width would ideally match that of the finger stop width. The most significant deviation between design spec and measured spec was in the dial inner diameter. While this was not
originally planned, the dial inner diameter had to be changed in order to create a proper press fit between the dial and bearing. In reality, the dimensions of the measured specification created a good press fit, which helped keep the yoyo together. The elevation of the thermoform part depression was very good, allowing for a high degree of detail in the thermoform part and does not have to change. Finally, the retaining ring outer diameter was designed to make an important press fit with the inner diameter of the base. However, the OD of the ring was not large enough to cause this fit to occur.

Cost Analysis:

-Prototyping (Additive Manufacturing)

We found additive manufacturing to be invaluable during our design process. However, when looking to use additive manufacturing to mass manufacture our YoYo it is impossible to compare. While the cost of additive manufacturing stays constant, the time required to print each components stays linear, thus making the production of millions of parts impossible. As you can see in the graph below the unit cost for AM never goes down. This is a huge issue when talking about mass manufacturing – the reason you decide to mass manufacture something is to reduce the cost per unit.

Figure 4.1: AM Unit Cost Using ABS Plastic

-Mass Manufacturing

The mass manufacturing style of producing our YoYos is noticeably more expensive in small production volumes, but eventually evens out becoming less expensive than the 2.008 style manufacturing at higher quantities.  This is because things such as tooling quality and machine run-time become important. Again, we had to make assumptions to make this analysis possible. The assumptions we used here were:

• Using high quality aluminum molds lasting > 1 million uses (Class 102)

• Assume 1% scrap on all components

• IM Machines cost $200,000 to purchase

• Thermoforming machine cost $350,000 to purchase

• Same cycle time as our 2.008 cycle time

Figure 5.1: Cost Breakdown

Figure 5.2: Total Cost

Reflection:

Looking back on our design methods and considerations, it is clear we had to take into consideration the 2.008 machines we would be using.  For example, both the Boy and Engel could only fit molds of a given size.  If we wanted to scale our YoYo production to a mass production level, we most likely would use an injection molding machine capable of support larger molds and as a results allowing us to injection mold multiple parts at once.  One of our components, the fingerstop, was already small enough so this could be done.  The fingerstop mold produced 4 fingerstops at once.  In addition to this we would most likely use better quality molds capable of lasting more cycles.  

Final, assembly of the YoYos manually was a long and tiring process.  To make just 50 YoYos took our human assembly line roughly 4 hours, which would not be acceptable at a mass production scale.  Instead an automated assembly line would be much more efficient.  Many steps could be automated with SCARA manipulators, because many assembly steps required only precision placement in the horizontal plane. An optical system that recognized holes and features for alignment could be used to ensure accuracy during placement.  The only step that would be troublesome would be placement of the spring that allows to dial to rotate and spring back.  A new design of the rotating mechanism could most likely be achieved to allow it to be compliant with an automated assembly line.

Figure 6.1: Human Assembly Line Diagram with Potential for Automation