Breakfast is served!
Key features of the plate include the snap-fit inner ring and the smoothed outer curvature of the plate edge. We consider both of these features a success because the snap-fit works perfectly with the waffle design, and the outer plate edge is smooth and comfortable to the hand.
Key features of the waffle include the snap-fit outer ring and the syrup storage cells. The snap-fit worked really well, and the part resembled a waffle. Opportunities for improvement include the color of the waffle (there was considerable variation) and warpage due to shrinkage on the sides of the waffle.
Key features of the butter include rounded edges to resemble real butter, and being small and a cute touch to the assembly. The thermoformed butter ended up with nice round edges and snapped well with the waffle and plate. They made the waffle look more adorable as well!
Key features of the assembled yo-yo include the two halves, string, axle spacer, and the shoulder bolts. We consider the final assembled yo-yo a success, particularly for beginners (like us). The waffle yo-yo is easy to use and spins well. Opportunities for improvement include adjusting the string gap and assembly design so that the yo-yo can perform sleeping and other tricks.
Design Specs Vs. Measured Specs
|Item||Design Value [in]||Measured Value [in]||Explanation of Discrepancies|
|Max Diameter of Yoyo (plate)||2.630||2.630||Within spec|
|OD of waffle||2.200||2.203||Within spec|
|OD of snap fit (ID of waffle)||1.950||1.951||The outer diameter of the snap fit averaged to be 1.951”, which is slightly larger than the design value, but still within specification for the part.|
|ID of snap fit (plate extrusion)||1.960||1.961||The inner diameter of the snap fit was actually out of the specifications that we had originally designed. However, the plate mold was not re-machined to fix this discrepancy because the snap fit with the waffle fit extremely well. Thus it was determined that the discrepancy was negligible.|
|Height of waffle||0.400||0.402||Within spec|
|Height of plate extrusion (nut)||0.200||0.167||The height of the plate extrusion was not within spec, mostly due to the shrinkage of the plastic after injection molding. This dimension was not critical to the function of the yo-yo, and the slightly out of spec final dimension is acceptable.|
|Height of rib from plate bottom||0.300||0.299||Within spec|
|Thickness of plate (inner)||0.090||0.092||Within spec|
|Thickness of plate (middle ring)||0.130||0.132||Within spec|
|Thickness of plate (edge)||0.080||0.084||Within spec|
|String gap||0.100||0.09||The average measured string gap was slightly smaller than the designed value, and this was due to a decision to tighten the assembled yo-yo’s shafts slightly more than expected, in order to create a more snug/secure assembly.|
|Total width of yoyo half||0.53||0.544||The discrepancy between the design value and the measured value for the total width of the yoyo half is likely due to slight deviations in the thicknesses of each component of the yoyo; although each of the components are within spec, together, the cumulative variations cause the total width to be out of spec. This was not seen as a problem.|
|Total mass||0.13 lbs||0.13 lbs||Within spec|
|Moment of Inertia||0.08 lbs*in^2||0.08 lbs*in^2||Within spec|
|Estimated Maximum Rotation Speed||1798.2 RPM||1798.2 RPM||Within spec|
|Tolerances (all tolerances not listed here are +/- 0.005”)|
|Location||(+) Value [in]||(-) Value [in]|
|OD of snap fit||0.005||0.00|
|ID of snap fit||0.0||0.005|
Findings about Plate from Written Deliverable 4
For the plate, we measured the inner diameter of the snap-fit between the plate and the waffle (which was the inner rib of the plate). It was determined that the average value for the inner diameter was 1.961”, which was actually slightly larger than the specified design value. Overall, the range of measurements for the inner diameter varied from 1.960” to 1.965”, and although these measurements were technically out of the limits set by the design specifications, no change in design or mold was made for the plates (since the plates fit extremely well with the waffle snap-fit). It was thus determined that the originally tight tolerances set on the snap-fit were not as necessary, and the range of inner snap-fit diameters achieved were acceptable.
Although a disturbance in the production system (an increase in the injection pressure) was introduced during the manufacturing of parts numbered 50-60, there was no detectable difference in the measurement of the inner snap-fit diameter. This led us to the conclusion that the designed part was decently robust and could withstand parameter changes in the manufacturing setup.
Summary of Cost Analysis for Prototyping Versus Manufacturing
For 100 Parts:
|10-24 Hex Nuts||$0.01||100||$1.00|
|Machining and Programming Time||$25.00||60||$1500.00|
|Injection Molding Run Time||$15.00||5||$75.00|
|Thermoforming Run TIme||$15.00||2.5||$37.50|
|Instruction and Overhead||$35.00||40||$1400.00|
For 100,000 Parts:
|10-24 Hex Nuts||$0.01||100000||$1000.00|
|Machining and Programming Time||$25.00||60||$1500.00|
|Injection Molding Run Time||$15.00||5000||$75000.00|
|Thermoforming Run TIme||$15.00||2500||$37500.00|
|Instruction and Overhead||$35.00||40000||$1400000.00|
Changes for Mass Production
For mass production, the waffle core and cavity would need to be adjusted. Currently, there is a lot of warpage due to shrinkage on the sides of the waffle due to uneven thickness. The core would need to be redesigned so that the entire waffle has even thickness. We also had an issue where the waffle would sometimes get stuck to the cavity mold. Currently, the issue was resolved by creating a burr on the core mold. This would not be an ideal solution in mass production. Instead, the molds could be designed with larger draft angles or the ejector pins could eject the waffle from the cavity. The plate and butter production required a significant amount of human/manual labor. For the butter, a lot of labor was put into cutting the thermoforming material, placing it into the machine, and cutting out the final part. For mass production, the entire process could be automated to save time and human capital.
Team’s Constructive Recommendations
We all agreed that we found the most useful and interesting aspect of 2.008 to be the labs and applying what we were learning during lecture to design and manufacture 50 yo-yos. It was well-planned that the machining, injection molding, and thermoforming lectures were towards the beginning of the semester so we could directly apply what we were learning to designing the molds and adjusting injection molding and thermoforming parameters. In contrast, the general organization of 2.008’s pop-quizzes, problem sets, and quizzes has a lot of room for improvement. The pop-quizzes were often on small details in the reading. If the pop-quizzes were actually meant to just check attendance and reading completion, the questions could have been better tailored so that anyone who did the reading could answer the questions, not just those with extraordinary memory. The problem sets were sometimes frustrating in that some of the material was not covered during lecture. Particularly for micromachining and thin films, the lecture went through the process very quickly for a subject that not many students have had prior exposure to. This topic definitely could have been addressed more in depth. Additionally, in the future, the materials needed for studying for the quizzes should be posted well in advance, not a couple hours before the exam (solutions to problem sets covered in Quiz II). Finally, the quizzes given would seriously benefit from some proofreading by either the professor or the TAs — we were consistently given quizzes with multiple errors, typos, and confusingly worded questions that the TAs would sporadically alert us to during the actual taking of the quiz. We think that this manner of writing and administering quizzes is extremely unprofessional, and we hope that the administration of the class spends some time to improve this aspect of the class for the future.