Monday, May 15, 2017

Thanks for a great semester: Thoughts on 2.008

Overall I thought this class was pretty enjoyable and interesting.  However more office hours for pset and understanding deliverables would be nice, the office hours this semester all conflicted with my schedule. I also think it would have been cool to get experience with other manufacturing tools beside the lath, mill, injection mold and thermometer machine. Like possibly doing some welding, 3D printing or composite laying.

I enjoyed the yoyo project and the class overall. However, I think the psets were a bit too long given all of the deliverables that were due for the yoyo project. I also agree that office hours should be held during different hours (maybe after 5pm). This would probably be beneficial to a lot more students. I thought the notes for the class were sometimes difficult to follow because they did not always explain enough. For example, sometimes equations were given and the variables in these equations were not labeled. I enjoyed experiencing injection molding and therrmoforming, but I would’ve also liked to experience other processes if possible like different types of welding. Lastly, I enjoyed attending the factory tour and found it very useful to see some of the material we had learned in class implemented in a real world setting.

Thank you for an incredible semester!

Introducing...50 completed yo-yos

The completed yo-yo:

Our yo-yo has four individual pieces that come together to create each side of the yo-yo. There is the base, the ring, the cover, and the gears. During the first few attempts at production, we ran into many problems with fit. The base was too large and the ring was too small for a press fit. Additionally, the internal hole for the gears were too large and the pegs were too small for a press fit. However, after a few iterations we came to a final mold pattern that consistently created parts that were able to press fit to create a completed part. At the end, we were successful with the press fit, specifically between the gear and pegs. There is room for improvement with the press fit between the base and ring. While we were able to join them all, it required some effort and a hammer.

Base Ring Cover

50 yo-yos:


Design specification of Critical Dimension
Average of
Critical Dimension
Percentage of Parts
In specification
Large Gear
Press fit worked perfectly every time, though design specification limits might have been too wide
Medium Gear
Press fit worked perfectly every time, though design specification limits might have been too wide
Small Gear
Press fit worked perfectly every time, though design specification limits might have been too wide
While the cover ended up being too large and exceeded the specification limits, it ended up still working in the assembly because the ring still fit around it. We designed the cover such that there was significant clearance between the ring and the cover for the assembly to still fit. Because the only constraint of the cover’s inner diameter was to be bigger than the overall diameter of the gear system, and the cover’s inner diameter was bigger than specified, the cover still fit the gears. Our design specification of mean +/- .01 was probably too strict.
The ring ended up shrinking slightly more than expected causing decent amount of the parts to be below the design specification limits. However the base also shrunk more than expected, in the end none of the rings were actually too small.
The base shrunk much more than expected causing pretty much all the the base parts to be below the the lower specification limit which was the specified mean -0.005. As mentioned before the the ring also was slightly small. In the end the two parts were still able to press fit together abiet with considerable effort. The design specification for this part where also probably too strict considering the parts still assembled even though none of the parts were within spec

Gears - Process capability and run charts

Process Capability
Large Gear
Medium Gear
Small Gear

Overall, the gears holes had similar process capabilities (Large = 1.437, Medium = 1.452, and Small = 1.367). All dimensions were close and fit within the upper and lower control limits.
Team Mike Presents:

Sunday, May 14, 2017

Costs and Analysis

Total cost table:




Total cost of 100 parts




Fix Costs




Marginal Cost per 100




Cost per part




Marginal Cost per part





Costs graph:

Cost analysis for prototyping versus manufacturing
The prototyping cost per unit part is relatively high. For the first 100 yoyos, the cost per part is $33.42. The is due to the following reasons:

  • The hours invested working in CAD for the drawing of the parts and their assembly are labor intensive and expensive
  • High labor costs. The initial stages involve the development of a concept and design each of the components making the yoyo.
  • The molds of each part need to also be drawn with the appropriate dimensions to account for any shrinkage or press fits required.
  •  A large amount of hours is also invested in Mastercam, to be able to fabricate the molds.
  •  Prototyping requires the iteration of molds and the running of several trials to ensure the quality and matching assembly of the yoyo.
  • The cost of labor per hour is one of the highest of the entire process (as seen in the cost chart) This results in a very high cost for the prototyping stage
The manufacturing cost per unit part is relatively low. The marginal cost per unit is only $5.03. In addition, the total cost per unit is only a few cents above this prices, as for a batch size of 100,000 parts, the fixed costs of $2,839.13 becomes almost negligible.
-       The manufacturing costs of large batch sizes are mostly dominated by the material costs, the machines run time (costs of running the injection molding and thermoforming machine) and the overhead costs.
-       The manufacturing costs is relatively low for a profit to be made

Assumptions on the cost estimation:
  •  No prototyping or modification of molds required after the first 100 parts (after finalizing the first batch of yoyos)
  • The costs of the machinery and tools has not been included as part of the fixed cost (since it was not given as a variable in the available spreadsheet)


Our team decided to make three injection-molded parts (the gears, ring, and base) and one thermoformed part (the clear cover) instead of two injection-molded parts and two thermoformed parts because there was only one thermoformed machine available in lab. We knew this would be the bottleneck since this machine is not quite as automatic as the injection molding machines. Thus, we saved time and decreased our cost.

Due to time limitations (fast approaching deadlines) our team had to make decisions quickly. Thus, we focused on one design instead of multiple like it is usually done in an ideation process.

We were limited to certain ejection pin diameter configurations. Therefore, we considered creating an outer ring concentric with our ring with runners to the ring so that we could eject our part properly. Fortunately, we were able to find an ejector pin diameter configuration and there was no need for this extra ring.

Changes for Mass Production


We would add an assist to our thermoformed part for mass production. During our production run for 100 runs, the covers were produced with small deformations on its apex due to air. This was difficult to address, especially due to time constraints. The assist would therefore help eliminate these deformations.


We observed that the ring displayed sink marks characteristic of shrinkage at the middle section of the thickest cross section. We would also consider decreasing the thickness of the ring to minimize the amount of shrinkage.


The use very little material and have a low cooling time which is ideal for mass production. The limitations that we encountered was that the gears, due to shape and mold design, often stuck to the cavity. In mass production this could be a large problem because it would require human intervention. The problem could be mitigated by creating a taper along the gears holes. Another consideration is during assembly. Due to small size and shape, assembling the gears on a peg becomes a problem of specific placement. The taper along the holes could also help with less precision during placement for gear assembly.


We had limited range of ejector pin placement, where in a factory we could generate our own ejector pin framework. This significantly impacted our design, forcing us to re-machine our molds to comply with existing framework. Besides the ejector pin constraints, the design gave no issues in low-scale manufacturing.