The Race Ends!
Our Story
Final Yo-Yo Description
Our final yo-yo is an assembly with four main parts as well as a stand that it rests on. The assembly consists of two body parts that screw into each other and two custom-made parts that snap onto the body parts. Inspired by the theme of the fable The Tortoise and the Hare by Aesop, one of the custom parts is a turtle shell with the head insert-molded, and the other is a hare body with the tail insert-molded. The yo-yo is entirely injection-molded and the stand is thermoformed.
The turtle half of the yo-yo consists of a lime green body part and a caramel-colored shell. The shell was probably one of the hardest parts to make because the intricate pattern of the mold for the shell had to be 3D-machined on the Haas. It took multiple tries for the machining to work, and the machine cycle time for the engraving of the shell was around five to six hours. We cut out felt pieces for the head because our vision was to have something light sticking out of the piece that would exactly mirror the hare’s tail, choosing felt instead of a thicker piece of plastic that would be an extension of the injection-mold so that it wouldn’t interfere with the rotational dynamics of the yo-yo. In order to create the injection-molded shell with the head inserted into it, we added a pocket to the mold that the felt piece would easily slide into. At first, we made the pocket too deep, so that plastic would also fill in the gaps that the felt did not take up, and had to redo the core of the mold to have a shallower depth. The shallower pocket made it difficult at first to attach the felt. However, we figured out a trick of dipping our finger in a bit of water and adding it to the pocket, which is just enough to cause the felt to stay in place instead of falling out. This trick allowed us to successfully run an injection-molded shell with the felt as an insert-mold.
The hare half was made in much the same way. It consists of a white body part with a white hare that would snap fit on top. The detail on the hare included a face and ears, which were colored in with a permanent marker, and included an insert-mold of a felt tail. The core for the hare part was the same as the one we used for the turtle part; we just switched out the cavities. The process for inserting the felt tail was also the same process as inserting the turtle head.
Finally, the stand was made via thermoforming. The stand is a rounded platform that the yo-yo can rest comfortably on, with designs of turtles and hares all around it. Like the fact that our turtle and hare on the yo-yo chase each other around in an infinite race as the user twirls the string, the turtles and hares on the stand also chase each other around in a never-ending race. The stand is spray-painted green so that multiple stands adjacent to each other would resemble a field of grass, and finished off with additional black-and-brown detail.
Our injection-molded parts are slimmer and smaller than our 3D-printed parts. When we 3D-printed the parts, we realized that they were quite large and did not fit easily in the palm of a hand. In fact, they were larger than the permitted size to easily fit on a mold blank. They were also very spherical. These were things that did not seem glaringly obvious in Solidworks, so it really helped to have physical prototypes that we could feel to help us realize that we needed to make some changes. After 3D-printing, we went in and edited the designs to that they were slimmer and smaller, and that came out in the final injection-molded yo-yo.
Design Specifications
Design Specification | Measured Specification | |
Body Part Inner Diameter | 2.000” +- 0.005” | 2.011 +- 0.011 |
Turtle Part Outer Diameter | 2.000” +- 0.005” | 2.020 +- 0.005 |
Hare Part Outer Diameter | 2.000” +- 0.005” | 2.025 +- 0.005 |
Base Outer Diameter | 2.740” +- 0.005” | 2.73 +- 0.01” |
The body, turtle, and hare parts are not within specification because we overestimated how much these parts would shrink. We used a scaling factor of 1.035 when accounting for shrinkage in our mold. This should be appropriately decreased so that our parts our closer to the design specification.
The thermoform part was smaller than the specifications because we did not account for shrinkage during cooling. The uncertainty change for the measured parts came from silicone release spray that was applied every 5 parts causing the parts to have varied ejection stress patterns.
Cost
- Cost model for additive manufacturing
We used the quotations obtained from Xometry and assumed that production will stop after 100 parts. This was under the assumption that the vendor would not ship any parts that exceed the volume of 100 units. We could not get consistent quotations beyond this point and that is the reason for choosing this value. Please refer to cost estimation model am_cost.mat for comprehensive matlab script.
Cost model for lab manufacturing
We used the breakdown of tooling cost, equipment cost, material cost and overhead cost for the model. Here we assumed to be using molds with a single cavity/core, non dedicated equipment, the lab times of team members foo design/machining/production and also manual spray painting and ignored minor cost such as sharpies and paper. Please refer to lab_cost.mat for comprehensive matlab script.
Cost model for mass manufacturing
Here we assumed to be using the same equipment used in the lab but with greater efficiency i.e less time per part and less material costs due to volume of parts used. We also added a industrial scale spray painter to the model since hand spraying is not a viable method of painting at the given volumes. Please refer to mass_cost.mat for comprehensive matlab script
b.
50 yoyo’s using additive manufacturing techniques
Part
|
Cost
|
Stand
|
19.14
|
Body
|
50
|
Hare
|
46.61
|
Turtle
|
48.38
|
Total cost
|
164.13
|
The costs of this technique cannot be broken down into its individual components. The major cost fro this part is split between the Hare part, the Body part and the Turtle part. This is due to its complexity and the volume of the parts.
50 yoyo’s using lab techniques
Cost component
|
Cost
|
Tooling cost
|
13.36
|
Equipment cost
|
0.9004
|
Material cost
|
1.6766
|
Overhead cost
|
0.7
|
Total cost
|
16.63
|
The major contribution for cost comes from the tooling. The production volume is not high enough to justify production of this volume using lab techniques. The tooling cost is quite significant relative to the other components as we are using non dedicated machinery very cheap materials and also shared manufacturing space.
50 yoyo’s using industrial techniques
Cost component
|
Cost
|
Tooling cost
|
0.668
|
Equipment cost
|
0.9303
|
Material cost
|
1.2110
|
Overhead cost
|
0.7
|
Total Cost
|
3.509
|
At this scale the major cost contribution comes from the material used. This is because we are using non dedicated machinery, shared production facilities and also high volume manufacturing that divided tooling cost across a large number of parts.
c.
Units
|
AM cost
|
Lab production cost
|
1
|
188.05
|
697.52
|
2
|
188.05
|
350.13
|
3
|
188.05
|
132.3347
|
4
|
188.05
|
176.43
|
5
|
188.05
|
141.69
|
6
|
188.05
|
118.53
|
7
|
188.05
|
101.99
|
8
|
188.05
|
89.58
|
9
|
188.05
|
79.93
|
The crossover occurs at 4 parts. This is also dependent on the AM vendor used. Here we used Xometry as the vendor. If we used Shapeways as the manufacturer, the crossover point will be at a volume of 13 parts.
Conclusion/Comparison
In order to fit the constraints of the manufacturing equipment in the 2.008 lab, our design was modified.
We made sure that all of the features in our yoyo, specifically the turtle part, were machinable with the given bit sizes. We decided to machine the pockets of the turtle cavity instead of the lines between them to accomodate for the smallest size bit.
If we were changing the design for mass production we would reduce the size of our part so that the cooling times would be shorter and the part would take less time to make. We would probably reduce the complexity of the cavities for both the turtle and hare parts to reduce the machining time. For example, the turtle could have fewer, larger patches and the hare could have the eye feature as paint instead of a physical feature. We would also want to make the yoyo flatter (instead of the more spherical shape that it is) to reduce the machining time and lower the cost of the part. Another change would be to either remove the felt or find a way (perhaps through automation) to insert the felt tail automatically instead of having to place it by hand each time. This would reduce time and cost.
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