2.00YoYo: Yoyos yoyos and more yoyo!

Hi Everyone!

For the past few days, our yoyo team has been eating, sleeping, and breathing yoyos! As the semester is wrapping up, so is the yoyo project. Today, we just finished assembling all our yoyos and they look fabulous. We assembled 56 yoyos with one special limited Clare edition.

Assembled yoyos

Clare edition yoyo

Our yoyo is composed of
1. The (injection molded) body structure holding
2. The (laser cut) compass face on the needle pole
3. And the black (laser cut) needle.
4. The (injection molded) cap keeps the needle on the pole. There is enough spacing between the cap and the needle to allow the needle to spin!
5. The compass face is held down by a (thermoformed) clear dome
7. And snap ring.
The colors were chosen accordingly to follow the steam-punk theme. When assembling the yoyo and keeping aesthetics in mind, the compass face north arrow were all chosen to align with the weld line. For functionality, the star design on the inner part of the yoyo body allows the string to wrap easily around the yoyo axle and do tricks.

Yoyo body with star design

While assembling, we faced some expected as well as unforeseen difficulties.
The expected difficulties come form the poorly manufactured parts (process change) to simulate malfunction in the manufacturing process. For example, some of the yoyo bodies were almost too small to hold the compass faces. This was due to the shortening of cooling time for the 10 defective yoyo bodies. In addition, some of the thermoformed dorms did not allow the snap ring to fit flush with the body. Due to a decrease in heating time for 10 of the thermoformed domes, the plastics' draw angles were too shallow for the snap ring to fit properly.

The unexpected manufacturing difficulty came in the laser cut compass face. We did not anticipate the acrylic to warp as a result of being overheated. This increased assembly time tremendously. Each compass face had to be painstakingly separated from the acrylic sheet. While doing this tedious task, some of the compass faces broke. Also, the edges on the non broken face were rough and not aesthetically pleasing. If were were to ever redo this assembly process, we will be cognizant of warping and the fragility of thin (1.8mm) acrylic cut out to fine designs.

Now that we are done with the yoyo adventure we decided to reflect and see how much things have changed from the original design. The major changes in design were very due to one mistake described in earlier blog posts (cutting the yoyo core too deep). Lucky for us, the mistake led to a redesign that eliminated an unneeded part (washer) and extra assembly time. Below is a more detailed assessment of how the final yoyo compares to our original design.

Item	Dimension	Design Value	Design Tolerance	Actual	Reason
Assembly	Max Diameter	2.464"	± 0.003"	2.510"	The shrink rate was not as large as we expected. Therefore, the yoyo body is larger than our design.
	String Gap	.15"	± 0.005	.15"	On Target!
	Weight	.15 lbs	± 0.02 lbs	.13 lbs	On Target

Compass Face	Outer Diameter	2"	± 0.005"	2.005	On Target
	Thickness	.0625"	± 0.005"	.0625"	On Target
	Inner Hole Diameter	.25"	+ 0.005"	0.495"	This was made larger to fit over the redesigned compass needle pole.

Extrusion Pole for Needle	Outer Diameter	0.23"	0.005"	0.490"	The pole was made larger as a result of the core accidentally being machined too deep. Instead of re-machining the yoyo body to fit with our old washer design, we decided to change the compass pole.diameter
	Inner Diameter	0.20"	0.005"	0.23"	The pole was made larger as a result of the core accidentally being machined too deep. Instead of re-machining the yoyo body to fit with our old washer design, we decided to change the compass pole diameter.
	Height	0.22"	0.005"	0.35"	This change was due to cutting the core mold too deep

Thermoform Cap	Cap Outer Diameter	1.76"	+ 0.005"	1.813"	The draw angle is not as sharp as an ideal CAD drawing. Therefore, the actual outer diameter is slightly larger than the CAD designed outer diameter

Yoyo Body	Cavity Diameter	2"	- 0.005"	2.014"	The shrink rate was not as large as we expected. Therefore, the yoyo body is larger than our design.

Snap Ring	Outer Diameter	2.040"	+ 0.005"	2.035"	On Target!
	Inner Diameter	1.845"	+ 0.005""	1.829"	The shrink rate for the snap ring was larger than expected.

Before assembling, we graphed run charts and xbar charts for critical dimensions. These charts and findings are discussed in more detail in lab deliverable 4. Continuing with the yoyo body Odyssey, we decided the inner cavity diameter was the critical feature due to the fact that the snap ring, dome, and compass face all had to fit flush within (we don't want the yoyo to fall apart!). Fortunately, the yoyo body did not show much variation (no process change could be detected) and stayed within in a range of 0.005". The Cp calculated for the yoyo body was 0.4289 indicating some yoyos fell outside our upper limit and lower limit range. If a yoyo was out of USL and LSL range, it was usually below specifications. This makes sense as the process change was a shorting cooling time leading to a shrunken yoyo. However, despite some variation, we still had enough bodies to make at least 50 yoyos.

Our team had fun assembling the 60 yoyos.

We will end the blog post on a happy note! Team boding! (thank you James for providing us with macaroons to get through this tedious task!)
Note: Photographer Clare Zhang not pictured.