Summary and Reflection

 The bulk of our material cost is in the ball bearings embedded into the body. The ball bearings cost $2/ea, making up 66% of our yo-yo's $3/ea material cost. Adding in energy and labor costs, and based on our assumptions, our total variable cost per yo-yo is $4.83, which is pretty inexpensive compared to other yo-yos from our class.

Of course the real cost driver in a manufacturing run is amortizing the fixed costs of capital. Injection molding machines, thermoforming machines, and CNC lathes + mills are very expensive pieces of machinery, and we estimate approximately $700,000 could be spend on those machines.

This indicates that were we to enter into production, we'd likely be better off working with a contract manufacturer on their line rather than investing in our own machines, until we get to around 100,000 parts. At that point, we're dropping below $10/yo-yo, and starting to level off, so some consideration could be made about what advantages having our own factory would bring.

A log-log plot of projected cost per yo-yo vs number of yo-yo's manufactured. Fixed costs amortization causes the cost per yo-yo to asymptotically approach the variable cost.

When designing our yo-yo, we did so with the tool capabilities of 2.008 in mind, so we didn't have to compromise much on our designs. Had we been able to accommodate a 3 or more part mold, we could have incorporated some different snap joint designs, but our existing design worked well enough. Embedding more components would improve assembly time, and could be possible with more complex mold tooling. In particular, we could embed a small metal post into the spinner to replace the plastic nub, which would make pressing into the bearing less delicate and smoother. The bearing itself could be insert-molded into the body at the time of injection, which would reduce secondary assembly time. Locating features like pegs and holes could be molded into parts to improve the assembly process. Steel and/or actively cooled molds would improve our cycle time.

2.008 was a tremendously valuable experience for our team diving into the manufacturing mindset, and that mentality will help us be better designers upstream of manufacturing. Simply knowing what different kinds of processes exist and what sorts of parts they're good for is very valuable. Likewise, being able to do a QC analysis on samples to adjust tolerances and/or control limits is critical to having a smooth production run. 

The most value is gained from experience, and if I had to focus on one aspect I think could be improved, it would be the earliest labs introducing how to use MasterCAM. One alternative to consider would be having some tutorial videos as homework before lab, and having lab be the time to generate and verify toolpaths for the paperweights. More experience using the tool to build our own parts could be helpful.

Final Production and Assembly

The Yo! Yo-Yo is Finally a Reality!

The 4 parts of the Yo-Yo manufactured in-house: (Left to Right: Body, Spinner, Window, Ring)

The Body

The body, injection-molded with red polypropylene, is the largest part of the Yo-Yo and has 3 key interfacing features. 

The backside of the body has a small cylindrical recess in it to hold a metal spacer. At the base of the recess, a nut is embedded into the center of the body. This embedded nut allows the two halves of the Yo-Yo to be joined with a set-screw.

On the frontside of the Yo-Yo, a precisely dimensioned outside diameter interfaces with the Ring. An interference of a few thousandths of an inch allows the two parts to mate together and stay together after assembly without any fasteners.

Near the center of the frontside of the body, another small cylindrical recess is precisely dimensioned to fit a 6mm bearing. In final assembly, the small 6mm bearing is press-fit into the body, joining the spinner and the body together.

The body shape worked well and had a relatively quick production time. Even though the injection-molding machine had to be run in semi-automatic mode to insert-mold the nut, the overall cycle time was only 24 seconds. Note the ejector-pin marks on the frontside of the body. These marks are hidden by the spinner.

Though the bearing press-fit easily into the body, the bottom of the bearing recess occasionally interfered with the inner racer of the ball-bearing, increasing friction between the body and the spinner. A secondary recess, or a curved floor of the recess would eliminate this interference. The body also did not shrink as much as we expected it to. This caused difficulties in assembly, because it took a lot of effort to press the ring onto the body.

The Spinner

The spinner has only one key interfacing feature. On the backside of the spinner, a 3mm peg protrudes from the spinner. In final assembly, a ball-bearing is press-fit onto the spinner. When the bearing is press-fit into the body, the spinner and body are constrained together, while still allowing the spinner to rotate independent from the body.

The spinner is originally injection-molded with black plastic. Afterwards, white-out was used to paint the top surface of the spinner and highlight the letters. The backside of the spinner is asymmetrical, causing one side to weigh more than the other. This causes the spinner to naturally orient itself so that the letters are upright.

The 3mm peg on the backside of the spinner did not shrink as much as we expected. During assembly, we had trouble pressing the bearing onto the peg, but with a little help from a press, we were able to do it. Future design iterations should be used to improve this interface. Also, increasing the weight on one half of the backside of the Yo-Yo would improve performance of the spinner so that the Yo! was always upright.

The Window

The Window is thermoformed out of 0.030" High Impact Polystyrene. The window is slightly oversized so that it snaps into the ring and does not move around during assembly. This surface also has a slight draft angle to ease mold removal.

A small lip around the edge of the window is sandwiched between the body and ring after final assembly, keeping everything in place.

The window dimensions were very consistent and worked well in assembly. The window was a little bit cloudy after thermoforming and scratched very easily. It also had a few pock marks on a few of the parts. Though these defects are only cosmetic and do not inhibit the performance of the Yo-Yo, the window could really be cleared up in future iterations.

The Ring

The Ring, injection-molded out of red polypropylene, has 3 important interfacing features.

The window snaps into the inner-most diameter of the ring. The ring has another very important inner diameter, where the ring has an interference fit of a few thousandths of an inch with the body. These two features are important for the assembly of the Yo-Yo. However, the ring has an even more important interfacing feature.

The most important interfacing feature of the Yo-Yo is the outside surface of the ring. This is how the user interfaces with the Yo-Yo and a lot of thought was put into ensuring this surface felt good as the Yo-Yo was used.

The Ring fit on the body and kept the entire Yo-Yo together very well. During assembly, it took more effort to press the ring onto the body than we wanted. Therefore, future design iterations could enlarge that dimension, decreasing assembly time. Though we were able to cut off the runners easily, it would be a great improvement if the runners could be placed on a surface that was not the outer surface where the user touches the Yo-Yo.

	Target	Tolerance	Actual Measurement	Explanation
Complete Assembly
Diameter	2.514"	+/- 0.005"	2.513"	We optimized the shrinkage of the Ring so that this dimension was consistent.
Total Width	1.195"	+/- 0.005"	1.225"	The string gap was about 0.020" larger than the design specifications, which affected the overall width.
String gap	0.075"	+/- 0.025"	0.093"	It appears that shavings of plastic became lodged between the nut and the spacer, probably because the spacer recess was slightly smaller than the spacer OD.
Part 1: Body
Interference Fit Diameter	2.220"	+ 0.000"/-0.005"	2.217"	The interference is slightly larger than designed, because the body did not shrink as much as we expected.
Maximum Thickness	0.350"	+/- .005"	0.351"	This dimension is small compared to the diameter, and wasn't affected by the poor shrinkage.
Part 2: Spinner
Peg Diameter	0.118"	+ 0.000"/-0.005"	0.124"	This dimension did not shrink as much as we expected it to, because it is such a small part.
Peg Depth	0.118"	+/- .005"	0.112"	It appears that air got trapped at the bottom of the peg hole on the mold, because the peg had a concave shape every time.
Outside Diameter	1.860"	+/- .01"	1.835"	This dimension was not extremely critical, and it was ok that is shrank more than expected.
Part 3: Window
Outer diameter	2.030"	+ 0.000"/-0.005"	2.028"	We optimized the thermoforming parameters so that these dimensions were precise.
Inner diameter	1.970"	+ 0.000"/-0.005"	1.968"	We optimized the thermoforming parameters so that these dimensions were precise.
Part 4: Ring
Interference Fit Diameter	2.200"	+ 0.005"/-0.000"	2.218"	When we designed the ring mold, we accounted for shrinkage towards the center upon cooling. However, the ring appears to have shrunk outwards on the ID.
Window Inner Diameter	2.029"	+ 0.005"/-0.000"	2.030"	This dimension has a significant draft angle, so the shrinkage had less of an effect on the inner diameter.

Earlier we discussed the optimization of the Spinner part. This turned out to be the part with the most difficulty in manufacturing, because of the great variation in feature. The most critical dimension was the peg diameter. The average peg diameter was 0.124", which was 0.006" larger than the target dimension. Even though the peg diameter was off by 0.006", there was very low variability in the process. The injection molding process had a C_p value of 6.41 relative to the tolerance range of 0.005".

During our production run, we purposefully introduced a small parameter change. However, it had very little effect on the peg diameter.

For more information on the variation in critical dimensions during our production run of 50 Yo-Yo's, view our statistics report.

Overall, the assembly of the YO! Yo-Yo went very well and the finished product feels nice in the hand, making it a fun toy to play with. We learned a lot about process control and how to make well-fitting interference-fitted parts.