Reflections

See a summary of our cosmic 2.008 journey in our final video HERE!

Cost Analysis

Cost per yoyo (prototyping 100): $28.21

Cost per yoyo (manufacturing 100,000): $1.92

These numbers were calculated according to the Ashby Cost Model, which includes material, equipment, tooling, and overhead costs.  However, we overlooked the cost of student labor in our calculations.  

Materials:

m = part mass (0.086 kg)

Cm = material cost ($3.9/kg)

f = waste fraction (0.2)

Tooling:

Ct = tool cost [$/tool]
nt = tool lifetime [parts]

n = total runs [parts]

Machinery: 

n = production rate (0.2 parts/min)

Cc = equipment cost ($500,000)
L = load factor (0.9)
t = lifetime (10 years)

Overhead:

Coh = overhead costs, energy, labor, etc ($50/hr)

n = production rate (60 parts/hr)

In the cost analysis, we accounted for mold manufacturing, cost of resin pellets for injection molding, polystyrene thermoform material, axle sleeves and hex nuts, design labor, staff labor, and machine production run time.  As the number of parts manufactured increases, the cost per yoyo decays, as seen below: 

Cost per Yoyo [$] vs Number of Yoyos Manufactured

# Yoyos Produced

Mass Production

In adapting our design for mass production, we might make fewer sacrifices in terms of color.  We would have liked more time to experiment with colors for our yoyo parts, especially the ring and body.  For example, when producing the body, we experimented with marbling but eventually stuck with pure black due to time constraints.  With more injection molding machines for mass production, however, we would be able to vary colors of our yoyo parts more easily and be able to offer customers more options for space yoyos without worrying about the crunch for time that comes with having to flush out previous colors from the injector.   

The molds would have to be made out of a more robust material like steel in order to withstand more production runs.  Additionally, we would redesign our molds to produce perhaps hundreds of the same part of the time.  This might involve redesign of the injection molded parts to adapt for the necessary runners between parts.  Our themoforming process would be unsustainable in mass production.  We would need a rollfeed system into the thermoform machine and then an automatic punch.  To further increase cycle time, we might think of using a thinner polystyrene sheet in the thermoforming process.  This would require redesign in the injection molded moon and star disc and the moon and star thermoform part to adjust the affected dimensions.

Course Reflections

Our favorite parts of 2.008 were the interactive components of the lectures and our time working in the lab.  Applying knowledge from lecture material to in-class examples with small teams was a great way to quickly grasp the concepts of the material.  Additionally, the in-class demonstrations put on by Dave and Dave were really interesting and much appreciated.  Having access to and being trained on the neat manufacturing machines in the lab was great.  Hands-on experience, making molds, optimizing production runs, and cranking out yoyos was fun and extremely useful for learning about industrial fabrication processes.  With the relatively large size of the teams, it could be possible to squeeze another type of manufacturing process into the requirements for the yoyo design in addition to injection molding and thermoforming. 

In terms of the class structure, we felt the lectures were often long and over-stuffed.  A structure with Monday/Wednesday/Friday lectures one hour long could be nicer.  Additionally, we covered so many interesting topics within manufacturing that for some parts it would have been nice to choose fewer so as to delve deeper into the subject material.  This could be done more smoothly in accordance with the lab assignments, relating the lectures to the yoyo project more tightly.  The reading quizzes were very good (we actually did the readings!) but not very fun to have every lecture.  And the readings were usually great preparation for the lectures, especially the textbook readings.  The research articles towards the end of the semester seemed too complex and unnecessarily stressful for the scope of the lectures and reading quizzes that followed them.

Turning in blog assignments was a fun addition to the course structure, as was the poster session - a nice wrap-up and celebration.  This has been a great class.  Thank you to Professors Hart and Culpepper and Sanha Kim for the semester.  We are so proud of what we were able to accomplish!  We felt very well-prepared for the yoyo project and appreciate the great lab instruction - thank you Dave and Dave!  

The Final Product!

Individual Parts

The yoyo body is shown below.  The key feature of this part is the inner diameter, as this is crucial for the snap fit with the green ring.  We're very happy with the radius we added to the outside rim.  Reducing the sharpness of this edge makes catching the yoyo much more comfortable.  One opportunity for improvement lies in the coloring of the body.  We would have liked to experiment more with marbling black and silver.  The majority of these parts are pure black, which is still nice because it connects with the black inside moon and star disc. 

The first part to be added to the body in the assembly is this white moon and star thermoform part.  The key features on this part are the outer dimensions of the moon and star since this piece has to fit into the injection molded moon and star disc.  The texture on this part turned out perfectly!  We are especially excited about how the craters on the moon turned out.  Also, the stripes on the comet tail were a later addition and we're really glad we added that detail.  An opportunity for improvement on this part is in the tiny webbing we were still unable to eliminate between the star and comet tail, although this didn't interfere with the fit into the injection molded disc.



Next to be added to the assembly is the injection molded moon and star disc.  The key parameters on this part are the inner dimensions of the moon and star so as to fit nicely over the white thermoformed part above.  These dimensions fit extremely nicely - the thermoformed part snaps beautifully into the part without gaps.  The black and white contrast without gaps really brings out the texture of the moon and star.  One thing to improve on this part is on the outer diameter - the part did not shrink as much as expected and so does not slide into the body as well as hoped, but this tighter fit is not a problem in our assembly.

Then we place a laser-cut acrylic rocket on top of the moon and star layer.  The rocket 'flies' loose around the yoyo, trapped by the window.  A key feature is the thickness of the acrylic since we wanted the rocket not to be able to slide between the window and surface below.  We chose 0.11" acrylic for the job according to the window dimension.  We are really happy with the material colors we chose for this part.  We decided to make the rocket colors asymmetrical, one transparent red rocket and another mirror rocket on the other yoyo half.  A potential point of improvement for these parts is gluing them down.  We will do this with a small fraction of the yoyos, but based on feel, the small change in moment of inertia has no significant effect on the rotation of the yoyo.

 

Next, we place the window into the assembly.  The key dimension here is the window diameter, since it needs to fit inside the ring.  The windows turned out really nicely - they are very clear, without many bubbles, not obstructing the inside view of space.  The optimization for reducing bubbles on the surface of the window was pretty tedious and we ended up blowing air on the plastic before every run in order to clear bubble-causing dust. 

The last piece of the assembly process is the ring.  This piece really holds everything together.  The snap fit between the inner diameter of the body and outer diameter of the ring is crucial.  This parameter turned out well, and the ring has a great fit between the body and window.   We are so happy with the color we were able to get.  The bright green really brings some pop and brightens the yoyo which is otherwise black and white.  We experimented with the color a lot, first trying a marbling with glow-in-the-dark then with black, but never getting satisfactory results.  Also, doing a run on the black moon and star discs prior to this made flushing the black out of the green really difficult resulting in some putrid greens.  You can see the evolution in the second picture below!

 And finally...the assembled product! 

Meeting Design Specifications

Below are histograms which show control limits and the measured critical dimension for each part.

Yo-Yo Body: Inner Diameter target dimension = 2.000in(+0.000in/-0.010in)

Almost all of our parts had a larger diameter than our target range. This was most likely because the effect of shrinkage was not as great as we estimated it to be. However, during our optimization runs the parts fit perfectly with the rings, so we did not make any changes. We did not have any problems with the snapfits during our final assembly. 

Thermoform moon and stars piece: target dimension for distance from opposite edges of moon: 0.092in

Injection molded moon and stars piece: thickness target dimension = 0.108in

We were within our tolerances for this part. There were no critical dimensions other than making sure it fit with the thermoform part. 

Ring: Outer Diameter Target = 2.000in (+0.010in/-0.000in)
Several parts were outside of our tolerance range, possibly due to improper prediction of shrinkage. However, this did not affect our final assembly, as there were no problems getting the rings to snapfit into the yo-yo.

Window: Outer Diameter  = 1.800in
While almost all parts were smaller than the target dimension, this was acceptable because the critical factor was that the window was able to fit inside the ring.

String gap: Target dimension = 0.075in

Our string gaps were much larger than our target. This could be due to placement of the nut inside the body, and how tightly we screwed the shafts in during assembly. We also noticed that body parts that had 2 colors had a smaller string gap.  Our yo-yo works fine despite the deviance from our intended design.

Findings: Injection Molded Moon and Star Part

 

The target value for the thickness of the injection molded moon and star part was 0.108".  The mean value of the 100 parts made was 0.10778".  According to the Shewhart X-bar chart, the majority of our parts were within the upper and lower control limits, with only a few exhibiting more or less shrinkage due to disturbances in cooling times. 

There was less shrinkage in the injection molding process for this part than expected.  Initially, we had accounted for 8% shrinkage with a target value of 0.1" but when it was found that the parts that did not shrink as much fit better into the yoyo assembly, the target value of this part was changed to 0.108" for the production runs.        

In summary, the production run responded as was expected to disturbances in cooling times, resulted in a set of 100 parts which were all usable despite a minority which were a little bit out of specification, and resulted in a good process capability of 0.490 and relatively small standard deviation of 0.003".   Scrolling to the bottom of the link below will lead you to the run chart, histogram, and Shewhart x-bar chart for the injection molded moon and star part:

Paper Deliverable 4