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