After numerous hours injection molding parts, we met as a team to measure critical dimensions and trim the sprue from each part.
The windows were trimmed to size and stacked in order of production. After optimizing cooling time and other thermoforming parameters, the windows were clear and free of bubbles.
The bow tie production run had some hiccups, mostly due to the parts not releasing from the core mold. Sometimes, the ejector pins would dent or completely puncture the parts. A spray of mold release was all it took to mitigate the problem for 30 or 40 shots.
The press fit was slightly tighter than we had anticipated (~.003" on the radius). Fortunately, it was not so severe as to deform or noticeably diminish the performance of the ball bearings. However, it did require that we use an arbor press to fit the bearings.
Body production went smoothly. We recut the cavity mold for the bodies after noticing that our first run had significant dishing. The dishing caused our string gap to be larger at the center of the yo-yo than at the radius. This made the yo-yo difficult to wind. The core mold modification fixed the problem noticeably.
A test shows that the bow tie does indeed spin:
Below is a list of dimensions on which we gathered data. A range of data was gathered and averaged for the critical dimensions, while other dimensions are taken from a single part. Critical dimensions appear highlighted in the table.
Although most of our parts were not within the initially specified tolerances, most of the errors were mean shifts with relatively small standard deviation. Ultimately, we modified the offending parts until our yo-yo became functional. Noncritical dimensions were treated as noncritical. As a result, they were allowed to vary as we modified more critical dimensions to fit the specifications.
Table of Specifications | ||||
Specification | Value (with units) | Measuring Method | Measured Value | Explanation |
Yo-Yo Diameter | 2.500 ± 0.005 inches | Digital Caliper | 2.514 inches | overestimating shrinkage during mold design |
Yo-Yo Outer-Gap Diameter | 2.368 +0.000 | Digital Caliper | ||
- 0.005 inches | ||||
Yo-Yo Inner-Gap Draft Angle | 2 ± 2 degrees | Digital Caliper, tangent of two sides | ||
String Gap | 0.075 ± 0.005 inches | Digital Caliper | ||
Yo-Yo Width | 1.700 ± 0.005 inches | Digital Caliper | 1.604 inches | poor estimation |
Yo-Yo Cavity Diameter | 2.092 ± 0.005 inches | Digital Caliper | 2.113 inches | overestimating shrinkage during mold design |
Bow Tie Max Length | 2.046 ± 0.005 inches | Digital Caliper | 2.053 inches | close to spec |
Bow Tie Inner Diameter | 0.315 +0.000 | Digital Caliper | 0.309 inches | close to spec |
- 0.005 inches | ||||
Inner Shaft (for ball bearing) Width | 0.197 +0.005 | Digital Caliper | 0.202 inches | within spec |
- 0.000 inches | ||||
ID of Ball Bearing | 0.197 +0.000 | Digital Caliper | 0.195 inches | within spec |
- 0.005 inches | ||||
OD of Ball Bearing | 0.315 +0.005 | Digital Caliper | 0.313 inches | close to spec |
- 0.000 inches | ||||
Yo-Yo Wall Thickness | 0.1875 ± 0.005 inches | Digital Caliper | 0.185 inches | close to spec |
Retaining Ring Outer Diameter | 2.378 +0.005 | Digital Caliper | 2.385 inches | close to spec - extra tight fit |
- 0.000 inches | ||||
Retaining Ring Inner Diameter | 2.2185+0.000 | Digital Caliper | 2.135 inches | noncritical dimension varied as a result of adjusting linked critical dimension |
- 0.005 inches | ||||
Window Extrusion Outer Diameter | 2.185+0.005 | Digital Caliper | 2.155 inches | thermoforming inaccuracy |
- 0.000 inches | ||||
Mass of Yo-Yo | 0.166 pounds | Scale | 0.171 pounds | |
Volume of Yo-Yo | 4.10 inches cubed | Mass/Density | ||
Max RPM of Yo-Yo | 142.29 RPM | Tachometer sensor | 2700 | invalid assumption |
Inertia in X and Y direction | 0.0952 pounds*inch2 | Calculations | ||
Inertia in Z direction | 0.1345 pounds*inch2 | Calculations |
Our report can be found here.
The bowtie showed a very controlled run. The critical dimension is linear in time with the noise making up one or two thousandths of variation. Our shift in cooling time from 30 seconds to 20 seconds is not visible in this data.
A histogram of critical measurements shows a well behaved distribution. The specification for this measurement was 0.315" +0.000" -0.005". Here, the mean landed slightly below the minimum accepted dimension of 0.310" that we had decided on in the beginning. The bearing interference averaged at 0.013" rather than the specified 0.010". This proved to be benign as the bearing did not experience too great of a compressive force and still spun freely.
The C_pk calculated for this process was .4978.
Although that qualifies as "wildly out of control," it is mostly a reflection of the mean shift of our data.
Calculating C_p without considering the mean shift yields a value of .9436 - not great by industry standards but reasonable by 2.008 standards.
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