Deliverable 4

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.