Monday, May 9, 2016

Deliverable IV: Final Product

Manufactured Components

Hacker's Map Face 

The key feature of the hacker's map face of the yo-yo is the detailed contouring of the map design.  The edges of the map features are smooth, and the surface of the part shows the milling tool paths of the mold.  The main challenge was obtaining a consistent surface finish, as the thickness of the part varies.  Some dimpling is evident on the part; however, through process optimization, the defects were minimized to preserve the aesthetics of the part.  The part could be improved by removing all visual blemishes and dimples by adjusting the part dimensions and process optimization parameters. For a more streamlined yo-yo design, we should have made the surface of the hack map flush with the outer face of the body shell.

Dome Face

The dome face has several layers which provide a sense of three-dimensionality to the face of the yo-yo.  The complex face design and non-uniform thickness of the part caused problems during injection molding, as poor air flow caused holes to burn at certain locations in the part.  The part was successfully optimized by lengthening the cooling time as well as drilling small air holes in the molds.  The dome face could be further improved by ensuring a consistency in color/transparency in the production run.  More air holes could be added to the molds to eliminate minor surface blemishes by improving air flow in the part.

Body Shell

The body shell has the shape of a right cylinder, giving the yo-yo an aesthetic uniformity.  The inner diameter of the dome shell was critical for a tight press-fit with the two injection molded faces (the hacker's map and dome).  The body shell was optimized to provide a consistent inner diameter dimension.  We also adjusted the length of the shaft used to hold the nut in place during injection molding - the shortening of this shaft resulted in a slightly larger string gap that was more optimal for yo-yoing.  For further improvements on this part, we would redesign the gate location of the mold.  The current gate location resulted in little nubs on the outside of the part that had to be manually removed with clippers.  After removal with clippers, the parts showed small defects.  Moving the gate location to the inner diameter would be ideal to minimize external defects.

Spinning Hack Ring

(Hack ring shown with bearing)

The key feature of the hack ring is the cylindrical pocket in the center of the part, which has the correct inner diameter to press fit with the outer diameter of a small bearing.  Originally, the pocket had a diameter that was too small such that the bearing would not fit into the ring.  Instead of re-machining the thermoform mold, we performed a time-saving "hack" - press-fitting a metal washer on the mold's small cylinder and then manually reducing the washer size to the desired diameter.  The "hacked" mold was a success: the press-fit of the thermoformed hack ring and the bearing started to work.  Another small problem arose when cutting the rings - the punch cutter created small burs on the edge of the ring that prevented it from spinning freely in the yo-yo assembly.  We corrected this problem by filing the burs off the edge of the part.  The hack ring could be improved by perfecting the cutting process to provide a clean edge on the part.  This would reduce the time to produce the part since filing would not be required.

Assembled Yo-yo

The key features of the assembly were the press fits of the two faces on the body shell and the press fit of the spinning hack ring with the dome face.  During our prototyping phase, we realized that our press fits between both faces did not fit - so we adjusted our molds to provide a tight press fit.  The hack ring does not spin freely as we had planned, but it still turns, allowing the user to "hack" their own dome.  The yo-yo's press fits were a success, and the yo-yo does not break apart when it falls from the hands of a user (approximately 4 ft).  To further improve assembly, we could use an adhesive to attach the faces of the yo-yo to its body.  This joining method would provide even more durability if the yo-yo experiences an impact.

Table of Specifications

Critical Dimensions
Target Value
Expected Tolerances
Measured Value
Total Mass
The actual total mass was smaller than the designed mass since we decided not to use shims.
Max Rotational Velocity
106.3 rad/s
+/-.005 rad/s
329.9 rad/s
 Mass of yo-yo was altered
String Gap
Our string gap was smaller than anticipated due to the length of the shaft we used to insert the nut in the injection molded body shell.  After our production run of 100 body shells we tried to create a slightly larger string by shortening the shaft.
Dome Injection Mold Thickness
Shrinkage may have been off
Dome Injection Mold Snapfit OD
 Consistent with tolerance
Hacker’s Map Injection Mold Thickness
Shrinkage may have been off
Hacker’s Map Injection Mold Snapfit OD
Consistent with tolerance
Hacking Ring Injection Mold Thickness
Consistent with tolerance
Shell Dome Injection Mold Thickness (Thinnest)
Consistent with tolerance
Shell Dome Injection Mold Snapfit ID


Increased the diameter of all parts when snapfits did not work.

More about the Dome Face

For this part, we found the average outer diameter (the critical dimension for the snap fit with the body shell) to be 2.401 in.  We chose the size of our rational subgroup to be 20, which placed our control limits at 2.3992 and 2.4034.  This subgroup allowed most of our measured part data to fit within the upper and lower limits, leaving few outliers.  The parameter change we made in our production run was a reduction in cooling time from 10 seconds to 5 seconds.  The change is detectable in the chart below from parts 51 to 60 in the production sequence.  There is a sudden plunge in the outer diameter of the part after the cooling time was decreased (due to increased shrinkage with lower cooling time) around part 51.  The part dimensions return to the desired dimensions when the cooling time is restored to 10 seconds at part 60.  The process capability for the production of this part given the specification range is 0.231. 

Cost Analysis

For this cost analysis, we estimated the cost to produce 100 yo-yos using 2.008 materials and processes. We considered three main cost factors, material, tooling, and overhead costs. Capital costs were neglected since machines were borrowed as was shop space. The main variable costs were in the form of materials and overhead costs which includes machine run times. Tooling we considered a fix cost for the duration of 100 yo-yos since production of the molds was only done once through the process. To determine the cost per volume for a 100,000-part yearlong production run, we assumed the same processes and materials as the prototypes. It can be seen that the cost significantly reduces over the course of 100,000 parts to about $7.88 per unit as compared to $34.09 per unit for the 100 yo-yo prototype run. For a longer course of production, some fluctuation in the unit price will be evident as the machines and molds will require changing, so the unit price will spike around those points. 

For a large scale 100,000-part production run using high-volume production tooling, a separate cost analysis was done. In this, material, tooling, capital, and overhead costs were factored in. It can be seen that the final unit price for the high-volume production run came out to around $7.16 per yo-yo compared to the $7.88 using 2.008 processes. The overhead cost is much lower for the high-volume production run and for a high-volume 2.008 processes and equipment run than for the 100 yo-yo prototype run. This is because the overhead cost is split over many more parts when producing at high volumes. Further, in the high-volume production tooling, it is assumed that workers can work round the clock and that the machines have higher efficiency and reliability.  Both high-volume production types seem cost effective if enough parts are produced when compared to the $34.09-unit price on the prototype run.

Cost Factors
50 yo-yos with 2.008 processes and equipment
100000 yo-yos with high-volume production tooling
Material Unit Cost
Tooling Unit Cost
Capital Unit Cost
Overhead Unit Cost
Total Unit Cost


Within the 2.008 Constraints

Our yo-yo design was adapted to meet the constraints of the 2.008 machining and production equipment.  We were limited to the injection molding and thermoforming machines for the production of our 100 yo-yos.  We decided to make the molds for these manufacturing processes from aluminum, as it was a soft enough material to easily machine on the CNC lathe and CNC mill and was cheaper than other materials like steel.  We chose a yo-yo design with two different faces - the MIT hacker's map on one side and the MIT dome with its hacks on the other - to give the yo-yo an interesting design.  For the ease of manufacturing, we designed the body shell to be the same for both faces of the yo-yo.  We had attempted to use the same core molds for the faces of the yo-yo (the hacker's map and dome), but this was less practical in practice since we decided to make molds for the faces that would be easy to eject from the injection molding machines, choosing to invert the cavity and core sides for the hacker's map.  In the early stages of prototyping, we decided to remove the thermoformed plastic cover on the dome face due to a problematic snap fit with the dome face and body shell.  Our other thermoformed part was a simple circle that could be easily cut to dimension with the 2.008 punch cutter.  In total, we produced 4 unique parts from 3 injection molds and 1 thermoform mold.

The parts were designed for maximum machinable resolution, preserving detail on the hacker's map and dome face but within the limits of the sizes of the tools used to make the molds.  A draft was added to the molds to improve the ejection of the parts.  We used a shrinkage factor between 1.7% and 2% for all the parts based on their unique geometries.

The yo-yo was designed to have two main body pieces attached by a shaft, rather than one piece.  Reducing the part size of injection molded parts reduces cooling time, thereby improving the efficiency of the process.  If the yo-yos were one piece, the chances of defects and warping would increase significantly since the piece would have a non-uniform thickness.  The yo-yo was also designed to be easy to assemble, fitting together with snap fits.  The only labor-intensive aspect of assembly was aligning the circular stickers (decorated with hacks) on the spinning rings.  We did not have the resources to join or bond materials through other methods, which could have increased the durability of the yo-yo assembly.  The yo-yo design was cost-effective: While our yo-yo required a bearing, which increased the cost of the product, the two-faced yo-yo design was made such that only one bearing was required per yo-yo.

Another limitation was the types of materials available in the shop for production.  We designed the ring of hacks to spin freely in the assembly, but the ring was made from a thin, lightweight plastic that did not have enough inertia to spin during the yo-yo's trajectory.  The ring may have also hindered from spinning freely due to clearance issues within the assembly.  We decided that a freely-spinning wheel of hacks was not necessary for our "Hack Yo Dome" yo-yo: Users of the yo-yo can now customize their own yo-yo by turning the wheel with their finger.

We had considered other features for our yo-yo, including light and mirror displays, but decided that these features were not cost or time effective given the limitations of the 2.008 shop.

Ready for Mass Production?

For a larger production run, we would probably want to make our molds out of steel.  The cost of the molds would be less of an object, since the cost-per-part would be reduced in mass production.  Steel molds would be more durable to prevent any dents on the mold that could result in surface defects on the manufactured parts.

For mass production, we would want all of the machines to run in automatic mode.  Therefore, we would like to use a thermoforming machine with automatic capability and possibly multiple molds to improve process efficiency.  We would need to design a mechanism within the injection molding machine to insert the shaft-nut assembly in the body shell of the yo-yo.  We could potentially use human labor to create the shaft-nut assembly and remove the shafts from the injection molded parts or could use machines to perform these tasks.

The design of the parts themselves seem suited for mass production.  We could try to reduce the thickness of the parts - while this may decrease the quality of the yo-yo, it would reduce the cooling time of the process and improve efficiency of the production process.

Constructive Recommendations for 2.008 Class

We found the class to be informative and useful but think that improvements could be made in the course organization to minimize the chaos.  The lab instructors, Dave and Dave, were extremely helpful during lab time to help us machine and fix our molds and parts.  Because there were eleven students in our lab section with plenty of questions, we sometimes felt like there were not enough "Daves" to go around and would recommend the addition of one or two undergraduate or graduate lab assistants.  Lab assistants could be helpful to teach machine usage and process optimization. 

As far as organizational improvements, a larger share of the course workload should be shifted to the front of the semester.  The workload seemed to be concentrated at the end of the semester - and while our team could have made better use of working time for our yo-yo in the middle of the semester, the beginning of the semester was spent on the paperweight assignment.  The paperweight assignment was valuable to learn how the use the CNC mill and lathe, but it was a straight-forward assignment and did not require 3 weeks of lab time to complete.  (Note that we were also in the Tuesday lab section and lost a week or two to holidays, so this may have contributed to our own time crisis in our last two weeks.)  In addition to streamlining the lab schedule to maximize time with the yo-yo project, the 2.008 lecture/problem set/test schedule could be improved to reduce student stress.  During our final production week for the yo-yo project, we also had a problem set due and an exam.  Spreading out the workload over the semester would be preferable. 

With regard to content, we found the discussion of machining, injection molding, and thermoforming processes to be interesting and relevant to the yo-yo project.  All of the lectures on other machining processes - metal casting, additive manufacturing, etc. - were also helpful to understand manufacturing systems as a whole.  We did not have any strong feelings about topics we disliked.  Certain lectures, like the lecture on statistics with respect to manufacturing variation, covered material that some of us had already seen.  We understand that it can be difficult not to repeat information covered in other classes when not all of the students have the same course background.  We also think that the in-class physical demonstrations were valuable to learning and entertaining.

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