Lab Report 1Marwa Al Alawi, Ben Gutierrez, Lily Mueller, Jonathan Sampson

2.008 Spring 2019

I. Final Design

Final Yo-yo Design:

Figure 1: Side view and perspective view of full yoyo

Figure 2: Cross-sectional view of yoyo.

Figure 3: Cross-sectional view of yoyo. This view is taken perpendicular to figure 2.

Figure 4: Full yoyo cross-sectional view. Note that the shoulder bolt is longer in picture than the bolt that will be used in the yoyos. The shoulder bolt will be cut to length.

Figure 5: Exploded view of yoyo half

II. Assembly Plan We plan to start our fabrication process by machining the goggle strap mold and the

yellow body mold as these two parts have a more complicated interface (snap fit and two-pin-press-fit) than the interface between the yellow body and blue body. We will make the goggle strap mold first as that allows us to measure the actual shrinkage of the pegs (and compare it to our estimated values) before machining the corresponding press-fit hole features into the yellow body mold. After we successfully interface the goggle strap and yellow body we will machine the blue body mold, fabricate the thermoforming dies for the goggle glasses and white sclera, and laser cut the pupils from black acrylic. We will use the 1.074” thermoforming punch to cut out the goggle glass part and white sclera part from their respective sheets of formed plastic.

Once all parts are fabricated, we will start the assembly process by stacking the eye on top of the yellow body part. We will stack the white sclera, then the pupil, then the thermoformed dome. Then we will capture those pieces by snapping the goggle strap to the upper ledge of the yellow body part. We will first snap the goggle strap to the yellow ledge then we will press fit the pegs on the strap into the matching holes on the yellow body. While the eye and goggle are being assembled we will use the spacer and set screw to attach two blue pieces together, leaving a space of 0.075” between the two blue parts for the string. We will then snap two fully assembled yellow bodies with goggles and eyes to the two blue pieces to complete the yo-yo.

III. Snap Fit Tolerances Our design has two traditional snap fits and one pin press fit. The yellow body piece

snap fits around the lip of the blue body piece. There is a 0.005” radial (0.01” diameter) overlap between the yellow and blue part for this snap fit meaning the outer diameter of the blue lip is 2.35” and the inner diameter of the yellow part is 2.34” so that the yellow will have to snap onto the blue. Additionally the snap fit surfaces are 0.125” high which is the same height as the snap fit surface on the LMP body part. The second snap fit occurs where the gray goggle strap snap fits around the outside of the upper ledge of the yellow body part. There is a 0.002” radial overlap (0.004” diameter) between the goggle strap and the yellow body part. The inner surface of the goggle strap has a diameter of 1.146” and the outside of the yellow ledge has a diameter of 1.15” so that the goggle will snap onto the outside of the yellow ledge. This snap fit is also 0.125” high. Lastly, the goggle strap has two identical pegs on the underside to press fit into holes on the yellow body part in order to securely hold the goggle straps in place. The diameter of the holes on the yellow body part are 4% smaller than the diameter of the pegs on the goggle in order to account for shrinkage of the pins (discussed in Section IV) and also allow the pegs on the goggle to be press-fit into the holes on the yellow body.

IV. Shrinkage Compensation PlanTo account for shrinkage, we found a few parts in the lab classroom that had similar

features and sizes to those in our yoyo design.

Yellow Body:

This orange piece found in the shrinkage kits was very close to our intended diameter of

2.5 inches. The mold had a diameter of 2.509 inches, and these part diameters were, on average, 2.4333 inches in diameter on the outside edge. This translates to a little over 3% shrinkage. Our important dimensions (i.e. those used for snap-fitting and to lay the gray goggle strap piece in) are affected by this radial change in diameter. Therefore, our initial molds will be approximately 3% bigger to ensure our snap fits will still line up.

Blue Base:

The blue base to our yoyo is another important piece in the functionality of our yoyo. This piece is also intended to have a diameter of 2.5 inches. The parts measured had an average outer diameter of 2.192 inches, which is approximately 1.5% smaller than the mold diameter of 2.227 inches. To ensure that this piece’s snap features line up with the yellow body, we’re going to have the mold diameter of our blue piece be around 2.53 inches.

Goggle + Strap (snap feature):

These semi-flower pot parts were of similar outer diameter as we expect our goggle diameter to be. The goggle diameter is currently 1.306 inches, and the parts in the kit had average outer diameter of 1.475 inches, flange included. The mold diameter was 1.497 inches, meaning the shrinkage incurred was around 1.5%. We will scale the mold for this part to be 1.5% larger than the intended size of the part, meaning the mold will be about 1.325 inches in diameter.

Goggle + Strap (pegs):

The pegs from the goggle strap to go into the holes in the yellow body present an interesting shrinkage challenge. We want them to fit snugly with the holes in order to provide some structural support and cohesiveness with the yellow body away from the central circular features. If the pegs shrink too much, then there is no interference between their outer diameter

and the inner hole diameter, and the feature has little actual purpose. The shrinkage between these circular extrusions and the mold ranged from 2% to 4%. We will initially plan for 4% shrinkage, but we plan to adjust this number with a lot of trial and error.

V. Injection Molding DFMWe considered the strengths and limitations of the injection molding process when

designing our yo-yo. We made the injection molded pieces a uniform thickness of 0.08” so that the parts will cool evenly without sinks or warpage. We also chose 0.08” because this thickness gives our parts enough strength and stiffness to work properly while also minimizing the cooling time in the mold. Since cooling time is the rate-limiting step in the injection molding process, it is crucial to design the parts as thin as possible while not sacrificing any necessary mechanical properties in order to increase the production rate thereby decreasing the cost of the manufacturing process. Additionally, we put a 5° draft angle on the surfaces of our parts that will tend to hug the core side of the mold after cooling. This draft angle will help with part ejection and will ensure our parts don’t get stuck in the mold. We also avoided undercuts in our part designs so that they will come out of the mold. Lastly, we chose to extrude the decorative hair and mouth features of our minion out of the face of the yellow body part so that the molds for this part will be easier to make. The features will formed by making small cuts in the aluminum mold rather than having to make raised skeleton features which would’ve required more material removal and would’ve taken longer to fabricate.

VI. Yo-yo Performance Estimation

Center of Mass:

The center of mass of the yoyo from Fusion 360 is in the center of the yoyo’s horizontal axes, and about ⅓ of the z-direction height of one yoyo half. This is probably not the exact center of mass of the yoyo as the CADed components are not of the correct density, and the mass will change as a result. However, because the thickness is the same throughout and the sides are symmetrical, we can expect the center of mass to be in the axial center. Once the two halves of the yoyo are put together, the center of mass should balance in the space between them, directly in line with the string.

Yoyo Weight:

All parts together, including the screw and nuts, the yoyo weighs 66.59 grams. To calculate this we measured the volume of the individual components in Fusion and multiplied it by the density of Polypropylene of 0.91 g/cm^3 (found on the RTP Company spec sheet for Polypropylene). The mass falls within the suggested range of ~60-70 grams for an ideal yoyo weight.

VII. Measurements & Drawings