Monday, March 13, 2017

The Design

Our Design

Our Yoyo design is inspired by a sugar cookie.
It consists of three main manufactured parts:
  1. The cookie - this will be the main part of the yoyo. This part will be injection molded.
  2. The frosting - this will cap off the cookie and also have holes so that the sprinkles can poke through. This part will also be injection molded.
  3. The sprinkles - this will add a bit of decoration to the cookie. This part will be thermoformed.

For each part, the mold will be machined out of aluminum. The main cookie, frosting,  and sprinkles will be connected to each other through a snap-fit assembly. The main cookie and frosting parts will have dye added to the injection molded part for color, while the sprinkles piece will be spray-painted to get a variety of colors.

The two cookies make up the two halves of the yo-yo. The two halves will be attached with a ¼” axle. Additionally, there will be several weights spaced into the cookie cavity that will be injection molded into the part for extra strength.

Half.JPGHalf-Iso.JPG

Production Details: Injection Molding

Several design for manufacturing principles were used when modeling our yo-yo. These included, but were not limited to, having a uniform wall thickness, draft angle on all the important faces, and having fillets on the edges of all parts so nothing gets stuck in the mold. In order to minimize warpage on the part due to the number of cavities, a uniform (and relatively thin) wall thickness was used, allowing the parts to cool evenly in the mold. We went through several design iterations of the wall thickness, and eventually ended up removing the thickest parts for fear of warpage.

Our yo-yo design also includes draft angles and filleted edges so that the yo-yo is easier to remove with the ejector pins and does not get stuck.

Shrinkage tolerances were also taken into account because there are several snap fit pieces. Based on previous yo-yo parts that were shown to us in lab, the plastic has a tendency to shrink 1-2% upon cooling. The dimensions and tolerances for our parts reflect this.

Production Details: Thermoforming

All design for manufacturing details were included in the thermoformed sprinkles part. These included having a uniform wall thickness, avoiding undercuts, rounding/filleting all corners and edges, and using a draft angle. Because of the sharp angle to which our sprinkles protrude through the frosting layer, we anticipate having to use a 2 part thermoforming mold (core and cavity) to achieve our tolerances.

Specifications


Dimensions
Target Value
Expected Tolerances
Measuring Methods
Manufacturing Method
Exterior Shell - OD
2.5"
(+/-) 0.005
Calipers
Injection Molded
Exterior Shell - Snap Fit OD
1.7"
(+0.005/-0.000)
Calipers

Exterior Shell - Inner Hole Diam.
1.5"
(+/-) 0.005
Calipers

Exterior Shell - Width (1/2 yo-yo)
0.5"
(+/-) 0.005
Calipers

Exterior Shell - Snap Fit Depth
0.1"
(+0.005/-0.000)
Calipers

Exterior Shell - Inner Hole Depth
0.2"
(+/-) 0.005
Calipers

Exterior Shell - Inner Diameter
2.2"
(+/-) 0.005
Calipers

Exterior Shell - Screw Hole
0.2"
(+/-) 0.005
Calipers






Frosting - OD
2.2"
(+/-) 0.005
Calipers
Injection Molded
Frosting - Snap Fit ID
1.7"
(+0.000/-0.005)
Calipers

Frosting - Snap Fit Depth
0.1"
(+0.000/-0.005)
Calipers

Frosting - Sprinkle Hole Width
0.05"
(+/-) 0.005
Calipers

Frosting - Sprinkle Hole Length
0.15"
(+/-) 0.005
Calipers






Sprinkles - OD
1.55"
(+/-) 0.005
Calipers
Thermoformed





String Gap
0.1"
(+/-) 0.01
Calipers











Mass




Assembled Cookie
35.64 grams/half
(+/-) 1 gram
Scale

Steel Rim Weights




Moments of Inertia (I xx)
0.34 pounds*in^2



Moments of Inertia (I yy)
0. pounds*in^2



Moments of Inertia (I zz)
0. pounds*in^2








Volume of Assembled Cookie
2.02 cubic inches



Frosting
0.34 cubic inches



Cookie
1.64 cubic inches








Density of High Impact PolyStyrene
1.08 g/cm^3




Max Rotation Speed: 1400 (+/-) 150 rpm

Starting height: 48”
Starting velocity: 0”
Ending height: 0”
Spool Radius: 0.5”
Total Mass: ~ 75 grams

Using the following equations:
v = ((2gx)/(1+J/mr^2))^0.5
J = 0.5*mass*R^2 = 8.78 * 10^-6 (from solidworks)
V = 4.892m/s = 146.91 rad/s = 23.38 rot/s = 1402.88 rpm

This will be measured using a Tachometer.

Division of Labor and Timeline

See Gantt chart here: https://docs.google.com/spreadsheets/d/1GtLDWsaaeVXUo7GcY40s-b1mtdcodMV7nvzyx4qV2X0/edit?usp=sharing

Monday, March 6, 2017

Meet the Team!

Division 1 Yo-Yo Squad

About the Team:

We are a diverse group of MIT students, united by Yo-Yos and 2.008.

About the Players:

Christopher Harmon,  Major 2,  Year 2017
Jerome "JJ" File,  Major 2,  Year 2017
Zhishen "Juju" Wang,  Major 16,  Year 2017 
Sam Belden,  Major  3,  Year  2018
Samuel Matthews,  Major 2,  Year  2017