Say hello to BB8!
Figure 2: Section view of yoyo.
Figure 3: Yo-yo head with slots for string and magnets
Figure 4: Sand dune base.
Our yo-yo is based on the Star Wars character BB-8. The character’s head moves freely while the body rotates. This inspired us to design a yo-yo with a free spinning body and a head that independently hovers above (see Fig. 1). The body is divided into two hollow hemispheres, each with a press fit inner face. Each inner face piece has an insert molded hex nut and radial ribs to increase strength (see Fig. 2). The head is a smaller, solid hemisphere with a smooth hole through the center (see Fig. 3). The string is threaded through the head and wound in the gap between the two body pieces. In order to keep the head close to the body while the yo-yo is spinning, we will insert mold a steel shim into the outer hemisphere and a magnet into the head hemisphere, as shown in Figure 3. The entire yo-yo will sit on top of a “sand dune” base, which is shown in Figure 4.
We have 3-D printed a prototype of our yo-yo to test our design. We plan to injection mold the body hemisphere and inner face and the head. The body and head will be spray painted, with 3D printed stencils. We plan to thermoform the yo-yo base and spray paint the surface to look like sand.
Design for Manufacturing
To incorporate for design for manufacturing principles, we reduced the number and complexity of our parts to just 3 unique Injection Molded parts and one thermoformed part.
Our most complex parts are Injection Molded, which allows us great flexibility. Our hemisphere has an almost totally uniform cross section, which will allow for very consistent results, and the shape is very easy to make a tool for with the lathe. Our inner face has several ribs for increased stiffness, and incorporates both the nut and the shim as molded-in inserts for easier assembly and tougher finished products. Our head part was carefully considered to produce a geometry that can be injection molded reliably, by using a tapered alignment pin in the center hole.
Our simplest part (the sand base) will be thermoformed, because it does not need the structural integrity of the rest of the yoyo. Drafted angles in the base allow for a simple release of the thermoformed part from the mold.
A final remark is that our project requires complex decals. Instead of using several colors of polypropylene, we are choosing a pad printing process to allow us to quickly print the decals on each yoyo with minimal effort.
Table of Specifications
Total Mass of Yo-Yo: 0.0554 kg
Mass of Base: 0.05 kg
Est. maximum rotation speed: 781 RPM
Please see attached calculations and uploaded MATLAB script.
Critical Dimension
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Target Dimension mm (inches)
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Expected Tolerance ±mm (±inches)
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Measurement Method
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String gap
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2 (0.080)
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±0.5 (±0.020)
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Calipers
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Depth of sleeve recess
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3.2 (0.125)
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±2.5 (±0.010)
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Calipers
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Body shell dia. (for press fit)
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50.8 (2.000)
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+0.13/-0 (+0.005/-0.000)
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Calipers
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Inner face OD (for press fit)
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50.9 (2.005)
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+0/-0.13 (+0.000/-0.005)
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Calipers
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Dia. of head string hole
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2.5 (0.100)
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±0.25 (±0.010)
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Gauge pins
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