Jetspinner Yoyo
Challenge
The objective for 2.008 - Design and Manufacturing II was not only to design a product, but also the processes required to mass-produce it. In a team of 4, we were tasked with designing a multi-part yoyo, making the tooling, and producing 50 full yoyos using mass production techniques.
With that brief, the challenge now was to create a killer yoyo that looked good and felt great to use, while making sure it could be made using injection moulding and thermoforming techniques.
Design
We decided on a visual theme of a jet turbine for our yoyo. We also identified a number of key characteristics we wanted it to have:
- Stable, high spin speed - to give the yoyo that ‘snap’ when thrown
- Open face - to truly look like a real jet turbine, we needed to have the ‘blades’ open and not enclosed behind a clear cover
- Durability - snap-fits were a requirement of the class, but we wanted our yoyo to withstand a strong throw to the floor
To achieve our first characteristic, we set a target weight of 75g. We also kept the weight close to the center line by insert-moulding metal washers into the body, to minimise wobble.
We created our first CAD model of the yoyo from our sketches by splitting the work between us, after agreeing on some key interfacing dimensions. I worked on the body of the yoyo, for which there were a number of design considerations:
- Predicting shrinkage, reducing warpage: The central hub diameter was a critical dimension, as it had to press-fit with the fan, to a tolerance of +0.010”-0.000” (+0.025mm-0.00mm). By keeping the wall thickness throughout the cross section close to constant I reduced the risk of warpage on cooling, allowing good process control.
- Insert moulding: Moulding the metal washer into the plastic of the body simplified assembly and reduced part count. I inset magnets into the core of the mold to hold the washer in place during injection.
Our original fan design proved too complex to create moulds for with the tooling available. This caused us to mimic turbine design even further, by creating the nose cone and blades as separate pieces that dovetailed together. Once our design was finalised and we had accounted for part shrinkage, we created the moulds in Solidworks, before exporting to Mastercam to create the CNC toolpaths.
Production
Our yoyo required three two-part IM moulds, a thermoform mould, and a die for cutting out the ‘stator’ shape from the thermoform. The IM and thermoform moulds were turned and milled out of aluminium using the CNC toolpaths we created, and the cutting die was milled from mild steel. We tested the moulds on the IM machine to identify where dimensions needed to be remachined to achieve the tight tolerances required for the interference fits. It also let us work out the shot size and other injection parameters to get a good finish.
Once we were happy with our moulds and the injection parameters, we started our production runs of 100 parts (800 for the fan blades). Work was effectively split between the team, before we all came together for the final assembly on a mini production line.
Performance
Our Jetspinner yoyo was able to live up to its name - using high speed photography we were able to record a maximum speed of 6000rpm, and the fan blade produced a very satisfying ‘whoosh’ at high speed. We had good process control indices for all of our parts, resulting in a quick and efficient assembly process.
We presented our yoyo to the rest of the class, and produced a short video cataloging the process from start to finish, which you can watch below.