Overall Design and Specifications

We will be spending the next six weeks preparing breakfast: a golden-baked waffle on a plate, complete with a pad of butter. Our yoyo will be breakfast you can spin at any time of day, making mealtime unlimited fun! Each half of the yoyo consists of one waffle, one plate, and one slab of butter placed in the middle of the waffle.

Each half of our waffle-inspired yo-yo contains three standalone parts: the injection molded base plate, the injection molded waffle, and the thermoformed butter. Specifically, the injection molds for the base plate will be machined entirely on the lathe, the injection molds for the waffle will be machined on both the lathe and the mill, and the thermoform mold for the butter will be machined on the mill. Both the plate and the waffle will be molded with polypropylene, and the thermoformed butter will be molded with clear polystyrene. Once molded, the plate, butter, and waffle come together in a sandwich-like fit — the butter is inserted into the assembly from the underside of the waffle, which allows it to have long thermoformed flanges and supports that will be hidden from view. The end of the butter’s flanges rests on the rim of the plate, and the waffle snaps into place around the same rim, effectively sandwiching the butter’s supports between the waffle and the plate. Once each half of the yo-yo is completed, the two halves are held together by a sleeve.

In order to make sure our breakfast assembly would fit together well, we considered tolerances and shrinkage in our dimensioning. Draft angles and part thicknesses were taken into account across each component to ensure consistent manufacturability and to minimize unwanted effects in our final parts.

The plate is designed to be manufactured by injection molding. The bottom of the plate

is a filleted extruded part, which creates the surface parallel to the windings of the yoyo string. The fillet prevents the plastic from slicing through the string or any wayward fingers. The curvature of the plate was established to emulate the silhouette of a dinner plate. On the surface of the plate we’ve built a circular rib, sized to press-fit with the waffle part. We also built an inner cylinder to surround the hex nut, where our shaft will screw in. We’ve removed material between the cylinder and the rib, where we would expect to see material deformation had we not removed as much material as possible. And lastly, we drafted the inner edge of the rib at three degrees which will ease its removal from our mold.

The waffle part was carefully scaled down from the dimensions of the real waffle while also taking into account the limitations of our mill tools. The waffle holes and filleted edges are all machinable with a 1/16” minimum tool diameter while maintaining the delicious look of a real waffle. Draft angles were incorporated in all of the holes and edges to assist in part ejection. To minimize warping, the thickness of the part was kept to a consistent 0.1”.

The butter part was designed while carefully considering all the parts around it and also the properties of the thermoforming process. The butter extends into a disk of radius 0.97”, so that it could be sandwiched between the waffle and the plate, which have radii of 0.98” at the point where they snap together. The disk also has 8 ribs to make it more sturdy and for the fit to be more snug. The height of the butter above the disk base is 0.25”, which is slightly taller than the waffle, which is 0.24” thick. This allows the butter to pop out a little above the waffle to give it a more realistic look.

Table of Specifications

Item Value [in] Measurement Method
Max Diameter of Yoyo (plate) 2.500 Calipers
OD of waffle 2.200 Calipers
OD of snap fit (ID of waffle) 1.950 Calipers
ID of snap fit (plate extrusion) 1.960 Calipers
Height of waffle 0.400 Calipers
Height of plate extrusion (nut) 0.200 Calipers
Height of rib from plate bottom 0.300 Calipers
Thickness of plate (inner) 0.090 Calipers
Thickness of plate (middle ring) 0.130 Calipers
Thickness of plate (edge) 0.080 Calipers
String gap 0.100 Calipers
Total width of yoyo half 0.53 Calipers
Total mass 0.13 lbs Scale
Moment of Inertia 0.08 lbs*in^2 Solidworks
Estimated Maximum Rotation Speed 1798.2 RPM Hand calculation
Tolerances (all tolerances not listed here are +/- 0.005”)
Location (+) Value [in] (-) Value [in]
OD of snap fit 0.005 0.0
ID of snap fit 0.0 0.005

Our project timeline and progress are recorded in this Gantt chart and Google Doc!

2.008 Yoyo


Anjali Krishnamachar and Kira Schott will assume the task of designing and machining molds for the injection molded base plate, as well as carrying out the injection molding process to manufacture one hundred base plates. Grace Li and Wen Zeng will assume the task of designing and machining molds for the injection molded waffle, and they will manufacture one hundred waffles. Finally, Tracy Cheng will assume the task of designing and machining the thermoformed butter mold, and she will manufacture one hundred thermoformed butter parts.