IEBPROJECT CASE STUDY
21 / 47
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A four armed fidget spinner in marbled red, blue and yellow plastic with four steel ball bearings and a red printed cap at the centre

Course design projectDrawing 21 of 47

Fidget Spinner
Design

For ME 210 at UT Austin, I took a fidget spinner from a sketch all the way to an aluminium injection mold, and I finished every step of the design package myself. Unfortunately, the part that came out of the mold still didn't work.

Sheet 02 of 08/Brief

The assignment

The assignment was to take a fidget spinner from a sketch through CAD, GD&T, an FEA drop test and a real aluminium injection mold. It ran about twelve weeks in total, and the mold and CNC work was the biggest single chunk at three weeks.

A SolidWorks assembly drawing of a three armed fidget spinner showing four orthographic views and a shaded isometric view in a titled drawing frame
PL 02My assembly drawing, with three arms and one centre bearing. The front, side, top and isometric views are all on one sheet.

Sheet 03 of 08/Design

The design and the drawings

I drew it with three arms around a centre hub and four bearings in total, one in the middle and one in each arm. The three arm bearing pockets have a position tolerance referenced back to the centre bore. It's the drawing's way of saying how far off centre a pocket is allowed to sit.

An engineering drawing of the spinner body with geometric dimensioning and tolerancing callouts, feature control frames and a datum symbol
PL 03The body drawing, which has the tolerancing on it. Datum A is on the centre bore, and the three bearing pockets have a position tolerance referenced back to it.

Sheet 04 of 08/Analysis

Drop test

In the FEA drop test, the stress in the body peaked at about 4,600 psi, around 71 microseconds after impact.

An exploded assembly drawing of the fidget spinner with numbered instructions and a bill of materials table in the corner
PL 04The exploded view and assembly instructions, plus the four line bill of materials: one molded body, four bearings and two printed caps.

Sheet 05 of 08/Mold

Cutting the mold

Half of what makes a part manufacturable, like how much the plastic shrinks as it cools, never shows up in a model on a screen. You only run into it once the metal is being cut.

A finite element stress plot of the spinner in dark blue with green and orange bands around the inner edges of the arms, beside a von Mises colour scale
PL 05The FEA drop test, 71 microseconds after impact. The stress peaks at about 4,600 psi where the arms meet the hub, which is exactly where it looks thin.

Sheet 06 of 08/Failure

What came out

My group's spinner didn't work. The mold that got cut and the part that came out of it have four arms and five bearing pockets, but my own drawings called for a three arm part.

Two machined aluminium blocks side by side on a workbench, each with a butterfly shaped cavity milled into it
PL 06The two aluminium mold halves after CNC milling, split down the middle of the part.
Two red 3D printed discs joined by a short shaft, resting on a concrete surface

Sheet 07 of 08/Next time

What I'd change

  • If I did it again, I'd split the mold differently. One half would be the spinner body and the other half a cap. That way the plastic would shrink more evenly and the mold would come apart more easily.
  • I'd also use three bearings instead of four, mainly to save time.

PL 07The two bearing caps, 3D printed and pressed together. They're the only parts of the assembly I made myself.

Sheet 08 of 08/Source

Inspect the work