IEBPROJECT CASE STUDY
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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 33

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, finishing every step of the design package myself. The part that actually came out of the mold still didn't work.

Sheet 02 of 08/Brief

The point of 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, about twelve weeks total, with the mold and CNC work eating the biggest single chunk, 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: three arms, one centre bearing, front, side, top and isometric on one sheet.

Sheet 03 of 08/Design

The design and the drawings

Three arms around a centre hub, four bearings total, one in the middle and one per arm, with the three arm bearing pockets carrying a position tolerance referenced back to the centre bore, which is 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, where the tolerancing lives: datum A on the centre bore, and a position tolerance on the three bearing pockets referenced back to it.

Sheet 04 of 08/Analysis

Drop test

The FEA drop test on 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 04Exploded view and assembly instructions, plus the four line bill of materials: one molded body, four bearings, two printed caps.

Sheet 05 of 08/Mold

Cutting the mold

Half of what actually makes a part manufacturable, like how much the plastic shrinks as it cools, never shows up on a model on a screen, only once the metal is actually 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 05Drop test FEA at 71 microseconds after impact. 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, and the mold that got cut and the part that came out of it are a four arm shape with five bearing pockets, not the three arm part my own drawings called for.

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/Again

What I would change

  • Split the mold differently: one half the spinner body, the other a cap, for more even shrinkage and an easier release.
  • Use three bearings instead of four, mainly to save time.

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

Sheet 08 of 08/Source

Inspect the work