Variable Moment of Inertia Fidget Spinner

An assignment for my manufacturing class gone overboard. I was tasked with coming up with a unique design for a fidget spinner and I wanted its uniqueness to be more than just aesthetic. The idea I came up with was to create a spinner with a variable moment of inertia so that the velocity over time behavior for its spins was altered. I wanted to mimic what I used to do as a kid on swings, which was to twist the ropes up and let myself spin, then tuck my legs in as I was going to increase my angular velocity. Surprisingly my spinner actually worked and I even proved the difference in behavior versus fixed mass spinners through spectrograms of audio recordings of spins. A brief summary is attached as a pdf if you wish to read about the process in full.

To make this you’ll need:

a) [1x] The Body – 3D printed (the STL and CAD are attached)

b) [2x] The Grips/Holders – 3D printed (the STL and CAD are attached)

c) [1x] 608 skateboard bearing

d) [1x] 3mm diameter metal tubing or rod cut to size in 3 pieces (or you can adjust the hole size in the body’s CAD for something different, or even try printing these pieces (I just wanted metal on metal to ensure minimal friction)

e) [30x] ¼” inner diameter, 1” outer diameter fender washers

f) [2x] #9 Hillman Compression Spring, cut them in half and you’ll need 3 of those halves

g) [3x] ¼”O.D x ½”length, 6-32 Thread, aluminum spacers

h) Bondo body filler (or probably just about any cement/bonding agent/glue)

Here are some links for the specific items I used:

c) https://www.amazon.com/gp/product/B07VH6XT6Z/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

d) https://www.amazon.com/gp/product/B00S50NRLQ/ref=ppx_yo_dt_b_asin_title_o04_s06?ie=UTF8&psc=1

e) https://www.hillmangroup.com/us/en/Fastening-Solutions/Fasteners/Washers/Fender-Washers/Zinc-Fender-Washers/p/290012

f) https://www.hillmangroup.com/us/en/Home-Solutions/Home-Accessories/Springs/Compression-Springs/Hillman-Compression-Spring/p/540027

g) https://www.hillmangroup.com/us/en/Fastening-Solutions/Specialty/Spacers%2C-Bushings-%26-Bearings/Spacers/Threaded-Aluminum-Spacers-Assortment-%286-32%2C-8-32-%26-10-32%29/p/545SC60

Note that these are just the items I used based on what I had on hand and could cheaply get through a quick run to my local ACE Hardware. I encourage you to search around, be creative and use whatever is easiest for you.

To assemble the masses use a vice or clamp to hold ten washers together. Apply Bondo to the outside of an aluminum spacer and push it into the washers. Make sure it has sufficient boding agent to stick, then wipe away excess. Let cure. Repeat for the other two masses.

Video of spinner disassembly/assembly: https://youtu.be/lhV_asJLJz4

Video of it spinning: https://www.youtube.com/watch?v=ELkqryU4Dow