This is my take on the venerable Dyna Bee gyroscopic exercise ball that came out in the late 1970's. It uses 48 pennies (US) as the flywheel mass. As a twist, I've added a geared zip starter to get the rotor spinning up to speed. From there, you keep it going with pure wrist power.
Aside from the 3D printed parts, you will need a 0.125" diameter steel rod and some superglue to put it together.
UPDATE: I've modified the shell to make it easier to align the zip starter with the slots in the side.
Printer:
UDIO
Rafts:
No
Supports:
No
Resolution:
0.2mm
Infill:
20%
Notes:
Print to two of the shell.STL, two of the rotor.STL, one of the ring.STL and one of the gear.STL. I've included two versions of the rack.STL (the zip starter) - one about 11" long and a shorter one about 7.5" long that should fit on a 250x250mm print bed.
You will also need a 0.125" diameter steel shaft (available from most hardware stores) that is 2.465" long, and also 48 US pennies, preferably new.Assembly
Use
The only real annoying thing with this design is that you have to align the rotor so that the gap in the center with the gear lines up with one of the zip starter slots in the top or bottom shell. You can use the tip of the zip starter to poke through one of the slots to rotate the rotor and ring into place.
Once aligned, insert the zip starter, teeth towards the center of the ball, into the slot - it should engage the central gear. Shove the starter all the way in and then pull it back out smartly. This should give you enough speed to start accelerating the rotor using a wrist wobbling motion. Check out this video to see it in use: https://youtu.be/0vgsZctOkaI