Gyroscopic Exercise Ball

No

0.2mm

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.

Post-Printing

Assembly

1. After printing, sand the top and bottom surfaces of the ring smooth. Also sand smooth the the surface of the shell that the ring rubs against.
2. Insert 6 US pennies into each cavity of both rotors (48 total). Use pennies all of the same type because different versions (with different back designs) have slightly different weights. Use a few drops of superglue to hold the pennies in place.
3. Insert your 0.125" diameter steel shaft into one of the rotor halves and position it so that it sticks out of the bottom side by exactly 0.750". Superglue the shaft in place.
4. Slide the gear over the other end of the shaft and superglue it in place too.
5. Slide the other rotor half over the shaft and superglue it to the gear, too.
6. Place the ring in one of the shell halves, and then place the rotor assembly such that the ends of the shaft fit into the notches in the ring.
7. Assemble the other shell half to the bottom half. If you are lucky the two will fit together with a satisfying click and will stay together. If the fit is a little loose, Temporarily tape the two shell halves together until you know that everything works properly. Once you are sure it all works, you can glue the two shell halves together.

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