Fun simple print to play or teach about gears and gear ratios.
Every gear adds a 3:1 gear ratio, 60:20. I stopped at 5 gears. It is still easy to spin by hand but you can certainly tell the torque required to spin starts to increase with every gear. Too many gears and you might notice it begins to get difficult to spin.
The print profile will come with 10 cm long rod, which is more than enough to begin with. You'll likely want to shorten these to a length below depending on how many you want to add.
These lengths should make it loose enough so that the gears don't rub the walls, but tight enough to prevent excessive play between gears and all the gears still mesh well.
2 gears - 38mm rods
3 gears - 46mm rods
4 gears - 54mm rods
5 gears - 62mm rods
6 gears - 70mm rods
etc.
Rods are 5mm should you choose to upgrade to another material.