Hardware Per Gear:

- 1x  Bearing (3mmID*10mmOD*4mmH)
- 1x  M3 Nut (standard nuts work, but i suggest using low-profile ones)
- 1x  M3*8 Hex Socket Head screw
- 3x 10mm magnets (5mm height max). 6x magnets are needed for 1 large gear

Features: 

- No plastic-on-plastic rubbing
- TPU feet - no sliding
- Designs are modular and customizable
- Zero backlash
- Extremely durable (cannot be broken by me, and will certainly withstand a child)
- Press-fit design with no glue - broken parts can simply be re-printed

Why?:

- Interactive, and tactile gears allow for learning of rotation, direction, teeth count. I suggest printing a Small, Medium and Large gear, and attach an arrow on them; Connect the gears in a line, and demonstrate how size (teeth count) affects how quickly the gears spin. How many times do you need to turn each gear to make all of the arrows face the same direction again?

- Have your child place a certain number of gears in a row and have them guess which direction a gear will spin when one is turned.

- Teach your child that an odd number of gears attached in a circle will always cause the mechanism to jam; why? 

Printing Tips:

- The softer the TPU, the better it will stick to the fridge (or metal surface)
- Glue M3 nut on the hex socket screw (or use threadlocker)
- Use a smooth PEI plate
- Print all parts with ABS and join with a chemical that dissolves the plastic slightly. Otherwise, ordinary CA glue might not be strong enough, causing the topper to become a choking hazard.
- Before gluing, scratch the balls out of your surface with a knife to increase surface area