Pitch diameter demonstration

Outer diameter of the arrows on the gears shows that pitch diameter of a gear is the diameter where the gears actually mesh. While you can measure the outer diameter and get 59.85mm, this is due to tolerances and the actual pitch diameter is 60mm (as is written in the back).

The arrow direction shows that when two gears mesh, their angular velocities are opposite.

This model is small, so that all components can be printed by any printer with at least 120mm x 120mm bed size.