A summary of typical gear data:
 * Pitch circle is the theoretical circle of a gear, the circle that rolls without slipping at the same linear speed with the pitch circle of the mating gear.
 * Gear Ratio is number of teeth driven_gear/drive_gear. For example 20/10 is gear ratio 2:1.
 * Involute is the shape of the teeth. It is the curve described by the end of a chord, when the chord is un-winded from a base circle.
 * Base circle, see involute.
 * Module = pitch diameter / number of teeth. The module of mating gears should be the same value.