spur gear training tool

This set of spur gears is intended as a training tool for spur gear theory.
- large teeth to allow easier observation of tooth contact
- two thread holes are provided to show that gear tooth contact follows the 20 degree pressure line from the intersection of the pitch diameters.
- One face of the tooth follows one string, the opposite face follows the other string.

Fundamental gear parameters (in inches)
diametral pitch (DP) = 3 inch
number of gear teeth (N) = 12
pressure angle (phi) = 20 degrees
then using standard spur gear equations:
pitch diameter (PD) = 4 inch
outer diameter (OD) = 4.666... inch

Note that the units above are provided in inches however the .stl files are provided in millimeters.

references:

1. a .pdf from Boston Gear discussing both spur (& other) gear theory:
http://www.bostongear.com/pdf/gear_theory.pdf

2. another .pdf providing additional information and clarification on both spur and spiral gears: "Parametric Design of a