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:

a .pdf from Boston Gear discussing both spur (& other) gear theory:
http://www.bostongear.com/pdf/gear_theory.pdf
another .pdf providing additional information and clarification on both spur and spiral gears: "Parametric Design of a Spiral Gear Process" by Jeffery Baker, Jason Reynolds and Stephen Tecce of Worcester Polytechnic Institute:
https://www.wpi.edu/Pubs/E-project/Available/E-project-042811-130543/unrestricted/Final_Report-Parametric_Design_of_a_Spiral_Gear_Process.pdf