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 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

print two of each part:
gear_4_DP3_N18_150116a.stl
gear_4_DP3_N18_150117_link.stl

use care when gluing the parts together to avoid binding the gears.

• see the link picture
• place glue (I used acetone) only on top of the post and in the socket hole
• assemble parts
• insert string as shown in the gear assembly picture to show line of contact for teeth