The original Gear_Blank parts were downloaded under a GNU-GPL license:

Source: http://www.thingiverse.com/thing:8077
License: GNU-GPL
Author: Matt Ruggles: http://www.thingiverse.com/msruggles/about
Date Downloaded: 2017-01-26

Files included in original package:
  Design_Table.xlsx	(48923 bytes)
  Gear_Blank.SLDPRT	(529408 bytes)
  Gear_Blank_V2.SLDPRT	(559104 bytes)

spur-gear.SLDPRT was derived from Gear_Blank_V2.SLDPRT.

spur-rack.SLDPRT was drawn separately.

The generated DXF files use a 2 mm module, so a 20 tooth gear has a pitch
diameter of 40 mm and a pitch radius of 20 mm.  The pressure angle is the
default 20 degrees.

E.g. a 2:1 reduction can be obtained by driving a 20 tooth gear from a 10 tooth
gear.  The pitch radii are 20 mm and 10 mm, which can be added to find the axis
separation of 30 mm.