Behold and fear my Skulltruder! This is a set of spur gears for the Printrbot or any printer using Wade's accessible extruder. 13:43.
Reworked from Brook Drumm's Printrbot's involute herringbone gears, (which were ultimately derived from works by Wade and Frost) http://www.thingiverse.com/thing:16990 and Cleaned Skull by ssd (http://www.thingiverse.com/thing:4126 .
Watch one come to life:
New 4mm drive gear!
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UPDATE: There are now THREE versions of the Skulltruder - a 5/16 inch and and an 8mm to fit your favorite hobbed bolt, in addition to a straight spur designed (tooth profile poached from thing #41808 by jameswood).
I ditched the small skull on the small gear. It never printed well with the hole through the forehead and the reduction in size. The NEW! small gear has some nice features, including cleaned up teeth, and a strengthened area for the set screw. The small gear designed by jameswood is as his original.
Thanks to Emmett, I have a better understanding of involute gear geometry, and I should mention that these two gears are designed to work together. If you use a different drive gear (on the stepper) you may have issues with smooth meshing because of different variables in the tooth profile. They were both ultimately derived from Greg Frost's 9/43t herringbone extruder gear drive.
If you change gears on your extruder, you will need to change your mm/rotation calibration in firmware / slicing software.