My redesign of the extruder gears for the printrbot to be stronger and easier to print.
The teeth are fatter and stubbier so that infill reaches into the teeth themselves much more completely. In making this happen I changed the gear tooth count; it's now 10 and 33 instead of 13 and 43, changing the ratio from ~0.3023 to ~.3030. This is pretty slight (0.23%), so we didn't bother to recalibrate our extruder, and the tooth counts are still coprime.
The base of the small gear has been widened so that the gear can be printed base-down instead of teeth-down, which seems to give better teeth. The wider base also means the nut trap won't crack as easily --- which failure is what led to our printing new gears to begin with.
Last but not least, the gear teeth are helical, making alignment less of an issue than with the herringbone gears but still making for very smooth motion.
Git version at https://github.com/zack112358/printrbot-extruder-gears. The involute gears library is included because I wound up editing it slightly to get the tooth shape I wanted.
If the gears are too large or small for your printer, there's a spacing parameter in the scad file to adjust the gear size. Otherwise, just print and install as usual for Wade's-style gears.
We used an 8mm M3 screw as a set screw; that turned out to be a little short so we had to file a touch of plastic off for good fit. Since they don't make 9mm M3 screws I should probably reduce the radius of the base by a millimeter at some point; until then you'll need to use an actual set screw with no head or else file it down a little.
I messed up and made the nut trap too big, but then I decided I liked it that way. You will need to glue the m3 nut or shrink the nut trap.