This is the second half of my http://openscad.org gear sets.
I couldn't get the top and bottom parts of the double helical to union properly, so I ended up offsetting the bottom piece by 0.1mm :( It skeins and prints fine though.
This thing is part of a set:
UPDATE: I'm printing some gears and noticed a couple mistakes on the openSCAD script: the variable 'orientation' wasn't doing anything and the value 'extrudeInDiam' was in fact being used as radius. They're both fixed now and I uploaded a new version of the script.
The openSCAD file accepts parameters for height, pitch diameter, shaft diameter, double helical, teeth (number of teeth, addendum, dedendum, tooth width, angle, orientation), shape (solid, star, circles), extrude in and out (diameter, height), rome (pitch diameter, gear height, number of teeth, addendum, dedendum, tooth width, rotation).