Derived from triffid-hunter's herringbone gears .scad http://www.thingiverse.com/thing:10707, and The Hack a Day badge http://www.thingiverse.com/thing:1562... a one-off mashup. Cause I like herringbone gears. And skulls.No instructions other than to lookie-look. As I am not completely satisfied with the skull faces, I will probably do some more edits and re-print with .35 nozzle and finer layer height. (Pictured one was done with .5 tip @ .36 layer, 1 perimeter and 70% infill)Uploaded the .stl I used. I haven't had the opportunity to get back and work on polishing it up, so print knowing in advance that the skull faces print a little messy. The gear itself is beautiful. You will need to print the small gear of your choice to match using triffid-hunter's .scad script.Also, be forewarned... 47 teeth pitch diameter is at the absolute size limit for a standard Greg's Wade extruder on a "flat-top" x-carriage. Test very carefully before use! You may need a thin (.5mm - 1mm) shim under your extruder body to get clearance.