First off, thanks to Airtripper for a great compact extruder design! This is my J-tripper -- a remix to the Airtripper design to allow for DIRECT mounting of a J-head (or similarlly slotted hot end) to the bottom of the Airtripper direct-drive extruder. I made this design to replace the finicky parts that I encountered on the Mk 7 / QUBD extruder designs. I built mine using a Makerbot MK7 Drive Gear. Visit http://airtripper.com/?p=367 for extruder body build details. Once the extruder body is assembled and mounted to the stepper motor (as in Airtripper's instructions), simply slide the J-head hot-end into the bottom slot. I consider this version "Beta" as I have only been using it for a few days. But it seems reliable so far, so I'm releasing it to the public. As the design is proven and updated to the Airtripper V3 core, I will also release the OpenSCAD file. This is the reference extruder design for the F3 CubeMax (come see us at MakerFaire San Mateo 2013).Assemble using Airtripper's instructions here: http://airtripper.com/?p=367 Then slide in your hot end!