My reworked design of a Printrbot extruder. While there are many other extruder designs the ones for a bowden fed hotend and manufacturable without a 3D printer are rare.This one I made on my Chinese 6040 router to convert it to 3D printer.See it in action here: https://www.youtube.com/watch?v=E38WfRnVwgAIn the prototype I used 3mm and 6mm Tufnol but 1/8" and 1/4" should be okay. Be sure to drill the filament channels to the outer diameter of your PFTE tubing after assembly such that the tubing reaches right up to the hobbed bolt.If you like a challenge then make the small gear out of 6061 Aluminum like I did; it requires a 1mm end mill (and probably more than one...) Otherwise glue a few bits of 6mm stock together and use that.To date I've run about 2kgs of 1.75mm filament through this extruder quite convincingly. I'm planning on re-using it on my 1.5 Mini Kossel currently under construction.Look here for an assembly manual:http://help.printrbot.com/Guide/Extruder/38