This extruder (cold end only) was developed to provide a low cost entry level extruder for use in RepStraps. It uses no printed parts and should cost less than $10 to make from mostly hardware store parts.
The core of the extruder is a 3" wood screw that is used as a worm drive to feed the filament. The wood screw is held by 2 small 90 degree angle brackets and turned by a small stepper motor. The filament is held to the top of the wood screw with a small cross board. The cross board is held by 2 machine screws which go through the mounting board to T-nuts. the cross board has a slot cut into it that is only slightly larger than the wood screw and about 1/2" deep. The filament is fed above the woodscrew.
The wood screw is turned by the stepper motor with a standard screwdriver ratchet adapter. The adapter needs to be fixed to the motor shaft relatively securely.
3/11/2014 status update: A working version of the wood screw assembly has been created. The stepper moter hasn't yet been mounted. The current version consistantly feeds 3mm PLA filament when the wood screw is turned with a hand or electric screwdriver.