Update v2: https://www.youmagine.com/designs/nema23-direct-drive-extruder-v2This is directly inspired by the popular extruder by whosawhatis http://www.thingiverse.com/thing:15718.  So popular in fact that MB is trying to patent it: http://www.freepatentsonline.com/y2014/0120196.htmlHis design is great but I wanted to use some of the bigger motors I had around so as to reduce their heat.  This one also uses the common 608 skate bearing and hobbed pulley (I made my own but 12mm OD ones should work).  1/4x1" springs should have been enough but I found I wanted more pinch force so went with even longer ones.  I recommend printing at least the 'Motor Attachement' in PLA since it seems to hold a thread much better than ABS.In case you haven't heard elsewhere it is very important to turn down your motor current until is is close to slipping during extrusion.  With too much power it is very easy to strip the filament... especially so with this beefy motor.