I really liked the simple system used on the new Replicator extruder set up so decided to re-create it for the Printrbot and generic RepRap style machines.
This unit is designed to fit a UBIS style hot end, but can be easily modded to take a J head or whatever else.
This will bolt straight onto a Prusa i3, and should fit any other generic Reprap style machine, maybe with minor mods..
This is a new iteration of my previous design with improvements.
Make sure you print the main "block" in ABS. I tried it in PLA and it went rubbery and loose after about 20 prints.
The 1.6 includes a re-designed extruder mount, a filament guide, better spring mount, a cleaner drawing and a slightly tweaked lever. A new change is a much stronger shaft area around the 608ZZ bearing. Earlier versions had a tendency to flex and bend.
You'll need a 608ZZ bearing and some sort of spring. This one I salvaged from a dead printer, which are full of cool springs. You want a fair bit of tension on the lever, so you'll need a pretty strong spring.
There's a few M4 nuts and bolts there too. I used allen screws on the attachment to the head because it's easier getting an allen key in there rather than a phillips head, plus they look cooler.
The new version now mounts the extruder on 2 x M4 screws. I found the M3's on the original were just too wimpy to hold the extruder firm for a long time and had to be re-tightened on a regular basis. The new M4's solve this.
This is a heavily remixed version of this unit - http://www.thingiverse.com/thing:155711
This is also a GREAT extruder, I was running it for a while and it's a huge improvement over the stock piece.