I have changed one major thing and several minor things on the original design.
Major change - The original gear on the Makerbot meant for gripping the plastic can be used on this extruder. This translates to superb grip compared to using only the screw's threads to grip the plastic.
-Flipped the mount, so Stepper motor doesn't run out of cable length.
-Extruder holes moved, mounts DIRECTLY onto the Makerbot.
-Small gear center hole larger to fit on Makerbot's Stepstruder motor.
The increased grip added to the geared extruder means you can get up to some nice speeds on your Makerbot!
Better Pictures coming soon!
First make sure you have a screw that can fit through the original gear on the Makerbot.
You will also need three 608 or 606 Bearings. (The scad contains an extra module for the 606 bearings, right now the stl and scad are set to a default of 608 bearings.)
The screw will most likely not fit in the 608 bearings tightly, so you might need to print a small washer like thing to make them fit tightly. Basically just a small cylinder with a slightly smaller cylinder cut-out in the middle. I trust you can do this on your own to match your screw size and fit your printer settings.
NOTE: It might be easier to just take a slice of paper and wrap it around the screw to act as a filler between a screw too small for the bearings/gears. Currently doing this for the Makerbot gear because its about 1mm too large for my screw. Works well for keeping the makerbot gear centered on the screw.
NOTE 2: Grinding down the screw on one side in the right spot might make it easier for the gear, as it will have a flat surface to grip to.
NOTE 3: Suggest flipping about x-axis, so the stepper motor for the thingomatic is closer to the entrance where the cables go in.
Print. Assembly is same as Greg's Extruder.