Lego RepStrap CartesianBot
Use Lego as much as possible. At least use Lego for all parts that aren't off-the-shelf standard hardware, and all moving parts.
OK to use non-Lego for large rigid parts that would require a vast quantity of lego bricks, i.e. the base-board and the stand / pedestal that mounts the Z axis and print head.
X,Y and Z axes have smooth movement and no noticeable play. OptoEndstop_2.1 works and senses limits. The Lego extruder can drive a feed rod. Next step is to finish the extruder by adding the hot parts and not melting my Lego bricks.
Will submit details of Lego construction once the entire machine is verified to print accurately.
The arrangement of motors and axes is based on the McWire_Cartesian_Bot_1_2_(Death_March:_Do_not_build!!!) . The object is printed onto a stage that moves coplanar to the ground plane in X and Y; the print head moves in Z only (ie, up and down).
Extruder driven by Lego technic motor. Lots of information about Lego technic motors here.