These x ends are designed to minimize Z axis binding. They do that by adding a second set of bushings into the X axis idler. These bushings allow the X shafts to move a small amount in the X direction while remaining rigid in the Y & Z directions. This allows the distance between the two x-ends to increase and decrease as the height changes, helping to minimize Z axis binding.
Note: The leadscrews these weere designed to be compatible with are no longer available. You can still use these with a printable nut adapter such as these: http://www.thingiverse.com/thing:23326
Update: Added a variant to use with select 3-hole lead screws such as the new ones sold by Maker's Tool Works ( http://store.makerstoolworks.com/motion/z-axis-lead-screw-and-nut-set-of-2/ ) and the PBC leadscrews sold by Trinity Labs. These files are names X End [part] for MTW Leadscrew. Source file is also updated to the latest versions.
Uses eight SDP self aligning bushings, available from Stock Drive Products (For example http://shop.sdp-si.com/catalog/product/?id=A_7Z41MPSB10M, also available from http://store.makerstoolworks.com). Four in the idler end for the anti-binding feature, and four for the Z axis bushings. It is compatible with either 8 or 10mm bushings in either bronze or PTFE-filled acetal. Use bronze with precision shafting or PTFE with drill rod for best results.
The motor end clamp and the delrin nuts all attach with M5-20mm screws and nuts.
Update: Added an alternate version that clamps on both ends. This eliminates the anti-binding benefits, but is easier to print reliably. With the easily adjustable Z axis shaft spacing of the MM 1.5, the non binding features are less important than they were on earlier versions. This version uses the same clamp as the motor end.
Update: Added an optional lead nut adapter to use with select 3-hole leadscrews such as the new ones sold by Maker's Tool Works ( http://store.makerstoolworks.com/motion/z-axis-lead-screw-and-nut-set-of-2/ ) and the PBC leadscrews sold by Trinity Labs.