The Z axis stage is basically identical to the Y axis stage for ease of assembly and in order to simplify part production. The only real difference between the two is that the Z axis stage has ten counter bored holes towards the end of the plate which are used for attaching various tool heads.
The Z axis stage uses four linear ball bearing blocks, two per each rail, as opposed to the X and Y stages which each use two. This was implemented in order to give the Z stage a larger degree of rigidity when subject to transverse loads. In the above image, the through holes on the Z beam are clearance for the screws that fasten into the linear bearing blocks. Speed is not an issue for the Z axis because it moves very small distances (~.425 mm) only intermittently (once per 3D printed layer). So, this stage uses 3/8″-12 (1 start) ACME threaded lead screws. This gives the Z axis a high resolution for traversing to each incremental layer while a part is being printed. Layer height is crucial when 3D printing high quality parts.
Here the Z beam, with the newly attached Z axis stage, is mounted onto the Z axis support structure. Once again, the Z beam is positioned with laser cut jigs so that when the plastic extruder is attached, its nozzle will be perfectly centered over the build base.