X then Z

Now the basic frame is complete the next stage is to build the X axis assembly. This moves the print head along two steel bars, driven by a motor using a toothed belt. The first stage leaves the belt off and was fairly simple. One fiddly bit is the way bolts (or are they screws?) fix into the plastic mouldings. The steel M3 screws (bolts?) would chew through any threads in the plastic, so Prusa have created little slots that intersect the holes for the bolts that a square, flat nut gets pushed into. It is a simple captured nut, but is allows the bolt thread to engage steel not plastic. The assembly that will eventually hold the print head has five of these captured nuts in it, but there's no bolts in them yet so as the fitting gets moved around it is easy for the not-yet-captured nuts to fall out, usually needing a search on the floor to find the escapee.

The second part of the job is the Z-axis assembly. The previously assembled Y and X axis units are moved by toothed belts each driven by a single stepper motor. The Z axis is driven by two threaded bars each with its own motor. I think this is to allow very fine control over slight unevenness in the print bed. The threaded shafts were already fitted to the motors and assembly was easy. Now the X axis assembly can be added, being careful not to lose those loose nuts, then the toothed belt was added. Tensioning these toothed belts seems a bit of a black art, but it seems that the calibration process at the end of the build should check that they are at the correct tension.

The section of the manual looked quite long for the Z axis assembly, but it turned out that there were alternative ways of working for older or newer parts so the relevant part was much shorter.

Now with the X and Z axis assemblies in place

The next section is the E axis. A 4th axis? We'll see.