The X axis assembly was pretty straightforward. I'd seen a lot of comments about the Igus bearings being difficult to mount, and easy to split - I can certainly see where this might be the case. I managed to get all four in place with only minor hacking using my trusty needle file.
The linear bearings didn't want to go onto the rods at first, so I spun them in a drill, whilst running
wire wool up and down the length. Once you get over the fact that you look like you're engaging in power tool foreplay, it's worth doing this for all the rods as everything will run a bit smoother. Just remember that if you use this method on the threaded rods, when the drill is spinning clockwise, if you're holding a rag/wire wool tightly, the rod will attempt to mash your fingers into the drill chuck...so go slow! I finished off all of my rods with a healthy coating of WD-40 (substitute your favorite lube), and felt very satisfied with the results. Oh yeah.
There are three recessed nuts in the X assembly - the first of these can be seen in the picture here, between the LM6UU linear bearings. This one went in with some persuasion - I filed away as much material as I could without compromising the structure, and then pushed it in the rest of the way using a flat head screwdriver and some gentle taps. This nut holds an M3 socket screw, which will eventually be used to tension the belt - it works...but it could be a lot better. See below for further details on this step.
Those rods really do go a looong way into the mounts and I took a while to make sure that they reached the ends by dropping a thin screwdriver into the hole, pushing to the end and measuring on the screwdriver the depth. You can of course do the same with your Vernier caliper if it has depth measurements - I didn't this time because the depth rod was too thick for the holes. I then just taped the rods with the measurements so that I could judge when they were fully inserted. This proved to be a good move, because I kept hitting rough PLA as the rods were inserted, which made me think that they were home already.
The belt is fitted before the mount caps are screwed into place - I didn't take a picture for some reason, but do follow the instructions on this stage - you won't be able to get the belt in easily (on the motor assembly at least) with the mounts fully assembled. The idler end could probably be done, but it would be fiddly. For the motor assembly - forget it, you need access to route the belt.
Here's the belt tension adjuster system I was talking about above - like I said it works, but barely and only once.
The idea is that you fold the belt back on itself and stuff it into the recess on the right hand side. The left side has a shallower opening, with teeth which grip the belt firmly. Before stuffing the belt in, you run a loop of chromed wire through the belt loop, and under the socket screw. Once the X axis assembly is installed into the frame, and you have set the exact width using the bolts protruding from the idler mount, you twist the chromed wire to tension the belt. Its a simple solution granted, but the wire broke as I was twisting it, making it useless for later adjustment.
If you've come up with a better solution for this, please let me know!!
The final two recessed nuts can be seen in this picture, in place, with rods threaded into them. The two bolts took me nearly two hours to get in place and involved a lot of filing, swearing and ultimately brute force. I did manage to get them in straight though which is a relief.
Quick tip - leave the rods in place for the Z axis next step - I took them out and ended up having to do some disassembly on the next stage, which was my own fault for not paying enough attention to the instructions.