There are a couple of design decisions that might deviate somewhat from the 'standard' router configurations and might be worth discussing:
First, I intend to have the target 'cutting surface' positioned below x-axis shaft level. In a sense, the router will "cut down below". The idea is that this will decrease the required height of the gantry and it works in conjunction of the second point below. In effect, the lower y-axis shaft could be at around the same height as the x-axis shafts - or even below. This will reduce the all-over gantry height and bring everything closer to the x-axis mounting points. It helps, as I am using a fairly tall z-axis (shaft 400mm; z-axis movement range 210mm).
Second, I intend to avoid the "bridge" configuration, where the x-axis drive setup, eg, belt drive or threaded bolt, is centered below the floating cutting surface and connected across to both sides of the gantry. This is probably my greatest fear that it could go wrong (precision issues, or worse, lock-ups). The x-axis will be driven on the side, not the centre as my theory is that side versus centre improves alignment only by a factor of two (you are encouraged to disagree with some maths), and I found the ball bearing setup to resist alignment problems very well.
Cnc y-axis gantry. Work in Progress...
For reference, y-axis shaft length is 600mm.
If you look closely at the design, you will also notice that some of the board positioning has been chosen very carefully to be able to reach screws during assembly, for example, the inner mounting screw for the rear x-axis ball bearings. On that note, have a guess how I assembled the z-axis' nut that sits quite invisibly under the small square mdf board and is held in place by a piece of small mdf board.
Cnc y-axis gantry - from behind.
In other news, I bought a few nuts and bolts today to assemble idlers from my wide ball bearings. I think this will work very well.