I would change the joint for the top Y-plate that holds two of the Y-axis bearings to make it easier to adjust the rails and bearings so that they are parallel. Currently you're relying on the plate at the back being both precisely the right height and having parallel edge faces, which is risky. If you change the joint round so the back plate is bolted to the aforementioned Y-bearing plate, then that plate with the bearings will self align to the correct position.

I strongly advise using timing pulleys (most likely 5M HTD) at the very least on X and Y since as you have chosen 5mm pitch ballscrews, you may need to use a 2:1 ratio between the motor and ballscrew to get decent feedrates for cutting wood. Since you're planning on mainly cutting aluminium, that's not so big a deal, however pulleys also help damp vibrations, so reduce resonance problems. Using pulleys also means you don't need motor couplings, which are often a weak point as you can see from the number of threads about which ones to choose. Pulleys easily tolerate a lot more misalignment than oldham couplings, so it arguably makes mounting the motors easier.

Have you considered not having gantry sides and making the machine frame such that the tool operates below the level of the X-rails, rather like your first machine? This tends to be much stronger as it replaces the flimsy gantry sides with the steel frame, which can be made as strong as you want since it's not moving. This is especially important since you want to cut aluminium, which as you no doubt know requires substantially higher stiffness than your previous machine. If you stick with the current design, then most of the time the Z-axis will be operating at near full extension, so the force imposed on the gantry due to cutting forces will be significant. Even if the material is raised up, you still have gantry sides which aren't as strong as a steel frame.

Quote Originally Posted by Ross77 View Post
I would still be tempted to widen the y axis to create a squarer beam, as this will improve the torsion resistance when the cutter is in he middle of the beam.
I would too.