Starting to design out my build, looking for critique

[AndyUK]

Cool, changes look to be in the right direction. Z axis will be much more stable - even more so if you can space those bearings apart a bit.

Add a mounting point for the Z-Axis NEMA 34 behind where the Y-Axis ballscrew currently connects to the Y-Axis sled.

Woowwww there cowboy! Hold up a second! A Nema 34 for the Z axis? That's a little overkill. Get a much lower inductance Nema 23, gain yourself some performance by not having a big inertia to overcome (and take the extra mass from the moving carriage), and give yourself more space up on the Z. Something like 2-3Nm - even that will be more than sufficient.

Substitute the Aluminium Profile for Steel Box section and weld it. The warping should be controllable in a machine this size, is what I am hearing.

Certainly do-able. I planned to do the same myself initially. But the welding will warp it, and mild steel box section is nowhere near flat to begin with, so either way you'll need to mill the faces you want to mount rails to. Or, you can epoxy them, but that won't be easy to get the two faces parallel. Then you have to consider how to attach the box section to the gantry end plates, which probably means capping them off and tapping them etc.

Rigidity? Look at my build thread. I simulated a steel L shape out of a box and a rectangle, then I simulated a slightly smaller Ali HD profile. Although the young's modulus of Ali vs Steel says you'll get more deflection from Ali, when you take the profile shape into account its really minimal. I think I went from 3um deflection in steel to 4um in Ali (but the real numbers are in the build log). Either way, its nothing to be concerned with.

Profile Ali is flat enough to mount straight to and just shim where and if needed. It also has nice ends which can be tapped in about 5mins and bolt directly onto your end plates.

Weight? Every extra kilo is lost acceleration!

Next question is cost - In my case, the aluminium was only marginally more expensive. Really not much in it - Approx £40 for steel, and £50 for Ali. The ease of Ali just won the day for me by a long margin.

Change the Y-Axis gantry to a design that has a Profile up top and one below with the ballscrew sandwiched in between. The rails would go to the front and both profiles would be held together at the sides, like they are now, and with an extra plate at the back. Alternatively manufacture this out of steel section.

So, two tubes with a large gap between? From memory of my deflection calculations that's a far less rigid structure. The L shape is good because it resists deflection both up and down, as well as twisting around it's own axis. I think that design would perform poorly in both situations.