Gantry Aluminium thickness guidance

[dachopper]

If you don't have need for tall material or long tools then wouldn't go that route as your losing stiffness for no reason. It's not more difficult building it the other way around.

Also have you realised that you'll need to space the rails off the plate or machine channel down centre to allow clearence for the ballscrew/bearings.?
Best if you take little from each plate depending on the ball nut mount the total need machining off is the distance from bearing face to ballscrew flat which is 45mm. Profiled rail and bearings total aprox 30mm depending on make, (Hi-Win HGH-20CA are 30mm) so I'll let you do the math.?
How you devide it upto you, I just split it in half.

I'm planning on using up to 200 mm z height, so really needed a design that maximizes this, when I put the first plate with back to back bearing blocks, it becomes hard to do them up ( I have to offset them , in gives less stiffness, down I loose z clearance, and up I have a greater z overhang for the same clearance. I can always bolt angle onto the spindle plate sides if on the future there is some play going on.

My plan is for the rails to be mounted flat on the back plate, I will add 15 mm spacers between the bearing block and the spindle plate to get the extra clearance as at the moment making a rail spacer, or counter sinking is far more work.