I don't know the dimensions but, that kind of bearing assembly and ballscrew should let you put the spindle closer to the gantry. It will be hard to have it all really parallel. I quite like that you use the linear rail as part of the structure of the Z component. Check the deflection gantry it seems that it will be huge. It would be better if you could make a square beam using aluminium plates (aluminium tubing has too little wall thickness) and have the same assembly procedure as you have right now. Because the gantry is not moving, you can make it as rigid as long as the table/floor can hold it. In my case, I wanted a compact and easy to tune CNC so I went with a moving gantry but that design has lots of problems. Like: chip management, sealing against dust all axis, limited mass on the moving axis, big enclosure, difficult to assemble, hard to water cool all of the moving members, 4 motors for 3 axis, 4 encoders, etc.

Im certain you are going to see a lot of vibration issues with the design, especially when cutting deep. You should try to alter the design so there is a more rigid connection with the rails and the spindle, especially because its sticking out

3 Issues I see with this design. 1: Wet noodle. When this is extended more than a few inches, you'll be able to move the spindle visibly with your own strength. I'd guess you need to stiffen this by at least a factor of 10 to have it usable for more than drilling. 2: Accuracy issues. Those types of linear rail are not guaranteed to be straight. They need to be screwed down and and held against a reference (flat) surfaces. 3: Not much clearance. In the configuration you show this most of the time, with the axis as far up as it will go without coming out of the lower block, the spindle shaft sticks a couple inches below the gantry clearance, and any tool inserted in it will just stick out further. If you move the spindle up the rails, then the rails will have to protrude below the gantry when at it's lowest position. Linear rails are not a good substitute for a real frame.

Love it! Excellent concept and explanation of its advantages. Are there any new disadvantages to this approach over the more traditional ways to build Z-axi?

Did this for 2 years ago you will get vibration and a no rigidity

Ive made mine like that 25 years ago using a 16mm thick steel plate to suport the rails and its not as rigid as having the rails fixed to the gantry ,so i will change it for a fixed rail version. the max depht of cut in steel using a 10 mm end mil is about 1mm not good enough for me. So go for the fixed rail if you can its much more rigid.