hi

thee is also the possibility of extending it
.

If so, you may wish to consider the rotating ballnut design for driving your X

now toying with a 4th axis to make columbs instead of building another machine.
Have you played much with the software, because there are options for 2 sided machining, and it would be a lot easer to do columns with 2 sided machining than rotating it. 4th axis is well worth doing, but if you specificaly want colums, or anything of that weight, you would be better doing a vertical machine just for them.

I would have a play with some of the software, meshcam ( free trial ) is a good bit of software to start with if you want to see what 2 sided machining can achieve, the learning curve of the software is reletivly easy as well

MeshCAM - CNC Software - CAD/CAM Software