My first CNC machine has a long z axis. You can see pictures of it on:
The Z axis steel tube measures 36" and the z travel axis is about 24" to 30" ( I have not measured it). In order to make the frame stiff, I used thick mild steel beams. I welded some of the frame pieces together and bolted down the rest. Since this is my first CNC machine and I have not yet finished it, I do not know how good my design is.