I have been designing a couple of machines for a while now, design changes are all free when you have most of the parts still to buy - I only have a 600mm ballscrew so far (intended for Z axis use on one design).
I intend to build a small 3 axis one (about A4 or smaller) for PCB engraving and 3D printing and a large 4/5 axis one (1.2mx2.4m) for wood
I would also like to use the frame on this one for hot wire foam cutting, probably involve a separate set of motors at least partially - not given too much thought about integrating this yet.
Loads of information gathering, research, etc. The large one has evolved from something along the lines of "Joe's JGRO", through "Joe's 4x4" into something else entirely, frame made of Unistrut, gantry, etc from aluminium extrusion, all being done as cheaply as possible - finds on ebay, etc all help here.
I will start a build thread soon and let you see the current design (all done in sketchup), still playing with ideas for the extra rotational axes.