Building a CNC router table is such a great idea. I've been building furniture for a long time but I'm a better designer than a patient woodworker.
I keep pondering having an 8' gantry that travels 4'. Here's my newbie thinking; The gantry will be heavy due to it's length and extra support to compensate it's span but it's driven by two screws. You will need less screw length; two 4' sections in the X and one 8' in the Y for a total of 16' compared to 20' if the axis were reversed.
I'm not going to dive into such a large bed for my first build. I'll start with a smaller machine to learn from and use it to build a larger one. That brings me to my second question. Can you cut aluminum with a CNC router that was intended for wood?
Saving 4 ft of screws is not a good enough reason to double the length of the gantry. You'll now need to build an 8 ft long beam that doesn't sag or flex at all. A 4 ft beam would be much stiffer.
Also, When you get over 4ft, you might want to look at rack and pinion. You need much bigger screw diameters for an 8ft screw, and the cost goes up quickly.
Yes, you can cut aluminum. But, how fast you can cut it depends on the rigidity of the machine.
Mach3 2010 Screenset
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)