I built myself a cnc router of my own design.  I constrained myself to a footprint of just one square foot and was able to create a fairly capable machine.

I used 20mm aluminum extrusion, nema-17 stepper motors, and a 400w DC spindle. The gantry style design keeps the moving components clear of debris and coolant.  A TinyG controller is connected to a BeagleBone Black to wirelessly stream g-code to the machine.

The work area is rather limited at only 150x150mm, but I have found many uses for the machine.  It can cut circuit boards in a snap and moves through acrylic at a quick pace.  Aluminum is a bit of a challenge, but simple 2.5-D jobs can be run with conservative feed rates and depth of cut.