David,
What types of parts are you looking to cut? That plays a big factor. My dad built a
CNC routing table with a 24"x12" usable table when I was a little kid, which still runs and works great. We use it mostly to make
one-off signs & cribbage boards (they make great gifts!). It can cut wood & plastics at 20 inches/minute, which is pretty slow (the sign in the upper right of second link took nearly an hour to cut), but passable. 1276 borrowed our school's ShopBot table (8'x4') to make
these handouts in 2007.
www.cnczone.com is a great resource. I was too young to participate in the building of the one we have now, but almost everything he learned in the construction process was from that website. You've got some reading to do!
