The only problem with gas welding is the distortion. Whenever you heat metal, it shrinks in the area heated and pulls everything out of square. That'll make it really hard to get a good CNC frame done! I have a little 110 Century MIG that my mom got me when I graduated high school (thanks mom!), and in the last 7 years I have welded everything from 22 ga, to 1/4", though the latter is very tough to get good penetration on. I think that it cost somewhere around $400, and would be just right for welding a 3/16" tube frame together. By the way, the gas bottle is almost never included! Don't learn this the hard way. I would like to suggest though, that you bolt together your first frame. This will get you up and running, and alignment won't be so difficult. Even professional welders can have a hard time welding together a steel frame that is true to even 1/16". Its hard enough to build your first machine, without learning to weld at the same time. Don't take it the wrong way though! Welding is an invaluble skill, that most people love, but get to know it well first. That way you can save the surprises for later when you know how to handle them! And the fewer surprises, the less expensive anything is.