I've followed a similar path the Neale, the first machine was a useless plywood and MDF contraption that at least proved I was capable of puling all the components together into a working machine. The current design is a steel frame which is mostly bolted together though the second gantry I built for it is welded using a dirt cheap stick welder that I bought for the job having never welded before. I used a technique I call 'bird poo' welding due to it's appearance. Grinding down the lumps and smoothing things off with car body filler followed by a coat of paint hides the evidence! It works fine.

I managed to source all the steel from the local tip so it was a cheap option for me! The design of the machine was partly dictated by the available material.

I followed Neale's excellent advice about making everything adjustable and the trick then is working out how to measure all the errors in a systematic way to isolate each source of error. There's still more to do but it's getting better in stages. I'm only cutting wood so my ultimate target accuracy is to put the tip of the tool where I want it be within a whopping great margin of 100 microns.