I am pondering building from scratch and have what is (apparently?) a rather odd idea.
I am thinking that if a gantry-style machine was built to only use tiny bits, then the stress on it would be much lower, and it could be built of steel 3" x 1.5" x 1/8"-thick rectangular tubing (hand-scraped flat) instead of solid iron. Much cheaper + easier to move around as well....
Also since the stress of cutting is intentionally kept so low, would it be possible to use split nuts instead of ballscrews? The ballscrews I'd need would probably cost ~$500+ alone. If precision acme screws were used instead, the price drops to ~$100 or so.
It would cut a lot slower than larger+heavier machines, but for hobbyist use that's not a huge drawback. -Unless there's something else I don't know about it. The longest 1/8" end mills I've found have a 1" depth of cut, which would be one limitation. It would also be possible to build the spindle to accept bits up to 1/4" or so, and just limit the toolpaths to light cuts. That way you could get bits that had a depth of cut up to 2" or so.
I have a Grizzly G0619 mill (SX-3) that can be converted, but the work envelope is too limited. I'd want the CNC machine to have a X-Y-Z of maybe 30" x 12" x 6", and to hold maybe .001" over most of that.
I haven't found a lot of people who have built 3-axis metal-cutting machines from scratch, so maybe I haven't been using the right search terms. All I find is people making routers (with a limited Z-axis) or plastic extruders (much less stress and precision needed), or people converting existing manual milling machines. Does anyone do this much?