3d Metal Printer
A just-for-fun-idea (at least until I build a much less ambitious project like a [CNC machine] or plotter.
Aluminum wire is pretty cheap and not a lot of special supplies are needed for arc welding. It may be possible to feed a wire in to deposit a weld bead. If this were done in a near vacuum, it should come out pretty clean. It looks like TIG welding is closes to what I need. Possible a pure nitrogen atmosphere at near vacuum would work.