3d printer is probably the wrong technology anyway. They're cool and I would love to have one but something in CNC would get you something better, cheaper and more reliable.
All you need to do is change the print head out for a CNC cutting head, and (assuming you have the right software) you're good to go.Nope that wouldn't be as efficient, a 3D Printer isn't sturdy enough for milling and a mill isn't fast enough for printing. You need both until you figure out how to do it. Some professional SCARA robots might qualify but those have high end material and precision requirements. It's not out of reach but there is quite some road ahead to get there.
I've seen an industrial robot arm repurposed as a 3d printer. That would handily run a mill. But yeah, your average makerbot isn't going to be able to handle chewing through block aluminum (much less steel). Casting from a printed positive, though, is still a very appealing possibility to me.
