So how many machines are you planning to build? If you started a thread for each one, it would be easier to understand what you're asking about. If you want a large mill for cutting steel, what most people here do is retrofit an old cast-iron mill with new electronics. That's a lot easier than starting from scratch. I don't understand why you'd want to use 4 servo motors for the Y axis, though.

If you want to build a machine that will additively produce metal objects, that's a whole other topic. I'd suggest you get a MIG welder and try to build something freehand with it, if you think that's a good way to go. Or try aiming a TIG at a pile of metal powder and see what happens, before getting involved in a project to automate these processes.