The Makerbot uses a thermoplastic filiment as feedstock, which it melts, extrudes, and deposits to build up a form. Unfortunately, metal just doesn't act the same way. If you've got a MIG welder, try it yourself - you can't just lay down lines of metal and expect them to pile up neatly on themselves. What you get is a puddle.
There are machines that build objects directly in metal, but they use a different principle, starting with a bed of powdered metal and fusing a line in it with a laser or other concentrated heat source. Then another layer of powder is laid down and the process repeats. Look at EOS Metal Laser-Sintering Systems for more info on how this process works.
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