I don't think many 3d printers are really suitable to machine metal beyond milling a pcb, which is closer to engraving anyway. I'd go for a dremel with an extension so you move the weight off the machine if you really want to give it a go.
The prusa i3 and clones are not stiff enough to get the end mill dig into the material with sufficient stiffness and force to get good chips. It will deflect and rub more than cut, which heats up the endmill and material leading to melting material and endmills loosing their hardening and breakage. The nema 17 steppers are more optimized for speed than for torque.
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Kind regards
Imqqmi
NFAN CoreXY printer:
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reprap.org]
