Firstly, I'm a welder, not an electrician so sorry about not knowing too much. The appliance is a very old sewing machine. The motor does run on AC too, but I think it's cool that it can use both, and I would like to design a system to run it from some batteries provided it wouldn't be 100% a death trap like some friends (who are also not electricians) say. I am redoing the wiring and already have grounding points cleaned and/or drilled in the motor housing, the metal foot pedal, and the cast iron body of the machine. Obviously the worry is that since the whole thing is metal, it could become live if one of the wires wears through. If it comes live at 100-110v DC (with 0.6amps) is it more likely to kill someone touching it than AC? If I understand correctly, something like a GCFI wouldn't work on that kind of circuit, because it's DC and because of the batteries. Are there other safety solutions? Or would it just be a case of keeping an eye on the wires and not using it near water?

(I know I could use an inverter to give it AC power from batteries but that's not what this question is about)

 

If it is running on batteries and there is no connection ground/earth then it is perfectly safe to touch either terminal of the battery. So a short from one of the wires to the metalwork is harmless. A fuse should be used so that if two wires, one from each side of the battery, short to the meatalwork the fuse would blow rather than lots of current damagin the battery or wiring.