I remember seeing a demonstration of a seemingly magic process at an engineering open house decades ago, in which a soft metal bit carved detailed shapes into far harder metals. It's called electrochemical machining (ECM), and it's so simple in principle that you can do it at home with a drill press, a battery charger and a pump for a garden fountain.
ECM is basically electroplating in reverse. In electroplating, you start with a solution of dissolved metal ions and run an electric current through the liquid between a positive electrode and the object you want to plate (the negative side). The ions deposit themselves as solid metal onto the surface of the object.

This process is used industrially to create extremely delicate, detailed shapes in very hard metals. Since there is zero force exerted on the part being machined, it's possible to make fine shapes that would break if you tried to cut them with a milling machine.
Without the precise current control of those commercial systems, my home setup produced a disappointingly blurry copy of the earring's shape. But it's still amazing that in a contest between a tin earring and hardened steel, the earring won.
Achtung! When your hands are wet with saltwater, even 12 volts is potentially dangerous. Wear rubber gloves.
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How does this differ from EDM? Popular Science had a project for a home EDM machine back in the 60's.
EDM and ECM both use Electricity to machine metal.
EDM = Electric Displacement Machining,
ECM = ElectroChemical Machining.
The difference comes in how they are applied. ECM Means that you will have a pattern that is not conductive and the part being "Machined" is in electrical conductive solution and the current etches the design. EDM's typically use a wire or a steel wheel that the electrical conductive solution flows over. By moving the wire or wheel the solution forms the shape and cuts it out of the material. My dad operates a Wheel EDM to cut Stainless Tubing into sharp points without burrs.
from Miami Beach, FL
This is an interesting demo, but without a mechanism to better control the movement of the "cutter" it is pretty useless, except for making blurry blobby indentations.
I am sure there is an increase in electrical current before it actually starts arcing, so using that increase to control a servo driving a screw drive would seem to be fairly simple compared to making your own cnc milling machine.