BB Video: Carve Steel with Saltwater, Electricity and a Tin Earring (Popsci)
Boing Boing Video teamed up with Theo Gray and Popsci.com to produce this video that demonstrates how you can mold steel with electrochemical machining, using a soft, cheap piece of tin -- without any physical contact. Theo is the author of the book Mad Science, in which many other experiments like this are featured. Theo says:
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.Read the whole HOWTO over at popsci.com: Carve Steel with Saltwater, Electricity and a Tin Earring
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.
Image below: "The tin peace-sign earring acts as an electrode, etching away the metal in the hardened steel washer [left]. The imperfect results are due to the difficulty of manually maintaining an exact thousandths-of-an-inch distance between the two. Commercial electrochemically machined pieces, like this microturbine for a water pump, use sophisticated electronics to monitor the current flow and carve precise pieces [right]. (Courtesy ECM Technologies BV/ ECM Productions BV; Mike Walker)