A machine that cuts hardened steel without ever touching it. Electrical discharge machining removes metal with controlled lightning. The workpiece and a shaped electrode sit submerged in dielectric fluid, separated by a gap of roughly 10 to 100 micrometres. A pulse generator raises the voltage until the dielectric breaks down. A plasma channel forms, reaching temperatures above 8,000 degrees Celsius. The metal does not shear. It melts and vaporises. Then the voltage collapses. The plasma implodes, dielectric floods back into the gap, and the molten pool is flushed away as microscopic debris. What remains is a crater a few micrometres across. Repeat tens of thousands of times per second and you have a machining process. The consequences are strange. - Hardness is irrelevant. Tungsten carbide, hardened tool steel and Inconel erode at similar rates to mild steel, because removal depends on melting point and thermal conductivity, not mechanical strength. - Cutting forces are effectivel...