How does ambient temperature variation affect EDM machining tolerances?

Although EDM operates as a force-free cutting process capable of holding sub-micron positional increments, thermal expansion of workpiece steel can quickly exceed tight drawing tolerances. Steel expands at a coefficient of approximately 6.3 millionths of an inch per inch per degree Fahrenheit (roughly 11 to 12 microns per meter per degree Celsius), meaning that a standard ten-inch steel block expands by over six ten-thousandths of an inch with only a ten-degree temperature rise. Handling a finished piece with warm bare hands or running unchilled dielectric oil causes thermal drift that throws high-precision parts out of tolerance, making continuous dielectric chillers and climate-controlled rooms necessary for close-tolerance jobs.