Sinker EDM is a nontraditional machining method of removing metal by eroding a part with electrical discharges between an electrode and the workpiece. The EDM can erode parts regardless of its hardness as long as the part is conductive.
Sinker EDM is typically used in the mold die industries, but also used in production parts, aerospace and medical industries in which production requirements are relatively low. EDM is commonly used for parts which are made from materials that are difficult to machine and/or intricate cavities or deep ribs. If all other machining processes fail, your answer is probably a sinker EDM.