The machining parameter settings installed at CNC EDM machines are developed under optimum process conditions. Standard workpiece and electrode materials are used traditionally by machine manufacturers to establish the EDM parameter settings. However, this is not the usual situation of the tooling industry, where many different grades of workpiece and tool electrode materials are used. Consequently, the customers are required to develop their own process parameters, which normally demand many experimental tests. According to the aforementioned argument an experimental investigation on the EDM of AISI P20 tool steel under finish machining has been carried out. The tests were performed with graphite and copper as tool electrodes. Important EDM electrical parameters that influence the process performance were investigated. The measured technological outputs were the material removal rate Vw, volumetric relative wear J and workpiece surface finish Ra . The main conclusions can be summarized as follows: the best results for material removal rate Vw were reached when EDM with negative graphite electrodes. Graphite and copper tools presented similar results of Vw for positive polarity. For graphite and copper tools the lowest values of volumetric relative wear were achieved for positive polarity. The best surface roughness Ra was obtained for copper electrodes under negative polarity