the plane pole surface of a magnet 14 for magnetic attachment of the tracer unit 2 to the work piece 6 whereby distance bosses 16 on the side surfaces of the cavity 4 provide a precisely defined attachment position the electronic discharge wire 18 of the machine is shown to extend vertically an upper guide head 20 and a lower guide head 22 of the wire 18 are represented schematically from the free side surfaces of the tracer unit 2 extend outwardly a first tracer cylinder 24 and a second tracer cylinder 26 the first tracer cylinder 24 has an axis 28 extending in the y direction of the machine and the second tracer cylinder 26 has a longitudinal axis 30 extending in the x direction of the machine the first tracer cylinder is provided with an eccentric fixing shaft 32 on the tracer unit 2 sic and the second tracer cylinder 26 has an eccentric fixing shaft 34 on the tracer unit 2 by rotation of the tracer cylinders 24 26 around the fixing shaft 32 and respectively 34 concerned the pertinent perpendicular tangential surface 35 and respectively 36 can be caused to extend flush with the pertinent side surface 12 and 10 respectively of the work piece 6 or at a defined distance from the said side surfaces 12 and 10 respectively the tracer cylinders 24 26 are composed of hard metal at least on.