In a spark erosion machine in which the tool electrode comprises a tensioned flexible wire or tape, machining induced force due, for example, to the magnetic and electric fields induced by the sparks, act on the tool electrode and may cause it to vibrate. To reduce the tendency for such vibration, a compensating signal is passed through the tool electrode or applied between the tool electrode and workpiece so as to produce a magnetic or electric field which compensates for the machining induced forces. The compensating signal is preferably adjustable in magnitude and/or frequency (the latter assuming it is an alternating signal) and the output of the generator which produces it is decoupled from the output of the spark pulse generator. The spark pulse generator and the compensating signal generator may be interdependently synchronized by a synchronizing signal lead connected between them.