The appts. for a controlled material removal action, by spark erosion, has a power supply connected to the workpiece and the tool. A control unit (3) contains a spectral evaluation unit (6) which studies the actual values of voltage and/or current from the frequency 0 of different frequency spectrums, to pass a setting signal through a control amplifier (5) to control the output values at the power supply. This is to maintain an output signal from the evaluation unit (6) and the arc intensity as constant as possible.
