A shredding machine for domestic or office use having a feed passage 3 leading to a cutting mechanism 10, 11 powered by an electric motor, has a thickness measuring device 15 for measuring the thickness of bundles of paper fed through the feed passage and the machine is controlled by a microprocessor which receives signals from the thickness measuring device and prevents the cutting mechanism from being energised if the thickness measured is above a threshold determined by the microprocessor. The microprocessor varies the threshold in accordance with electrical supply voltage, the electric motor temperature and the electric current drawn by the motor during a previous shredding operation, so that the maximum thickness the shredder will accept can be reduced automatically when motor temperature increases or as the effectiveness of the machine deteriorates throughout its working life.
