according to one preferred embodiment the devices 7 by means of which the segments or flaps are pivotally journalled in the housing 1 preferably have the form of flap adjusting devices which enable the flaps 5 at said one end to be adjusted radially towards and away from the mantle surface of the rotor 4 thereby enabling the grinding gap formed between the flap and the mantle surface of the rotor 4 to be adjusted to a basic setting in order to enable fibre material or other material to be worked in the grinding gap of the apparatus to be delivered to the gap the apparatus includes a material inlet 2 which communicates with a central channel 15 surrounding the rotor 4 the fibre material is dogged or otherwise entrained to the material outlets 3 by rotation of the rotor 4 as shown in fig2 while being worked between the flaps and the mantle surface of the rotor 4 the material leaving the apparatus through outlets 3 although the illustrated embodiment is shown to have four grinding flaps or segments which cover the major part of the mantle surface of the rotor 4 it will be understood that the number of stationary grinding segments or flaps 5 can be varied without departing from the inventive concept several material.