each chip clearance protrusion is preferably formed with a profile having a sloping leading edge e g as indicated by 12c whose profile forms an obtuse angle with the profile of the preceding portion of the periphery e g angle u03b8 in fig1 this sloping leading edge of each chip clearance protrusion is important for the operation of the apparatus as described in detail hereinafter the overall shape of each chip clearance protrusion is blunt as viewed in profile i e the chip clearance protrusion do not perform a penetration function during shredding this function being carried out entirely by the cutter teeth in this embodiment the chip clearance protrusions each have a profile as viewed along the direction of rotation of a disk in the form of three sides of a trapezoid with one of these sides being the sloping leading face referred to above each chip clearance protrusion in a pair of protrusions is positioned to lead the cutter tooth of that pair with respect to the direction of rotation of the cutter disk on which the protrusions are formed it is an essential feature of the present invention that the angular position of the roller shafts are precisely arranged such that as indicated in fig1 as the two roller shafts counter rotate each cutter tooth on a cutter disk.