crushed parts 8 being pulverized along the edges of the holes 10 and subsequently after being of small enough size passing through the holes 10 to drop onto a subjacent chute 11 that transports the scrap parts 8 out of the hammer mill 1 and onto a conveyor belt 12 a hood 15 with lateral shield 16 is secured to the framework 2 and extends around the grate assembly 9 at a distance thereto to direct scrap 8 passing through the grate assembly 9 toward the chute 11 as shown in fig1 the bottom grate 29 extends transversely to the axis 6 of the striking tool 5 so that an elongation of the bottom grate 29 does not affect the lateral shield 16 of the hood 15 turning now to fig2 a there is shown a cutaway view of a grate bottom grate ejection door or ceiling grate of a conventional grate assembly 9 39 which is formed with a symmetric disposition of square holes 10 39 in longitudinal and transverse directions to define webs 19 39 20 39 during extended use of the grate 9 39 these webs 19 39 20 39 cause an elongation of the grate 9 39 in direction of arrow 17 in order to prevent elongation in both directions the deck 9 has a grate assembly which according to the cutaway plan view of fig2 b has formed therein holes 10 exhibiting a hexagonal configuration and so positioned in an offset relation as to define webs 19 which deviate from a straight line in longitudinal direction as well as transverse direction such a configuration is applicable for the bottom grate 29 ejection door 13 and ceiling grate 14 in a peripheral area of.