an apparatus for breaking solid objects includes a box - shaped support open at the top and at the bottom , a substantially horizontal rotor mounted in the box and carrying at least one knife which turns with it while passing between backing knives , an active face of which is turned toward the rotor , and preferably has teeth , the backing knives being movable independently of each other and having their own return means , for example by oscillating the backing knives on a pivot parallel to the rotor . the knives may be set in forced oscillation , and may be individually removable .