A comminution apparatus for refining various materials into smaller pieces of a desired size, the apparatus having a pair of generally circular disks disposed on a common axis and arranged generally parallel to one another forming a comminution region there between. Each disk incorporates a plurality of removable cutting bars radially disposed on the disk face, one disk having an even number, the other having an odd number of cutting bars. Process material is fed into the comminution space between the opposing disks and comminuted by counter-rotation of the disks which shears material between opposing cutting bars as they pass. Spacing between the disks is adjustable to suit the input process material. Tapered disk faces control the flow of material during the comminution process, permitting only material that has been sufficiently reduced in size to migrate toward the disk perimeter where a skirt ring structure controls the size of the granulated material output.