A cutter assembly is presented for mounting to one or more hammers of a rotor assembly of a grinding machine. The cutter assembly includes a base, a tip and a fastener securing the base and the tip to the hammer The base includes a key having side walls extending upwardly from a surface of the base and spatially oriented as sides of a pyramid that is truncated before reaching their apex. The tip includes a keyway having side walls extending inwardly from a lower surface into a body of the tip and spatially oriented as sides of a pyramid. The keyway mates with the key of the base. The tip includes at least one, and preferably two, cutting edges disposed on opposing ends of the tip. In one embodiment, the tip is reversibly mountable to the base such that the cutting edges may be selectively used within grinding operations. In one embodiment, at least a portion of side walls of the truncated pyramid key and keyway are curved in a plane parallel to the surface of the base.