Abstract The cutting energies utilized by rock-working machines are reviewed in relation both to the rock compressive strength and the laws of comminution. The conclusion is drawn that the energy required for breaking into rock is much greater than for breaking it off. The quotient of the rock compressive strength divided by the rock cutting specific energy has proved roughly constant for given designs of rotary machine and forms a comparative parameter for machine efficiency. A mathematical model of the operation of a rock-working machine is derived: it comprises equating this parameter to the relative efficiency of the method of fracture multiplied by the relative debris size produced.
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