Feb 13, 2017 Critical speed graph. Example A mill with 4.5 m dia (15) N obtained 20 rpm. If N 15 rpm nc obtained 75 %. Ball Mill Lining. The mill lining can be made of rubber or different types of steel (manganese or Ni-hard) with liner types according to the customers requirements. A Ball Mill Critical Speed (actually ball, rod, AG or SAG) is the speed at which the centrifugal forces equal gravitational forces at the mill shells inside surface and no balls will fall from its position onto the shell. The imagery below helps explain what goes on inside a mill as speed varies. Use our online formula The mill speed is typically defined as the percent of the Theoretical ...