A Ball Mill is a rotating cylinder containing steel balls, which act as the grinding media. The material to be ground is put into the cylindrical barrel, which is rotated at a speed between 4 and 20 revolutions per minute, depending on the diameter of the mill. The larger the diameter of the cylindrical device, the slower the rotation speed. The rotation produces centrifugal forces which lift the balls to a given height causing them to fall back into the cylinder and onto the material to be ground. The product is mixed and crushed to powder by the grinding media (steel balls) as a result of the rotation. If the speed of the mill is too great, the cylinder will act as a centrifuge, causing the balls to stay on the perimeter of the mill instead of falling back. The point at which a ball mill acts as a centrifuge is called the "critical speed". The operating speed of ball mills is usually between 65% and 75% of the critical speed