ROD MILL (STRM -240/300) Working Principle: A rotating drum causes friction and attrition between steel and ore particles. But note that the team ‘rod mill’ is also used as a synonym for a slitting mill, which makes rods of iron or other metal. Rod mills are less common than ball mills for grinding minerals. The rods used in the mill, usually a high-carbon stee, can vary in both the length and the diameter. However, the smaller the rods, the larger is the total surface area and hence, the greater the grinding efficiency.