Abstract Results of experiments investigating the motion of balls deep within the charge of a rotary grinding mill are presented. The true centre of mass of the entire charge was determined. The angle of repose of the charge was found to be uniquely defined by the centre of circulation of the charge. The angle of the cascading region is determined by the angle of repose of the charge. The inner part of the cascading region and the toe region are responsible for the majority of the grinding work. The face angle of the lifter bars was found to influence both the maximum impact energy and the amount of work done by the charge. Radial segregation of the balls takes place throughout the charge according to both the mass and size of the balls, even in a mill with smooth end faces.