The driving force of the mill is transmitted via the liner to the charge. At relatively low speeds, or with smooth liners, the medium tends to roll down to the toe of the mill and essentially abra- sive comminution occurs. This cascading leads to finer grinding, with increased slimes production and increased liner wear. At higher speeds the medium is projected clear of the charge to describe a series of parabolas before landing on the toe of the charge. This cataracting leads to comminution by impact and a coarser end product with reduced liner wear. At the critical speed of the mill the theoretical trajectory of the medium is such that it would fall outside the shell. In practice, centrifuging occurs and the medium is carried around in an essentially fixed position against the shell.