Tower mills are considered to be appreciably more energy efficient than ball mills. Why this should be so is a question which can be explored by using DEM to simulate one machine of each type with similar breakage capabilities. This paper reports on a comparison between a pilot scale tower mill and a small ball mill in terms of the power required to produce reasonably similar distributions of normal and tangential impacts. While the tower mill produces quite a narrow spectrum of normal energies, the ball mill produces a wide distribution. Hence, the ball mill can be expected to be Much more "forgiving" of variable feed conditions but much less efficient in terms of utilization of the energy from media interactions. (C) 2009 Published by Elsevier Ltd.