Cusps and scallops of hardened steel moulds produced by high-speed milling using a ball-nose end mill were mathematically modelled, characterised by microscopy and experimentally validated. The experimental results show that the part material is crushed or ploughed near the cutter centre, where the cutting speed is very low. This kinematic singularity, associated with tool feed, compresses and bends the ball-nose end mill axially. Because of this double effect, the end mill marks on the part at the end of the milling path cause surface damage and dimensional errors to the hardened mould. A mathematical model may predict the formation of the cusps and scallops and be of use in computer numerical control or computer-aided manufacturing programming to obtain the desired part topography.