A worm and worm wheel gearing is widely used in a geared motor unit for the convenience and safety of an automobile. For mass production of a high quality worm, the current rolling process is substituted with the milling process. The milling process offers comparatively accurate machining quality and high production capacity for worm manufacturing. Moreover, since the milling process enables the integration of all operations of worm manufacturing on a CNC lathe, production efficiency can be remarkably improved. However, there are several important factors to be considered for producing high quality worms such as cutting force, tool-workpiece interference, and others. Planetary milling and side milling are generally applied to machine worms. In this study, the cutting characteristics of worm machining on an automatic lathe are investigated for two types of milling processes and those processes are compared with each other. A tool-tip trajectory model based on tool-workpiece interaction is proposed, and then tool-workpiece interference and cutting force are simulated with the model. The simulation results are verified through numerous experiments. The experimental results show the cutting characteristics of each milling process and the efficiency for mass production of a high quality worm.