Turn-milling is relatively a new cutting process which combines two conventional manufacturing processes; turning and milling. This promising technology becomes an alternative to turning due to its advantages such as higher productivity and lower cutting temperatures, which provide longer tool life. Intermittent characteristics of turn-milling helps maintaining lower cutting temperatures and making high cutting speeds possible. In this study, the objective is to build a process model for turn-milling operation. Two possible methods used in turn-milling, orthogonal cutting and tangential cutting are considered. The developed model includes cutting geometry and force calculations. In addition, analytical expressions for circularity are presented as well. Finally, a comprehensive process model is obtained for both orthogonal and tangential turn milling operations. This model is used to compare different types of turn-milling operation and optimize the process. Experiments were conducted to verify the force model on a mill-turn CNC machine tool where the cutting forces were measured by a rotary dynamometer.