<p>The current research investigated the influences of tool surface engagement (TSE), material hardness, and cutting parameters on the machining force and roughness when milling free-form surfaces with a ball-end cutting tool. The frequent alteration of the TSE (as typically encountered in this milling process) also alters the dominant cutting mechanism, either ploughing or shearing. The phenomena occurring in such machining processes are still not fully understood, and their comprehension is fundamental for the development of any robust model to predict such processes. Machining experiments were conducted by varying the TSE (angle φ) and workpiece hardness (AISI H13 steel: annealed 227 HV and hardened 495 HV). A geometric study allows identifying a critical geometric cutting condition - the ξ angle, which corresponds to the angle of chip extension. It is driven by the cutting parameters together with the TSE. The ξ angle allows identifying the cutting condition in terms of force and roughness. When the ξ angle is altered, the predominance of the machining mechanism is also altered between ploughing and shearing. Thus, a new specific Ks coefficient according to ξ angle was established, Ks_(φ) and Ks_(ξ), to estimate the machining force according to the tool position along the trajectory. The ξ angle, together with material hardness, allowed the identification of distinct surface damage patterns that affect the surface roughness Sz.</p>