Milling of free form geometries is an usual machining operation in dies and moulds industry. A ball-end cutting tool is frequently used because its geometry allows the finishing and semi-finishing milling operations of any complex shape. However, unlike the ordinary milling machining, the tool-surface contact alternates constantly what makes the process unstable. Due to the geometrical characteristics of this process, the value of the effective tool diameter depends on the depth of cut and the surface curvature, which alters the lead angle (angle formed between tool axis and surface normal direction). The effective tool diameter alternates constantly along the machining. Moreover, it can be zero when the tool is using its centre to remove material, what makes the cutting speed null. A preview work has shown the severe alteration on the Cartesian components of the machining force when milling free form geometries. Such force oscillation is still not predictable by the up methods, and its cutting phenomenon is still not clear. The current work aims to quantify the influences of the lead angle and the engagement of the tool tip centre into the cutting region, which may lead to improvements on the choice of milling strategy in machining research and application. To do so, the machining force and its Cartesian components were investigated according to the process variables, such as: (1) tool path direction, (2) cutting way, (3) cutting speed, and (4) tool-surface contact (lead angle); when milling free form geometries with a ball-end cutting tool. Geometrical analyses together with milling experiments were carried out. The results show that the tool-surface contact had the greatest impact on the forces, because it can either be related to the effective tool diameter or with the tool tip on the cutting zone. In this area, the material is removed part by shearing and part by ploughing (plastic deformation), which increases the forces. 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