Recently, the utilization of five-axis machining centers (5MC), which integrate linear and rotary axes, has expanded rapidly. 5MC facilitates the rapid and precise fabrication of complex third-dimensional surfaces, including propellers, and die-casting molds, etc. by means of synchronous control. In recent years, demand has increased for enhancing both the dimensional accuracy and surface finish of freeform surfaces. In our previous research, we sought to improve the quality of finished surfaces by synchronizing two linear axes and one rotary axis of a 5MC, thereby maintaining a constant feed speed vector at the milling point between the tool and the workpiece. A notable characteristic point of this machining method is that the positional relationship between the tool and the workpiece remains constant throughout the machining process. As a result, the direction of the machining force remains constant, enabling the high-quality surface finishing without being affected by the structural or rigidity anisotropy of the machine tool. Typically, side milling using an end-mill is known that the deflection of tool due to machining force causes the slope angle of machined surface. Therefore, this research focuses on the fact that tool deflection can be uniquely determined, owing to the constant feed speed and consistent machining force at the milling point. The deflection of the tool during machining is calculated from the machining force obtained using the instantaneous cutting force model. Leveraging the characteristic that the tool and workpiece maintain a fixed relative position during machining, this research proposes a method to enhance the slope angle of machined surface by adjusting the tilt of the 5MC’s rotary axis. The findings of this research demonstrate that machining forces can be estimated via the instantaneous cutting force model, thereby enabling prediction of the slope angle of machined surface caused by machining forces. Furthermore, the slope angle of machined surface can be effectively suppressed by appropriately tilting the rotary axis of the 5MC.