Forced vibration is one of the factors setting a limit to the productivity when milling thin-walled parts. Their inherently very low stiffness and damping properties are added to the highly interrupted cutting taking place under some operations, which depending on the machining parameters, can result in large amplitude vibrations. By defining a simple unidirectional milling model, this paper studies the effect of engagement conditions on forced vibration regardless of other process parameters or part dynamics. In this way, the superior performance of up-milling strategy in terms of forced vibration is demonstrated. Moreover, it is demonstrated that even if it is not possible to totally cancel the force harmonics, it is possible to find an optimal engagement for up-milling that provides the lowest forced vibration, whose expression uniquely depends on the cutting force ratio and the number of flutes. Finally, the findings are validated through numerical simulations.