Cutting forces analysis is one of the most important means of improving machining quality and productivity. In 3-axis milling, the frequency domain analysis of the cutting forces provides information which can be adopted for both predicting and monitoring the cutting process. However, the evaluation of the cutting forces spectra requires the application of specific tools (i.e., Fourier’s Transform) to the time domain representation of the same cutting forces. This paper presents analytical formulations to directly evaluate the cutting force spectra starting from the tool geometry and the cutting parameters. The proposed formulations start from the expression of the cutting forces as a Fourier series and enhance the predictions showed in previous works by reducing the overall number of calculations needed. Then, the proposed formulations are applied to identify both radial and axial depths of cut exploiting the normalized cutting force spectrum. Finally, the proposed formulations and their application were numerically and experimentally validated. This work represents a preliminary step in the development of a monitoring system to identify cutting conditions in 3-axis milling.