Researchers have typically used a flexible model in an attempt to accurately simulate peripheral milling forces. The approach generally described in the literature divides the model into manageable pieces by modeling a cutter as several disk-like elements in its axial direction and by dividing the turning cutter into several angular increments. When the forces calculated at each differential element are integrated, it is important to consider the angle (δλ) measured back as the flute engagement wraps up the helix. Therefore, flute engagement is very influential in determining the milling force profile and magnitude, and thus the effects of engaged flute length in the milling operation is of interest. In this paper, the concept of flute engagement is presented, and the equations calculating the engaged flute length (FL e ) are derived. In addition, a method for determining the number of engaged flutes caused by the radial and axial depths of cut is presented. It is felt that the results of this research may provide the basis for a machining force model.