In an effort to maximize the metal removal rate in end milling while avoiding excessive cutter deflection or breakage, both fixed gain and adaptive controllers have been implemented for on-line feedrate manipulation to maintain a constant cutting force. While such controllers have been able to increase the metal cutting efficiency, they have also exhibited performance problems when large changes in the process dynamics occur. To assist in controller design and evaluation through digital simulation, a new dynamic model of the end milling force response to changes in feedrate and/or spindle speed is presented. This model, based on chip formation mechanics, takes explicitly into account the effect of cutter runout and deflection on the chip load, permits variations in the axial and radial depths of cut to be modeled, and provides surface geometry predictions. Model predictions are shown to correspond well with experimental machining data.