A mechanistic force system model for the face milling process has been developed and implemented on the computer. The model predicts the force system in face milling over a range of cutting conditions, cutter geometries, workpiece, and process geometries including relative positions of cutter to workpiece, spindle tilt, and runout. Machining tests have been conducted for both fly cuting and multitooth cutting with polycrystalline diamond tools on plain surfaces. The 390 casting aluminum alloy has been used as the workpiece material. Force data from these tests were used to estimate the empirical constants of the mechanistic model and to verify its prediction capabilities. Data bases from flycutting tests have been used to predict forces under multitooth face milling and the results indicate good agreement with observed data from multitooth tests.