Contact forces play an important role in restraining workpiece motion during machining. In particular, frictional forces are often used to immobilize the workpiece against machining forces. This paper presents results of an experimental study of the effects of fixture, workpiece, and machining condition variables on workpiece-fixture contact forces in end milling. The effects of locator tip geometry, workpiece stiffness, workpiece surface roughness, and tool feedrate on the normal and tangential forces measured at a workpiece-fixture contact were analyzed statistically. It was found that the interaction of contact geometry and surface roughness has a significant effect on the tangential contact force. The normal contact force was found to be affected mostly by the machining condition and to a lesser extent by the workpiece stiffness. Experiments also suggest that the workpiece-fixture contact friction is considerably influenced by the cyclical nature of the milling forces.