Three-dimensional (3D) molecular dynamics (MD) simulation is performed to study the tool/chip interface friction phenomenon in machining of polycrystalline copper at atomistic scale. Three polycrystalline copper structures with the equivalent grain sizes of 12.25, 7.72, and 6.26 nm are constructed for simulation. Also, a monocrystalline copper structure is simulated as the benchmark case. Besides the grain size, the effects of depth of cut, cutting speed, and tool rake angle are also considered. It is found that the friction force and normal force distributions along the tool/chip interface in both polycrystalline and monocrystalline machining exhibit similar patterns. The reduction in grain size overall increases the magnitude of normal force along the tool/chip interface, but the normal forces in all polycrystalline cases are smaller than that in the monocrystalline case. In atomistic machining of polycrystalline coppers, the increase of depth of cut consistently increases the normal force along the entire contact area, but this trend cannot be observed for the friction force. In addition, both higher cutting speed and more negative tool rake angle do not bring significant changes to the distributions of normal and friction forces on the interface, but both factors tend to increase the magnitudes of the two force components.