The tool angles such as the normal rake angle and the inclination angle vary continuously along the cutting edges of a twist drill, furthermore the cutting velocities also change in magnitude and direction at any point on the cutting edges. There-fore the chips are produced with various chip flow and normal shear angles, and the cutting process is three dimensional. In this paper it is assumed that the chips at infinitesimal parts on the cutting edges are described by simple shear plane oblique cutting models, and the whole chip formation process is treated as an accumulation of the infinitesimal parts along the cutting edges. A cutting model, in which the condition of side curl of the chip is considered, is developed and the energy method is extended to the case of drilling process. Then they are used with the appropriate data obtained in orthogonal cutting to predict the chip formation along the cutting edges and cutting forces in the case of drilling with pilot holes. Preliminary examination shows that the proposed cutting model and the energy method give good qualitative explanations on the chip formation and the torque.