ABSTRACT PDC bit modeling has historically incorporated the assumption that cutter forces are constant for a full revolution of the bit. This assumption places considerable limitations on the model's ability to simulate drilling in nonhomogeneous rock and in cases where the bit whirls. Recent studies have shown that cutter damage due to impact loading is a primary cause of PDC bit failure. These impacts may be caused by both bit whirl and drilling through nonhomogeneous rock. The three dimensional model presented in this paper provides the capability to evaluate cutter loading for both of these conditions. It shows that the cutter loads for common drilling situations may be as much as ten times greater than those calculated with previously available two dimensional models.