ABSTRACT This paper presents the results of a study investigating PDC bit vibrations. The study shows that the most harmful vibrations can be attributed to a phenomena called "bit whirl". During whirl the instantaneous center of rotation moves around the face of the bit, and the bit backwards whirls around the hole. Cutters on a whirling bit can move sideways, backwards, and much faster than those on a true rotating bit. The impact loads associated with this motion cause PDC cutters to chip, which, in turn, causes accelerated wear. Lab and field results showing the detrimental effects of whirl on PDC bit penetration rate and life are included.