The mechanisms of abrasive grain machining processes are investigated based on thermo-elastic-plastic finite element simulations of orthogonal cutting with a negative rake. When machining of a steel of SK-5 with negative rake at low, intermediate, or fast cutting speed, bulge-type, ribbon-like, or serrated chips are formed, respectively. First, changes in the equivalent stress, temperature, plastic strain, and plastic strain rate observed at a spatially fixed point during a serrated chip formation are clarified. This indicates that a adiabatic shear occurred in the serrated chip formation. Next, changes in these cutting state parameters at some convective points of different types of chips are clarified. This shows the conditions of transition from a type of chip formation.