Abstract: A thermalmechanical orthogonal cutting finite element model (FEM) was established by Abaqus/Explicit to explore the sawtooth chip formation process and mechanism in hard turning AISI 52100 steel. The observation of stress distribution, temperature distribution and mesh distortions along cutting time in high and low speed machining processes contributes to demonstrate the adiabatic shear mechanism of sawtooth chip formation process of AISI 52100 bearing steel. Firstly, a short horizontal adiabatic shear band initiates in front of the rake face; the short band gradually extends to the free surface from the end of the shear plane as the rake face advances to push the work material more severely; sawtooth chip forms as the adiabatic shear band slides. “Adiabatic shear” is the primary mechanism to form sawtooth chip morphology in machining AISI 52100 bearing steel, crack initiates after the formation of adiabatic shear band; cutting speed is the driving force of the microcrack initiation in shear band and subsequent propagation to the tool tip. The friction condition in second shear zone strengthens the tendency of sawtooth chip formation process.