High speed machining (HSM) is an advanced machining technology with great future potential. It is widely oberved that the continuous smooth chip flow breaks down and serrated chip flow begins to form with cutting speed increasing, which is found to be ascribed to a repeated adiabatic shear band formation caused by thermo-plastic instability occurring within the primary shear zone. The occurence of serrated flow leads to the cutting force fluctuation, decreased tool life,degradation of the surface finish. Hence, understanding the emergence and evolution dynamimics of shear bands in HSM is very important. In this talk , our systematic HSM experimental results were presented . New theoretical model was developed to characterize chip unstable thermo- viscoplastic flow in HSM, the onset condition for adiabatic shear band and explicit expression of shear band spacing were obtained, where the material convection caused by chip flow was taken into account. Interestingly, the frequency of the serrated chip flow is found to be followed a power-law dependence on the Reynolds thermal number. In order to control shear banding, a novel dynamic large strain extrusion machining (DLSEM) technique is developed for suppressing formation of serrated chips. The corresponding theoretical model was also developed where the effects of extrusion constraint, material convection due to chip flow and momentum diffusion during shear band propagation are included. The analytical expressions for the onset criterion of adiabatic shear band and shear band spacing in DLSEM are obtained. The theoretical predictions are in agreement with the experimental results.