Automating machining processes require the functions of high-accurate process monitoring. However, additional sensors have problem that they lead to high-cost and reduce stiffness and reliability of mechanical systems. Authors proposed the sensor-less and real-time cutting force estimation methodology based on the disturbance observer theory, whose system is called cutting force observer. There are possibilities that control methods using the cutting force observer enhance the productivity in turning. One of the parameter which significantly-affects the cutting process is shear angle. Determining the shear angle is very important that it leads to minimum energy machining. The final goal of this study is to develop the automatic optimum cutting condition determining system. In this study, external sensor-less monitoring system of shear angle in turning is developed and its performance is evaluated. It remains possible that the developed turning system enable a shear angle to be controlled by adjusting the depth of cut and the cutting speed.