External sensor-less cutting force estimation using a load-side disturbance observer (LDOB) has potential to estimate the cutting force with high accuracy in both feed and cross-feed directions. However, the accuracy of its low frequency components in feed direction decrease due to effect of the friction and heat of a ball-screw-driven stage. In this study, DC and AC components of the cutting force is estimated by different methods; friction-compensated motor thrust force and LDOB, and the cutting force was estimated in real time by hybridizing them. In particular, regarding the friction model, the dynamic and static characteristics of the friction force in each axis (X, Y, Z) were identified from the idling test results. In addition to the model that depends on the velocity, the characteristics of the friction that depend on the position was also identified and considered when compensating for the motor thrust force. Then, a simple moving average filter with an appropriate window length is applied to the cutting force by LDOB and motor thrust force, and the DC component error of LDOB is corrected by that of motor thrust force. The validity of the proposed method was evaluated through end-milling tests. The experimental results showed that estimation accuracy of cutting force using the proposed method can be greatly improved in feed directions. On the other hand, in cross-feed direction, the cutting estimation was performed using the conventional LDOB.

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