A laminated model of cutting face for evaluating cutting quality has been built up on the basis of cutting experiments. The experimental results and the theoretical analysis of the model show that, the lower part of the cutting face, adjacent to the bottom edge, has the worst cutting quality due to its worst roughness. The quality of the bottom face is mainly depended on the temperature of the lowest part of cutting front. Therefore, the method with this roughness as the main criterion for quality evaluation and detection is reasonable. The behavior of sparks has also been investigated by side imaging the sparks jet. It is found that the sparks behaviors are related to both the cutting speed and the roughness of the bottom face. The maximum temperature at the exit of sparks occurs at the status of the best roughness of the bottom face, which relates to the optimal cutting speed. Based on the method, on-line quality monitoring of lowest part of cutting front has been realized.