The objective of this research is to study the relation between the straightness of aluminium
and the cutting force ratio during the aluminium turning. The dynamometer is installed on the turret of
CNC turning machine in order to monitor and obtain the cutting force. The relation between the
cutting force ratio and the straightness is investigated under the cutting conditions, which are the
cutting speed, the feed rate, the depth of cut, the tool nose radius and the cutting force ratio. From the
experimental results, when the cutting speed and the tool nose radius increase, the straightness will
improve. The relation between the dynamic cutting force and the straightness profile can be verified in
the frequency domain by using the Fast Fourier Transform (FFT). The results showed that the dynamic
cutting force and the straightness profile appear at the same frequency. Therefore, the cutting force ratio can be used to predict the straightness during the turning process. The relation has been proposed and model by applying the exponential function. The multiple regression analysis is utilized to calculate the powers of the model by using the least square method at 95% confident level. From the model, the feed rate was the most significant factor affecting the straightness. The model has been verified by the cutting tests that the straightness can be predicted by the high accuracy of 89.13%