The focus of this study is to investigate the effects of cutting speed, feed rate, hardness and cutting tool material on cutting region temperature and surface finish in hard turning process. A L36 Taguchi's standard orthogonal array was applied as experimental design. Machining tests were conducted under controlled conditions. AISI 4340 alloy steel which has numerous industrial applications was used to perform machining experiments. CBN, ceramic and cermet tools were employed to accomplish 36 tests with three random repetitions, resulting in 108 experiments. Main effects of the factors and their interactions are considered in this study using ANOVA (Analysis of variance) method. Also predictive models are derived by regression. Furthermore, optimal factor levels are obtained through S/N (signal to noise) ratio analysis in order to increase the machining efficiency. Finally, the results of confirmation tests are presented to verify the adequacy of the predictive models.