Abstract: Aiming at the problems of varying chip thickness and depth due to the complex motion in orthogonal turn-milling, the mathematical models of centric and eccentric chip formation were proposed based on the orthogonal turn-milling theory, respectively. Consequently, the mathematical expressions of chip depth and thickness for both the circumference edge and the face edge were obtained. The theoretical model of orthogonal turn-milling chip was validated by comparing the geometric chip formation with the experimental one under the same cutting condition. The chip model may provide a theoretical foundation and reference for the research on orthogonal turn-milling mechanisms.