Abstract: A groove (Da) extending in a Y-axis direction of a workpiece (D) is finish machined with an offset tool (T) that has a cross-section with a shape (L3) offset in a reducing direction by only a predetermined offset amount relative to a finished shape (L1) of the groove (Da) and that rotates around an axis of rotation parallel to a Z axis. Each time an incising step in which the offset tool (T) is relatively moved within the X-Z plane only by a predetermined distance on the circumference of an imaginary circle (C) having an initial position as its center and the offset amount (o) as its radius is carried out, a cutting step in which the entire length of the groove (Da) is cut while relatively moving the offset tool (T) in the Y-axis direction is carried out repeatedly.

Abstract: A groove (Da) extending in a Y-axis direction of a workpiece (D) is finish machined with an offset tool (T) that has a cross-section with a shape (L3) offset in a reducing direction by only a predetermined offset amount relative to a finished shape (L1) of the groove (Da) and that rotates around an axis of rotation parallel to a Z axis. Each time an incising step in which the offset tool (T) is relatively moved within the X-Z plane only by a predetermined distance on the circumference of an imaginary circle (C) having an initial position as its center and the offset amount (o) as its radius is carried out, a cutting step in which the entire length of the groove (Da) is cut while relatively moving the offset tool (T) in the Y-axis direction is carried out repeatedly.