The invention relates to an improved method and structure of a roller wiper, and discloses a method and a device for machining symmetric inclined serrated lines on a narrow window slot side of a tubular part. Symmetric inclined serrated slots are milled in the narrow window slot side of the tubular part by relative movement between a disc milling cutter rotating on a horizontal milling machine spindle and the tubular part. Two parallel forming disc milling cutters are simultaneously mounted on a milling cutter bar, the requirement of the distance between the serrated slots is met by the aid of a spacer sleeve, and cutting blades can synchronously move. Owing to the method for milling by the double cutters, two pairs of parallel slots can be machined once, and milling efficiency is improved. The method can be popularized to similarly form a window in a circumference and mill the slots in optional shapes in two side walls of the window.