Abstract: This paper studies the application method of motion controller in CNC retrofitting of milling machines. First, the working process of the CNC system is introduced, then its hardware composition and software design are analyzed in detail. The expected functions of the CNC code are realized through experiments, demonstrating that the application of this general-purpose embedded motion controller in the retrofitting of economical CNC machine tools is feasible and has broad prospects. Keywords: Milling machine; Motion controller; Embedded; CNC 1 Introduction CNC machine tools can achieve automated machining, improving production efficiency compared to traditional machine tools, and producing parts with high precision and small dimensional dispersion. China has a vast market for CNC retrofitting of machine tools. This paper uses a general-purpose embedded motion controller for the CNC retrofitting experiment of a vertical milling machine X8126. The original spindle system and cooling system were retained during the retrofit, and a stepper motor drive system was used to perform CNC retrofitting of the milling machine's X, Y, and Z axes. After this retrofit, the step distance is 0.001 mm/pulse. 2. CNC Basics 2.1 Working Process of CNC System (1) Input the part machining program, control parameters and compensation data into the CNC system. (2) Machining program decoding and data processing. (3) Interpolation. The motion trajectory is the result of multi-axis coordinated motion. In order to achieve the desired trajectory, the motion of the relevant axes must be controlled. The direct method is to determine the motion of each axis in advance and store it in the computer's memory. When reproducing the trajectory, the axis is controlled according to the stored data. However, this means storing a large amount of data, which is not practical in actual applications. In fact, the contour or motion trajectory is generally composed of straight lines and arcs. For some non-circular curve contours, straight lines or arcs are used to approximate them. Based on a small amount of basic data (the starting point and the ending point can uniquely determine a straight line, and the arc only needs to be given the starting point, the ending point, the radius and the direction to be determined), the contour or motion trajectory of the workpiece can be described by calculation. While calculating, feed instructions are issued to each coordinate according to the calculation results. This is the basic idea of ​​interpolation: interpolation calculation involves the CNC system densifying data points between the start and end points based on the given curve type (such as a straight line, circular arc, or higher-order curve), start point, end point, and speed. Of course, interpolation does not exist for single-axis motion. For details, please click: Research on CNC Milling Machines Based on Embedded Motion Controllers