In applications with low automation requirements, relatively fixed machining objects, and few speed changes, the lathe spindle speed change does not need modification and can still use manual speed change. Feed device modification is a key aspect of economical CNC retrofitting. Feed device modification generally involves removing the original transmission mechanism and using a stepper motor to drive a ball screw via a toothed belt or gear reducer, which in turn drives the tool post's longitudinal and transverse movements. The longitudinal and transverse motors are mounted on the bed and saddle, respectively; the longitudinal and transverse nuts are mounted on the slide box and the intermediate slide, respectively, as shown in Figure 1. If specifically used for machining non-ferrous metal workpieces or for finishing, to simplify the mechanism, reduce motion inertia, and minimize intermediate steps that cause positioning errors, the motor and lead screw are generally directly connected. However, during roughing, to achieve the required pulse equivalent and increase torque, a reduction mechanism is generally used to connect the motor and lead screw. The design of the feed drive device mainly involves selecting four parts: the motor, lead screw, guideway pair, and tool post. 1. Stepper Motor Design and Selection When selecting a stepper motor, it is generally desirable to have high output torque, high starting frequency, and small step error. However, there is a certain contradiction between increasing torque and rapid operation, and between high performance and low cost. Practice has proven that the following design and selection method is more reasonable: 1) First, ensure positioning accuracy. The smaller the pulse equivalent, the higher the positioning accuracy, but the lower the feed rate. Under the condition of meeting accuracy requirements, try to select the largest possible pulse equivalent. For example, for finishing, a longitudinal pulse equivalent of 0.01mm and a transverse pulse equivalent of 0.005mm are generally selected; for roughing, a longitudinal pulse equivalent of 0.01mm and a transverse pulse equivalent of 0.01mm are sufficient. 2) Select the step angle and the transmission ratio of the transmission mechanism. Figures 2 and 3 show the starting torque-frequency characteristic curve and the operating torque-frequency characteristic curve of the stepper motor, which are important bases for selecting a stepper motor. Based on this, the motor torque can be easily determined. The rapid feed and cutting feed of the lathe require different frequencies and torques, and the selected motor should meet both requirements simultaneously. Given that the feed torque during cutting is T_e, the maximum cutting speed is V_e, and the rapid traverse speeds are T_k and V_k respectively; substituting these into equation (1), we can obtain the maximum feed frequencies f_e and f_k: [Full text download of the practical design for economical CNC retrofitting of lathe feed device]