The ball mill used in the ore dressing of Daye Iron Mine, a subsidiary of Wuhan Iron and Steel Group, requires a large quantity of grinding balls in several sizes, including 60mm, 80mm, 100mm, and 125mm. Production is undertaken by the Daye Iron Mine Machine Repair Plant. Due to the large annual demand for grinding balls (approximately 70 tons/year) and the high requirements for product dimensional consistency, we upgraded an existing C620 horizontal lathe using economical CNC technology. The increased automation significantly improved production efficiency, reduced production costs, and ensured product quality. 1. Upgrade Principle To achieve automated machining under microcomputer control, we adopted a single-chip microcomputer-based CNC device to perform open-loop control of the lathe's longitudinal and transverse feed systems. Stepper motors were used as the drive elements, ball screw pairs were used in the transmission system, and an automatic indexing tool post was used. The structural principle after the upgrade is shown in the figure. The machine tool CNC system consists of a CNC unit, a stepper servo drive unit, and stepper motors. The CNC unit is a single-chip microcomputer. The control program of the CNC unit is the core for realizing various functions. In the workpiece machining program, specific machining length, movement direction, and feed rate are given. With the support of the central processing unit, the control program calculates and processes the input machining program data, sends out the required pulse signals, and after power amplification by the drive power supply, drives the stepper motor. The stepper motor then drives the mechanical load, realizing automatic machining of the machine tool. During tool change, the microcomputer sends a tool change signal, the relay in the tool holder control box activates, the motor rotates forward, and through the reduction and lifting mechanisms, the upper tool body is raised to a certain position, driving the tool body to rotate to the selected tool position, and then positioned, completing the tool change action. In the machine tool CNC system, we selected the K-15T machine tool CNC system from Nanjing Dafang Co., Ltd., in which the CNC unit is an MCS-51 single-chip microcomputer. It has many advantages such as small size, light weight, multiple functions, and convenient operation and maintenance. For the stepper motors, we selected the Ding BF type and JBF-3 type special geared stepper motors from Nanjing Dafang Co., Ltd. The number of motor phases is 3, and the rated currents are 7A and 6A respectively. The electric tool post is the LD4-C6140 type four-station automatic tool post produced by Changzhou Xinshu Machine Tool CNC Equipment Factory. 2. Modification measures To improve transmission efficiency and transmission accuracy and prevent low-speed crawling, ball screws are used in both longitudinal and transverse directions. First, disconnect the power connection between the screw and the feed box, remove the small slide plate, and install the main stepper motor at the tail of the main screw to drive the saddle screw. Then, install a stepper motor at the tail of the middle slide screw to drive the middle slide screw. Finally, install the automatic tool post at the small slide plate. 3. Online debugging (1) Connect the stepper motor, electric tool post and other devices through the corresponding sockets on the control device, and set the power amplifier switch to the off position and turn on the system power switch. (2) After the current is turned on, the CNC unit should work normally. At this time, the operation of the axial flow fan should be checked. It is strictly forbidden to work when the fan is stopped. (3) Turn the power amplifier switch to the on position and use the manual method to check whether the servo drive unit and stepper motor of the system are working normally. Use the "manual" tool change key to check the correctness of the rotation and selection of the automatic indexing tool post. (4) Try to input the workpiece machining program according to the program input steps, run the machine tool without a load, and check whether the actions of each part of the machine tool are correct and the correctness of the program. (5) Repeatedly process the same workpiece, measure and analyze the machining error of the workpiece, and eliminate the error in a targeted manner.