According to the data, China has more than 4 million machine tools, most of which are ordinary machine tools produced over many years. These machine tools have low automation and low processing accuracy. It is not feasible to replace the existing machine tools with a large number of highly automated equipment in a short period of time, whether from the perspective of funds or the production capacity of China's machine tool manufacturers. However, it is imperative to realize the automation and precision transformation of some of China's existing ordinary machine tools as soon as possible. Therefore, how to transform them has become an important issue that China's existing equipment technology transformation urgently needs to solve. 1 Analysis (1) Technology In the past few decades, the basic operating principle of metal cutting machine tools has not changed much, but the social productivity, especially the application of microelectronics technology and computer technology, has developed rapidly. This is reflected in the machine tool control system. It can improve the automation level of machine tools and improve the processing accuracy. Some enterprises have already made beneficial attempts in this regard. Practice has proved that the transformed machine tools not only meet the requirements of technological progress and higher productivity, but also have a greater adaptability due to the improved product accuracy and increased surface processing range. This further highlights the necessity and urgency of CNC technology transformation on old machine tools. (2) Economy: Due to the high price of new machine tools and the huge one-time investment, if all old machine tools are replaced with new ones, the country will have to spend a lot of money. The replaced machine tools will be idle and wasteful. If the technology is used to modernize them, more than 50% of the funds can be saved. In my country, the price of an economical CNC device is only 1/3 to 1/5 of that of a full-function CNC device, which is affordable for most users. This has opened up a new path for the technological development of small and medium-sized enterprises with tight funds, and has also generated great economic attraction for large enterprises with strong strength, playing a positive role of achieving twice the result with half the effort. (3) Market: According to domestic statistics, the delivery cycle of ordering new CNC machine tools is generally long and often cannot meet production needs. Therefore, the CNC transformation of machine tools has become the main supplementary means to meet market demand. (4) Production: In the production of the machinery industry, multi-variety, small-batch and even single-piece production are the basic characteristics of modern machinery manufacturing and account for a considerable proportion. To complete these production tasks, one can choose general-purpose machine tools, special-purpose machine tools or CNC machine tools. CNC machine tools are best suited to meet these production needs. From the above analysis, it's clear that CNC technology for machine tool retrofitting is based on the combination of modern microelectronics and traditional technology. Through theoretical derivation and practical application, the introduction of microcomputer CNC systems into machine tool retrofitting has the following advantages: ① High reliability; ② High flexibility; ③ Easy to achieve mechatronics integration; ④ Considerable economic benefits. Therefore, CNC retrofitting of old machine tools can improve their performance, reduce production costs, and achieve higher machine tool performance and greater economic benefits with less capital investment. The ZA5025/I type ordinary drilling machine is widely used in my country's machining industry for its low purchase cost and simple, practical operation. The transmission system of this type of machine tool mainly consists of two parts: one is the main motion transmission chain, which transmits the motion of the power source (main motor) to the spindle, causing the spindle to drive the drill bit to rotate. The spindle can have 12 speeds controlled by a handle. Secondly, the feed motion transmission chain, also powered by the main motor, allows for 4 feed rates via the handle, but manual control is also possible. Modifying this type of machine tool can improve its accuracy and automation, enabling rapid adjustments while maintaining its versatility. It can also enhance its functionality, allowing for accurate workpiece machining using microcomputer numerical control (CNC). 2. Selection Based on the principles of minimizing machine tool modifications, ensuring strong anti-interference capabilities in the control system, and minimizing modification costs, and according to relevant technical requirements, the machine tool's performance and data should be thoroughly understood before modification. This allows for determining the post-modification accuracy and scope based on the machine tool's inherent precision requirements. Generally, the CNC modification of a conventional vertical drilling machine mainly involves two parts: first, designing a simple microcomputer CNC x-Y worktable and fixing it to the drilling machine's worktable; second, removing the handle controlling the drill bit's up-and-down movement and replacing it with a microcomputer-controlled stepper motor that uses a single-stage reduction gear to move the drill bit up and down. According to the performance requirements of CNC drilling machines, the machining accuracy of general drilling machines is between 0.1 and 0.02. Then the pulse equivalent of the stepper motor of the drilling machine can be 0.01 ram/step, and the step angle of the stepper motor is 1 5. 3 Scheme (1) Determination of system motion mode CNC systems can be divided into point-to-point control system, point-to-point linear system, and continuous control system according to motion mode. If the workpiece does not cut during the movement of relative to the tool, the point-to-point control mode can be selected. The drill bit does not perform drilling during the movement of the CNC drilling machine table, so the CNC device can adopt the point-to-point control mode. The requirement for the point-to-point system is rapid positioning and ensuring positioning accuracy. (2) Selection of servo system The servo system realizes position servo control in three ways: open loop, closed loop, and semi-closed loop. The open loop control servo system has the basic problem that the control accuracy cannot reach a high level. However, the stepper motor has a strict correspondence between angular displacement and input pulse. This design prevents step error accumulation; maintains a strict correspondence between rotational speed and input pulse frequency; and remains unaffected by fluctuations in current, voltage, load size, or environmental conditions within its load capacity range. Furthermore, open-loop stepper motor control systems, lacking position detection and feedback control issues, have a simpler structure, facilitating system implementation and debugging. With advancements in electronic and computer control technologies, significant progress has been made in improving stepper motor control performance. Therefore, within certain limits, this type of open-loop servo system using a stepper motor as the drive actuator can meet machining requirements and is suitable for applications in general CNC systems where high precision is not required. While closed-loop and semi-closed-loop control offer the possibility of achieving high-precision position servo control, the addition of position detection, feedback comparison, and servo amplification in specific systems not only increases the workload and complexity during installation and debugging but also significantly increases the difficulty of achieving good steady-state and dynamic performance of a closed-loop system from a control theory perspective. Therefore, considering that the modification is carried out on the Xitong vertical drilling machine, the accuracy requirement is not very high (the system resolution is 0.01). In order to simplify the structure and reduce the cost, this paper adopts the stepper motor open-loop servo system. (3) Determination of the transmission mode of the 45-mechanism To ensure the transmission accuracy and working stability of the CNC system, when designing the transmission assembly of the mechanism, the requirements of low friction, low inertia, high stiffness, no backlash, high resonance and appropriate damping ratio are usually put forward. Therefore, the following points should be considered in the design: ① Use low friction transmission and guiding elements as much as possible. For example, use ball screw nut transmission pairs, rolling guides, etc. ② Eliminate transmission backlash as much as possible. For example, the transmission gear on the stepper motor adopts an eccentric bushing type backlash elimination structure. ③ Shorten the transmission chain. Shortening the transmission chain can improve the transmission stiffness of the system. Reduce the transmission chain error. Preload can be used to improve the transmission stiffness of the system. For example, use preloaded rolling guides and ball screw transmission pairs, design the screw support to be axially fixed at both ends and add a pre-tensioned structure to improve the transmission stiffness. The x-Y table transmission adopts a ball screw nut transmission pair and rolling guide rail. (4) Selection of computer system The computer numerical control system is generally composed of a microcomputer part, I/O interface circuit, opto-isolation circuit, servo motor drive circuit, detection circuit and other parts. In simple numerical control systems, most microcomputers with 8-bit microprocessors are used. How to use a microcomputer application system composed of Z80 CPU or MCS-51 microcontroller. Z80 CPU has the characteristics of low chip price, strong versatility and convenient maintenance. MCS-51 microcontroller has the characteristics of high integration, good reliability, strong function, fast speed and high performance-price ratio. Through comparison, it is recommended to use MCS-51 series microcontroller as the main controller for simple numerical control machine tools. 4 Implementation ① Keep the original machine tool main transmission chain, keep the drilling machine table and control table moving handle, and install a set of microcomputer numerical control x-Y table on the original worktable. Since the weight of the moving parts of the x-Y table and the cutting force are not large, a double V-shaped ball guide rail with preload is selected. Using rolling guides can reduce the difference in dynamic and static friction coefficients between two relatively moving surfaces, thereby improving motion smoothness and reducing vibration. Considering that the motor step angle and lead screw can only be selected according to standards, gear reduction transmission is required to achieve a resolution of 0.01. ② The handle controlling the drill bit feed motion is removed and replaced with a stepper motor. The stepper motor, through a first-stage reduction, makes the gear mesh with the rack on the sleeve, thereby realizing the downward feed or upward retraction of the main shaft. ③ A boss is left at each of the four corners of the lower base plate of the x and y worktables, and a slot is cut in each slot. Bolts are inserted into the slots of the original workpiece table and into these slots and tightened with nuts. In summary, the overall scheme of this modification is determined as follows: an 8031 ​​microcontroller is used to calculate and process the data, and step pulses are output from the I/O interface. After the stepper motor is reduced by a first-stage gear, it drives the lead screw to rotate, thereby realizing the longitudinal and transverse movement of the workpiece. At the same time, in order to prevent accidents and protect the microcomputer and other equipment, alarms and emergency stop circuits are also set. The system block diagram is shown in the attached figure.