Abstract: CNC machine tools generally consist of three parts: an NC control system, a servo drive system, and a feedback detection system. This paper discusses several aspects of how the position control system affects the machining requirements of CNC machine tools. Keywords: CNC machine tool control system , servo drive system I. Machining Accuracy The servo performance requirements of the position system for CNC machine tools include: positioning speed and contour cutting feed rate; positioning accuracy and contour cutting accuracy; surface roughness of finishing; and stability under external disturbances. These requirements mainly depend on the static and dynamic characteristics of the servo system. For closed-loop systems, high dynamic accuracy is always desirable, meaning that when the system has a small positional error, the moving parts of the machine tool will react quickly. Accuracy is a performance indicator that machine tools must guarantee. The positional accuracy of the position servo control system largely determines the machining accuracy of the CNC machine tool. Therefore, positional accuracy is an extremely important indicator. To ensure sufficient positional accuracy, one aspect is to correctly select the open-loop amplification factor in the system, and another is to impose accuracy requirements on the position detection element. Because in a closed-loop control system, it is difficult to distinguish between the error of the detection element itself and the deviation of the detected quantity, the accuracy of the feedback detection element often plays a decisive role in the system's accuracy. It can be said that the machining accuracy of CNC machine tools is mainly determined by the accuracy of the detection system. The smallest displacement that a displacement detection system can measure is called the resolution. The resolution depends not only on the detection element itself but also on the measurement circuit. When designing CNC machine tools, especially high-precision or large and medium-sized CNC machine tools, the detection element must be carefully selected. The resolution or pulse equivalent of the selected measurement system is generally required to be one order of magnitude higher than the machining accuracy. In short, a high-precision control system must be guaranteed by a high-precision detection element. For example, the accuracy of the linear inductive synchronizer commonly used in CNC machine tools can reach ±0.0001mm, i.e., 0.1µm, with a sensitivity of 0.05µm and a repeatability of 0.2µm; while the accuracy of the circular inductive synchronizer can reach 0.5N, with a sensitivity of 0.05N and a repeatability of 0.1N. II. Open-Loop Amplification Factor In a typical second-order system, the damping coefficient x = 1/2(KT) - 1/2, and the steady-state speed error e(∞) = 1/K, where K is the open-loop amplification factor, often referred to as the open-loop gain in engineering. Clearly, the open-loop gain of a system is one of the important parameters affecting the static and dynamic performance of a servo system. Generally, the gain of a CNC machine tool servo mechanism is taken as 20–30 (1/s). Servo systems with K < 20 are usually called low-gain or soft servo systems, mostly used for point-to-point control. Systems with K > 20 are called high-gain or hard servo systems, applied to contour machining systems. If, to avoid affecting the surface roughness and accuracy of the machined parts, it is desirable that the step response does not oscillate, i.e., a larger value is required, and the open-loop gain K should be smaller; if the system's speed is prioritized, a smaller x is desired, i.e., the open-loop gain should be increased, and simultaneously, increasing the K value can improve the steady-state accuracy of the system. Therefore, the selection of the K value must be considered comprehensively. In other words, a higher gain is not always better. When the input speed changes abruptly, a high gain may cause drastic changes in output, subjecting the mechanical device to a large impact, and in some cases, may even cause system stability problems. This is because in high-order systems, system stability has requirements regarding the range of K values. Low-magnification systems also have certain advantages, such as easier system adjustment, simpler structure, less sensitivity to disturbances, and better surface roughness. III. Improving Reliability CNC machine tools are high-precision, high-efficiency automated devices. Failures result in greater losses, making improved reliability crucial. Reliability is one of the main quantitative indicators for evaluating reliability, defined as the probability that a product will perform its intended function under specified conditions and within a specified time. For CNC machine tools, specified conditions refer to environmental conditions, working conditions, and operating methods, such as temperature, humidity, vibration, power supply, interference intensity, and operating procedures. The intended function mainly refers to the machine tool's functionalities, such as its various functions and servo performance. Mean Time Between Failures (MTBF) is the average time from one failure to the next for a repairable device or system that can continue operating after repair or parts replacement. CNC machine tools commonly use it as a quantitative indicator of reliability. Since the adoption of microcomputers in CNC devices has significantly improved reliability, the reliability of servo systems has become particularly prominent. Its failures mainly originate from servo components and mechanical transmission parts. Generally, the reliability of hydraulic servo systems is lower than that of electrical servo systems. Electromagnetic components such as solenoid valves and relays have poor reliability and should be replaced with contactless components whenever possible. Currently, the reliability of CNC machine tools is not very high due to limitations in component quality, process conditions, and cost. To make CNC machine tools more acceptable to factories, their reliability must be further improved, thereby increasing their value. When designing a servo system, components must be selected according to the design's technical requirements and reliability, and rigorous testing and inspection must be conducted. Close attention must be paid to mechanical interlock devices and other aspects to minimize failures caused by mechanical components. IV. Wide Speed ​​Range In CNC machine tool machining, the servo system requires a sufficiently wide speed range for the feed drive to simultaneously meet high-speed rapid traverse and single-step jogging. Single-step jogging is often used as an auxiliary working method for adjusting the worktable. For the servo system to achieve smooth feed at low speeds, the speed must be greater than the "dead zone" range. The term "dead zone" refers to the phenomenon where, due to static friction, the motor cannot rotate under very small inputs because it cannot overcome this friction. Additionally, mechanical backlash can also cause the motor to rotate while the slide remains stationary; these phenomena can also be described using the term "dead zone." Let the dead zone range be 'a'. The minimum speed Vmin should satisfy Vmin ≥ a. Since a ≤ dK, where d is the pulse equivalent (mm/pulse) and K is the open-loop gain, then Vmin ≥ dK. If we take d = 0.01 mm/pulse and K = 30 × 1/s, then the minimum speed Vmin ≥ a = 30 × 0.01 mm/min = 18 mm/min. The selection of the maximum speed for the servo system must consider the machine tool's mechanical limits and actual machining requirements. While higher speeds improve productivity, they also place higher demands on the drive. Furthermore, from a system control perspective, there is also the issue of detection and feedback, especially in computer control systems, where sufficient software processing time must be considered. Since fmax = fmax/d, where fmax is the pulse frequency of the highest speed (kHz), vmax is the highest feed speed (mm/min), and d is the pulse equivalent (mm), and D is the speed range (D = vmax/vmin), we get fmax = Dvmin/d = DKd/d = DK. Since the reciprocal of the frequency is the interval between two pulses, the reciprocal of the highest frequency fmax is the minimum interval tmin, i.e., tmin = 1/DK. Clearly, the system must complete the position detection and control operations within tmin using hardware or software. For the highest speed, the value of vmax is constrained by tmin. A good servo system often has a speed range D of 800–1000. The most advanced level today is continuous adjustment of the feed speed from 0 to 240 m/min under the condition of a pulse equivalent d = 1 µm. V. Conclusion The above aspects analyze the servo performance requirements of CNC machine tool position servo systems and propose reliability indicators for stable system operation. The research results can be used for the design of servo CNC systems and for the retrofitting of existing CNC machine tools to improve their working accuracy.