The configuration and function selection of a CNC system are crucial components of a CNC machine tool. Choosing the right CNC system and its functions are key concerns for both machine tool manufacturers and end-users. CNC System Configuration, Servo Control Unit Selection, and Position Control Methods : Open-loop control systems: These use stepper motors as the drive unit and lack position and speed feedback devices. They are simple to control and inexpensive, but have low load capacity, poor position control accuracy, and low feed speed. They are mainly used in economical CNC devices. Semi-closed-loop and closed-loop position control systems: These use DC or AC servo motors as the drive unit. A semi-closed-loop system can be constructed using a pulse encoder and rotary transformer integrated into the motor as position/speed detection devices. Alternatively, a high-precision fully closed-loop system can be constructed using an optical encoder or inductive synchronizer directly mounted on the worktable as the position detection device. Due to pitch error, the change in the lead screw rotation angle fed back from the position detector in a semi-closed-loop system cannot accurately reflect the linear motion position of the feed axis. However, after compensation for pitch error by the CNC system, they can achieve fairly high position control accuracy. Compared with fully closed-loop systems, they are cheaper, have better sealing of the position feedback device installed inside the motor, are more stable and reliable in operation, and require almost no maintenance, so they are widely used in various types of CNC machine tools. DC servo motors are relatively simple to control and cheaper, but their main disadvantages are the mechanical commutation device inside the motor, easy wear of carbon brushes, and high maintenance workload. They are also prone to sparking during operation, making it difficult to increase the motor's speed and power. AC servo motors are brushless, require almost no maintenance, and are relatively small in size, which is beneficial for increasing speed and power. They have largely replaced DC servo motors. Types of Servo Control Units Separate servo control units are characterized by the CNC system and servo control unit being relatively independent. This means they can be used with various CNC systems. The NC system provides commands in the form of DC voltage (e.g., 0-10V) related to the axis movement speed, while the machine tool returns axis movement position detection signals matched to the NC system (e.g., encoder/inductive synchronizer output signals). Servo data settings and adjustments are performed on the servo control unit side (using potentiometers or digital input). Serial data transmission type servo control units are characterized by bidirectional data transmission between the NC system and the servo control unit. Command data, servo data, and alarm signals related to axis motion are transmitted via corresponding clock signal lines, strobe signals, transmit data lines, receive data lines, and alarm signal lines. The position encoders return information to the NC device such as the actual position and status of the moving axes. Network data transmission type servo control units are characterized by densely packed axis control units powered by a common DC power supply unit. The NC device connects in series with the SR and ST points of the network data processing modules of each axis control unit (substation) via the SR and ST connection points of the network data processing module on the FCP board, forming a servo control loop. The position encoders of each axis are connected to the axis control unit via two high-speed communication lines, providing feedback information including the position of the moving axis and related status information. Servo parameters for both serial and network data transmission type servo control units are set digitally in the NC device and loaded into the servo control unit during power-on initialization, making modification and adjustment very convenient. Network data transmission type servo control units (such as the Okuma OSP-U10/U100 system), with corresponding control software, have real-time adjustment capabilities. For example, in the Hi-G type positioning acceleration/deceleration function, a corresponding function can be calculated based on the speed and torque characteristics of the motor, and then used to control the acceleration and deceleration during high-speed positioning, thereby suppressing vibrations that may be caused during high-speed positioning. Increasing the positioning speed can shorten non-cutting time and improve machining efficiency. Similarly, in the Hi-Cut type feed rate control function, after reading the part machining program, the system can automatically identify the shape of the part (arcs, edges, etc.) required by the CNC instructions and automatically adjust the machining speed to optimize it, thereby achieving high-speed, high-precision machining. With the advent of fully digital AC servo systems using high-speed microprocessors and dedicated digital signal processors (DSPs), hardware servo control has become software servo control, and some advanced algorithms from modern control theory have been implemented, greatly improving the control performance of servo systems. Servo control units are components in CNC systems directly related to the machine tool; their performance is closely related to the machine tool's cutting speed and positional accuracy, and their price accounts for a large portion of the CNC system's cost. Relatively speaking, servo components have a higher failure rate, accounting for over 70% of electrical failures. Therefore, selecting the right servo control unit is crucial. Servo failures are not only related to the reliability of the servo control unit but also closely related to the machine tool's operating environment, mechanical condition, and cutting conditions. For example, excessively high ambient temperatures can easily cause overheating and damage to components; inadequate protection can lead to water ingress into the motor, causing short circuits; poor lubrication of the guide rails and lead screws or excessive cutting loads can cause motor overcurrent. A jammed mechanical transmission mechanism can further damage power devices. Although the servo control unit itself has some overload protection capabilities, severe or repeated failures can still damage the components. Some CNC systems have real-time load display functions for the spindle and feed axes (for example, the "Current Position" page of the Okuma OSP system not only displays the real-time position data of the axis movement, but also simultaneously displays the real-time load percentage of each axis. Users can use this information to take measures to prevent accidents). The output torque of the feed servo motor is an indicator of its load capacity. As shown in Figure 2, in continuous operation, the output torque decreases with increasing speed; the better the motor's performance, the smaller this decrease. When configuring a motor for the feed axis, the output torque at the highest cutting speed should be sufficient. Although no cutting occurs during rapid feed and the load is smaller, the starting torque at the highest rapid feed speed should still be considered. Excessive drop in output torque at high speeds will also affect the control characteristics of the feed axis. The selection of the spindle servo motor... Output power is an indicator of the spindle motor's load capacity. As shown in Figure 3, the rated power of the spindle motor refers to its output power when operating within the constant power range (speeds N1 to N2). Below the basic speed N1, the rated power cannot be reached; the lower the speed, the lower the output power. To meet the power requirements at low spindle speeds, gearbox transmission is generally used, ensuring the motor speed at low spindle speeds is above the basic speed N1. However, this results in a more complex mechanical structure and increased costs. In CNC machine tools where the spindle and servo motor are directly connected, there are two methods to meet the power requirements at low spindle speeds. One is to select a spindle motor with a lower basic speed or a higher rated power. The other is to use a special winding-switching spindle servo motor (such as the YMF type spindle motor from Okuma, Japan). In this type of motor, the three-phase windings are connected in a star configuration at low speeds and in a delta configuration at high speeds, thereby improving the low-speed power characteristics of the spindle motor and reducing the cost of the spindle's mechanical components. It is important to note that while high-speed machining is an effective way to improve the production efficiency of CNC machine tools, high-speed and high-precision cutting places higher demands on servo drives and computer components, inevitably increasing the cost of the CNC system. Another important application area for high-speed machining is the processing of light metals and thin-walled parts. Therefore, the spindle and feed motor speeds should be selected according to the actual needs of the machine tool. Selection of Position Detection Devices The mechanical origin is the reference point for all coordinate systems of a CNC machine tool. The stability of the mechanical origin is an extremely important technical indicator for CNC machine tools and a fundamental guarantee for stable machining accuracy. There are two methods for establishing the mechanical origin: In CNC machine tools using relative position encoders, inductive synchronizers, or optical gratings as position feedback devices, the CNC system uses the first zero-point mark signal generated by the position feedback device after the zero-return deceleration switch (or marker) of each feed axis as the reference point. These types of machine tools must perform a zero-return operation on each feed axis after each power outage or emergency stop; otherwise, the actual position may shift. Improper adjustment of the relative position of the zero-return deceleration switch and its stop block can also cause instability in the mechanical origin position. These are all issues that should be taken seriously. In CNC machine tools that use absolute position encoders as position feedback devices, the absolute position encoder can automatically memorize the position of every point within the full stroke of each feed axis, eliminating the need for a zero-return switch. After each power outage or emergency stop, there is no need to reset the reference point. The reference point position is permanently set and stored in a dedicated memory for the absolute position encoder. This is particularly suitable for setting the zero point position of rotary tables with ratchet positioning, offering not only good stability but also significant convenience for operation and adjustment. [b]Selection of Mechanical Design Scheme[/b] Machine tools consist of both mechanical and electrical components. When designing the overall scheme, the implementation of various machine tool functions should be considered from both electromechanical perspectives. The mechanical requirements of CNC machine tools and the functions of CNC systems are complex, so electromechanical integration is crucial to leverage strengths and avoid weaknesses. Examples are given below. Example 1: Spindle speed adjustment can be achieved through two methods: automatic stepless speed regulation using a servo motor or frequency converter, and manual gear shifting using a standard three-phase asynchronous motor. Machining centers use various cutting tools to perform continuous cutting operations of different types (milling, drilling, boring, and tapping, etc.), so the spindle speed changes frequently and must be automatically controlled by the S-code of the machining program. Automatic tool changing also requires spindle orientation, necessitating an automatic stepless speed control system with orientation functionality. For ordinary CNC milling machines where high spindle speed requirements are not critical, tool changes are usually manual. Furthermore, the opportunity to select different speeds for the same tool during machining is infrequent, so manual speed changes during tool changes have minimal impact on production efficiency. Therefore, mechanical gear-based stepless speed regulation with manual shifting is commonly used. Compared to using servo motors for stepless speed control, this approach significantly reduces production costs, saves energy, and simplifies maintenance, making it a very practical choice. Example 2: When using a horizontal machining center to perform multi-faceted machining on parts, it is often necessary to change fixtures and perform multiple clamping operations, which inevitably occupies valuable machine tool running time. Choosing a horizontal machining center with a dual-station automatic pallet changer (APC) device can significantly save machine time occupied during part clamping, thereby improving machine tool production efficiency. Moreover, this function is controlled by a PLC control program; apart from using a few more input/output control points, the cost increase of the CNC system is minimal, making it a high-performance/price ratio choice. Example 3: The tool change time of a machining center has a significant impact on production efficiency, and the tool change speed is closely related to the mechanical structure. For example, the tool change time of a hydraulically controlled robot is generally over 10 seconds. Robots that can complete the tool change action within 2-3 seconds are generally driven by servo motors and equipped with cams and internal/external hydraulic cylinder tool release mechanisms. A tool change speed that is not commensurate with the mechanism may increase the failure rate. Selecting a reasonable cutting path, using high-quality tools, and optimizing cutting conditions are also important means to improve production efficiency and should be considered comprehensively. [b]Selection of CNC Functions[/b] In addition to basic functions, CNC systems offer users a variety of optional functions. The basic functions of CNC systems from various well-known brands are largely similar. Therefore, rationally selecting optional functions suitable for the machine tool and abandoning those that are unnecessary or impractical is highly beneficial for improving the product's function/price ratio. Below are a few examples for reference. **Animation/Trajectory Display Function: ** This function simulates the part machining process, displaying the actual cutting path of the tool on the workpiece. It allows simultaneous display of two different planes in a Cartesian coordinate system, or a three-dimensional display from different perspectives. Real-time display can be performed during machining, or the machining process can be quickly sketched under mechanical locking. This is an effective tool for verifying part machining programs, improving programming efficiency, and real-time monitoring. **Floppy Disk Drive:** This data transfer tool allows the system to save the debugged machining program to a floppy disk for archiving. It can also be used to save machining programs generated on other computers into the NC system, reducing the time spent on machine program input. Furthermore, it can be used for backup or storage of various machine tool data, greatly facilitating programmers and operators. **DNC-B Communication Function:** As is well known, programming parts composed of non-circular curves or surfaces is extremely difficult. The usual method involves using a general-purpose computer to subdivide these curves into tiny three-dimensional straight line segments before writing the machining program, resulting in a very large program size. In mold making, such programs, often hundreds of kilobytes long (4 kilobytes is approximately equal to 10 meters of paper tape), are frequently encountered. However, the basic program storage capacity provided by a typical CNC system is only 64-128 kilobytes, which poses a significant challenge to mold making. The DNC-B communication function has two operating modes: one is bidirectional program transfer between the personal computer and the CNC system's program storage area; the other is transferring the program from the personal computer to the CNC system's buffer memory segment by segment, processing and transferring simultaneously until completion. This completely solves the problem of machining parts with large program sizes. Although using this function incurs an additional cost, it is indeed a high-value option. Of course, expanding memory capacity is also an effective way to solve the machining of curved molds. For example, the maximum runtime buffer memory capacity of the Okuma OSP system is 512KB. The program memory capacity can be expanded to 4096KB, which can meet the needs of most mold machining. Compared with the DNC-B method, its advantage is that it eliminates the need for a personal computer, making the operation more reliable and convenient. [b]Simplified Programming Functions[/b] To improve programming efficiency, shorten the length of machining programs, and give full play to the potential of program memory, CNC systems provide some methods to simplify program compilation. Fixed Cycles Common machining operations (such as drilling, boring, tapping, and cavity and peripheral machining, etc.) can be programmed into parametric fixed cycle programs. During programming, the user fills in the corresponding data (such as base surface, hole depth, depth per cut, spindle speed, and feed rate, etc.) to complete the predetermined machining operation, which can be reused multiple times. Coordinate Calculation Function Utilizing the real-time computing capabilities of the CNC system, hole machining operations distributed according to various rules (such as oblique lines, circles, and grids) can be programmed into parametric fixed-loop programs. During programming, the user inputs the corresponding data (such as angles, radii, number of holes, number of rows, and number of columns) to complete the predetermined machining operations. **Subroutine Function:** Users can program the same machining operation used in multiple places on a part into subroutines, which can be called at the corresponding locations, thus shortening the length of the machining program. **User Macro Program:** Users can use various arithmetic, logical, and functional operators, as well as various branching statements provided by the system, to compose mathematical expressions describing the shape of the machined part. During program execution, the CNC system performs calculations and outputs results simultaneously, enabling the machining of special curves and surfaces with a very short program. Rigid Tapping Function Rigid tapping requires a servo motor to drive the spindle, necessitating not only a position sensor but also strict requirements on the backlash and inertia of the spindle drive mechanism. Therefore, the cost of this function cannot be ignored. For users without special requirements (such as high speed and high precision, special materials, or large-diameter hole machining), flexible tapping on a general spindle using a telescopic chuck can meet machining requirements, eliminating the need for rigid tapping. [b]Tool Life Management Function[/b] Whether to select tool life management on a machining center depends on factors such as the batch size of machined parts, the consistency of tool and blank quality, and the tool magazine capacity. Otherwise, it will not only cause many human errors and affect normal production, but the tool positions occupied by spare tools will also greatly reduce the effective capacity of the tool magazine, making it impossible to machine some complex parts due to insufficient tool positions. [b]Automatic Tool Radius/Length and Workpiece Measurement Function[/b] The tool movement trajectory in the machining program is usually written according to the tool center and tool tip, so the corresponding tool radius and length must be input before program execution, which is especially important for machining centers. Tool radius and length can be measured manually with ordinary measuring tools or with a specialized tool measuring instrument. The operator can compensate for the length offset by using the positional difference of each tool tip in the Z-axis direction relative to the same "tool setting face" on the machine tool. This can be achieved using the "semi-automatic tool length measurement" function provided by the CNC system itself, inputting the length compensation value relative to the "standard tool". Automatic tool radius/length and workpiece measurement functions require dedicated contact sensors, laser probes, and signal receivers. When selecting this function, the following points should be noted: The contact sensors and signal receivers are installed within the machine tool's working area; their protection is crucial. They are not suitable for machine tools with large cutting volumes or those using spray cleaning. The above measurements require machining time, potentially affecting machine tool efficiency. The general purpose of the workpiece measurement function is to measure the position of the center of the reference hole or other reference points on the workpiece blank, replacing manual "tool setting." Its accuracy will not exceed the positioning accuracy of the machine tool itself. (Edited by: He Shiping)