This paper mainly describes several commonly used CNC maintenance methods, including PLC input/output recording method, hard disk backup method, spare parts rapid replacement method, key point maintenance method, and defect improvement method. Basic methods for CNC retrofitting include using PLC, commercial CNC, industrial PC, and PC-based NC. The following describes the basic methods in two aspects: CNC maintenance and retrofitting, for your reference. 1. Basic Methods of CNC Maintenance 1.1 "Collective Consultation" Method: This method is led by the supervisor, convening personnel from process, equipment, and operation departments for a consultation. Each person proposes their own solution, and problems are checked and resolved one by one. This method is frequently used in foreign-invested companies. 1.2 PLC Input/Output Point Recording Method (PLC I/O LED photo record): In past maintenance, we have found that many equipment failures occur when the machine is turned on. In fact, the equipment does not have a hardware fault; it is often due to improper operation or other reasons causing the PLC input signal to be lost. Although the fault is not serious, it is still quite time-consuming to find. Therefore, we use a digital camera to photograph the input/output indicator lights on the PLC when it is normal, print them out, and stick them next to the PLC. In this way, if a similar problem occurs, maintenance workers can quickly identify the issue by comparing the photos, and sometimes even operators can find the problem based on this information. The advantage of using a digital camera is that the images can be stored on a computer, facilitating classification, management, and long-term preservation. We also use this photo-based comparison method on some adjustable valves, limit switches, and mechanical positioning devices, which has proven very helpful in guiding operation and maintenance. 1.3 Hard Disk Backup Method Many existing devices use industrial microcomputers as host computer control devices or human-machine interfaces. More than 80% of microcomputer failures are software failures. Previously, software failures often required professional computer technicians to repair, typically involving a system reinstallation, which was very time-consuming. Our current hard disk backup method involves purchasing a new hard drive identical or similar to the original machine, and using Ghost or similar software to copy all data from the original hard drive, including the system, to the new hard drive as a backup. If a software failure occurs, simply replacing the backup hard drive with the backup can be done easily, greatly shortening repair time. It also prevents actual hard drive failure. This work can generally be performed by a maintenance worker. 1.4 Rapid Spare Parts Replacement Method Due to the rapid development of electronic components, some electrical spare parts are updated and replaced very quickly. High-tech products such as frequency converters are updated almost every two to three years. Therefore, it is difficult to purchase spare parts that are exactly the same model as the original, and sometimes products from different companies may be purchased. Replacing these spare parts is generally challenging for maintenance workers, especially with frequency converters. If a different manufacturer's product is used, not only are the hardware wiring different, but the parameter settings are also different. To ensure that maintenance workers can replace parts accurately and quickly, we create a direct replacement operation flowchart and parameter setting table based on the technical parameters of the frequency converters used on-site and those in the spare parts inventory. This flowchart is distributed to each maintenance worker and posted next to the frequency converter on-site. Similar methods are used for other complex spare parts, effectively shortening the spare parts replacement time. 1.5 Key Point Checking and Maintaining Method How to maximize the normal operation of equipment using a limited number of maintenance personnel is a research topic for our equipment management personnel. Many large pieces of equipment and production lines have numerous and widespread potential points of failure, making it difficult for maintenance workers to cover everything during routine inspections. Conversely, many maintenance workers often lack a clear understanding of the key points and waste considerable energy on minor issues. To address this, we implemented a work method called "point-based maintenance" to guide maintenance workers in their daily inspections and repairs. This method involves identifying several critical areas based on the specific characteristics of each piece of equipment, presenting them in chart form to the relevant maintenance workers, and requiring them to conduct focused inspections of these areas daily, promptly repairing any abnormalities. This approach has yielded positive results. When creating the point-based maintenance checklist, the following two points must be considered: 1) The critical points must be appropriately and accurately identified. We determined these points by combining the equipment's technical manual, the opinions of experienced maintenance technicians, and past equipment failure scenarios. 2) The critical points should be "precise." Generally, a large piece of equipment or a 500 m² production line should have no more than 10 critical points; otherwise, it may become merely a formality. 1.6 Defect Retrofitting Method In long-term maintenance work, we found that some equipment frequently experienced recurring faults with a high degree of randomness. Further analysis revealed and confirmed that the faults stemmed from design and manufacturing defects that could not be eradicated through repair alone; modification was necessary to eliminate the hidden dangers. A typical example is a refrigerator liner vacuum thermoforming machine imported from Italy. Its heating control section used a single-chip microcomputer system for network data exchange with a host computer. A frequent fault was poor communication between the microcomputer and the host computer. However, powering back on or replacing the microcomputer motherboard might restore normal operation; one fault even burned out the host computer's COM1 port. After researching, we discovered that the problem was caused by an unreasonable communication method in the design. The microcomputer and host computer were directly connected via RS232C without isolation, and the cable length exceeded 30 meters. We know that RS232C communication has poor anti-interference capabilities, and the communication distance is generally less than 15 meters. To address this, we modified the communication cable, adding ADAM's 7520-RS485/RS232 optical isolators to both ends, enabling data transmission via RS485 over most distances. Since then, the fault has not recurred. 1.7 Real-time Monitoring CNC machine tools frequently experience soft faults, making diagnosis difficult. In such cases, data acquisition cards can be used to collect and record key signals in real time for comprehensive analysis during diagnosis. The emergence of "virtual instruments" provides the possibility for real-time monitoring. It should be noted that this monitoring system and the CNC are independent of each other; the monitoring system is custom-made by the end user. 2. Several Options for CNC Retrofitting 2.1 Using PLC for CNC Retrofitting In printing, packaging, and other applications where positioning accuracy requirements are not too high, as well as in some production line control applications, CNC retrofitting tasks can be completed using a PLC. In this case, a general-purpose PLC is used, rather than a CNC-specific PLC such as FANUC PMC. Currently, general-purpose PLCs are quite powerful, including positioning modules, A/D, D/A, and I/O modes, enabling many industrial control functions. 2.2 Retrofitting Commercial CNC (PLC) and Servo Systems: This is a popular retrofitting method. CNC system manufacturers are providing increasingly convenient debugging methods and software tools to facilitate machine tool manufacturers and retrofitters. Debugging software mainly includes servo system adaptation, ladder diagrams, and machine tool parameters. CNC manufacturers typically provide CDs and debugging manuals. For example, Siemens provides a free debugging CD and a concise debugging guide for the SINUMERIK 802D when selling its economical CNC model. AB (Allied Power Systems) in the US, when selling its 9-series CNCs, requires users to purchase an ODS (offline development system) floppy disk and manual. This ODS allows for ladder diagram development, machine tool parameter setting, and part machining program editing on a personal computer, communicating with the CNC via a 232 port. 2.3 Choosing an Industrial PC for CNC Retrofitting This applies not only to CNC retrofitting of metal cutting machine tools, but also to process control, sequential control, and other industrial control applications. The reliability of industrial PCs can be leveraged by selecting appropriate software tools or developing custom software for retrofitting. In the early 1990s, we used STD industrial PCs and four DC servo systems to develop our own software to retrofit a domestically produced 5040 CNC milling machine. The industrial control computer consisted of three 8088 CPU STD computers connected by a communication interface. Each STD computer was composed of several modules, serving as hardware support for distributed computer CNC. 2.4 Choosing PC-based CNC PC-based CNC is the current implementation method for open CNC. However, it is still far from truly open CNC. Currently, there is no universally accepted definition of "open architecture." Some users believe that open means accepting locally used communication protocols; while others believe that open means all controller operating interfaces are completely consistent. For machine tool application engineers, open means having standard input/output interfaces for tool holder movement, sensors, and logic control. For research engineers at large companies and universities, open architecture means that all of the above comes from readily available building blocks. Driven by pressure from users and integrators, the development of open architectures is progressing and will continue. A positive outcome is PC-based CNC. Current implementations consist of a personal computer and a motion control card in hardware. The software primarily consists of a computer software platform (such as WIN or DOS) and CNC system software. Delta Tau's PMAC multi-axis motion control card development has progressed more rapidly in this area, having developed PMAC cards for PC bus, VME bus, and STD bus. The entire motion control is handled independently by the PMAC. The software running on the host computer is merely a development environment, responsible for display, debugging, part programming, and communication. During PMAC operation, the foreground handles axis motion control, while the background handles the PLC. It uses a Motorola 56001 DSP as the CPU, which can connect to DC, AC, and stepper motors. Feedback elements can include incremental/absolute encoders, rotary transformers, and linear scales. Another alternative approach is to reference the architecture of a personal computer in terms of hardware and software, but with modifications. For example, the Siemens 840D, following the current integrated MMC, CNC, and PLC CNC control machine model, is essentially a standard IPC industrial computer on the MMC, but uses the Flex OS operating system. The CNC and PLC are integrated into the NCU, with peripheral interfaces including the spindle, feed module, and I/O ports. The connection between the 840D and the 611D drive is via a bus (the 802D uses the PROFIBUS fieldbus), more closely resembling the information transmission method of a computer. Furthermore, due to the use of the Flex OS operating system, its user interface is very similar to that of a regular personal computer. The PMAC motion control card and the Siemens 840D mentioned above have done a lot of work in CNC based on personal computers, but they still have a long way to go before achieving true openness. The ideal model should be an industrial computer + real-time operating system + development environment + CNC function library. The CNC control software should be configurable, with the configurable objects being the library functions within the CNC function library. The development environment provides users with development tools, integrating library functions into the real-time operating system. Just as PLC program development shifted from CNC manufacturers to machine tool factories and CNC integrators, the formation of CNC systems can also shift from CNC manufacturers to machine tool factories, integrators, and even end users. This is the ultimate meaning of open CNC. 3. CNC Retrofit Process The CNC retrofit process is as follows: 1) When ordering, selection is mainly based on indicators such as spindle power, feed torque, installation, and I/O points. It's best to refer to the original machine tool's documentation if available; otherwise, on-site testing is necessary. 2) Hardware wiring should primarily refer to the CNC manufacturer's hardware wiring manual. 3) Ladder diagram development should primarily refer to the manufacturer's PLC programming manual. 4) Debugging is mainly based on the parameter manual and debugging guide. In summary, the hardware wiring manual, PLC programming manual, debugging manual, and parameter manual provided by the CNC manufacturer must be thoroughly studied, especially before the first retrofit. As CNC systems become increasingly "personal computer-like," software is commonly used for CNC debugging. For example, with the Siemens 840D, before debugging, the PG programmer and the PG port of the 840D should be connected using an MPI cable (multi-terminal communication interface). Debugging involves two parts: one is the application of basic PLC programs—activating the MCP (Machine Tool Control Panel) and axis enable; the second is the adjustment of machine tool data (MD, i.e., machine tool parameters). 4. The Relationship Between CNC Repair and Retrofitting In China, most CNC retrofitting engineers come from CNC repair or CNC system backgrounds. However, those who have done both repair and retrofitting have found that while there is a relationship, there are also many differences. Some say CNC retrofitting is a more advanced form of repair; others say that those who do retrofitting may not be good at repair, and most repairers have never done retrofitting. It should be said that both are part of CNC technical services, but they are ultimately two different things. Given the situation in China, CNC repair is more difficult than retrofitting. This is mainly because the repair market is immature, spare parts are difficult to obtain, and repair is a broad discipline with many uncertainties, outdated diagnostic technology and equipment, and lower costs. While CNC retrofitting requires a higher overall level of knowledge, it involves fewer uncertainties and has a relatively higher success rate. Currently, domestic CNC service companies are increasingly prioritizing retrofitting over maintenance. However, the most pressing and widespread need of end users—CNC maintenance—remains unmet.