1. Concept of Fault Diagnosis and Repair of Controlled Machine Tools
Numerically controlled machine tools employ computer numerical control (CNC) systems, hence they are also called computer numerical control machine tools or CNC machine tools. CNC machine tools integrate multiple technologies such as computers, automatic control, precision measurement, modern mechanical manufacturing, and data communication. They are typical mechatronic equipment in the field of machining, suitable for processing complex parts in small to medium batches with a wide variety of products.
Maintaining the uptime of CNC machine tools requires higher standards for their reliability, maintainability, and availability. The main indicator for reliability is the Mean Time Between Failures (MTBF). MTBF is calculated by taking N faults during a CNC machine tool's operation, measuring the working time before each fault (t1, t2, ..., tN0), and then calculating the MTBF as T/N (where T is the sum of t1, t2, ..., tN0). Maintainability is measured by the Mean Time To Repair (MTTR). MTTR is the ratio of the total corrective repair time to the total number of faults repaired at any given maintenance level under specified conditions and within a specified time period. Simply put, it's the average actual direct repair time required to eliminate a fault: MTTR = (Σti)/n (ti is the time of the i-th repair, and n is the number of repairs). Availability is the ability of a machine tool to perform its specified functions under specified conditions and within a specified time interval, provided that the required external resources are guaranteed. It is a comprehensive reflection of product reliability, maintainability, and maintenance support. Reliability improves product availability by extending its uptime, while maintainability improves availability by reducing downtime due to maintenance.
In recent years, the MTBF (Mean Time Between Failures) of domestically produced CNC systems has mostly exceeded 10,000 hours, while the MTBF of advanced international CNC systems has reached 80,000 hours. Although my country's machine tool industry has made significant progress, with an annual output of over a thousand units, the overall level of Chinese machine tools remains low and incomplete. Some users are still unable to make timely and accurate diagnoses and troubleshoot CNC machine tool malfunctions, and manufacturers' after-sales service cannot provide timely on-site support. Currently, the average uptime of CNC systems across various industries in China is only around 25%.
2. Types and characteristics of CNC machine tool failures
A CNC machine tool malfunction refers to an event or state in which the machine tool cannot perform its intended function. In other words, it loses the ability to complete its prescribed function. Based on the consequences, malfunctions can be divided into fatal and non-fatal malfunctions. Fatal malfunctions prevent the product from completing its intended task, potentially leading to significant losses of personnel or property, and ultimately causing task failure. Non-fatal malfunctions do not affect task completion but may lead to unplanned maintenance. Based on statistical characteristics, malfunctions can be further divided into independent malfunctions and dependent malfunctions. Independent malfunctions are those not caused by a malfunction of another product, while dependent malfunctions are caused by a malfunction of another product. According to the frequency of CNC machine tool malfunctions, they can also be divided into early malfunctions, random malfunctions, and wear-out malfunctions.
As shown in the figure, early failures refer to malfunctions caused by design or production reasons during the initial use of a machine tool. During this period, the machine is in a break-in phase, and mechanical parts or electronic components may fail to withstand the initial stress and become damaged. Therefore, the frequency of failures is relatively high. Random failures refer to malfunctions that occur after the machine tool has been in use for a period of time, when the product's failure rate drops to a low level but is essentially in a balanced state. At this point, the failure rate can be considered constant. During this period, the machine tool's failures are mainly caused by random factors and represent the main service life of the machine tool, generally 7-10 years. Wear-out failures are...
Failures that can be predicted through advance detection or monitoring are caused by the gradual degradation of machine tool specifications over time. Wear-out failures can be prevented through preventive maintenance to extend service life, or timely replacement before the wear-out period to ensure the service life of the machine tool.
Characteristics of CNC machine tool failures: CNC machine tools generally consist of a CNC system, a servo system including servo motors and detection feedback devices, a high-voltage control cabinet, the machine tool body, and various auxiliary devices.
The complexity of CNC machine tools makes their malfunctions complex and unique. There are many factors that can cause CNC machine tool malfunctions. We cannot just look at the symptoms of the malfunction; we must look beyond the symptoms to examine the comprehensive factors that cause the malfunction, find the root cause, and take appropriate measures to eliminate it.
3. CNC Machine Tool Fault Diagnosis and Troubleshooting Methods
Fault diagnosis of CNC machine tools generally includes fault detection, fault identification and isolation, and fault location. Diagnostic methods typically include reset, spare parts exchange, functional self-diagnosis, vibration tapping, and principle analysis.
3.1 Reset Method: For alarms caused by momentary faults, a hardware reset can be used to troubleshoot the problem. If the system operating area is affected by unstable voltage or garbled/disorganized screen characters, the system must be initialized and cleared. Before clearing, data and programs should be copied.
3.2 Replacement method: Replace the faulty circuit board with a spare circuit board and perform initial startup to enable the machine tool to start running quickly. This is currently the most common method for troubleshooting.
3.3 Functional self-diagnosis method: By utilizing the machine tool's self-diagnosis function and based on the alarm information displayed on the CRT, the general cause of the fault can be determined.
3.4 Vibration and tapping method: The faults in the CNC system are intermittent. The cause is often poor soldering or contact of electronic components. Gently tapping the suspected area with an insulating material may reproduce the fault.
3.5 Principle Analysis Method: Based on the composition structure of the CNC system, the voltage and related parameters of each point are logically analyzed. Then, multimeters, oscilloscopes, signal generators, logic probes, etc. are used to measure, analyze, and compare them to locate the fault.
4. CNC machine tool fault management
Machine tool reliability is measured by the frequency of failures. Analysis, evaluation, and improvement of machine tool reliability all rely on fault information. The purpose of establishing a fault reporting, analysis, and corrective action system is to ensure the accuracy and completeness of fault information and to utilize this information promptly for analysis and improvement, thereby increasing product reliability.
Fault Reporting: Any fault occurring at any functional level of the machine tool during the specified inspection and operation period should be reported to the designated management level. Fault reports should be in written form, such as fault cards or documents, for review and archiving. The fault reporting system can make full use of existing information management systems to avoid unnecessary duplication. Fault reporting should be timely and accurate. Fault reports should have good traceability.
Fault Analysis: After a fault occurs, the on-site supervisor responsible for inspection and operation should promptly notify relevant personnel to conduct an on-site investigation, verify the fault, and perform engineering and statistical analysis in order to take targeted corrective measures.
Fault Correction: Corrective measures must be analyzed, calculated, and verified through necessary experiments to prove their feasibility and effectiveness before implementation. After the fault correction activity is completed, a fault analysis and correction report should be prepared, compiling all data and information generated during the fault analysis and correction process. Faulty components should be properly stored for further research and analysis.
