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To maintain the integrity of CNC machine tools, it is required to put forward higher standards for the reliability, maintainability, and availability of CNC machine tools. The main indicator of reliability is MTBF, which is the occurrence of CNC machine tools during use. N times of failures, each time the fault is repaired and put into use again, the duration of the work before each fault is measured as t1, t2...tN0, and the average fault clearance time MTBF=T/N. (T is the sum of t1, t2...tN0). The measure of maintainability is the mean time to repair (MTTR). The MTTR is the ratio of the total time of remedial maintenance to the total number of failures of the repaired product at that level, at any given level of maintenance for the machine under the specified conditions and for a specified period of time. Simply put, it is the average of the actual direct repair time required to troubleshoot, MTTR = (Σti)/n (ti is the ith repair time, n repair times), and the availability is based on the prerequisite that the required external resources are guaranteed. The ability to perform a prescribed functional state under specified conditions and within a specified time or time interval. It is a comprehensive reflection of product reliability, maintainability and maintenance guarantees. Reliability increases product availability by extending its normal operating hours, while maintainability increases availability by reducing downtime due to repairs.
In recent years, the MTBF of domestically-manufactured numerical control systems has mostly exceeded lO000h, but the MTBF of internationally advanced enterprise numerical control systems has reached 80,000h. Although China's machine tool industry has made great progress and its annual output has reached more than 1,000 units, most of our country's machine tools are at a low level. And there is a lack of door. Some users can't make correct judgments on the faults of CNC machine tools and accurately discharge the faults. The after-sales service of the manufacturer can't get on-site service in time. At present, the on-boarding rate of CNC systems in various industries in China is only about 25% on average.
2. Types and characteristics of faults in CNC machine tools Faults in CNC machine tools refer to events or states in which the machine tool cannot perform a predetermined function is called a fault. That is, the completion of a specified function is lost. According to the result of a fault, it can be classified into lethal and non-fatal. Failure, the former will make the product can not complete the specified task can lead to significant loss of people or things, and ultimately make the task fail; the latter does not affect the task is completed, but it will lead to unplanned maintenance. According to the statistical characteristics of the fault can be divided into separate faults and subordinate faults. The former refers to a failure that is not caused by another product failure, and the latter refers to a failure caused by another product failure. In accordance with the level of failure frequency of CNC machine tools can be divided into early failure, accidental failure and loss of failure.
As shown in the figure, the early failure refers to the failure of the machine tool due to design or production at the initial stage of use. During this time, the machine is in a running-in phase and the mechanical parts or electronic components cannot withstand the initial test and are damaged. So the frequency of failures is relatively high. Accidental failure means that after the machine tool is put into use for a period of time, the failure rate of the product is reduced to a low level, but it is basically in equilibrium. At this time, the failure rate can be considered as a constant. During this period, the failure of the machine tool is mainly caused by accidental factors. The accidental failure caused is the main period of use of the machine tool, generally 7 to 10 years. Loss-of-use failures are caused by prior detection or monitoring of predictable failures due to the gradual decline of the machine tool's regulations over time. Depletion failures can be prevented through preventive maintenance, prevent failures, extend service life, or until the end of the depletion period. Updated to ensure the service life of the machine.
Control machine fault features: CNC machine tools are generally composed of numerical control system, servo system including servo motor and detection feedback device, strong electric control cabinet, machine body and various auxiliary devices.
The complexity of CNC machine tools makes its faults complex and specific. There are many factors that cause NC machine tool failures. You can't just look at the phenomenon of failures. You must examine the comprehensive factors that cause the failure through the phenomenon and find the root cause of the failure. Reasonable ways to exclude.
3, CNC machine tool fault diagnosis and troubleshooting methods CNC machine tool fault diagnosis generally includes fault detection, fault determination and isolation and fault location. Diagnostic methods generally use the reset method, exchange spare parts method, function self-diagnosis method, vibration hammering method and principle analysis method.
3.1 reset method: due to transient failure alarm, hardware reset can be used to eliminate the fault, if the system work area due to voltage instability or screen garbled, chaotic, the system must be initialized to clear. Before copying, copy and copy the data and programs.
3.2 Exchange Spare Parts Method: Replace the circuit boards diagnosed as faulty by the spare circuit board, and initialize and start the machine, so that the machine tool can run quickly. This is the most commonly used method for discharging faults.
3.3 Function self-diagnosis method: Using the self-diagnosis function of the machine tool, according to the alarm information displayed by the CRT, the approximate cause of the fault can be determined.
3.4 Vibratory tapping method: sometimes the CNC system fails. Most of the reasons are due to the presence of soldering or poor contact of electronic components. If the insulator is gently tapped at the point of doubt, the fault may reappear.
3.5 Principle analysis method: according to the composition structure of the numerical control system, the voltage and related parameters of each point are analyzed logically, and then measured, analyzed and compared by a multimeter, an oscilloscope, a signal generator, a logic pen, etc., so as to locate the fault. .
4. The reliability of CNC machine tool fault management Machine tool reliability is measured by the frequency of fault occurrence. The analysis, evaluation and improvement of the reliability of the machine tool are inseparable from the fault information. The purpose of establishing a fault report, analysis and corrective action system is to ensure the correctness and completeness of the fault information, and to use the fault information in a timely manner for analysis and improvement to achieve the reliability growth of the product.
Fault report: Any failure at any function level of the machine tool during the specified inspection and operation shall be reported to the management department at the specified level. Failure reports should be written in the form of failure cards, documents, etc. for review and archiving. The fault reporting system can make full use of existing information management systems to avoid unnecessary duplication. Fault report work should be timely and correct. Fault reports should have good traceability.
Failure Analysis: After the occurrence of a fault, the responsible person in charge of inspection and operation shall promptly notify the relevant personnel to conduct on-site investigations, verify the faults, conduct engineering analysis and statistical analysis, so as to take targeted corrective measures.
Fault rectification: Corrective measures must be analyzed, calculated and verified by necessary tests to prove that they are feasible and effective before they can be implemented. After completion of the fault rectification activities, fault analysis and correction reports shall be prepared, and various data and information formed during failure analysis and correction shall be collected. The fault parts shall be properly kept for further research and analysis.
1. The concept of fault diagnosis and maintenance of controlled machine tools Numerically controlled Machine Too1 uses a computerized Numerical Contro1 system and is therefore also referred to as a computer numerical control machine tool or a CNC machine tool. The CNC machine tool has comprehensively applied various technologies such as computer, automatic control, precision measurement, modern machinery manufacturing and data communication, and is a typical electromechanical integration equipment in the field of mechanical processing. It is suitable for the processing of complex parts of many varieties and medium and small batch sizes.