The CNC machine tool is an automatic machine tool that integrates machine, electricity, liquid, gas, light, etc., and through the execution functions of each part, finally completes the movement, rotation, clamping, loosening, shifting and tool changing of the mechanical actuator. Actions to achieve cutting tasks. When working, the functions are combined with each other, and when the fault occurs, they are mixed together. The fault phenomenon and the cause are not simple one-to-one correspondence. There may be several different causes of a fault phenomenon. Most faults appear in a comprehensive form. The crawling and vibration of CNC machine tools is an obvious example.
When the moving parts driven by the CNC machine feed servo system are running at low speed, the moving parts can not start, and then the acceleration movement is suddenly started after the start, and then pauses, and then accelerates, so that the moving parts are stopped. The phenomenon of sudden jump, slow and slow movement is called crawling; when it is running at high speed, the moving parts show obvious vibration. This failure phenomenon is a typical creep and vibration failure of the feed system.
There are many reasons for this type of failure. It may be caused by a fault in the mechanical part, or it may be caused by a fault in the electrical part of the feed system, or it may be caused by a combined fault of the mechanical part and the electrical part, or even Crawling failures can also occur due to incorrect programming.
I. Analysis of mechanical parts and countermeasures Because the crawling phenomenon of CNC machine tools at low speeds often depends on the characteristics of the mechanical transmission part. The vibration at high speed is usually related to the pre-tightening force of the motion pair in the feed drive chain. The creeping and vibration faults may be in the mechanical part.
If in the mechanical part, the rail pair should first be checked. Because the frictional resistance of the moving parts is mainly from the guide rail pair, if the dynamic and static friction coefficient of the guide rail pair is large and the difference is large, it will easily cause creep. Although the guide rails of CNC machine tools are widely used with rolling guides, static pressure guides or plastic guide rails, if the guide rail gap is not well adjusted, it will still cause creep or vibration. For the static pressure guide pair, it is important to check whether the static pressure is in place. For the plastic guide rail, it is possible to check whether there is any foreign matter or foreign matter hindering the movement of the guide rail. For the rolling guide, check whether the pre-tightening measures are good. Paying attention to the lubrication of the guide rail pair also helps to analyze the crawling problem. The lubrication status of the guide rail is not good. (Source: http://) The lubricating oil of the guide rail is not enough, causing the slide to crawl. At this time, it is a very effective measure to add lubricating oil and use the rail lubricating oil with anti-climbing effect. The rail lubricant has a polar additive that forms a layer of oil film that is not easily broken on the surface of the rail, thereby improving the friction characteristics of the rail to prevent creep.
Second, check the feed chain. Because in the feed system, the servo drive must move between the moving parts through the transmission chain consisting of gears, screw nut pairs or other transmission pairs. Decreased positioning accuracy and increased backlash will also cause the table to creep during the feed motion. By adjusting the pre-tightening force of the bearing, the screw nut pair and the lead screw itself, adjusting the loosening link and adjusting the compensation link, the torsion and tensile rigidity of the transmission chain can be effectively improved (ie, the transmission rigidity is improved), The accuracy of the movement and the elimination of the crawling are very beneficial; in addition, the transmission chain is too long, the diameter of the transmission shaft is small, and the rigidity of the support seat is also a factor that causes creep. Therefore, in the inspection, we must also consider whether these aspects are defective, and check them one by one.
2. Analysis of the causes and countermeasures of the feed servo system If the cause of the fault is in the feed servo system, the relevant links in the servo system must be checked separately. The crawling and vibration problems of CNC machine tools are speed problems, which are closely related to the feed speed. Therefore, it is inseparable from the analysis of the speed loop of the feed servo system. Checking the speed regulator fault is a given signal, and the second is the feedback signal. It is the speed regulator itself failure. Check whether the motor or tachogenerator surface is smoothed according to the fault characteristics (such as whether the vibration period and the feed speed are proportionally changed); also check whether the system interpolation accuracy is too bad, check whether the speed loop gain is too high; Whether the system parameter setting has error or not; whether the short-circuit bar or potentiometer setting of the servo unit is correct; whether the gain potentiometer is adjusted for deviation or not, and whether the line of the speed control unit is good, these items should be checked and sorted out one by one.
Third, other factors Sometimes the fault is neither the mechanical part nor the reason for feeding the servo system, there may be other reasons such as programming error. For example, the FANUC 6M system CNC machine tool is overloaded and crawled during one cutting process. After careful verification, it was found that the motor failure caused overload, the replacement motor overload was eliminated, and it could crawl or exist. First, we started to find the cause of the fault from the machine tool. As a result, we verified that there was no problem in the drive chain, and checked the servo system to confirm that there was no fault. Then we checked the machining program and found that the machining of the workpiece curve was realized by fine segmentation arc approximation. The G61 command is used in the programming, that is, the in-position stop check is performed once for each processing, so that the machine tool is crawling, and the G61 is changed to the G64 command for continuous cutting, and the crawling is eliminated.
If the fault has both the mechanical part and the feed servo system, it is difficult to distinguish the main contradiction that caused the fault. This is an important factor that restricts us from quickly detecting the cause of the fault. Faced with this situation, it is necessary to carry out various aspects of testing, using comprehensive knowledge of mechanical, electrical, hydraulic and other aspects, taking comprehensive analysis and judgment to eliminate faults.
CNC machine tools are technology-intensive and knowledge-intensive equipment. The fault phenomena are diverse, and their manifestations are not followed by simple rules. This requires maintenance technicians to have electronic technology, computer technology, electrical automation technology, and detection technology. Mechanical theory and practical technology, hydraulic and pneumatic and other comprehensive technical knowledge, also requires comprehensive analysis and problem-solving capabilities.
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