About the misunderstanding of high machining accuracy of CNC machine tools (original) I have been engaged in the design of CNC machine tools for some time, and found that many mechanical practitioners and even professional engineers and technicians have many misunderstandings about the machining accuracy of CNC machine tools. It is often heard in production that this is processed by a CNC machine tool, and how accurate it is is actually unscientific. As a professional technician, I think it is necessary to explain whether the accuracy of CNC machine tools is really as elusive as the legend, or there is something else. So the so-called proposition lies in the machining accuracy of CNC machine tools. . . My humble opinion is that it is a fact, but it is an incomplete fact. First of all, we have to figure out what accuracy is. Usually, the precision of machining mainly refers to four points: 1, dimensional tolerance 2, shape tolerance 3, position tolerance 4, and surface smoothness (as for other concepts of maximum solid size, you can refer to the undergraduate textbook, which is mentioned in tolerance teaching, and he uses spline annotation when giving examples; However, if you look at "Course Design of Mechanical Design" and "Handbook of Mechanical Practical Design", he did not use these concepts in the annotation of splines, which shows that he is actually a icing on the cake concept that can be used or not. Beginners may find it strange. How is it possible? In fact, many concepts in machinery are interchangeable, such as parallelism tolerance, or it can be said that two surfaces have tolerances on the surfaces perpendicular to them at the same time. ) Back to the question of accuracy, since the concept of accuracy is now clear, the next question is whether CNC machine tools do these accuracies well. First of all, we must understand the characteristics of CNC machine tools. Numerical control machine tool is actually to install numerical control system (NC) on the machine tool, so it is called CNC. Many good machine tools in our country are transformed by numerical control, that is, adding numerical control devices and feeding systems to ordinary machine tools successfully (of course, ball screws will be added to improve the accuracy of lower spindle bearings, but according to my experience, the improvement of the accuracy of mechanical parts has little effect on the improvement of the machining accuracy of the whole machine bed, because of the influence of tools, this will be discussed later). So what accuracy has been improved by adding numerical control devices and feed systems? Position tolerance! ! ! Is it helpful to improve other tolerance levels? The result of my research is negative! ! ! ! This is the first point I want to elaborate, that is, the core of CNC machine tools (that is, people are most deified). CNC devices and feed systems improve the position tolerance of machine tools (in fact, this is also the main improvement of the whole CNC machine tool compared with ordinary machine tools, which will be discussed later). Because you think about what numerical control improves, whether it is the accuracy of cutting tools or worktable in feeding or walking, the position tolerance will be improved if the position is accurate. But can he improve other tolerances? No Can you improve the surface finish by keeping the correct posture? No way, what is the guarantee of fluency? First of all, tools and machine tools have good rigidity. Under vibration, you can improve them by installing numerical control devices and feed systems. No, it is impossible for CNC lathes, boring and milling machines and even machining centers to achieve the surface smoothness of grinding machines. The shape tolerance is the same. The so-called shape tolerance means that his plane is flat and the circle is not a circle. Was the position of the workbench guaranteed at that time? It is out of the question. After all, it depends on whether your surface is smooth or not. Do you think the surface is not only flat, how can a plane be flat? As for dimensional tolerance, it doesn't matter. In fact, the high-precision dimensional tolerance is guaranteed by the accuracy of the mechanical part of the machine tool, and has nothing to do with the electrical control (high-precision machining), so you can roughly introduce that the machining accuracy of the CNC machine tool is 0.0 1mm, also called 1 wire. To know that your instrument lathe can also achieve this tolerance, the grinder is more casual. Some people may question that the spindle components and spindle supports of CNC machine tools have high rotation accuracy and good rigidity, and the ball screw is used for feed transmission, and the directional clearance is eliminated by pre-tightening, so the mechanical parts have high accuracy. I can tell you, first, the improvement of the accuracy of mechanical parts has nothing to do with electrical appliances. In other words, any machine tool can improve its accuracy by doing so, so why must it be numerically controlled? Secondly, in fact, these have little influence on the smoothness and shape tolerance of the machine tool, because the key to finishing is metal cutting, and the last link of cutting lies in the tool. The high precision of grinding machine lies in the essential difference between grinding wheel and cutter. The quality difference caused by this difference can never be changed by improving the accuracy of the lower transmission chain, just like what a strong tiger can eat. No way. He's a tiger, too. He is a tiger at best, and can't become an elephant. In other words, it is impossible for your CNC lathe, boring machine or even machining center to produce the quality of grinder, such as smoothness, shape and so on. Having said that, I am afraid that some people will say that I am opposed to advanced production. No, I hope you can know what is the most suitable part for CNC machine tools. In fact, the most typical part is a hole-like part with multiple holes, and there is a position tolerance relationship between each hole. However, the surface smoothness and roundness tolerance requirements of the box body, whether it is the hole or the installation datum, are not very high, which can be achieved by general finish milling and boring, so that the characteristics of high position tolerance in CNC machine tools can be brought into play, and it does not need to be changed after programming. On the other hand, for some workpieces, the machining curve is very simple, which requires high surface smoothness (0.8 requires a grinder), and there is no special position requirement (such as the shaft of a reducer), so it is completely unnecessary to go to a lathe. It's a waste of life and money, not worth the loss. Technology is a complicated and serious matter and a scientific matter, but now many technicians are not knowledgeable and irresponsible, so they use CNC machine tools. But everything goes both ways. If we blindly emphasize simplicity and rely on numerical control, it will inevitably lead to high production cost of parts. You should know that the price of numerical control equipment is very expensive, far exceeding ordinary equipment. CNC machining personnel generally have a college degree or above, and the labor cost is also high. The power and spindle speed of CNC machine tools are much higher than those of ordinary machine tools, and the running cost of machine tools is also amazing. Therefore, my opinion is that in the production process, we should try our best to avoid the increase of equipment use cost, and we must choose low-cost processing methods under the same conditions, because enterprises are profit-oriented, and those who clamor for no cost are self-deceiving bookworms, which is unrealistic. Therefore, numerical control must not be regarded as an encyclopedia and a machining center, which is unfavorable to the development of enterprises, the maturity of products and the promotion of the market. In fact, numerical control should become a sharp knife in the production process (good steel is used on the blade), and it should not be synonymous with lazy technology or a business card that raises prices like many companies. That is the sorrow of mechanical practitioners. I created it myself. It's awesome! ! !