People engaged in the processing industry often have a strong desire to win in terms of precision, as if it is easy to achieve μm-level processing precision. But in fact, high-precision processing is a rigorous technical field. Many people do not even know the common sense of the impact of temperature on precision, but they talk about precision, which is really frustrating! Next, this article will provide you with a more comprehensive popular science.
PART 1 Basic common sense: The impact of temperature changes on materials Everyone knows that materials generally have the characteristics of thermal expansion and contraction. In the process of precision processing, the temperature problem must not be ignored! The temperature difference can be called the "enemy" of precision. If this key factor is not taken seriously, how can we talk about precision? After all, most machines are made of steel and cast iron, which will change shape and length with the room temperature and the heat generated by the machine itself. The specific deformation amount of the material due to thermal expansion and contraction depends on the material's own characteristics and the temperature change range. The following is a table of expansion coefficients of steel and copper. Taking steel as an example, its linear expansion is manifested as a change of 12μm per meter in length when the temperature changes by 1°C. The expansion coefficient of steel is shown in the figure below:
For example: If the length of the workpiece is 200mm and the temperature changes by 10°C, the expansion value is 0.02mm. The expansion coefficient of copper is shown in the figure below:
For example: When the electrode length is 200mm and the temperature changes by 10°C, the expansion value is 0.05mm. PART 2 Temperature causes detection errors If the workpiece and the instrument and gauge used for detection are made of different materials and are not under standard temperature (20°C) during detection, then the deviation from the standard temperature will become an important factor in the detection error. Detection error caused by temperature
For example, taking detection as an example, if a 100mm long steel block gauge is heated by the palm temperature and its temperature rises by 4°C, it will change in length by 4.6μm. In addition, when measuring high-precision parts, higher-precision measurement methods must be available. If the accuracy index of the measuring instrument or equipment itself is not high, how can high-precision measurement be achieved? PART 3 Important processing concept: Maintain thermal stability Take a steel part with a size of 100x30x20mm as an example. When the temperature drops from 25℃ to 20℃, its size will change: at 25℃, the size is 6μm larger, and when the temperature drops to 20℃, the size is only 0.12μm larger. This is a thermal stabilization process. Even if the temperature drops rapidly, it takes a certain amount of time to maintain stable accuracy. Generally, the larger the object, the longer it takes to restore stable accuracy when the temperature changes.
Some factories without experience in precision machining often attribute the cause of unstable accuracy to equipment accuracy problems when performing precision machining. Experienced factories know that it is basic common sense to pay attention to the thermal balance between ambient temperature and machine tools. They understand that even if the machine tool has high accuracy, the stability of machining accuracy can only be guaranteed in a stable temperature environment and thermal balance state.
Maintaining thermal stability is an important concept that must be deeply understood in precision machining. Some people may be entangled in whether the temperature should be maintained at 20℃ or 23℃. In fact, the key is to keep the target temperature value stable. Theoretically, the temperature is generally required to be 20℃, but in actual workshops, the temperature is usually controlled at 22℃~23℃, as long as the temperature fluctuation is strictly controlled. PART 4 Correctly understand processing accuracy and analysis Generally speaking, processing accuracy can be divided into precision and accuracy. Through the figure below, we can have a more intuitive understanding. Precision (Precision) Precision refers to the reproducibility and consistency between the results obtained when the same spare sample is used for repeated measurements. Sometimes, high precision does not mean high accuracy. For example, the three results obtained by measuring with a length of 1mm as the standard are 1.051mm, 1.053mm, and 1.052mm respectively. Although the precision of this set of data is high, it is not accurate. Accuracy (Accuracy) Accuracy refers to the degree of closeness between the measurement result and the true value. When the measurement accuracy is high, it means that the system error is small, and the deviation of the average value of the measured data from the true value is small, but the discreteness of the data, that is, the size of the accidental error is not clear. The relationship between precision, accuracy and temperature is usually closely related to precision and accuracy. If the precision of the machined parts is high but the accuracy is insufficient, it may be that the workshop temperature fluctuates slightly but deviates greatly from the standard temperature; if the parts are high in precision but poor in accuracy, it is likely that the workshop temperature fluctuates greatly, resulting in large discreteness of accuracy; if the parts are neither precise nor accurate, it means that the workshop temperature deviates greatly from the standard temperature and control requirements. PART 5 Forgotten machine tool preheating When using precision CNC machine tools for high-precision machining in the factory, you may have had such an experience: every morning when the machine is turned on for machining, the machining accuracy of the first product is often unsatisfactory; the first batch of parts that are turned on for machining after a long vacation often have unstable accuracy, and the probability of failure is very high when performing high-precision machining, especially in terms of position accuracy. The machine tool can only ensure the stability of machining accuracy in a stable temperature environment and thermal equilibrium state. When high-precision machining is performed right after the machine is turned on, preheating the machine tool is basic common sense for precision machining. The machining accuracy varies greatly when the machine tool stops running for a long time and when it is in thermal equilibrium. This is because the temperature of the spindle and each moving axis of the CNC machine tool will be relatively stable at a certain level after running for a period of time, and as the processing time increases, the thermal accuracy of the CNC machine tool will gradually stabilize, which fully demonstrates the necessity of preheating the spindle and moving parts before processing. However, many factories ignore the preparation link of "warm-up exercise" of machine tools, or even know nothing about it. If the machine tool has been idle for more than a few days, it is recommended to preheat for more than 30 minutes before high-precision processing; if the idle time is only a few hours, preheat for 5 to 10 minutes. During preheating, the machine tool can be allowed to participate in the repeated movement of the machining axis, and it is best to perform multi-axis linkage, such as moving the XYZ axis from the lower left corner of the coordinate system to the upper right corner, and repeatedly walking diagonally. In actual operation, a macro program can be written on the machine tool to allow the machine tool to automatically and repeatedly perform preheating actions. When the machine tool is fully preheated, it can be put into high-precision processing production, and stable and consistent processing accuracy can be obtained at this time.
