Some Thoughts on Improving the Machining Precision of CNC Machine Tools

In CNC machine tools, the accuracy is mainly expressed by the size of the error. The specific accuracy is divided into two types, including static accuracy and dynamic accuracy. Among them, the static accuracy refers to the detection when the machine tool is not working or cutting. The main indicators are the geometric accuracy and positioning accuracy of the machine tool itself. This accuracy can only represent the original accuracy of the machine tool for the detection of machine tool accuracy, as the name implies, is the accuracy detected and achieved by the machine tool during the cutting process. The measurement of the accuracy value includes the original accuracy of the machine tool and the accuracy of the environment and process problems during the machining process. , Workpiece, vibration and other errors. In the production process of the machine tool, the dynamic accuracy of the machine tool cannot be effectively controlled. What can be guaranteed is the static accuracy of the machine tool, the accuracy of the original manufacturing, and the machining accuracy of the CNC machine tool. content.

1. Factors affecting the machining accuracy of CNC machine tools

1.1 The error of the machine tool itself shows in a large number of data and statistics that more than 65.7% of the machine tool cannot fully meet its relevant index standards during installation. 90% of CNC machine tools are in an inaccurate working environment and state during work Next, this situation determines the importance of the monitoring of the working state of the machine tool. The test of the accuracy of the machine tool is a necessary foundation for the guarantee of the accuracy of the machine tool, which can better guarantee the machining accuracy of the parts.

1.2 Influence of temperature in addition to the accuracy error of the machine tool, the operation of the machine tool in the workshop environment will also be affected, including the temperature fluctuation of the workshop, the heating of the motor, the friction of the loading and unloading, the influence of the medium, etc. Accuracy has a certain degree of influence, and the temperature change of the machine tool will lose the adjustment accuracy, which will affect the accuracy of the machine tool and the size and accuracy of the workpiece. At the same time, the increase in temperature also changes the bearing clearance, which in turn affects the machining accuracy. On the other hand, an increase in temperature makes the temperature distribution uneven, resulting in changes in the positional relationship between the parts or parts of the parts, resulting in displacement or distortion of the parts.

1.3 Reverse deviation the so-called reverse deviation refers to the error phenomenon caused by the reverse dead zone or reverse gap caused by the coordinate axis in the transmission process of the CNC machine tool. It can also become the reverse gap or the amount of stall. For CNC machine tools that use a semi-closed-loop servo system, the presence of reverse deviation will affect the positioning accuracy and repeated positioning accuracy of the machine tool, which will affect the processing accuracy of the product.

1.4 Clearance error the transmission chain needs to be used in the processing of CNC machine tools. The operation of the transmission chain will produce some gaps. These gaps are easy to cause errors, especially when the machine tool does not produce movement during the operation of the motor, which will cause the CNC machine tool. Oscillation or large errors.

 2. Measures to improve the machining accuracy of CNC machine tools

 2.1 Selection of machine tools because of the difference in accuracy of the machine tools, we need to pay attention to the choice of machine model and accuracy in the choice of machine tools. At present, the inspection of the position accuracy of CNC machine tools usually adopts the international standard ISO230-2 or the national standard GB10931-89 Wait. In the choice of machine tools, we must pay attention to the standard, because the difference in the standard will also cause the difference in accuracy.

 2.2 Control of parts the machine tool with sliding bearing can choose a bearing with better wear resistance to ensure the working accuracy of the machine tool.

 2.3 Control of the workshop environment to reduce heat sources: Emphasis is placed on the speed, clearance adjustment and reasonable preloading of the spindle bearings. For thrust bearings and tapered roller bearings, due to poor working conditions, the heat is relatively large. If necessary, thrust angular contact ball bearings can be used instead to minimize the friction and heating of certain parts. Insulation: Keep the heat source away from the main shaft, such as isolating the motor and transmission, and using separate transmission. Heat dissipation: strengthen lubrication and cooling, adopt oil cooling, air cooling and other methods to accelerate heat dissipation. Reducing the influence of thermal deformation: No matter which method is adopted, it can only reduce thermal deformation and it is difficult to completely eliminate thermal deformation, so measures should be taken to reduce the influence of thermal deformation.

2.4 Due to reverse deviation, the accuracy of the equipment becomes lower, and the longer the application time of the machine tool, the greater the wear and the greater the error, which requires the application of the machine tool Regularly detect and compensate for deviations, so as to reduce errors as much as possible and improve machine working accuracy.

 2.5 Error compensation refers to the corresponding record of the position of the fixed axis in the processing of CNC machine tools. In addition, the relevant recorded data is compared with the actual measurement results to understand the error value and be on the axis during operation. Select the measurement reference point, record the error value during operation, and input it into the relevant control system, so that the axis movement and error time at different points can be well controlled. If the number of measured points is greater, the error effect of the pitch compensation needs to be more obvious. The premise of this error compensation technology is established under the coordinate system of the CNC machine tool, and the important parameter of the coordinate system of the CNC machine tool is the reference point. Therefore, it must be ensured that the error value of the selected reference point is zero.

2.6 Due to the influence of backlash errors in CNC machine tools, full attention must be paid to backlash errors in the design of CNC machine tools and effective solutions must be taken. But it is undeniable that the gap exists, so what we have to do is to record the backlash of each point during the operation of the machine tool through the pitch error compensation technology, and directly error the reverse movement through the control system of the CNC machine tool Compensation operation, so as to reduce the error by using parameter settings and CNC system settings.

 2.7 The accuracy of machine tools is constantly being improved and improved with the development of technology. The precision of machine tools has improved from the original micron level to nanometer level. More research and development are needed, especially in bearing technology Avoiding the lag of bearing technology is one of the development problems. The phenomenon of lag is particularly important for the accuracy of positioning. It is found in the research that the hydrostatic bearing technology can solve the phenomenon of mechanical lag. A lot of applications.

3. Conclusion

In summary, in the processing of CNC machine tools, there are many factors that affect the accuracy of CNC machine tools. For these effects, we need to analyze them comprehensively to reduce errors as much as possible and achieve effective improvement in processing accuracy.