The accuracy of CNC machine tools directly determines the rate of finished products. High precision machine tools play a vital role in reducing defective products and material consumption. The accuracy of machine tools is an important indicator to measure the performance of machine tools. There are many factors that affect the accuracy of CNC machine tools. The accuracy of the machine tool itself is a major factor. What factors determine the accuracy of CNC vertical machining centers?
1、 The Accuracy Problem in NC Machining Center
1.1 Influence of position error on machining accuracy
Position error refers to the variation or deviation degree of the mutual positions between the actual surfaces, axes or symmetry planes of the machined parts relative to their ideal positions, such as perpendicularity, position, symmetry, etc. The position error of the machining center is mainly caused by the machining error caused by the clearance and elastic deformation during the transmission during the machining of parts, and the position error caused by the friction of the cutter head of the machine tool during the machining. In the open loop system, the position accuracy is greatly affected, while in the closed loop servo system, it mainly depends on the accuracy of the displacement detection device and the speed amplification coefficient of the system, which generally has little impact.
1.2 Machining accuracy error caused by geometric error
The geometric accuracy of the machine tool is affected due to the external factors such as external forces and heat generated during machining, such as tools and fixtures. The geometric deformation of the parts machined on the machine tool results in geometric errors. According to research, there are two main reasons for geometric errors: internal factors and external factors. The internal factors causing geometric errors of the machine tool refer to the geometric errors caused by the factors of the machine tool itself, such as the levelness of the worktable of the machine tool, the levelness and straightness of the guide rail of the machine tool, and the geometric accuracy of the tool and fixture of the machine tool. External factors mainly refer to geometric errors generated under the influence of external environment, thermal deformation and other factors in the machining process. For example, during the cutting process, geometric errors are generated due to thermal expansion and deformation of tools or parts, which affect the machining accuracy of the machining center and the machining accuracy of parts.
1.3 Machining accuracy error caused by machine tool positioning
Through long-term data analysis and practical operation of parts processing, it can be seen that machine positioning has a great impact on the machining accuracy of CNC machining centers. In terms of structure, the machining errors of machining centers are mostly caused by positioning accuracy, among which the feed system of the machine tool is the main link affecting the positioning accuracy. The feed system of machining center is usually composed of mechanical transmission system and electrical control system. The positioning accuracy is related to the mechanical transmission system in structural design.
2、 Effective methods to improve machining accuracy
In the process of NC machining center, the precision of the processed parts directly affects the quality of the products. Some mechanical parts and equipment parts have very high requirements on the machining precision. Improving the machining precision of the machining center is the key to solve the problem. Through extensive research and analysis, it can be concluded that there are roughly the following methods to improve the machining accuracy of the machining center:
2.1 The machining accuracy can be improved by controlling the original error of the machining center
During the machining process of the machining center, the error itself is inevitable, and there is an inevitable error between the machined part and the machining center, which is called the original error.
To improve the machining accuracy, it is one of the important methods to control the original error of the machining center. Systematic analysis shall be carried out for the possibility of producing original errors, and corresponding improvement measures shall be formulated according to the causes and types of errors. During the machining process of mechanical parts, the position accuracy and geometric accuracy of the machining center have an important impact on the machining accuracy of parts. The influence of position error and geometric error on parts should be reduced through position control and geometric accuracy control. For the deformation error generated in the processing, it is necessary to use air cooling, water cooling and other methods to control the thermal deformation in the processing to reduce the impact of the thermal deformation error on the processing accuracy. At the same time, for the position error, it is necessary to reasonably select the tool suitable for the part material to avoid tool deformation. At the same time, it is necessary to select a reasonable fixture according to the shape of the blank of the part to be processed. If necessary, it is necessary to specially design the fixture according to the shape and size of the parts to avoid position error.
2.2 Reasonably design the core components of the machining center to avoid positioning errors
The positioning accuracy of the machine tool has an important impact on the accuracy of the parts. It affects the straightness and levelness of the core components of the machine tool positioning accuracy, feed system, guide rail, worktable, etc. When designing the CNC machining center, it is necessary to reasonably select the core components. For example, when selecting the ball screw that is widely used in machine tools, it is necessary to fully consider the accuracy of the ball screw, and the relatively mature ball screw technology for selection and installation. The support of the ball screw can also not be ignored. The support of the ball screw is closely related to the transmission accuracy of the system. The support of the ball screw is mainly determined by the axial load and rotation speed, which has an important impact on the processing accuracy of the CNC lathe. Usually, high-precision fixing and support methods are used. In addition, the bearing capacity of the ball screw should be checked strictly during the design process.
2.3 Effectively improve the machining accuracy of CNC machining center through real-time monitoring technology
With the continuous improvement of numerical control technology, the machining center is monitored in real time throughout the machining process to timely adjust the error in the machining process, and the error data in each link of the machining process is collected and fed back to the control terminal. Through the error data, the corresponding error compensation mechanism is adopted for timely error compensation, which can effectively improve the machining accuracy of parts.