In mechanical manufacturing, the machining accuracy of parts determines the quality of mechanical products. Mechanical machining accuracy refers to the actual geometric parameters of parts after machining, including the size, shape, and mutual position of mechanical parts, and the degree of conformity with ideal geometric parameters. The higher the degree of conformity, the higher the machining accuracy. In the process of mechanical processing, due to various factors, the processed parts cannot fully meet the ideal requirements. The deviation of the actual geometric parameters (size, shape, and mutual position) of this processed part from the ideal geometric parameters is called machining error. From the perspective of ensuring the performance of the product, it is not necessary to process every part with absolute precision, and a certain degree of machining error is allowed. Therefore, to meet the performance requirements of the parts, the allowable error range in the dimensions, shapes, and mutual positions marked on the part drawings is called tolerance.
Analysis of Several Factors Affecting the Quality of Mechanical Processing
To analyze the factors that affect the quality of mechanical processing, it is necessary to first understand the physical and mechanical essence of various factors that affect the original errors of mechanical processing, as well as the laws of their impact on processing accuracy. As long as the methods that affect mechanical processing errors are mastered and controlled, the expected processing accuracy can be obtained, and if necessary, methods and ways to further improve mechanical processing accuracy can be found.
1.1 Nature and types of machining errors
Various original errors may occur during the machining process of parts, which can cause changes in the positional relationship between various links in the process system and result in machining errors. There are positioning errors caused by the fixture when clamping workpieces, as well as clamping errors caused by clamping force. Before and after installing the central workpiece, adjustments must be made to the machine tool, cutting tools, and fixtures. After trial cutting several workpieces, precise adjustments can be made to maintain the correct relative position between the workpiece and the cutting tool. Since adjustments cannot be absolutely precise, adjustment errors may occur. In addition, manufacturing errors in machine tools, cutting tools, and fixtures already exist before machining. This type of raw error is called the geometric error of the process system. Due to the generation of cutting forces, cutting heat, and friction during the machining process, they will cause force deformation, thermal deformation, and wear of the process system, which will affect the relative position between the workpiece and the tool obtained during adjustment, resulting in various machining errors. This type of raw error generated during the machining process is called the dynamic error of the process system. During the machining process, it is also necessary to measure the workpiece in order to determine whether the machining is qualified and whether the process system needs to be readjusted. Any measurement method, measuring tool, or quantity cannot be absolutely accurate, so measurement error is also a significant original error that cannot be ignored.
1.2 Processing principle error
The processing principle error refers to the error generated by using approximate forming motion or approximate blade profile for processing, usually in the form of shape error, such as cutting involute gears with Archimedes worm gear; Using linear interpolation or circular interpolation methods to process complex surfaces on CNC machine tools and machining imperial threads on ordinary metric screw lathes often leads to complex machine tool structures and difficulty in tool manufacturing, resulting in reduced processing efficiency in actual production. By approximating the forming motion or blade contour, the process can often be simplified, the design and manufacturing of machine tools and cutting tools can be simplified, productivity can be improved, and costs can be reduced. However, the principle error caused by this must be controlled within an allowable range (generally, the principle error should be less than 0.1%), and the existence of principle error is allowed.
1.3 Parallelism error of front and rear guide rails
When processing machinery, if the front and rear guide rails of the lathe are not parallel and twisted, the tool holder may tilt during production. Therefore, the parallelism error of the front and rear guide rails of the lathe and external grinding machine has a significant impact on the machining accuracy. In addition to manufacturing errors in the guide rail itself, uneven wear of the guide rail and installation of the machine tool are also important causes of guide rail errors.
2 Ways to Improve and Control the Quality of Mechanical Processing
2.1 Reduce spindle rotation error
For the convenience of analysis, the motion error of the spindle rotation axis can be decomposed into three basic forms: pure radial runout, pure axial displacement, and pure angular oscillation. Due to the constantly changing actual rotation center of the spindle, the actual error is a momentary value generated by the combination of the three motion forms mentioned above. It is a fixed part that comes into contact with different parts of the inner surface of the bearing. Therefore, when there is a roundness error in the inner hole of the sliding bearing, the main shaft will produce radial jumping during the rotation process, causing a roundness error in the impact hole. The roundness error of the main shaft neck itself has a relatively small impact,
2.2 Reduce the impact of sliding bearings on rotational errors
The impact of raceway shape error on different machine tools using rolling bearing structure is different. For lathe type machine tools, the roundness of the inner raceway of the rolling bearing is the main factor affecting the spindle rotation accuracy, as the position of the bearing bearing bearing area remains basically unchanged. For lathe type machine tools, the roundness of the outer raceway of the rolling bearing is the main factor affecting the spindle rotation accuracy, as the position of the bearing bearing bearing area is constantly changing.
2.3 Control the influence of spindle rotation error on machining accuracy
The influence of spindle rotation error on machining accuracy depends on the changes in the instantaneous rotation center of the spindle relative to the tool tip position within different cross-sections. And this change should be analyzed with a focus on the impact on the sensitive direction of machining errors. For tool turning machine tools, the sensitive direction of machining errors and the direction of cutting force constantly change with the rotation of the spindle, such as a mantis bed; For workpiece rotary machine tools, the sensitive direction of machining error and the direction of cutting force remain unchanged, such as lathes. Taking lathe and lathe as examples, this article analyzes the impact of three basic forms of spindle rotation error on machining accuracy. The pure radial circular runout error of the spindle is significant for hole machining, where the drilled hole is an elliptical cylinder with constant or varying long and short axes. During turning, the pure radial runout of the spindle has little effect on the roundness error of the workpiece, and the surface of the machined workpiece is close to a true circle. The pure axial displacement error of the spindle has no effect on the machining of the inner and outer cylindrical surfaces, but when machining the end face, it will cause the machined workpiece end face to be non perpendicular to the inner and outer cylindrical axes, resulting in flatness errors and pitch errors when machining threads. The spindle axis produces pure angular oscillation, which results in small roundness errors in the same section of the workpiece during turning, but may cause cylindricity errors. When boring, pure angular swing causes the spindle axis to be non parallel to the worktable guide rail, resulting in an elliptical shape for the drilled hole.
2.4 Measures to improve spindle rotation accuracy
The following measures are usually taken to improve the spindle rotation accuracy: 1) Select high-precision bearings and improve the manufacturing accuracy of the spindle and housing, as well as the assembly accuracy of spindle components; 2) Make the rotation accuracy independent of the spindle. The rotary forming motion of the workpiece is not achieved by the rotary motion of the machine tool spindle, but by the rotary motion pair of the fixture. For example, when using dead center grinding external factors, improving the quality of the top hole and ensuring the coaxiality of the two top holes are very important for ensuring the shape accuracy of the workpiece.
2.5 Improve the accuracy of linear motion
In order to improve the quality of mechanical processing, methods such as scraping and grinding are often used to improve the machining accuracy and fitting contact accuracy of machine tool guides. Static pressure guides or plastic coated guides are used to improve the positioning accuracy and machine tool accuracy retention; Choosing a reasonable guide rail shape and guide rail combination form to improve the accuracy of linear motion, such as the 90 ° double triangle guide rail, has good retention of linear motion accuracy. However, the wear of this guide rail is mainly in the vertical direction, so it can maintain the original accuracy for some machine tools (such as horizontal lathes) that are error insensitive in the vertical direction for a long time.
2.6 Process Adjustment
In every process of mechanical processing, in order to obtain the size, shape, and positional accuracy of the machined surface, it is always necessary to adjust the process system in one way or another. Due to the possibility of absolute accuracy in adjustments, there may be adjustment errors. Trial cutting method is commonly used in single piece and small batch production. During processing, first test cut the workpiece, then measure and adjust it before cutting again until it meets the specified size requirements, and then formally cut the entire surface to be processed. In batch and mass production, the adjustment method (or sample template) is widely used. Pre adjust the relative position between the cutting tool and the workpiece, and maintain this relative position unchanged during the machining process of a batch of parts to obtain the required part size. In future processing, trial cutting is eliminated, which not only shortens the adjustment time but also achieves higher processing accuracy.
2.7 Implement ultra precision machining
Implementing ultra precision machining is also a measure to improve the quality of mechanical machining. Ultra precision machining refers to machining processes that exceed the highest tolerance standards currently used in terms of machining accuracy and surface quality. The boundary between precision machining and ultra precision machining is not fixed and is gradually moving forward with the advancement of science and technology. The main characteristics of precision machining and ultra precision machining are high machine tool precision, good rigidity, precise micro feed device, good low-speed motion stability of machine tool worktable, and good anti vibration performance of process system. In addition, they also have the following features: 1) Precision and ultra precision machining are both developed by closely combining precision components with precision components, and therefore cannot be separated from precision components for precision machining. The methods, equipment, and objects of precision machining are interrelated; 2) During ultra precision machining, the cutting edge is extremely small and requires both micro and ultra micro cutting, which places high demands on tool grinding, grinding wheel dressing, and machine tools; 3) Precision and ultra precision machining is a comprehensive advanced technology. To achieve high precision and high surface quality, the selection of machining methods, tools, and materials should be considered; The structure and quality of the processed material, the structure and technical performance of the processing equipment, the testing methods, and the accuracy of the testing equipment; The working environment of constant temperature, purification, and vibration prevention, as well as the positioning and clamping methods of workpieces and the techniques used, make precision machining and ultra precision machining a systematic project; 4) In precision machining and ultra precision machining, detection and machining are closely related, and precision measurement is a necessary condition for precision machining and ultra precision machining. It is necessary to have measurement technology that is compatible with the machining accuracy. Otherwise, it is impossible to judge whether the machining accuracy meets the requirements, nor can it point out the direction for further improvement of machining accuracy.
3 Conclusion
In short, in order to improve the quality and control of mechanical processing, in addition to the above processes, attention should also be paid to maintaining the thermal balance of the process system. This can enable the machine tool to operate at high speed without load. After the machine tool reaches thermal balance in a short period of time, processing can be carried out. When necessary, control heat sources can be installed at appropriate locations on the machine tool to artificially heat it up and achieve thermal equilibrium as soon as possible. In addition, during precision machine tool machining, it is advisable to avoid stopping midway. In addition, it is necessary to control the ambient temperature of the processing machinery. Precision machine tools are generally installed in constant temperature workshops, and their constant temperature accuracy is generally controlled within+1 ℃, with a precision level of+0.5 ℃. The constant temperature base is adjusted seasonally, generally at 20 ℃ in spring and autumn, 23 ℃ in summer, and 17 ℃ in winter. The above factors are comprehensive factors of mechanical processing quality and control, and none of them are indispensable.
