In mechanical manufacturing, the machining accuracy of parts determines the quality of mechanical products. Machining accuracy refers to the actual geometric parameters of the parts after machining, including the size, shape, and mutual position of the mechanical parts, and the degree of conformity with the 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 degree to which the actual geometric parameters (size, shape, and mutual position) of the processed part deviate from the ideal geometric parameters is called machining error. From the analysis of ensuring the performance of the product, it is not necessary to process every part with absolute precision, and there is a certain degree of machining error allowed. Therefore, to meet the performance requirements of the parts, the allowable error range of dimensions, shapes, and mutual positions marked on the part drawings is called tolerance.
Analysis of Several Factors Influencing the Quality of Mechanical Processing
To analyze the factors that affect the quality of mechanical machining, it is necessary to first understand the physical and mechanical essence of various factors that affect the original errors of mechanical machining, as well as the laws of their impact on machining accuracy. As long as the methods that affect machining errors are mastered and controlled, the expected machining accuracy can be obtained. If necessary, methods and approaches to further improve machining accuracy can be found.
1.1 Nature and types of processing errors
During the machining process of parts, various original errors may occur, which can cause changes in the positional relationships between various links in the process system and result in machining errors. There are positioning errors caused by the fixture during workpiece clamping, as well as clamping errors caused by clamping force. Before and after installing the central workpiece, it is necessary to adjust the machine tool, cutting tool, and fixture. After trial cutting several workpieces, precise adjustments can be made to maintain the correct relative position between the workpiece and the tool. As adjustment cannot be absolutely precise, adjustment errors may occur. In addition, manufacturing errors in machine tools, cutting tools, and fixtures already exist before processing. 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 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. The original error generated during the processing 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, whether the process system needs to be readjusted, and any measurement method, measuring tool, or quantity cannot be absolutely accurate. Therefore, measurement error is also an original error that cannot be ignored.
1.2 Processing principle error
The machining principle error refers to the error generated by using approximate forming motion or approximate blade profile for machining, which is generally shape error, such as using Archimedean worm to cut involute gears; Using linear interpolation or circular interpolation methods to process complex surfaces on CNC machine tools, such as machining imperial threads on ordinary metric screw lathes, often leads to complex machine tool structures and difficulty in tool manufacturing, resulting in reduced machining efficiency in actual production. Through forming motion or blade contour approximation, 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 the 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 machining machinery, when the front and rear guide rails of the lathe are not parallel and there is distortion, the tool holder tilts 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 of the guide rail itself, uneven wear of the guide rail and installation of the machine tool are also important reasons for guide rail errors.
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 runout, and pure angular oscillation. Due to the constant variation of the actual rotation center of the spindle, the actual error is an instantaneous value generated by the combination of the three motion forms mentioned above. When a fixed part contacts different parts of the bearing inner surface, when there is a roundness error in the sliding bearing inner hole, it will cause radial runout of the main shaft during rotation, resulting in roundness error of the collision hole. The roundness error of the main journal itself has a relatively small impact,
2.2 Reducing the impact of sliding bearings on rotational errors
In machine tools with rolling bearing structures, the influence of raceway shape errors on different machine tools is different. For lathe type machine tools, the roundness of the inner ring raceway of the rolling bearing is the main factor affecting the spindle rotation accuracy because the position of the bearing bearing bearing bearing area remains basically unchanged. However, for carbide type machine tools, the roundness of the outer ring raceway of the rolling bearing is the main factor affecting the spindle rotation accuracy because the position of the bearing bearing 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 instantaneous rotation center of the spindle relative to the position of the tool tip in different cross-sections. And this change should focus on analyzing the impact in the direction of processing error sensitivity. For tool rotation type machine tools, the direction of machining error sensitivity and cutting force direction constantly change with the spindle rotation, such as mantis beds; For workpiece rotation type machine tools, the sensitive direction of machining error and the direction of cutting force remain unchanged, such as lathes. Taking lathes and machining machines as examples, the impact of three basic forms of spindle rotation error on machining accuracy will be analyzed. The pure radial runout error of the spindle is an elliptical cylinder with unchanged or varying major and minor axes when machining holes. When turning, the pure radial runout of the spindle has little effect on the roundness error of the workpiece, and the surface of the turned 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 not perpendicular to the inner and outer circular axes, resulting in flatness error and thread pitch error during thread machining. The spindle axis produces pure angle swing, and during turning, the roundness error of the same section of the workpiece is small, but it will produce cylindricity error. When boring, pure angle swing causes the spindle axis to be not parallel to the worktable guide rail, resulting in an elliptical shape of the protruding hole.
2.4 Measures to improve spindle rotation accuracy
To improve the spindle rotation accuracy, the following measures are usually taken: 1) Select high-precision bearings and improve the manufacturing accuracy of the spindle and box, as well as the assembly accuracy of the spindle components; 2) Make the rotational accuracy independent of the spindle. The rotational forming motion of the workpiece is not achieved by the rotational motion of the machine tool spindle, but by the rotational motion pair of the fixture. For example, when using dead center grinding external factors, improving the quality of the tip hole and ensuring the coaxiality of the two tip holes are very important to ensure the shape accuracy of the workpiece.
2.5 Improving Linear Motion Accuracy
In order to improve the quality of mechanical processing, methods such as scraping and grinding are often used to improve the machining accuracy and contact accuracy of machine tool guides. Static pressure guides or plastic coated guides are used to improve the positioning accuracy of the feed and the accuracy retention of the machine tool; Choosing a reasonable guide rail shape and guide rail combination form to improve linear motion accuracy, such as the 90 ° double triangle guide rail, has good linear motion accuracy retention, and the wear of this guide rail is mainly in the vertical direction. Therefore, for some machine tools that are error insensitive in the vertical direction (such as horizontal lathes), the original accuracy can be maintained for a long time.
2.6 Adjustment of processing technology
In every process of mechanical processing, in order to obtain the dimensional, shape, and positional accuracy of the machined surface, it is always necessary to make adjustments to the process system in one way or another. Due to the possibility of absolute accuracy in adjusting, there is an adjustment error. Trial cutting method is commonly used in single piece and small batch production. When machining, first try cutting on the workpiece, then measure, adjust and try cutting again until it meets the specified size requirements, and then officially cut the entire surface to be machined. In batch and mass production, the adjustment method (or sample template) is widely used. Pre adjust the relative position between the 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, there is no need for trial cutting, 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 processing. Ultra precision machining refers to the machining process where the machining accuracy and surface quality exceed the highest degree of tolerance standards currently used. The boundary between precision machining and ultra precision machining is not fixed and constantly moving forward with the progress of science and technology. The main characteristics of precision machining and ultra precision machining are high precision and good rigidity of machine tools. Machine tools have precise micro feed devices, good low-speed motion stability of machine tool workbenches, and good vibration resistance of process systems. In addition, they also have the following characteristics: 1) Precision and ultra precision machining are both developed with precision components as processing objects and closely combined with precision components, so precision machining cannot be separated from precision components. 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, resulting in high requirements for tool edge grinding, grinding wheel dressing, and machine tools; 3) Precision and ultra precision machining is a comprehensive advanced technology. To achieve high precision and 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 accuracy of the testing equipment; The working environment with constant temperature, purification, and vibration prevention, as well as the positioning and clamping methods of the workpiece and the technique of insertion, are many factors. Therefore, precision machining and ultra precision machining are a systematic engineering; 4) In precision machining and ultra precision machining, detection and machining are closely related. Precision measurement is a necessary condition for precision machining and ultra precision machining, and it is necessary to have measurement techniques that are suitable for machining accuracy. Otherwise, it is impossible to determine whether the machining accuracy meets the requirements and point out the direction for further improvement of machining accuracy.
3 Conclusion
In summary, 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 and idle. When the machine tool reaches thermal balance in a short period of time, processing can be carried out. If necessary, control heat sources can also be set up in appropriate parts of the machine tool to artificially heat the machine tool and achieve thermal equilibrium as soon as possible. In addition, precision machining should avoid stopping midway as much as possible. In addition, it is necessary to control the environmental temperature of the processing machinery. Precision machine tools are generally installed in a constant temperature workshop, 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, it is 20 ℃ in spring and autumn, 23 ℃ in summer, and 17 ℃ in winter. The above factors are comprehensive factors of machining quality and control, and none of them are indispensable.
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