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Discussion on surface quality and precision control of metal material machining

The machined surface quality of metal materials are the key to improving product quality. Based on the characteristics of quality and precision control of the machined surface of metal materials, this paper summarizes its basic concepts, then discusses its impact, and finally puts forward targeted control measures in combination with the interference factors of machining quality and precision control of metal materials, to guide the improvement of machining quality of metal materials.

20230809093248 97690 - Discussion on surface quality and precision control of metal material machining

The quality of the machined surface of metal materials mainly refers to the micro-roughness of the machined surface after machining, which can also be called roughness. During the machining treatment of metal materials, no matter what method is adopted, the surface will inevitably show a more typical uneven knife mark shape. No matter how mature the metal processing technology is, it is difficult to ensure that there is no trace on the machined surface of metal materials. After inspection by magnifying glass and other methods, the roughness of the machined surface of metal materials can be observed very clearly. The overall quality of the surface of the metal material machine can directly impact the chemical, physical, mechanical, and other aspects of the performance of the manufactured products, which is also the key to determining the service life and reliability of the product itself.

During the machining of metal materials, the requirements of different products on the surface quality of metal processing are also significantly different. The more precise the products are, the higher the surface quality requirements of metal materials. Given this situation, it is necessary to combine the specific conditions of metal surface quality and select corresponding measures to strengthen the surface quality of metal processing. In recent years, with the proposal of China’s manufacturing strategy and the transformation and upgrading of the manufacturing industry to high quality, improving the surface quality of metal material machines has become a hot spot of concern in various industries. Therefore, it is very important to fully understand the factors affecting the surface quality of metal material machines and improve the accuracy through detail control.

1. The basic concept of surface quality and precision control of metal material machining

(1) Surface quality of metal material machine
The surface quality proposed in the metal material machine usually refers to the physical characteristics and shapes presented on the surface of the parts after processing. The material parts’ surface material properties and micro-geometric structure should be consistent with the processing objectives. However, it should be noted that although we can see that the surface of the processed material is very smooth with the naked eye if the inspection tool is used for microscopic inspection, it can still be found that the surface shows obvious unevenness and texture characteristics, which will greatly affect the quality of the entire component. In the process of processing, the geometric characteristics of the surface of the metal material machine will be presented. At the same time, it is necessary to pay attention to the following aspects: surface waviness, surface roughness, surface defects, and obvious processing texture on the surface. There is often a big difference between the expected and actual surface quality.
(2) Machining accuracy
The machining accuracy of metal materials mainly refers to the large gap between the pre-designed parameters and the actual parameters of the parts after the metal material machine is processed. The lower the gap is, the higher the machining accuracy is. This puts forward new requirements for the field of modern mechanical processing. The lower the error during processing and production, the higher the overall progress and reliability of the parts. According to the actual situation of parts processing, to improve the accuracy in an all-round way, it is necessary to deal with three different factors: size, shape, and position. This is because these three factors themselves have interrelated characteristics. From the scale point of view, the size is also the main factor leading to the large error between the machined surface and the design standard of metal materials. The shape factor will directly impact the geometric shape error of the surface of the metal material machine. The position gap between the reference and the surface of the processed material and the final machining error may affect the accuracy of the material parts.

2. The main factors affecting the surface quality and accuracy of metal material machine

(1) Technical level
During the machining process of metal materials, the level of artificial technology can have a direct impact on the surface quality of metal materials, which is embodied in the following two points:

  • 1) Different parameters show obvious characteristics of improper selection, which will directly impact the reliability of the processing results so that the metal material machine shows obvious surface quality differences. 
  • 2) During production, although the processing of metal material machines has been automated, sometimes it still needs manual operation. Therefore, once the operation does not meet the production requirements, it will inevitably lead to a decline in the surface quality of the metal material machine.

(2) Quality of raw materials
According to the relevant literature, the influence of the quality of raw materials on the quality of metal materials can reach more than 70 %. At the same time, the quality of materials is also an important basic condition to ensure the smooth development of mechanical processing. The factors that affect raw materials include:

  • 1) There needs to be more standardization in the selection of metal materials, which makes it difficult for the metal materials in the initial state to meet the processing requirements so that the surface quality of the formed materials changes abnormally. 
  • 2) The location of the metal material storage does not match the requirements, resulting in serious corrosion of the material, which affects the processing quality of the surface of the mechanical parts.

(3) Cold hardening
The cold work, hardening of the surface of the metal material machine, is also a key factor in the change of the mechanical surface. During the machine’s processing, the surface cold work hardening is inevitably accompanied by obvious metallographic structure and plastic deformation characteristics, which greatly affect the stability of the metal material processing structure. Moreover, after the successful completion of the machining process, under the interference of cold hardening factors, it is also easy to cause the shape and volume of the metal material machine to change significantly. This process has great uncertainty, and it is challenging to guarantee the machining quality effectively.
(4) Residual stress
Before the machining operation of metal materials, it is bound to be interfered with by various process factors. After these factors gradually disappear, their effects usually do not disappear immediately but remain in metal materials. These effects are the residual stresses we often mention. Residual stress is closely related to various factors, including feed rate, cutting speed, cutting force, etc. There is a significant correlation between residual stress and cutting speed, and when the cutting speed reaches 150m/min, the residual stress will reach the lowest value, about 670MPa. This shows that it greatly influences the surface quality of metal materials, and the risk of fracture of metal materials is also increased.
(5) Mechanical positioning error
Mechanical positioning error is the key factor affecting metal materials’ machining accuracy. During the mechanical positioning operation, the error cannot be completely avoided, which can be divided into two characteristics: the benchmark cannot be overlapped, and the positioning is not accurate. Among them, the non-coincidence of benchmarks mainly refers to the fact that in actual operation, the relevant design is not completed according to the design benchmark, and the processing size and fixed position of the parts involved will inevitably lead to the decline of the overall processing quality and accuracy of the finished products. In actual operation, the inaccurate positioning is mostly related to the poor accuracy of the positioning element used in the fixture. At the same time, this is also a key factor that makes it difficult for metal materials to achieve size specifications. Relevant statistical data show that the maximum fault tolerance rate can only reach 15 % when the metal material machine is positioned, which inevitably affects the accuracy of the metal material production results.

3. Metal material machining surface quality and precision control measures

(1) Reasonable selection of processing technology
According to the analysis results of the above influencing factors, the processing technology greatly influences the quality and accuracy of the metal material machine. Therefore, to improve the surface quality and accuracy of the metal material machine, the primary task is to strengthen the processing technology. With the help of modern information technology, the optimization of mechanical processing technology can be realized. On this basis, a new organizational management system can be created to guide the optimization of processing technology. In addition, the actual operation process of processing technology can be reorganized. For example, for the cutting process, the depth and speed of cutting must be re-determined during the combing process, and the main points must be clarified. Before the application of the process, the communication and exchange of personnel must also be implemented to help them fully grasp the specific operational requirements and reduce the interference caused by human factors.
(2) Strengthen the quality control of raw materials
For the material quality of the metal material machine, first of all, we must pay attention to the arrangement of the processing process, which includes precision requirements, processing benchmarks, durability requirements, etc. Based on this, a clear material procurement plan is formulated, refined to various procurement standards, procurement quantity, etc., requiring procurement personnel to strictly implement according to the content and create the first threshold of quality management. Secondly, attention should be paid to the storage management of metal materials. All incoming and outgoing ledgers should be established for all incoming materials, and materials should be arranged for production according to the principle of first in, first out. At the same time, all materials must be unified and centralized management. The warehouse needs to ensure drying and ventilation. In the case of more rainwater, attention should be paid to monitoring the temperature and humidity changes in the room and timely control and treatment.
(3) Reasonable selection of cooling conditions
To improve the problem of cold hardening, it is necessary to ensure optimal cooling conditions of production and processing. During the production period, coolant is usually used for cooling treatment to adjust the high temperature while grinding metal materials and preventing thermal expansion. Coolant will be directly sprayed on the surface of mechanical parts, resulting in more serious cooling problems. Therefore, the nozzle selection should be adjusted according to the part’s cutting pressure value to prevent the coolant’s waste under the premise of achieving the ideal cooling effect. In addition, we should also pay attention to the processing of the grinding wheel structure. Because of its complex characteristics, the coolant is easy to only attach to the surface, and it isn’t easy to control the internal temperature. Therefore, drainage management must be done during the spraying period to ensure that the structural cavity groove is fully cooled.
(4) Do a good job in the control of the processing process
The machining process includes multiple links, which require detailed processing for each link at different stages. In this process, we can cooperate with big data technology to complete the analysis and processing and then select 30 % 50 % of the value data to give technical guidance. In addition, we must also pay attention to the daily management of the team, the formation of on-site supervision and management team, mainly responsible for the supervision and guidance of the processing operation, with irregular spot checks to ensure that all aspects of the operation quality can achieve a higher normative. At the same time, it is also conducive to the timely detection of problems and adjustment of problems to realize the control of resource waste and the improvement of processing quality.
(5) Reducing the original error 
To improve the machining accuracy of metal materials in an all-round way, we must avoid and prevent the original error problem, which includes the fixture and measuring tool and the accuracy of the parts themselves. Therefore, it is necessary to cooperate with process technology to achieve the best error control effect. The error control scheme can be formulated according to different conditions during the production period. For example, during the surface forming processing of parts, it is necessary to improve the overall accuracy by controlling the tool error. At the same time, it is necessary to control the thermal processing and stiffness of the machine tool to ensure the precision of precision parts. In addition, the error offset method can also be introduced for error control. The sensitive points prone to errors are taken as the key management objects, and the interference degree of tight parts and core parts is mainly controlled, while the areas not prone to errors can be relatively relaxed. 

4. Conclusion 

In a word, the machined surface quality of metal materials is important to ensure the wear resistance, fatigue strength, and precision of metal materials. Improving the quality and accuracy of its machined surface is also a difficult problem for metal material machining enterprises. Combined with the actual production situation, to achieve this goal, it is necessary to attach great importance to the management of all aspects and strengthen the optimization of technology and error control to lay the foundation for enterprises to obtain high-quality products. 
Author: Yang Liuqing



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