Research on surface quality and precision control technology of metal material machining

With the continuous development of science and technology, mechanized production has already entered large and small enterprises and occupies an increasingly important position in social production. Machinery is composed of parts that determine machinery’s service life and efficiency. Therefore, the processing quality of parts is the guarantee of mechanical quality. This paper expounds on the influencing factors of quality and precision in the machining process of metal materials. It discusses ways to improve the processing level, hoping to provide reference suggestions for relevant departments.

1. Preface

With the continuous development of industrial production, the machinery manufacturing industry has gradually become a pillar of the national economy, affecting all aspects of people’s lives. However, the working conditions of many machinery parts are very harsh, and some even need to be used continuously in long-term high temperature, high pressure, and high speed environment, which puts forward higher requirements for the surface quality and accuracy of machinery. This or that wear will make mechanical parts scratches, cracks, etc., seriously reducing the machine’s service life and even leading to large errors. Therefore, we want to improve the accuracy of mechanical processing, which refers to the geometric parameters of the parts obtained after processing, such as shape, size, position, value, etc., and the quality and accuracy of the surface are affected by certain factors. In this paper, the main influencing factors are analyzed, and the technological ways to improve the surface quality and accuracy of metal material machining are analyzed.

2. Major factors affecting the surface quality and accuracy of machined metal materials

Below, we analyze the factors that affect the surface quality and accuracy of metal material machining from the following points.

2.1 The influence of machine tool’s error and wear on accuracy

Generally speaking, the surface quality and accuracy of metal material machining products are mainly caused by the geometric error of the machine tool. In the process of rotation, guide rail, and rotation chain of the machine tool’s spindle, the position and shape of the machined parts will produce certain errors, which may be caused by the instability of the spindle or the swing of the rotation direction of the shaft. This error is generally unavoidable, but it can be minimized by finishing. At the same time, in the process of high-speed machining of plastic materials, the generated debris is easy to leave scales on the surface of the parts, which also has a certain impact on the roughness of the mechanical surface.

2.2 The influence of tool geometric parameters and grinding on surface quality

Any tool will rub with the parts during the machining process, so there will be different degrees of wear. Due to the tool’s different materials, the machining’s roughness and accuracy will also be different. The cutting speed, feed rate, and feed rate will also have a certain impact on the surface quality of the parts. Among the geometric parameters of the tool, the secondary deflection angle, the main deflection angle, and the sharp knife arc can most affect the roughness of the mechanical surface. The roughness processed by the high-speed steel tool will generally be much larger than that of the cemented carbide tool, so it is unsuitable for processing iron tools. At the same time, the blade surface of the tool itself will have a certain impact on the cutting, and it is more appropriate to control the tool’s roughness value below the workpiece’s second level.

2.3 Effect of workpiece positioning error on surface quality and accuracy

The positioning error of the workpiece refers to the assembly error of the positioning fixture itself. It generally occurs when the positioning datum and the design do not match. In the processing, it is easy to cause under-positioning and damage the workpiece to varying degrees. At the same time, the plasticity and thermal conductivity of the workpiece material also affect the accuracy of the parts. For example, suppose a workpiece with large plasticity is used. In that case, it will be easier to block the mechanical grinding wheel, and using a workpiece with poor thermal conductivity is not conducive to the heat dissipation of the alloy parts.

2.4 Effect of grinding on surface quality and precision

First, the grinding wheel’s size determines the size of the scratches on the grinding surface. The finer the grinding wheel is, the smaller the scratches left on the surface of the workpiece will be. However, if it is too fine, it will also block the grinding wheel, and the resulting ripples will cause high temperatures, but the wear of the workpiece is more serious. Secondly, the grinding amount and speed also play a decisive role in the machining accuracy. When the rotation speed of the grinding wheel increases, the number of abrasive particles on the workpiece increases, but the metal thickness decreases. If the rotation speed of the grinding wheel is too low, it will increase the contact time with the part, and the heat release will also lead to a decrease in accuracy. At the same time, increasing the feed rate will increase the surface roughness of the part. To improve the grinding efficiency, you must use a smaller radial feed rate in time to reduce the surface roughness.

3. Process approach to improve the surface quality and precision of metal material machining

Below, we aim at the main factors affecting the surface quality and precision of metal material machining from the following aspects to improve the process, and ways are briefly analyzed.

3.1 Reduce the occurrence of the original error

Reducing or avoiding the original error can greatly improve the geometric accuracy of the machine tool, which not only needs to improve the accuracy of the fixture, measuring tools, and precision parts themselves but also relies on the control of the process to avoid the occurrence of original errors such as thermal deformation. First, the main causes of the original error should be analyzed, and the corresponding schemes and measures should be taken according to different situations. For example, for the processing of the forming surface of the parts, the accuracy should be improved by reducing the tool error, and the processing of the precision parts should be realized by improving the rigidity and hot processing of the machine tool. Secondly, the error transfer method can be used to reduce the error. It refers to transferring the more sensitive direction in the error so that it stays in the non-sensitive area and reduces its interference with the core and precision parts. It can be seen that although error is inevitable, the damage caused by the error can be reduced by the process means, and the quality and accuracy of the processing can be improved.

3.2 Using the correct cutting means

First of all, the geometric parameters of the tool should be changed as much as possible to reduce the height of the residual area of the tool. The roughness can be reduced by reducing the secondary deflection angle or directly using the finishing and fine planing knife. Secondly, it is necessary to control the cutting speed scientifically. The material and performance of the processed parts determine the choice of cutting speed. If the cutting speed is small for parts with large plasticity, it will lead to material deformation or scale. Heat treatment can be carried out before cutting to prevent damage during processing for materials with high hardness and low toughness. Again, correctly choose the cutting fluid; cutting fluid plays an important role in the process of metal material machining; it can play the effect of cooling, cleaning, and lubrication of parts; the correct use of cutting fluid cannot only reduce the roughness of the workpiece but also greatly prolong the service life, improve the performance. In addition, ultra-precision cutting methods can greatly improve process accuracy. It refers to using a tool with a surface roughness of Rn0.04μm. The blunt radius of the tool is required to be nanoscale; not only the amount of cutting tool is small, the cutting speed is high, and the residual scratch area on the part can be ignored. However, this method is slightly costly and is suitable for processing high-end precision parts.

3.3 Reasonable selection of cooling conditions

The coolant is used to alleviate the high temperature of the grinding area. Spraying the coolant on the surface of the heated workpiece can effectively cool it down. In general, the nozzle should be selected according to the pressure of the cutting fluid, which can increase the cooling effect and prevent the coolant from being sprayed elsewhere. At the same time, due to the complex structure of the porous grinding wheel and the oil-immersed sand wheel, it is necessary to pay attention to introducing the liquid into the cavity groove inside the center when spraying the coolant so that the inside and outside are evenly cooled, and the maximum effect of the coolant is exerted.

4. Conclusion

In summary, controlling surface quality and precision reading of metal material machining can improve the process quality and operation efficiency. There are many methods, such as the error control method, error transfer method, coolant method, and so on. At the same time, it is necessary to strengthen the machine’s quality and precision standard requirements, carefully analyze the causes of errors, avoid them as much as possible, make them run and work better, and produce high-quality qualified products. 
Author: Mother Hongjun