It is of great significance to correctly understand the surface quality of parts and analyze various process factors that affect the quality of machined surfaces during machining, to improve surface quality and improve product performance.
Machining surface quality refers to the microscopic unevenness of the machined surface after machining, also called roughness. The surface quality after machining directly affects the physical, chemical and mechanical properties of the machined part. The performance, reliability and longevity of the product depend to a large extent on the surface quality of the main part. Therefore, it is of great significance to correctly understand the connotation of the surface quality of the parts, analyze various process factors affecting the quality of the machined surface during machining, improve the surface quality and improve the performance of the products.
1. Factors affecting the surface quality of machined surface 1.1 Influence of machine performance on machined surface quality Affected by surface quality on the surface quality of a newly machined Capricorn pair, initially roughened on the surface At the peak contact, the actual contact area is much smaller than the theoretical contact area, and there is a very large unit stress in the contact portion, which causes plastic deformation, elastic deformation and shear failure between the peaks at the actual contact area, causing severe wear.
Effect of fatigue strength on surface quality Under the action of alternating load, the valley of surface roughness is likely to cause stress concentration and fatigue pattern. The larger the surface roughness value, the deeper the surface marks, and the lower the radius of the bottom, the worse the ability to resist fatigue damage. Residual stress has a great influence on the fatigue strength of the part. The residual tensile stress of the surface layer will expand the fatigue crack and accelerate the fatigue damage; while the residual stress of the surface layer can prevent the expansion of the fatigue crack and delay the generation of fatigue damage.
Effect of Corrosion Resistance on Surface Quality The corrosion resistance of a part depends to a large extent on the surface roughness. The larger the surface roughness value, the more corrosive substances accumulate in the valley. The worse the corrosion resistance. The residual tensile stress of the surface layer causes stress corrosion cracking, which reduces the wear resistance of the part, while the residual compressive stress prevents stress corrosion cracking.
1.2 Factors Affecting Surface Roughness Factors Affecting Surface Roughness by Cutting Process 1 Reflection of Tool Geometry When the tool makes a feed motion relative to the workpiece, the residual area of ​​the cutting layer is left on the machined surface, and its shape is a reflection of the tool geometry. .
2 Properties of the workpiece material When the plastic material is processed, the plastic deformation of the metal by the cutter is generated, and the tearing effect of the cutter forcing the separation of the chip from the workpiece increases the surface roughness value.
3 When cutting the brittle material, the cutting speed has little effect on the roughness; when processing the plastic material, the built-up edge has a great influence on the roughness.
The main factors affecting the surface roughness of the grinding process are the grinding wheel surface roughness, the grinding wheel hardness, the grinding wheel dressing, the grinding speed, the grinding radial direction, the feed amount and the number of times of grinding, and the circumference of the workpiece. Feed rate and axial feed, cooling lubricant, etc.
1.3 Factors Affecting the Physical and Mechanical Properties of Surface Layers During the surface layer cold work hardening, the plastic deformation caused by the cutting force causes the character to be distorted and distorted, and the shear slip occurs between the grains. The grain is elongated and the fiber is elongated. It can even increase the hardness and strength of the surface layer metal. This phenomenon is called cold work hardening (or strengthening). The main factors affecting the hardening of cold work are: the blunt radius of the cutting edge increases, the extrusion effect on the surface metal is enhanced, and the plastic deformation is intensified, resulting in chill and hardening. The flank wear of the tool increases, the friction between the flank and the surface to be machined increases, and the plastic deformation increases, resulting in chill and hardening. As the cutting speed increases, the action time of the tool and the workpiece is shortened, the depth of plastic deformation is reduced, and the depth of the chill layer is reduced. When the cutting speed is increased, the action time of the cutting heat on the surface layer of the workpiece is also shortened, which increases the degree of chilling. The feed rate is increased, the cutting force is also increased, the plastic deformation of the surface metal is intensified, and the chilling effect is enhanced. The greater the plasticity of the workpiece material, the more severe the chilling phenomenon.

Metallographic structure change of surface layer material When the heat of cutting causes the temperature of the surface to be processed to exceed the phase transition temperature, the metallographic structure of the surface layer metal changes. There are three types of grinding burns, quenching burns and annealing burns. There are two ways to improve the grinding of burns: one is to reduce the generation of grinding heat as much as possible; the other is to improve the cooling conditions and try to make the heat generated less to pass on the workpiece. The grinding wheel is selected correctly, and the cutting amount is reasonably selected to improve the cooling condition.
Surface layer residual stress Surface residual stress is caused by the following: First, residual stress is generated in the surface metal layer during cutting, and residual tensile stress is generated in the inner layer metal. Second, in the cutting process, a large amount of cutting heat is generated in the cutting zone. Third, the metallographic structure changes in the surface layer metal of different metallographic structures. The change of the specific volume of the surface metal is inevitably hindered by the matrix metal connected with it, and thus residual stress is generated.
2. Measures to improve the surface quality of machined workpieces 2.1 Formulating scientific and rational process specifications is the basis for ensuring the surface quality of workpieces Scientific and rational process regulations are the basis for processing workpieces. Only by formulating scientific and reasonable technical procedures can we provide a scientific and reasonable method basis for the surface quality of the machined workpiece to meet the requirements, so that the surface quality of the machined workpiece can meet the requirements. The requirements for scientific and rational process procedures are that the process flow should be short and the positioning should be accurate. When selecting the positioning reference, the positioning reference should be matched with the design basis as much as possible.
2.2 Reasonable selection of cutting parameters is the key to ensure the quality of processing Selecting reasonable cutting parameters can effectively inhibit the formation of built-up edge, reduce the height of the theoretical processing residual area, and ensure the surface quality of the machined workpiece. The selection of cutting parameters mainly includes the selection of cutting tool angle, the selection of cutting speed and the selection of cutting depth and feed speed. Tests have shown that the selection of a tool with a large rake angle can effectively inhibit the formation of built-up edge when machining plastic materials. This is because when the rake angle of the tool increases, the cutting force decreases, the cutting deformation is small, and the contact length between the cutter and the chip Shortening, reducing the foundation of built-up edge formation.
2.3 Reasonable choice of cutting fluid is the necessary condition to ensure the surface quality of the workpiece. Choosing a reasonable cutting fluid can improve the friction coefficient between the workpiece and the tool, reduce the cutting force and cutting temperature, and thus reduce the wear of the tool to ensure the processing quality of the workpiece. .
2.4 Main working surface of the workpiece The selection of the final processing method is essential. The selection of the final working method of the main working surface of the workpiece is crucial, because the residual stress left on the working surface of the final processing will directly affect the performance of the machine parts. The final working method of selecting the main working surface of the part must consider the specific working conditions and possible forms of damage of the main working surface of the part.
The surface quality of the workpiece is closely related to its performance. The performance of the workpiece is based on the design requirements to ensure the normal operation of the machine. Therefore, in the process of machining the workpiece, we should also consider the economic benefits and other aspects to ensure the surface of the workpiece. Quality, and avoid increasing the manufacturing cost of parts, causing unnecessary losses. Only by understanding and mastering the factors affecting the quality of machined surface can we adopt corresponding technological measures in production practice to reduce the processing quality problems caused by surface quality defects, thereby improving the performance, life and reliability of mechanical products.