It is of great significance to correctly understand the connotation of the surface quality of parts, analyze various technological factors that affect the quality of the processed surface during the machining process, and improve the surface quality and product performance.
The surface quality of mechanical processing refers to the micro unevenness of the processed surface of the part after machining, also called roughness, and the surface quality after processing directly affects the physical, chemical and mechanical properties of the processed part. The working performance, reliability and life of the product depend to a great extent on the surface quality of the main parts. Therefore, it is of great significance to correctly understand the connotation of the surface quality of parts, analyze various process factors that affect the surface quality of the machining process, and improve the surface quality and product performance.
1. Factors affecting the surface quality of machining
1.1 The impact of machine performance on the quality of machined surfaces
Effect of wear resistance on surface quality
Between the two contact surfaces of a newly processed friction clutch, the initial contact is at the peak of the rough surface. The actual contact area is much smaller than the theoretical contact area. There is a very large unit stress at the contacting parts, which makes the actual contact Plastic deformation, elastic deformation and shear failure between peaks occur at the area, causing severe wear.
Effect of fatigue strength on surface quality
Under the action of alternating load, the valleys of the surface roughness are likely to cause stress concentration and fatigue marks. The greater the value of surface roughness, the deeper the surface of the lines, and the more the radius of the base, the worse the ability to resist fatigue damage. Residual stress has a great influence on the fatigue strength of parts. The residual tensile stress of the surface layer will cause fatigue cracks to expand and accelerate fatigue failure; while the residual stress of the surface layer can prevent the expansion of fatigue cracks and delay the occurrence of fatigue damage.
Effect of corrosion resistance on surface quality
The corrosion resistance of a part depends largely on the surface roughness. The larger the surface roughness value, the more corrosive substances will accumulate in the valley. The worse the corrosion resistance. The residual tensile stress of the surface layer will produce stress corrosion cracking and reduce the wear resistance of the parts, while the residual compressive stress can prevent stress corrosion cracking.
1.2 Factors affecting surface roughness
Factors affecting surface roughness during cutting
① The reflection of the cutter geometry When the cutter moves relative to the workpiece, the residual area of the cutting layer is left on the machining surface, and its shape is a reflection of the cutter geometry. ②Properties of workpiece materials When plastic materials are processed, plastic deformation is generated by the extrusion of metal by the cutter, and the tearing effect of the cutter to separate the chips from the workpiece increases the surface roughness value. ③ When cutting brittle materials, the cutting speed has little effect on roughness; when processing plastic materials, built-up edge has a great influence on roughness.
Factors affecting surface roughness in grinding
The main factors that affect the roughness of the grinding surface are: the particle size of the grinding wheel, the hardness of the grinding wheel, the dressing of the grinding wheel, the grinding speed, the grinding radial direction, the feed amount and the number of smoothing, the circumferential feed speed and the axial feed amount of the workpiece , Cooling lubricant, etc.
1.3 Factors affecting the physical and mechanical properties of the processed surface layer
Surface work hardening
During the machining process, the plastic deformation caused by the cutting force distorts and deforms the character, produces shear slip between the grains, and the grains are elongated and fibrillated, and even broken, which will make the surface layer metal hardness and strength. Increase, this phenomenon is called cold work hardening (or strengthening). The main factors affecting the cold work hardening are: the blunt radius of the cutting edge is increased, the extrusion effect on the surface metal is enhanced, and the plastic deformation is increased, resulting in enhanced cold hardening. The wear of the flank of the tool is increased, the friction between the flank and the surface to be processed is increased, and the plastic deformation is increased, resulting in enhanced cold hardness. Increasing the cutting speed, shortening the action time between the tool and the workpiece, reducing the depth of plastic deformation expansion and the depth of the cold hard layer. After the cutting speed is increased, the action time of cutting heat on the surface layer of the workpiece is also shortened, which will increase the degree of cold hardening. As the feed rate increases, the cutting force also increases, the plastic deformation of the surface metal increases, and the chilling effect is strengthened. The greater the plasticity of the workpiece material, the more severe the chilling phenomenon.
Changes in the metallographic structure of the surface layer material
When the temperature of the processed surface exceeds the phase transition temperature due to cutting heat, the metallographic structure of the surface metal will change. There are three main types of grinding burn, quenching burn and annealing burn. There are two ways to improve grinding burns: one is to reduce the generation of grinding heat as much as possible; the second is to improve the cooling conditions and minimize the amount of heat generated to the workpiece. Choose the grinding wheel correctly, choose the cutting amount reasonably and improve the cooling conditions.
Surface layer residual stress
The reason for the surface residual stress: First, the residual stress is generated in the surface metal layer during cutting, and the residual tensile stress is generated in the inner layer metal. Secondly, during cutting, a large amount of cutting heat will be generated in the cutting zone. The third is that the metal on the surface layer of different metallographic structures produces a change in the metallographic structure. The change in the specific volume of the surface metal must be hindered by the base metal connected to it, so there is residual stress.
2. Measures to improve the surface quality of machined workpieces
2.1 The formulation of scientific and reasonable process regulations is the basis to ensure the surface quality of the workpiece
Scientific and reasonable process regulations are the basis for the method of processing workpieces. Only by formulating scientific and reasonable technological procedures, can we provide scientific and reasonable method basis for processing the surface quality of the workpiece, and make it possible to process the surface quality of the workpiece. The requirements for scientific and reasonable process regulations are that the process flow should be short and the positioning should be accurate. When selecting the positioning reference, try to make the positioning reference coincide with the design reference.
2.2 Reasonable selection of cutting parameters is the key to ensure the processing quality
Choosing reasonable cutting parameters can effectively suppress the formation of built-up edge, reduce the height of the theoretical processing residual area, and ensure the surface quality of the processed workpiece. The choice of cutting parameters mainly includes the choice of cutting tool angle, the choice of cutting speed and the choice of cutting depth and feed speed. Tests have shown that when a plastic tool is processed with a larger rake angle, the formation of built-up edge can be effectively suppressed. 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 tool and the chip Shorter, reducing the basis for the formation of built-up edge.
2.3 Reasonable choice of cutting fluid is a necessary condition to ensure the surface quality of the processed workpiece
Choosing a reasonable cutting fluid can improve the friction coefficient between the workpiece and the tool, and can reduce the cutting force and cutting temperature, thereby reducing the wear of the tool and ensuring the processing quality of the workpiece.
2.4 The choice of the final working method of the main working surface of the workpiece is crucial
The selection of the final working method of the main working surface of the workpiece is crucial, because the residual stress left by the final working surface on the working surface will directly affect the performance of the machine parts. When selecting the final process method of the main working surface of a part, the specific working conditions and possible damage forms of the main working surface of the part must be considered.
The surface quality of the workpiece is closely related to its performance. The performance of the workpiece is a design requirement based on the premise of ensuring the normal operation of the machine. Therefore, in the process of processing the workpiece, we should also consider from many aspects such as economic benefits to ensure the processing of the workpiece surface Quality, and avoid increasing the manufacturing cost of parts, causing unnecessary losses. Only by understanding and mastering the factors that affect the surface quality of mechanical processing can we take corresponding technological measures in production practice to reduce the processing quality problems caused by surface quality defects of parts, thereby improving the performance, life and reliability of mechanical products.