The damage of mechanical parts usually starts from the surface. The performance of the product, especially its reliability and durability, largely depends on the quality of the surface layer of the part. The purpose of studying the quality of machined surface is to master the influence laws of various technological factors on the quality of machined surface, so as to use these laws to control the machining process and finally achieve the purpose of improving surface quality and product performance.

First, the influence of machining surface quality on machine performance.

(a) the influence of surface quality on wear resistance 1. Influence of Surface Roughness on Wear Resistance Between the two contact surfaces of a newly machined friction pair, in the initial stage, they only contact at the peak of the surface roughness, and the actual contact area is much smaller than the theoretical contact area. There is a very large unit stress at the peaks in contact with each other, which causes plastic deformation, elastic deformation and shear failure between the peaks in the actual contact area, resulting in serious wear. Parts wear can generally be divided into three stages, initial wear stage, normal wear stage and serious wear stage. Surface roughness has a great influence on the surface wear of parts. Generally speaking, the smaller the surface roughness value, the better its wear resistance. However, the surface roughness value is too small, the lubricating oil is not easy to store, and the molecular bonding between contact surfaces is easy to occur, resulting in increased wear. Therefore, the roughness of the contact surface has an optimal value, which is related to the working conditions of the parts. When the working load increases, the initial wear increases and the optimum value of surface roughness also increases. 2. The influence of surface cold working hardening on wear resistance. The cold working hardening of the machined surface improves the microhardness of the surface metal of the friction pair, so it can generally improve the wear resistance. However, the higher the degree of cold work hardening, the higher the wear resistance. This is because excessive cold work hardening will cause excessive looseness of metal structure, and even cracks and spalling of surface metal, thus reducing wear resistance.

(II) Influence of surface quality on fatigue strength The fatigue damage of metal under alternating load often occurs under the surface and surface hard layer of parts, so the surface quality of parts has a great influence on fatigue strength. 1. Influence of surface roughness on fatigue strength Under the action of alternating load, the valley of surface roughness is easy to cause stress concentration and fatigue crack. The greater the surface roughness, the deeper the grain marks on the surface, and the smaller the radius of the grain bottom, the worse the ability to resist fatigue damage. 2. Influence of residual stress and cold working hardening on fatigue strength Residual stress has great influence on fatigue strength of parts. Residual tensile stress in the surface layer will enlarge fatigue cracks and accelerate fatigue failure; However, the residual stress in the surface layer can prevent the propagation of fatigue cracks and delay the occurrence of fatigue damage. Surface hardening is generally accompanied by the generation of residual stress, which can prevent the occurrence of cracks and prevent the propagation of existing cracks, which is conducive to improving fatigue strength.

(III) Influence of surface quality on corrosion resistance The corrosion resistance of parts depends largely on surface roughness. The greater the surface roughness, the more corrosive substances are accumulated in the valley. The worse the corrosion resistance. The residual tensile stress of the surface layer will cause stress corrosion cracking and reduce the wear resistance of the parts, while the residual compressive stress can prevent stress corrosion cracking.

(IV) Influence of surface quality on mating quality The surface roughness value will affect the mating quality of mating surface. For clearance fit, the greater the roughness value, the greater the wear and clearance, which will destroy the required fit characteristics. For interference fit, some surface peaks flatten during assembly, and the actual interference decreases, which reduces the connection strength between mating parts.

Second, the factors affecting the surface roughness

(1) Factors affecting surface roughness in cutting 1. Reproduction of tool geometry When the tool moves relative to the workpiece, the residual area of the cutting layer remains on the machining surface, and its shape is the reproduction of tool geometry. By reducing the feed, the main deflection angle, the auxiliary deflection angle and increasing the radius of the tool tip arc, the height of the residual area can be reduced. In addition, properly increasing the rake angle of the tool to reduce the plastic deformation during cutting, reasonably selecting lubricating fluid and improving the grinding quality of the tool to reduce the plastic deformation during cutting, and inhibiting the generation of burrs and scales are also effective measures to reduce the surface roughness. 2. The properties of workpiece materials When processing plastic materials, the metal is squeezed by the cutter, and the tearing effect of the cutter forces the chip to separate from the workpiece, which increases the surface roughness value. The better the toughness of the workpiece material, the greater the plastic deformation of the metal and the rougher the machined surface. When machining brittle materials, the chips are broken, and many pits are left on the machined surface due to the collapse of the chips, which makes the surface rough. 3. Cutting parameters

(2) The factors that affect the surface roughness in grinding are the same as the formation process of surface roughness in cutting, and the formation of surface roughness in grinding is also determined by geometric factors and plastic deformation of surface metal. The main factors affecting the grinding surface roughness are: grinding wheel granularity, grinding wheel hardness, grinding wheel dressing, grinding speed, radial feed speed and polishing times, workpiece circumferential feed speed and axial feed speed, and cooling lubricant.