Damage to metal workpieces generally begins with the surface layer. The performance of a mechanical workpiece, especially its reliability and durability, depends to a large extent on the quality of the surface layer of the workpiece. Studying the surface quality of machining is to grasp the law of the influence of various process factors on the surface quality of the machining process, and use these rules to control the machining process, and finally achieve the purpose of improving the surface quality of the workpiece and improving the performance of the product.

Effect of surface quality on fatigue strength
       The fatigue damage caused by the metal subjected to alternating load often occurs under the surface of the part and the chill layer of the surface. Therefore, the surface quality of the part has a great influence on the fatigue strength. Effect of Surface Roughness on Fatigue Strength Under the action of alternating load, the valley of surface roughness is not easy to cause stress concentration and fatigue cracking. The larger the surface roughness value, the deeper the surface marks, and the smaller the radius of the bottom, the worse the ability to resist fatigue damage.

Effect of residual stress and cold work hardening on fatigue strength
       Residual stress has a great influence on the working fatigue strength. The surface layer residual stress will expand the fatigue crack and accelerate the fatigue damage; while the surface layer cold work hardening can prevent the fatigue crack from expanding and delay the fatigue damage. Surface cold work hardening is generally accompanied by the generation of residual stress, which can prevent cracks from occurring and prevent the spread of existing cracks, which is advantageous for improving fatigue strength.

Effect of surface quality on wear resistance and effect of surface roughness on wear resistance
        The initial stage between the two contact surfaces of the newly machined motion pair is only in contact with the peak of the surface roughness, the actual contact area is much smaller than the theoretical contact area, and there is a very large unit stress at the peaks contacting each other, so that the actual contact area Plastic deformation, elastic deformation, and shear failure between the peaks occur, causing severe wear. Surface roughness has a large effect on the wear of the surface of the part. In general, the surface roughness has an optimum value, and its value is related to the working condition of the part. When the working load is increased, the initial wear amount is increased, and the optimum surface roughness is also increased.

Effect of surface cold work hardening on wear resistance
        The cold work hardening of the machined surface increases the microhardness of the metal of the friction pair surface layer, so that the wear resistance is generally improved. However, the higher the degree of hardening of the cold work, the higher the wear resistance. This is because excessive cold work hardening will cause the metal structure to be excessively loose, and even cracks and peeling of the surface metal may occur, so that the wear resistance is lowered.

Effect of surface quality on corrosion resistance
       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 are accumulated in the valleys, and 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.

Effect of surface quality on the quality of the compound
       The amount of surface roughness will affect the quality of the mating surface. For the clearance fit, the roughness value increases the wear and the gap increases, which destroys the required fit property. For the interference fit, a part of the surface peaks are squeezed during the assembly process, and the actual interference is reduced, which reduces the joint strength between the fittings.