Test everyone 24: Relationship between surface finish and surface roughness

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bg4.png Test everyone 24: Relationship between surface finish and surface roughness: Another term for surface roughness. Surface finish is based on human vision, while surface roughness is based on the actual micro-geometric shape of the surface. In fact, the two are mainly different in name and have corresponding comparison tables. Roughness has a calculation formula for measurement, while smoothness can only be compared with a sample gauge. Relatively speaking, using roughness to express it is more scientific and rigorous. Micro-geometric features consisting of smaller spacings and peaks and valleys on the machined surface. It is one of the issues in interchangeability research. Surface roughness is generally caused by the machining method used and other factors, such as the friction between the tool and the part surface during the machining process, the plastic deformation of the surface metal during chip separation, and high-frequency vibration in the process system. Due to different processing methods and workpiece materials, the depth, density, shape and texture of the marks left on the processed surface are different. Surface roughness is closely related to the matching properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, and has an important impact on the service life and reliability of mechanical products. Generally, Ra is used for labeling. Ra (arithmetic mean deviation of contour): the arithmetic mean value of the absolute value of the contour deviation within the sampling length L. Surface roughness refers to the small spacing and slight peak and valley unevenness of the machined surface. The distance (wave distance) between the two wave crests or the two wave troughs is very small (less than 1 mm) and is difficult to distinguish with the naked eye, so it is a microscopic geometric shape error. The smaller the surface roughness, the smoother the surface. The size of surface roughness has a great impact on the performance of mechanical parts, mainly in the following aspects: 1) Surface roughness affects the wear resistance of parts. The rougher the surface, the smaller the effective contact area between mating surfaces, the greater the pressure, and the faster the wear. 2) Surface roughness affects the stability of matching properties. For clearance fits, the rougher the surface, the easier it is to wear, causing the gap to gradually increase during work; for interference fits, the actual effective interference is reduced and the connection strength is reduced due to the flattening of microscopic convex peaks during assembly. 3) Surface roughness affects the fatigue strength of parts. There are large troughs on the surface of rough parts, which, like sharp corners and cracks, are sensitive to stress concentration, thus affecting the fatigue strength of the part. 4) Surface roughness affects the corrosion resistance of parts. A rough surface can easily allow corrosive gases or liquids to penetrate into the inner layer of metal through microscopic valleys on the surface, causing surface corrosion. 5) Surface roughness affects the sealing performance of parts. Rough surfaces cannot fit tightly together, and gas or liquid leaks through the gaps between the contact surfaces. 6) Surface roughness affects the contact stiffness of parts. Contact stiffness is the ability of the joint surface of parts to resist contact deformation under the action of external forces. The stiffness of a machine depends largely on the stiffness of the contact between the various parts. 7) Affects the measurement accuracy of parts. The surface roughness of the measured surface of the part and the measuring surface of the measuring tool will directly affect the accuracy of the measurement, especially in precision measurement. In addition, surface roughness will have varying degrees of impact on the parts' coating, thermal conductivity and contact resistance, reflective ability and radiation performance, resistance to liquid and gas flow, and conductor surface current flow.