The surface roughness (surface roughness) refers to the small spacing of the machined surface and the unevenness of the tiny peak valley. The distance between the two wave peaks or the two wave valleys (the wave distance) is very small (below 1mm), which belongs to the microscopic geometric error. The smaller the surface roughness is, the more smooth the surface is.

The surface roughness is usually formed by the processing methods and other factors, such as the friction between the tool and the parts, the plastic deformation of the surface layer during the chip separation, and the high frequency vibration in the process system. Because of the difference between the processing method and the workpiece material, the depth, the density, the shape and the texture of 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 important influence on the service life and reliability of mechanical products. Ra is used in general annotation.