Surface roughness is an important technical index to reflect the micro-geometrical shape error of part surface, and it is the main basis of inspecting the quality of parts. There are 3 kinds of methods to choose the surface roughness of mechanical parts, namely, computational method, test method and analogy method. In the mechanical parts design work, the most common is the analogy method, this method is simple, rapid and effective. The application of analogy method requires sufficient reference materials, and all the existing mechanical design manuals provide more comprehensive information and literature. The most common is the surface roughness which is suitable to the tolerance grade. In general, the smaller the size tolerances of mechanical parts, the smaller the surface roughness value of mechanical parts, but there is no fixed function relationship between them. For example, some machines, instruments on the handle, hand wheel and sanitation, food machinery, some of the mechanical parts of the modified surface, their surface requirements are very smooth processing, surface roughness requirements are very high, but the size of the tolerance requirements are very low. In general, there is a corresponding relation between the tolerance grade and the surface roughness value of the parts with the requirement of dimensional tolerances.