Surface roughness is an important technical index that reflects the microscopic geometric shape error of the part surface, and is the main basis for testing the surface quality of the part; whether it is reasonable or not is directly related to the quality, service life and production cost of the product.
There are three methods for selecting the surface roughness of mechanical parts, namely calculation method, test method and analogy method. In the design of mechanical parts, the most commonly used method is the analogy method, which is simple, rapid and effective. The application of the analogy method requires sufficient reference materials, and various existing mechanical design manuals provide more comprehensive materials and documents. The most commonly used is the surface roughness appropriate to the tolerance class.
In general, the smaller the dimensional tolerance requirement of mechanical parts, the smaller the surface roughness value of mechanical parts, but there is no fixed functional relationship between them. For example, handles, handwheels on some machines, instruments, sanitary equipment, and the modified surface of some mechanical parts on food machinery, their surfaces are required to be processed very smoothly, that is, the surface roughness requirements are high, but their dimensional tolerance requirements are very high. Low. In general, for parts with dimensional tolerance requirements, there is still a certain correspondence between the tolerance level and the surface roughness value.
In some mechanical parts design manuals and mechanical manufacturing monographs, there are many introductions on the experience and calculation formulas of the relationship between the surface roughness of mechanical parts and the dimensional tolerance of mechanical parts, and they are listed for readers to choose, but as long as you read carefully, you will It is found that although the exact same empirical calculation formula is adopted, the values in the list are not the same, and some of them have great differences. This creates confusion for those who are not familiar with the situation. It also increases their difficulty in selecting surface roughness for mechanical part work.
In actual work, for different types of machines, their parts have different requirements for surface roughness under the same dimensional tolerance. This is the stability problem of cooperation. In the design and manufacturing process of mechanical parts, for different types of machines, the requirements for the stability and interchangeability of the parts are different. In the existing mechanical parts design manual, the following three types are mainly reflected:
The first category is mainly used in precision machinery, which requires high stability of cooperation. It is required that the wear limit of the parts should not exceed 10% of the dimensional tolerance value of the parts during use or after repeated assembly. This is the main application On the surface of precision instruments, meters, precision measuring tools, and the friction surface of extremely important parts, such as the inner surface of cylinders, the main journal of precision machine tools, and the main journal of jig boring machines.
The second category is mainly used for ordinary precision machinery, which requires high stability of cooperation, requires the wear limit of the parts not to exceed 25% of the tolerance value of the parts, and requires a very good contact surface. It is mainly used in Such as machine tools, tools, surfaces mated with rolling bearings, taper pin holes, and contact surfaces with relatively high speeds such as mating surfaces of sliding bearings, working surfaces of gear teeth, etc.
The third category is mainly used for general machinery, requiring that the wear limit of mechanical parts does not exceed 50% of the dimensional tolerance value, and there is no contact surface of relative moving parts, such as box covers, sleeves, surfaces that require close contact, keys and keyways Working surface; contact surface with low relative motion speed, such as bracket hole, bushing, working surface with wheel shaft hole, reducer, etc.
Here we conduct a statistical analysis of various table values in the mechanical design manual, and convert the old national standard for surface roughness (GB1031-68) into a new national standard (GB1031-83) promulgated by the international standard ISO in 1983. ), using the preferred evaluation parameter, that is, the contour arithmetic mean deviation value Ra=(1)/(l)∫l0|y|dx. And using the first series of numerical values that are preferred by Ra, the relationship between the surface roughness Ra and the dimensional tolerance IT is deduced as
Class 1: Ra≥1.6 Ra≤0.008×IT
Ra≤0.8Ra≤0.010×IT
Type 2: Ra≥1.6 Ra≤0.021×IT
Ra≤0.8Ra≤0.018×IT
Class 3: Ra≤0.042×IT
List the above three relational expressions, as shown in Table 1, Table 2, and Table 3.
In the design of mechanical parts, when selecting the surface roughness value according to the dimensional tolerance, the corresponding table value should be selected according to different types of machines.
It should be noted that the Ra in the table adopts the value of the first series, while the limit value of the old national standard Ra is the value of the second series. When converting, there will be problems of upper and lower numerical values. In the table, we use the upper level for the values in the table, because it is beneficial to improve product quality, and the lower level for the individual values. The content and form of the table corresponding to the tolerance grade and surface roughness of the old national standard are relatively complicated. For the same tolerance grade, the same size, the same basic size, the surface roughness values of the hole and the shaft are different, and the values of different fit types are also different. , this is because the tolerance value of the old tolerance and fit standard (GB159-59) is related to the above factors. The current new national standard tolerance and fit (GB1800-79) has the same standard tolerance value for each basic dimension in the same tolerance class and the same size segment, which greatly simplifies the correspondence table between tolerance class and surface roughness, and also It is more scientific and reasonable.
In the design work, in the final analysis, the choice of surface roughness must proceed from reality and fully measure the surface function and process economy of the part in order to make a reasonable choice. The tolerance grades and surface roughness values given in the table can be used as a reference for design.
