The Relationship Between Surface Roughness and Tolerance Level in CNC Machining Industry

Surface roughness is an important technical indicator, it reflects the micro-geometric error of the surface of the part, and it is the main basis for inspecting the surface quality of the parts; whether it is reasonable or not, it is directly related to the quality, service life and production cost of the products.

There are three ways for selecting the surface roughness of mechanical parts, including calculation method, test method and analogy method. In the design of mechanical parts, the most common application is the analogy method, which is simple, rapid and effective. The application of analogy requires enough reference materials. The existing kinds of mechanical design manuals provide more comprehensive information and literature. The most commonly used is the surface roughness that is compatible with the tolerance level. Under normal circumstances, the smaller the dimensional tolerance requirements for mechanical parts, the smaller the surface roughness value of the mechanical parts, but there is no fixed functional relationship between them. For example, some machines, instrument handles, hand wheels, and sanitary equipment, food machinery, and some mechanical parts of the modified surface, their surface requirements are processed very smooth, that is, the surface roughness requirements are high, but their dimensional tolerance requirements are very low. Under normal circumstances, there is a certain correspondence between the tolerance level and the surface roughness value of parts with dimensional tolerance requirements.

In some mechanical parts design manuals and mechanical manufacturing monographs, there are many introductions to the experience and calculation formulas of the relationship between the surface roughness of mechanical parts and the dimensional tolerances of mechanical parts, and the list is for readers to choose, but as long as you read it carefully, you will found that although the same empirical calculation formula is adopted, the values in the list are different, and some are still very different. This has caused confusion for people who are not familiar with the situation in this area. At the same time, it also increases the difficulty of selecting surface roughness in the work of mechanical parts.

In actual work, for different machines, the requirements for surface roughness of their parts are different under the same dimensional tolerances. This is the stability problem of cooperation. In the design and manufacture 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 reflected:

The first type, it is mainly used in precision machinery, which has high requirements for the stability of the fit. It is required that the wear limit of the parts does not exceed 10% of the dimensional tolerance of the parts during use or after repeated assembly. This is mainly used On the surfaces of precision instruments, meters, precision measuring tools, and the friction surfaces of extremely important parts, such as the inner surface of the cylinder, the spindle neck of precision machine tools, the spindle neck of coordinate boring machines, etc.

The second type, it is mainly used for ordinary precision machinery, which requires high stability of the fit, requires that the wear limit of the part does not exceed 25% of the dimensional tolerance of the part, and requires a very tight contact surface. It is mainly used in machines, tools, surfaces that cooperate with rolling bearings, taper pin holes, and contact surfaces with relatively high speeds of movement such as sliding bearing mating surfaces, gear tooth working surfaces, etc.

The third type, it is mainly used for general machinery, which requires the wear limit of mechanical parts to not exceed 50% of the dimensional tolerance value, and there is no relative moving parts contact surfaces, such as box covers and sleeves, which require tight surfaces, keys and keyways’ working surface; contact surface with low relative movement speed, such as bracket hole, bushing, working surface with wheel shaft hole, reducer, etc.

we make statistical analysis on various table values in the mechanical design manual, convert the old national standard of surface roughness (GB 1031-68) into the new national standard (GB 1031-83) issued by ISO in 1983, and adopt the preferred evaluation parameter, namely the arithmetic mean deviation value of contour RA = (1) / (L) ∫ l0 ︱ y DX. Using the first series of numerical values preferred by RA, the relationship between surface roughness Ra and dimensional tolerance it is derived as follows

In the design of the machine, the surface roughness should be selected according to the surface roughness of the machine.

It should be noted in the table, Ra uses the first series of values, and the limit value of the old national standard Ra is the second series of values. When converting, you will encounter the problem of upper and lower values. We use the upper value in the table, because it is conducive to improving product quality, and the individual value is lower. The content and form of the table corresponding to the tolerance level and surface roughness of the old national standard are more complicated. For the same tolerance level and the same size, the same basic size is segmented. The surface roughness values of the hole and the shaft are different, and the values of different types of cooperation are also different. This is due to the fact that the old tolerance and tolerance standards (GB159-59) are related to the above factors. The current new national standard tolerance and coordination (GB1800-79) has the same standard tolerance value for each basic dimension in the same tolerance level and the same size segment, which greatly simplifies the correspondence table between the tolerance level and the surface roughness. More scientific and reasonable.

In the design work, all things must ultimately be based on reality when you are making choice of surface roughness and comprehensively measure the surface function and process economics of the parts in order to make a reasonable choice. The tolerance levels and surface roughness values given in the table can be used for reference for design.