Surface Roughness and Tolerance Class: Navigating the Critical Relationship in Quality Control

Surface roughness is an important technical index that reflects the microgeometric errors of a part’s surface and is a key factor in assessing surface quality. The selection of surface roughness is directly linked to a product’s quality, service life, and production cost.

There are three methods for selecting the surface roughness of mechanical parts: the calculation method, the test method, and the analogy method. The analogy method is commonly used in mechanical part design due to its simplicity, speed, and effectiveness. Sufficient reference materials are required for the application of the analogy method, and mechanical design manuals provide comprehensive information and literature. The most commonly used reference is the surface roughness that corresponds to the tolerance class.

In general, mechanical parts with smaller dimensional tolerance requirements have smaller surface roughness values, but there is no fixed functional relationship between them. For example, some mechanical parts, such as handles, instruments, sanitary equipment, and food machinery, require very smooth surfaces with high surface roughness values, while their dimensional tolerance requirements are low. Typically, there is a certain correspondence between the tolerance grade and the surface roughness value of parts with dimensional tolerance requirements.

Many mechanical parts design manuals and manufacturing monographs introduce empirical calculation formulas for surface roughness and the dimensional tolerance relationship of mechanical parts. However, the values in the lists provided are often different, causing confusion for those unfamiliar with the situation and increasing the difficulty of selecting the surface roughness for mechanical parts.

 

In practical terms, different types of machines have varying requirements for the surface roughness of their parts, even when they have the same dimensional tolerance. This is due to the stability of the fit. In the design and manufacturing process of mechanical parts, the requirements for the mating stability and interchangeability of the parts differ based on the type of machine. Existing mechanical parts design manuals reflect the following three main types:

Precision Machinery: This type requires high stability of the fit and mandates that the wear limit of the parts does not exceed 10% of the dimensional tolerance value, either during use or after multiple assemblies. It is mainly used in the surface of precision instruments, gauges, precision measuring tools, and the friction surface of important parts such as the inner surface of the cylinder, the main journal of precision machine tools, and the main journal of the coordinate boring machine.

Ordinary Precision Machinery: This category has higher requirements for the stability of the fit and necessitates that the wear limit of the parts does not exceed 25% of the dimensional tolerance value. It also requires a well-sealed contact surface and is mainly used in machine tools, tools, and rolling bearings to match the surface, taper pin holes, and contact surfaces with high relative movement speed, such as the mating surface of the sliding bearing and the gear tooth working surface.

General Machinery: This type requires that the wear limit of parts does not exceed 50% of the dimensional tolerance value and does not involve relative movement of the contact surface of the cnc milled parts. It is used for components like box covers, sleeves, the working surface of the surface, keys, keyways that require a close fit, and contact surfaces with low relative movement speed, such as bracket holes, bushings, and working surfaces with pulley shaft holes and reducers.

We carry out statistical analysis of various table values in the mechanical design manual, converting the old national standard for surface roughness (GB1031-68) into the new national standard (GB1031-83) in 1983 with reference to the international standard ISO. We adopt the preferred evaluation parameters, which is the average deviation value of contour arithmetic (Ra=(1/l)∫l0|y|dx). The first series of values preferred by Ra is used to derive the correlation between the surface roughness Ra and the dimensional tolerance IT.

 

Class 1: Ra≥1.6 Ra≤0.008×IT
Ra≤0.8Ra≤0.010×IT
Class 2: Ra≥1.6 Ra≤0.021×IT
Ra≤0.8Ra≤0.018×IT
Class 3: Ra≤0.042×IT

Table 1, Table 2, and Table 3 list the above three types of relationships.

When designing mechanical parts, it’s important to select the surface roughness value based on the dimensional tolerance. Different types of machines require different table values to be selected.

It’s worth noting that the table uses the first series value for Ra, while the old national standard uses the second series value for the limit value of Ra. During conversion, there may be issues with upper and lower values. We use the upper value in the table because it helps improve product quality, and the lower value is used for individual values.

The table corresponding to the tolerance grade and surface roughness of the old national standard has complex content and form. For the same tolerance grade, size segment, and basic size, the surface roughness values for the hole and the shaft differ, as do the values for different types of fits. This is due to the relationship between the tolerance values of the old tolerance and fit standard (GB159-59) and the factors mentioned above. The current new national standard tolerance and fit (GB1800-79) has the same standard tolerance value for each basic size in the same tolerance grade and size segment, simplifying the corresponding table of tolerance grade and surface roughness and making it more scientific and reasonable.

In design work, it’s important to base the choice of surface roughness on the reality of the final analysis and to comprehensively assess the surface function and cnc manufacturing process economy of the parts for a reasonable choice. The tolerance grades and surface roughness values given in the table can be used as a reference for design.

 

 

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