Surface Roughness, Classes of

the aggregate of surfaces grouped according to common numerical values of the main parameters of surface roughness. The main parameters— R_a, the mean arithmetic deviation of the profile from the median line x, and R_z, the height of irregularities at ten points (see Figure 1)—are determined from the following equations:

where l is the base length, h is the deviation of the points of the profile from the median, and R₁, R₂, . . . , R₉, R₁₀ are the distances of the five highest and lowest points of the profile along the base length parallel to the median line. The parameters of roughness are determined in practice under constant conditions for every class of surface roughness: the profile being measured must correspond to a normal section, and the measurements are performed in the direction of greatest roughness and over a base length that is standard for a given class. A particular surface may be included in a certain class of surface roughness when all three conditions are fulfilled and when the numerical values of the parameters R_a and R_z coincide with those in the range of the surface roughness class.

Figure 1

Fourteen classes of surface roughness have been established in the USSR (see Table 1). Classes 6 through 14 are further subdivided into three categories each (a, b, and c). Numerical values of the roughness parameters in this classification correspond to a series of normal standardized numbers based on a definite relationship. Because of technical difficulties in the direct measurement of R_z and the unavailability of instruments required to perform the measurements, the main parameter for classes 1 through 5, as well as 13 and 14, is R_a. In technical specifications surface roughness classes are designated by an equilateral triangle followed by the arabic numeral corresponding to the class. In addition, classes 6 through 14 are also followed by a letter (a, b, or c) designating the category—for example, ∇ 10c. Various production processes yield surfaces of different classes. For example, casting yields ∇ 3; turning, ∇ 5; grinding, ∇ 7. At the same time, the same class of surface roughness may be attained by various production processes: ∇ 7 may be achieved by fine turning and grinding.

Not all properties of two surfaces belonging to the same class of surface roughness must be identical when R_a and R_z coincide; therefore, surface roughness classification is an insufficient basis for concluding that the behavior of two parts during use will be identical.

The introduction of surface roughness classification makes possible effective study of surface quality and establishment of standards for it and recommendation of groups of surfaces, rather than specific surfaces, having common properties. In addition, it becomes possible to formulate general methods for the description of surfaces, to design instruments with standardized characteristics with respect to certain classes of surface roughness (which is one of the most important conditions for guaranteeing uniform measurements), and to develop and improve industrial processes for the production of surfaces of various types.