Surface Roughness

the aggregate of the irregularities that form the microrelief of a part’s surface. Surface roughness results mainly from plastic deformation of the surface layer of a work-piece during machining. It is caused by, for example, irregularities of a tool’s cutting edges, friction, the tearing of particles of material from the surface of the workpiece, and vibration of the workpiece or the tool.

Surface roughness is an important index in the specifications of a manufactured article. It affects the service properties of machine parts and assemblies, such as the wear resistance of rubbing surfaces, fatigue strength, corrosion resistance, and the maintenance of negative allowance in tight fits. Surface-roughness requirements are specified on the basis of the designated purpose of the surfaces of parts and the design characteristics of the parts. For a long time, a system characterizing the finish of a surface by means of classes of surface roughness was used in Soviet production. A new system, which was introduced on Jan. 1, 1975, replaced the classes of surface roughness that were previously used in the USSR.

The extensive set of parameters of the new system facilitates the specification of precise requirements for surfaces intended for various purposes. When numerical values of surface roughness are determined, reference is made to a single datum line, for which the mean line m of a surface profile is used. Measurements are performed within the limits of a roughness-width cutoff l, that is, the length of the surface segment chosen for the measurement of roughness without taking into account other types of irregularities, such as waviness, whose spacing is greater than l. The numerical value of the roughness-width cutoff may be 0.01, 0.03, 0.08, 0.25, 0.8, 2.5, 8, or 25 mm. Quantitatively, surface roughness is evaluated by means of one or more of the following main parameters: the arithmetical average deviation of the surface profile (R_a), the height of irregularities at ten points (R_z), the maximum height of irregularities (R_max), the average spacing of irregularities (S_m), the average peak spacing of irregularities (S), and the sampling length of the profile (t_p).

The numerical values of the roughness parameters and the types of roughness lays—for example, parallel, perpendicular, and circular—are specified by a standard. The choice of roughness parameters depends on the design of the parts and the designated purpose of their surfaces. For example, permissible values of R_a or R_z, R_max, and t_p, as well as the roughness lay, are specified for rubbing surfaces of critical parts, while R_max, S_m, and S are specified for the surfaces of critical parts subjected to fluctuating loads. Surface-roughness specifications are indicated by the numerical value or a range of values of one or more parameters and by the roughness-width cutoff. For noncritical surfaces, surface-roughness requirements are dictated by the requirements of industrial design, corrosion resistance, and manufacturing methods.

Nowadays, the above parameters are used in the USSR instead of the classes of surface roughness that were in use until 1980.

The surface roughness that parts have after their manufacture and assembly is altered when the parts are worn in. The surface roughness of parts after wearing-in is known as optimum surface roughness; it is produced by, for example, rolling friction or sliding friction. Optimum surface roughness assures minimum wear and is retained during the long-term operation of machinery. The parameters of optimum surface roughness depend on the design and material of parts that rub together, the quality of the lubrication, and other service conditions.

Surface roughness is usually measured by means of contact or noncontact methods. Stylus-type instruments, such as profile meters or profilographs, are used in the contact methods; optical instruments are employed in the noncontact methods. In machine building, a visual method is often used. In the visual method, a comparison is made between the surface under inspection and the surface of a specimen or part whose surface roughness has been certified.