Waviness is the characteristic form of topographical variations that are measurable as the profile of the part in an actual or imaginary cross-section. The term waviness implies a repetitive and essentially regular occurrence of topographical features, an assumption that is based on the typical surface topography of machined surfaces.  

Waviness width expresses the distance between adjacent crests of the essentially wavy profile. Waviness height is the distance, in a direction normal to the general surface, between the crests and the valleys of the waves.  

Roughness expresses the closely spaced digressions of the actual surface from its ideal form. These digressions are usually less regular in profile form and spacing than those termed waviness. Due to the technological circumstances from which roughness originates, the measurable digressions from a basic profile line are of higher frequency (closer spacing), yet of lower average amplitude (less height) than the waviness on which roughness is usually superimposed.  

The boundary between waviness and roughness is not distinct although, based on the investigation of typical machined surfaces, distinguishing characteristics are presumed in the standard.  

The diagram in Fig. 15-2A also shows a characteristic pattern: the essentially parallel ridges and valleys having a common direction that is termed the lay of the texture. That lay is illustrated to be normal to the cross-sectional plane in which the profile is observed. An oblique plane of observation (or oblique cross-sectional plane) causes the measurable distance between the consecutive profile features, the waviness width, to increase. A plane of observation parallel with the lay generally displays a substantially lesser amount of waviness, or no wave pattern at all. In Fig. 15-2 the cross-sectional plane is also shown as being perpendicular to the general plane of the investigated surface area. A cross-sectional plane at an incline would increase the measurable distance between the levels of the crests and of the valleys.  

The surface texture of cylindrical parts, as shown in Fig. 15-2B, is usually examined in an axial plane, that being approximately perpendicular to the lay produced by most of the conventional manufacturing processes.  

A few aspects of these interrelations between the lay and the orientation of the plane of observation are shown diagrammatically in Fig. 15-3.

Fig. 15-3. Diagrams of surface sections to visualize the interrelations between the direction of the lay and the orientation of the plane of observation.

It must be kept in mind that the conditions of the surface texture are illustrated here in an idealized manner in order to assist the understanding of the concepts on which most of the currently used methods of surface-texture measurement and assessment are based.  

Actually, the pattern of the surface texture is not always as regular in repetitiveness and lay orientation as shown in the diagrams. Nor is it always possible to select a plane for measurement that is normal to both the lay and the general plane of the surface. Finally, there are cases in which the direction normal to the lay is not the meaningful one with regard to the functional role of a particular surface.  

The effect of the direction and pattern of the lay on certain application requirements of technical surfaces is recognized in the American and several foreign surface-texture standards, by providing definitions and symbols for the specification of such conditions on product drawings. These specifications refer to the orientation of the predominant surface topography, as represented by the distinguishable grooves produced in the applied technological process. Some foreign standards also provide means to specify the profile of the machining marks or groove forms, because of the potential functional effects of these surface-texture characteristics.