General Background

Reasons to Measure Surface Topography. The primary reason to measure a surface is to try to predict the performance of the component. As an example, a bearing surface requires a level of surface texture that allows lubricant to be retained in small pockets and at the same time allows the bearing to roll with a minimum of friction. If the surface is too rough, wear can quickly develop; however, if the surface is too smooth, inadequate lubrication and seizure might occur.

The other principal reason to measure a surface is to control the manufacturing process. By measuring the surface topography during processing, an operator can detect changes in the surface finish and adjust the manufacturing process to ensure that the process remains in limit.

The Components of a Surface. The topography of a surface is made up of a combination of three features (Fig. 1).

Wavines$ spacing

Waviness

Waviness

Roughness spacing

Fig. 1 Illustration of the roughness, waviness, and general form of a surface

Roughness spacing

Fig. 1 Illustration of the roughness, waviness, and general form of a surface

Surface roughness refers to the high-frequency irregularities on the surface caused by the interaction of the material microstructure and the cutting tool action.

Surface waviness refers to the medium-frequency irregularities on the surface on which the surface roughness is superimposed. These forms can be caused by the instability of the cutting tool and errors in the machine tool guideway.

Surface form is the general shape of the surface, neglecting roughness and waviness, which are caused principally by errors in the machine tool guideway, and deformations due to stress patterns in the component.

Lay is another important feature of a surface. This is a machining pattern that has a distinctly directional characteristic, such as a parallel, circular, radial, or multidirectional pattern. The lay is an important consideration because surface topography measurements will differ depending on the direction from which they are taken. Measurement should be made perpendicular to the lay of the surface.

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