Surface Measurement

Any discussion of surface finishes will involve, among other things, comparisons of surface form. It is useful therefore to briefly describe the terminology and methods of measurement.

Roughness Average (Ra)

The most commonly used measurements confine themselves to the fine texture of the surface rather than more macro effects such as waviness. British Standard BS 1134 Part 1, ‘Assessment of Surface Texture’ describes the methods to be used and defines the various parameters. The most common of these is Ra, or Centre Line Average, and is defined as ‘the arithmetic mean of the departure of the roughness profile from the mean line’. The measurement is taken at right angles to the direction of the finish using a very fine stylus.

Measurements are quoted in either; thousands of a millimetre, microns (µm) or; millionths of an inch, micro-inch (µin). For all practical purposes 40µin = 1µm. In this manual all roughness measurements are in microns (µm).

The roughness values are very small compared with their horizontal spacing, the ratio can be 20:1. A trace graph taken with height and distance magnified by the same amount would show a very smooth surface. It is therefore necessary to magnify the height much more than the horizontal distance. This can lead to misunderstandings of the nature of a surface because of the spiky appearance of the graph.

Surface roughness measurements give accurate values for comparing similar finishes. Care must be taken when comparing totally different surfaces, such as a satin polish against a rolled surface as Ra, Rtm etc, only describe part of the topography of a surface.

Reflectivity

This is most helpful in quantifying a bright (mirror) polish, but can also be useful as part of the specifying package of a smooth satin type finish. The appearance of a bright finish is greatly affected by the light conditions in which it is viewed. Subjective assessment can therefore be influenced and flawed. Using reflectivity measurements introduces a qualitative, repeatable figure which is independent of the prevailing light conditions.

The amount of light reflected back from the surface is measured, usually at an inclusive angle of 120° (60° from the vertical). Different metals inherently reflect different amounts of light. For instance, Aluminium which is a white metal, will reflect more light than Stainless Steel. Each metal therefore requires its own specification range.