Roughness and waviness

A trace across a surface provides a profile of that surface which will contain short and long wavelengths (see Figure 4.11). In order for a surface to be correctly inspected, the short and long wavelength components need to be separated so they can be individually analysed. The long waves are to do with dimensions and the short waves are to do with the SF. Both can be relevant to function but in different ways. Consider the block in Figure 6.1. This has been produced on a shaping machine. The block surface undulates in a variety of ways. There is a basic roughness, created by the tool feed marks, which is superimposed on the general plane of the surface. Thus, one can identify two different wavelengths, one of a small scale and one of a large scale. These are referred to as the roughness and waviness components.

Roughness and waviness have different influences on functional performance. A good example illustrating the differences concerns automotive bodies. Considering the small-scale amplitudes and wavelengths called 'roughness', it is the roughness, not waviness, which influences friction, lubrication, wear and galling, etc. The next scale up from roughness is 'waviness' and it is known that the visual appearance of painted car bodies correlates more with waviness than roughness. The reason for this is the paint depth is about lOOum and it has a significant filtering effect on roughness but not waviness.

Roughness And Waviness Profiles
Shaped Block

Roughness wt

2D Profile with Roughness and Waviness Components

Roughness

2D Profile with Roughness and Waviness Components

Figure 6.1 A shaped block showing roughness and waviness components

6.2 Measuring the surface finish

The most common method of assessing the SF is by traversing a stylus across a surface. A typical stylus is shown in the scanning electron microscope (SEM) photograph in Figure 6.2 (courtesy of Hommelwerke GmbH). The stylus tip is made of diamond having a tip spherical radius of 5um and an included cone angle of 90°. Styli are available in a standard range of spherical radii of 2, 5 and lOum and included cone angles of 60° and 90° (ISO 3274:1996). The stylus is shown in contact with a ground surface that gives an indication of the scale of the surface features. The stylus is positioned at the end of a mechanical arm that connects to a transducer such that the undulations on the surface are translated into an electrical signal. This signal is amplified and eventually displayed on a PC screen along with the calculated parameters.

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  • ralf
    How to draw surface roughness on drawing?
    8 years ago

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