TABLE OF CONTENTS Considering the trace of a supposedly flat surface in Figure 4.11, the flat surface is far from a perfect straight line. Things related to the machine tool, such as vibrations and slide-way inaccuracies cause the long wavelength deviations where the undulations are of the order of millimetres. However, the figure also shows wavelengths of a much smaller magnitude. These deviations are the surface finish (SF). They are of the order of tens of microns and they are the machining marks. They are caused by a combination of the tool shape and the feed across the workpiece. In many instances the SF and texture can have a significant influence on functional performance (Griffiths, 2001).
The SF is normally measured by a stylus, which is drawn across the surface to be measured. The stylus moves in a straight line over the surface driven by a traversing unit. This produces a 2D line trace similar to that in Figure 4.11. A line trace produces an X Y set of data points that can be analysed in a variety of statistical ways to produce parameters. These parameters are descriptors of a surface. They can be used to describe the SF of a surface in much the same way as a dimension describes the form of a feature. In the same way that a dimension can never be exact, the SF, represented by a parameter, can never be exact. Tolerances also need to added to SF specifications. To ensure fitness for purpose, the...
