Surface Roughness (Metrology)

11.3.
On any finished surface, imperfections are bound to be there and these take the form of a succession of hills and valleys which vary both in height and in spacing and result in a kind of texture which in appearance or feel is often characteristic of the machining process and its accompanying defects. The several kinds of departures are there on the surface and these are due to various causes. Roughness or texture in the form of a succession of minute irregularities is produced directly by the finishing process employed e.g. while using a single point tool. The roughness on the surface tends to be both uniformly spaced and directional; whereas surfaces produced by straight and cylindrical grinding tend to have irregularly spaced but directional roughness and in the case of boring or lapping process the surface roughness tends to be irregular and non-directional in character. The characteristic roughness produced by the tool is not the only cause of roughness in case of machining operations, but the more openly spaced component or roughness are also produced from faults in the machining operation. In practice the complete roughness commonly represents a combination of irregularities of various kinds and magnitudes arising from several different causes and the individual effects of the separate contributing factors can’t always be readily distinguished. Thus for the complete study of the surface roughness, it is essential that the measurement and analysis of all the component elements and an assessment of the effects of the resulting combined texture be made. All this being very difficult and tedious job, in practice all that is essential is that a practical method of assessment be followed, the result of which can be readily compared with a specified requirement of quality, preferably on the numerical basis. It is done by analysing the form of profile revealed by a plane section through the surface. The measurement of surface roughness poses a problem in three dimensional geometry, but for simplification purposes, it is better to reduce it into two dimensional geometry by confining individual measurement to the profiles of plane sections taken through the surface. The direction of measurement is usually perpendicular to the direction of the predominant surface markings or ‘lay*. The shape of the hills and valleys which constitute surface roughness is best studied in a cross-section normal to the surface using a stylus instrument which plots a graph in which the minute irregularities of the surface are magnified to 50,000 times or even greater, whereas the length over which observations are made is magnified only by 50 to 300 times. Thus the above graph does not give a real representation of the surface nature, but a distorted view, but it gives very useful information. From this curve the height of the irregularities (total or average value), separation of the crests (separation) can be easily noted.
When the irregularities are comparatively uniform in shape and size, then the distance between the successive peaks is described as pitch or dominant spacing. It may be appreciated
here that the surface roughness is concerned both with the size and the shape of the irregularities e.g., in certain profile the height of departure from the nominal profile may be same but the spacing of the irregularities may be wider or closer, or the space of the irregularities may be of various forms. Thus realising that both size and shape (i.e. height of irregularities, their spacing and form) are important for specifying surface roughness, it is not considered possible to express the complete roughness characteristics by means of any single number. The normal practice is, therefore, to specify Ra value to define the quality or grade, and the manufacturing process which serves to produce the type of roughness.