Meaning of Surface Texture and Some Definitions (Metrology)

11.2.
Usually the concept of surface roughness is sensory and it is capable of being understood in an intuitive way. Generally the surface roughness has been experienced and understood by the sense of light and touch etc.
Therefore, to define surface roughness in terms of measurable attributes presents great difficulties. The definition of surface roughness could probably be best understood on a causative basis and independent of intuition.
As we know that any material being machined by chip removal process can’t be finished perfectly due to some departures from ideal conditions as envisaged by the designer. Due to conditions not being ideal, the surface produced will have some irregularities ; and these geometrical irregularities could be classified into four categories.
First Order. This includes the irregularities arising out of inaccuracies in the machine tool itself e.g. lack of straightness of guideways on which tool post is moving. This includes the surface irregularities arising due to deformation of work under the action of cutting forces and the weight of the material itself.
Second Order. Some irregularities are caused due to vibrations of any kind such as chatter marks and are included in second order.
Third Order. Even if the machine were prefect and completely free of vibrations, some irregularities are caused by machining itself due to characteristic of the process. This also includes the feed marks of the cutting tool.
Fourth Order. This includes the irregularities arising from the rupture of the material during the separation of the chip.
Further these irregularities of four orders can be grouped under two groups :
First group includes irregularities of considerable wave-length of a periodic character resulting from mechanical disturbances in the generating set-up. These errors are termed as macro-geometrical errors and include irregularities of first and second order and are mainly due to misalignment of centres, lack of straightness of guide-ways and non-linear feed motion. These errors are also referred to as Waviness or Secondary Texture.
Second group includes irregularities of small wavelength caused by the direct action of the cutting element on the material or by some other disturbance such as friction, wear, or corrosion. These errors are chiefly caused due to tool feed rate and due to tool chatter, i.e. it includes irregularities of third and fourth order and constitutes the micro geometrical errors. Errors in this group are referred to as Roughness or Primary Texture.
Thus any finished surface could be considered to be combination of two forms of wavelength (large wavelength for waviness and smaller wavelength for roughness) superimposed upon each other. One of the problems in measuring surface finish is to separate the waviness from the roughness.
The various terms used in connection with surface finish could now be shown on the diagram below (Fig. 11.1).
Terms for surface roughness.
Fig. 11.1. Terms for surface roughness.
Representation of surface finish
Fig. 11.2. Representation of surface finish.
These terms are represented systematically in Fig. 11.2 and this notation is followed universally. Roughness width may be sometimes omitted.


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