Some basic definitions of surface finish criteria
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Y=Roughness, S=Lay, V=Waviness |
When considering the concept of surface finish, we will disregard any sporadic surface defects, which have mechanical or metallurgical causes. Instead, we will concentrate on the minute, evenly distributed irregularities in the surface layer, which are characteristic of the different means of production and finishing of the steel product. In terms of strict definition, the concept "surface finish" can be said to describe the deviation from the ideal flat surface. This deviation is normally expressed in terms such as roughness, lay and waviness, which in turn may be defined as:
- Roughness represents the size of the finely distributed surface pattern deviations from the smooth surface.
- Lay represents the dominant direction of the surface pattern, such as grinding scores.
- Waviness represents deviations, which are relatively far apart.
Of these, waviness is the most difficult to detect with the naked eye.
Definition of Surface Roughness
Roughness is measured by special precision instruments that measure the vertical deviations when traversing the metal surface.
Ra is the most commonly used parameter to describe the average surface roughness and is defined as an integral of the absolute value of the roughness profile measured over an evaluation length:
The average roughness is the total area of the peaks and valleys divided by the evaluation length; it is expressed in µm (thousand of a millimetre). The total area of the peaks above the mean line should be equal to the total area of the valleys below the mean line.
Centre line average (CLA) and arithmetic average (AA) are older designations for average surface roughness.