THE METHODS OF
The general concept of measurement implies specific dimensions,
the extent or size of which must be determined and expressed in numerical
values based on standard units of measurement.
The commonly measured linear and angular dimensions are
usually delineated by specific elements on the part’s surface, and the
dimension to be measured is the distance between these boundaries. In the case
of surface-texture measurements, however, such readily perceivable and exactly
defined dimensions are not available. In the inspection of surface texture, the
dimension to be measured is the calculated or qualified extent of a
multiplicity of departures of the actual surface from the theoretical.
The number of such individual departures from the
theoretical is usually very large, even when a relatively small portion of the
surface is being examined. Not all these departures are equally significant
and, regardless, it would not be practical to measure all the variations of the
surface. Neither is it practicable to examine the entire surface of a technical
part for measuring its texture; therefore, only a relatively small area or a
short element of the surface is usually inspected and the results are
considered to be representative of the larger surface.
For assessing the functional effects of different types of
surface variations, not all imperfections are considered equally significant,
or even requiring informative measurements. Accordingly, the applicable methods
of surface-texture measurement are often chosen to supply reliable information
on the meaningful aspects only, neglecting, partially or totally, the other
types of surface irregularities.