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96 Part II: Metrology
Surface Metrology
Part II.E.1
The measurement of the difference between what a surface actually is and what
it is intended to be defines surface metrology. Surface measurement, however, is
involved with the relationship of a surface on the workpiece to a reference that
is not actually on the workpiece. The most common aspect of surface metrology is
the measurement of surface roughness as an average deviation from a mean center
line (Bosch 1984).
The quality of a surface finish is commonly specified along with linear and
geometric dimensions. This is becoming more common as product demands
increase, because surface quality often determines how well a part performs.
Heat-exchanger tubes transfer heat better when their surfaces are slightly rough
rather than highly polished. Brake drums and clutch plates work best with some
degree of surface roughness. On the other hand, bearing surfaces for high-speed
engines wear-in excessively and fail sooner if not highly polished, but still need
certain surface textures to hold lubricants. Thus there is a need to control all surface
features, not just roughness alone.
Surface Characteristics. The American National Standards Institute (ANSI) has
provided a set of standard terms and symbols to define such basic surface characteristics
as profile, roughness, waviness, flaws, and lay. A profile is defined as
the contour of any section through a surface. Roughness refers to relatively finely
spaced surface irregularities such as might be produced by the action of a cutting
tool or grinding wheel during a machining operation. Waviness consists of those
surface irregularities that are of greater spacing than roughness. Waviness may be
caused by vibrations, machine or work deflections, warping, and so on. Flaws are
surface irregularities or imperfections that occur at infrequent intervals and at random
locations. Such imperfections as scratches, ridges, holes, cracks, pits, checks,
and so on, are included in this category. Lay is defined as the direction of the predominant
surface pattern. These characteristics are illustrated in Figure 11.1.
Surface Quality Specifications. Standard symbols to specify surface quality are
included in Figure 11.1c. Roughness is most commonly specified and is expressed
in units of micrometers (mm), nanometers (nm), or microinches (min.). According
to ANSI/ASME B46.1-1985, the standard measure of surface roughness adopted by
the United States and approximately 25 countries around the world is the arithmetic
average roughness, R a (formerly AA or CLA). R a represents the arithmetic
average deviation of the ordinates of profile height increments of the surface from
the centerline of that surface. An approximation of the average roughness may be
obtained by:
y y y y
R = + + + ... +
a+ n
a b c n
where
R a+ = approximation of the average roughness
y a . . . y n = absolute values of the surface profile coordinates
n = number of sample measurements