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98 Part II: Metrology
Rms values are about 11 percent larger than R a figures. Some surface roughness
measuring instruments have a scale with numbers labeled “rms” and a scale calibrated
in R a values. On such instruments, the number called “rms” is a root-meansquare
value only when the profile is sinusoidal. Waviness height alone may be
specified, or it may be accompanied by a width specification. Thus, in Figure 11.1c,
the specification 0.05–50.8 mm (.002–2 in.) means that no waves over 0.05 mm
(.002 in.) high are allowed in any 50.8 mm (2 in.) of length. If no width specification
is given, it is usually implied that the waviness height specified must be held
over the full length of the workpiece. Other specifications shown in Figure 11.1c
are less common.
Part II.E.2
2. MEASURING FINISH
Describe and apply profilometers, profile
tracers, fingernail comparators, etc.
(Application)
Body of Knowledge II.E.2
Waviness and roughness are measured separately. Waviness may be measured
by sensitive dial indicators. One method of detecting gross waviness is to coat a
surface with a high-gloss film, such as mineral oil, and then reflect it in a regular
pattern, such as a wire grid. Waviness is revealed by irregularities or discontinuities
in the reflected lines.
Many optical methods have been developed to evaluate surface roughness.
Some are based on interferometry. One method of interference contrast makes
different levels stand out from each other by lighting the surface with two out-ofphase
rays. Another method projects a thin ribbon of light at a 45° angle onto a
surface. This appears in a microscope as a wavy line depicting the surface irregularities.
For a method of replication, a plastic film is pressed against a surface
to take its imprint. The film then may be plated with a thin silver deposit for
microscopic examination or may be sectioned and magnified. These are laboratory
methods and are only economical in manufacturing where other means are
not feasible, such as on a surface inaccessible to a probe.
Except for extremely fine surface finishes that require laboratory measurement,
most manufacturers measure surface texture at or near the workplace. A
variety of instruments called surface finish gages are commercially available in
either handheld or table-mounted models. These require only moderate skill, and
roughness measurements are displayed on a dial, digital readout, chart, or a digital
output for statistical process control, depending on the type of instrument
used. Most of these instruments employ a diamond-tipped stylus that is moved
across the surface of the part to sense the point-to-point roughness of the surface.
As illustrated in Figure 11.2, there are two basic types of gages, the skid and
the skidless type. The skid type shown in Figure 11.2a has a hinged probe that