Twelve common mistakes to know and avoid
Surface finish determines more than just how parts look and feel. It also influences how they wear,
conduct heat, distribute lubrication, accept and hold coatings, resist corrosion--even how they sound. In
spite of the capabilities of the new gages, surface finish remains a complex field that is subject to
misunderstanding. It is important that gage users and specifying engineers understand what can go wrong
to make sure that it doesn't. Here are a dozen common errors to avoid.
1.Not understanding parameters Parameters--the quantitative methods used to describe and
compare surface characteristics--are defined by the algorithms used to calculate a numerical value from
the raw measurement data. A surface finish requirement without a defined parameter is not specified. For
example, a requirement that states "roughness = 50 μ" is not acceptable today, because there are dozens
of parameters used to measure roughness. On the other hand, "Ra = 50 μ" or a symbol are acceptable,
because the Ra roughness parameter is specified.
The main body of the new ASME B46.1-1995 standard describes more than a dozen parameters, and
includes many more in appendices, while new ISO standards describe even more. Even more problematic
is the fact that past discrepancies between standards have made some parameters ambiguous. Three
different "Rz" parameters may be found on part prints: an old international version (ISO); an old Japanese
version (JIS); and a German (DIN) version that is now accepted by ISO and JIS.
Since ISO and JIS dropped their old Rz definitions and adopted the DIN version, there is now only one
current version. But there is still opportunity for confusion, if the specification is taken off an older part
print, if the specifying engineer relies upon an old standard, or if the inspector uses an old gage.
Confusion between two roughness parameters, Ra and Rq (formerly called AA and RMS, respectively), is
another source of error. When measuring a pure sine-wave profile, Rq results are 11 percent
higher than Ra results. For this reason, some gage manufacturers used to incorporate a switch on their
instruments, which simply applied a multiplication factor to convert AA to RMS (using the old
terminology). Some end-users who now own gages that measure only Ra have learned to apply that
multiplication factor manually if Rq is required. But whether this conversion is done automatically or
manually, it will generate inaccurate results when measuring any profile other than a pure sine wave
because there is no simple mathematical relationship between the two parameters. Real-world results on
manufactured parts can vary by more than 50 percent between the two parameters.
2.Selecting the wrong gage There are two basic categories of surface finish gages: "skidded" and
"skidless" types. In skidded gages, the stylus moves up and down relative to a pad attached to the bottom
of the probe that traverses along the surface of the part. Because the skid follows the general profile of
the part, the stylus registers only higher-frequency roughness characteristics--in other words, tool marks.
Thus, skidded gages are for roughness parameters only. In a skidless gage, the probe moves relative to a
reference surface inside the drive mechanism, so that the stylus is free to follow the full profile of the part,
including low-frequency geometry characteristics, such as out-of-straightness and waviness, as well as
tool marks. Skidded gages may use either a velocity-sensitive or a position-sensitive transducer, while
skidless gages use only position-sensitive devices. Mistakes can be made in purchasing, where a gage with
inadequate capabilities might be selected in an unwise attempt at economy. ASME B46.1 includes a useful
chart that classifies surface finish gages according to their measurement capabilities.
A gray area exists in the measurement of profile straightness, which can be measured by skidless profiling
surface finish gages, and by geometry gages with precision slides (i.e., "cylindricity" gages). The larger tip
radius of a geometry gage mechanically "filters" out most roughness and some waviness features, while a
profiling surface finish gage, with its smaller tip radius, responds to all features (roughness, waviness, and
straightness), and can filter out roughness electronically. A straightness callout that does not specify the
use of a particular electronic filter could be measured with either type of gage, but results would be
different, depending on whether waviness is included.
3.Reliance on default cutoff value The cutoff is the sampling length within which roughness
data is collected. The cutoff length must be long enough to provide sufficient data for the measurement,
but not so long that part geometry could impose extraneous data. A properly-specified cutoff usually
includes 10 to 15 tool marks. For most machined surfaces, this means cutoffs of .010", .030", or .100"
(0.25 mm, 0.8 mm, 2.5 mm). Don't confuse cutoff with stroke or traverse length, which is the total
distance the probe travels, and may be 2", 12" or even 18" long (50 mm, 300 mm, 450 mm), in order to
evaluate waviness and straightness. Detailed information about the choice of cutoffs is found in Chapters
3 and 4 of ASME B46.1.
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