Here is information sufficient to muddy the water. Some excerpts follow:
His Own Parameter
Custom surface finish parameters
can create more problems than they solve
--unless you know what to watch out for.
Ra, the most common surface finish parameter across most industries and
applications, is typically measured over five cutoffs of data.
Manufacturers of ultra-clean tubes have found this to be insufficient,
and want to know surface conditions of the whole surface. They have,
therefore, developed a special series of roughness parameters and
measurement methods to generate three Ra values.
RaCH (continuously high) continuously shifts the segment under
analysis by a single data point and identifies the largest Ra evaluated
across the entire part length.
RaCL (continuously low) continuously shifts the segment under
analysis by a single data point and identifies the smallest Ra
evaluated across the entire part length.
RaCA (continuously average) continuously shifts the segment under
analysis by a single data point and takes the average of all the
individual Ra measurements.
While this methodology is not recognized by any national or
international standards body, it has proven so useful that it has been
accepted as a de facto standard by this particular industry. In
response, at least one gage manufacturer developed special software to
perform these measurements, and incorporated it into existing surface
Parameter Confusion: Will The Real Rz Please Stand Up?
You might think that a simple surface finish parameter like Rz, mean
roughness depth, would be reasonably straightforward. It's not. A
classic example of parameter confusion, Rz has at least three flavors:
The old international (ISO) version --10-point Rz, which averages
the distance between the five highest peaks and the five deepest
valleys, based on an unfiltered profile
The old German (DIN) version, which is now the new ISO/ASME version
--an average of distances between maximum peaks and valleys in five
cutoffs, based on a roughness profile
The recent Japanese (JIS) version --10 point Rz which averages
distance between the five highest peaks and five lowest valleys based
on a roughness profile
This situation presents numerous opportunities for confusion:
The specifying engineer might write a specification based on an old
or a new standard.
The machinist may be working from an old part print or a new one.
The print may have originated in an engineering department operating
under a different national or international standard.
The measuring instrument might incorporate any or all of the
algorithms under the same or different names.
It's not clear how many potential errors that adds up to, but it's too
many in any case. You can be clear on one thing from this discussion,
however: If your print calls out an Rz value with no further
explanation, warning bells should go off. A time bomb may be ticking.
Human nature being what it is, engineers take great pride in inventing
new surface finish parameters specific to parts that their
organizations manufacture. Thus, out of something akin to pride of
ownership, new surface finish parameters are born, even though there
are many existing parameters that may have done the job satisfactorily.
There are currently more than 100 parameters related to
two-dimensional stylus-type measurements alone, with no end in sight.
Those parameters, most of which are included in various national and
international standards, describe a wide variety of surface conditions
and are capable of satisfying the great majority of metalworking
applications, including many of the most complex.
There is nothing wrong with custom parameters per se; however, when
they are substituted for standardized parameters that are already in
effect, they may harm the user and the industry in ways both subtle and
Reinventing the wheel
While some custom parameters help manufacturers come to grips with
unique problems, many others do not. New parameters are often created
simply because engineers and quality control personnel aren't familiar
with the large base of existing ones. This reinventing of the wheel
seems to be particularly prevalent outside the metalworking field,
where there is only a passing familiarity with surface finish
measurement. Some of these new parameters are merely redundant, while
others are somewhat less well-conceived than existing ones.
In the United States, it is often part designers and manufacturing
engineers who initiate the development of new parameters. The
responsibility for algorithm development, however, is often not in the
hands of a metrologist but those of a software engineer, who may not be
familiar with existing standards, appreciate their value, or have the
discipline or the specialized education to research and follow them.
While the PC has had a tremendously positive impact on manufacturing,
it has also made it just a little too easy to create algorithms for new
parameters and to incorporate those algorithms into the application
software of PC-based measuring instruments.
What's wrong with an overabundance of surface finish parameters?
Parameter proliferation may cause confusion among designers,
manufacturing engineers, machine operators and inspection personnel. It
can lead to economically significant disagreements between suppliers
and customers over what constitutes a proper testing method and what
kind of results are acceptable. Here are some examples:
Mean roughness depth. Rz is a widely used parameter to measure mean
roughness depth. Unfortunately, there are three different Rz parameters
still in use. When manufacturing engineers, machinists or parts
inspectors encounter the specification on a part print, they don't
necessarily know which one it refers to. This situation leads to all
sorts of confusion, some quite costly.
Ra and Rq roughness. There is frequent confusion over the Ra and Rq
roughness parameters. To start with, Ra used to be called AA or CLA,
and Rq used to be called RMS. Rq results happen to be 11 percent higher
than Ra results when measuring the test patch with a pure sine-wave
profile. However, there is actually no mathematical relationship
between the two parameters and, depending on the manufacturing process
and the resulting surface profile, the ratio between Rq and Ra can vary
from 20 percent to 200 percent (see Table 1).
Table 1: Ratio of Root Mean Square to Arithmetic Average Roughness
Nonetheless, there are numerous gages still used in the United States
that will "convert" actual Ra/AA measurement results to Ra /RMS results
by simply flipping a switch. However, the instruments are programmed to
apply a multiplication constant of 1.11, so the conversion is only
accurate when measuring a sine-wave test patch and inaccurate on all
real-world manufactured parts.
Some astute gage users noticed that the conversion occurred as a
constant factor, so they naturally concluded there was a direct
relationship between the parameters. Unfortunately, this has become a
commonly accepted fallacy. You'll occasionally find machinists or
inspectors who, when needing to measure the parameter other than the
one the gage measures, will manually apply the same multiplication
constant of 1.11 to make a conversion and get incorrect results with
There are numerous other examples of parameter confusion. As the
development of new standards has progressed, several basic terms and
parameters have changed "names" and symbols. In other instances, the
name of the parameter remained the same though the algorithm changed,
as is the case with Rz. Policies that require all in-use part prints to
be up-to-date with existing standards may help minimize confusion
within a company, but any time specifications are exchanged from one
company to another, there is a possibility for error.
What to do
The decision to develop new surface finish parameters should be
approached with great caution to avoid the introduction of superfluous,
or even erroneous, methods. The great majority of applications can be
satisfied with existing, standardized parameters, which should be used
As mentioned earlier, the decision to create a new surface finish
parameter is neither intrinsically wrong nor right. It all depends on
whether parameters that already exist are sufficient for the
application. Of course, it takes a bit of knowledge and some work to
identify the appropriate surface finish parameter. However, the
additional effort will eventually be rewarded, most likely with savings
in time and money and by avoiding some of the costly consequences of
"parameter confusion" outlined above.
On the other hand, if you have done a responsible search of existing
parameters and find them lacking for your purposes, by all means
customize one. Such a decision may prove to be far more than
self-serving. A technically sound new parameter developed by academic
or manufacturing entities to satisfy special requirements may
eventually find its way into broader use. This is good because it
improves the coherence of quality and engineering efforts worldwide.
In most cases, it's best to start out assuming that the parameter you
need already exists. Your organization, and the industries in which it
participates, will be all the better for it.
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