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Converting Common Surface Texture Measurements

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The irregularity of a machined surface is the result of the machining
process including the choice of tool, feed and speed of the tool,
machine geometry and environmental conditions. This irregularity
consists of high and low spots machined into a surface by the tool bit
or a grinding wheel. These peaks and valleys can be measured and used
to define the condition and sometimes the performance of the surface.
There are more than 100 ways to measure a surface and analyze the
results, but the most common measurement of the mark made by the tool
or the surface texture is the roughness measurement.

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There are several different methods of roughness measurement in use
today. The method used on any given part depends largely on where in
the world the part is manufactured and the measurement parameters the
manufacturer and the customer prefer to use. It is not uncommon for
different parties involved in the production to use different methods
for roughness measurement. In this article we will talk about only two
of the many methods of roughness measurement, how to convert between
these two methods and how to avoid the problems caused by the
inevitable use of more than one roughness measurement.

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In North America, the most common parameter for surface texture is
Average Roughness (Ra). Ra is calculated by an algorithm that measures
the average length between the peaks and valleys and the deviation from
the mean line on the entire surface within the sampling length. Ra
averages all peaks and valleys of the roughness profile, and then
neutralizes the few outlying points so that the extreme points have no
significant impact on the final results. It’s a simple and effective
method for monitoring surface texture and ensuring consistency in
measurement of multiple surfaces.

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In Europe, the more common parameter for roughness is Mean Roughness
depth (Rz). Rz is calculated by measuring the vertical distance from
the highest peak to the lowest valley within five sampling lengths,
then averaging these distances. Rz averages only the five highest peaks
and the five deepest valleys, therefore, extremes have a much greater
influence on the final value. Over the years, the method of calculating
Rz has changed but the symbol Rz has not. As a result, there are three
different Rz calculations still in use and it is very important to know
which calculation is being defined before making the measurement.

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There are rules
of thumb that can help clear up the confusion and convert Ra to Rz or
Rz to Ra. If the manufacturer specifies and accepts the Rz parameter
but the customer uses the Ra parameter, using a ratio range for
Rz-to-Ra = 4-to-1 to 7-to-1 is a safe conversion. However, if Ra is
used as an acceptance criteria by the manufacturer but the customer
accepts Rz to evaluate the part, then the conversion ratio would be
much higher than 7-to-1, possibly as high as 20-to-1. Keep in mind that
the actual shape of the part’s profile will have a significant impact
on these ratios.

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The approximate and sometimes questionable comparisons can be avoided
by developing an understanding of exactly what a parameter means, and how the various parties involved in the production plan to
check the surface. The best way for those involved in the production to be
in agreement on the parameters for measurement, is to have capable
evaluation equipment in both the manufacturer’s and customer’s facility
making the same check using the same method. If the manufacturer or the
customer uses conversion ratios, then both parties should be aware of
the use of the ratio and be comfortable with the ramifications.

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– Tech Tip courtesy of Mahr Federal Inc. Rz = Ra; Rmax

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