When working in metal finishing industries, you have most certainly heard about surface roughness. It measures the irregularities on the surface and is one of the characteristics of the surface texture, besides lay and waviness. The friction and wear of sheet metal is depending on the surface texture, thus is it important to examine.
How does a machined surface look like?
When you would zoom in on the surface of metal, it could look like the image below, with its peaks and valleys. This is called a profile.
How to measure surface roughness of metal?
There are numerous parameters to measure a profile’s roughness, of which the value Ra is the most common. Ra measures within a certain sampling length the arithmetic average of the peaks and valleys of the metal surface, including the deviations from the mean line. Measuring surface roughness is done with a profilometer or laser scanner. The greater the deviations, the rougher the surface and if the Ra is small, the surface is smooth. The Ra will be calculated in micrometers (µm) or microinches (µin.).
Although the roughness average (Ra) is a useful average, there are various roughness profiles that could still have the same roughness average, like on the image below.
Another roughness parameter is the Rz, which measures the difference between the highest peak and lowest valley within the sampling length of five lines. It shows the maximum height of the profile.
Moreover the Rq value can be used for the description of surface roughness. It measures the root-mean-square deviation of a profile.
Metal surface roughness measurement; example
In the video below (at 00:25), you can see how to measure roughness of a surface with a profilometer. The measuring instrument should always be placed across the lines, not along them. The measurement can slightly vary depending on where you place the meter, because the Ra is quickly influenced by deeper valleys or higher peaks. On the print you can see the measured surface roughness of the sampling length.
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How to achieve a certain Ra on stainless steel?
The right surface roughness can be achieved with the use of abrasive belts. The Ra and Rz value is depending on the grit size of the belt, which leads to the following results:
As can be seen, the finer the grit size, the smaller the Ra value. To get an even smoother surface, it is an option to add a brush from non-woven abrasive fleece after the belt. The brushes remove the remaining high peaks from the metal and make the surface smoother, like is shown below.
Why is surface roughness important?
A rough surface has a lot of irregularities and is more sensitive to friction, because if offers resistance to moving objects on the surface. Moreover, dirt could be stuck in the valleys of the metal and that makes it harder to clean.
Smooth surfaces on the contrary, are easy to clean, because of the smooth profile. That is also one of the reasons why surface roughness plays a great role in industries such as food processing and pharmaceutical industries. The surface is more hygienic and it decreases bacterial adhesion.
A rough surface is more likely to wear and has a larger amount of friction. The high friction coefficient means that there is more force needed to slide, than for a smooth surface finish.
In order to make the aluminium products’ surface more resistant to wear and corrosion, the electrolytic process, called anodizing, is used. It increases the thickness of the oxide layer on aluminium parts. However, this anodizing does affect the Ra, since the surface becomes slightly rougher than before the anodizing process.
Feel free to contact us whenever you have questions regarding our machines and your requirements for the roughness of the surface.
Source: Euro Inox 2014, Roughness measurements of stainless steel surfaces.
Partners: Hermes Schleifmittel / Werkzeuge and 3M