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 the surface of stainless steel 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 on stainless steel?
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 average of the peaks and valleys of the metal surface, including the deviation from the mean line. That can be 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 Ra is a useful average, there are various surface profiles that could still have the same Ra value, like in the image below.
Another value 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.
How to achieve a certain roughness 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. The brush makes 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 the surface harder to clean.
A smooth surface on the contrary is 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 the smoothness decreases the risk of 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.
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 surface roughness, while the surface becomes slightly rougher than before the anodizing process.
Source: Euro Inox 2014, Roughness measurements of stainless steel surfaces.
Partners: Hermes Schleifmittel / Werkzeuge and 3M