Surface roughness – roughness value (Ra)
The perfect measure for grinding and polishing
When grinding and polishing stainless steel, aluminium, brass etc., we have to take into consideration the roughness value. People can decide whether something looks nice or neat, but it will always remain subjective. With the roughness value (Ra) this becomes measurable. The purpose often dictates the required Ra value. For instance, the food industry often applies an Ra value of <0.8 and the pharmaceutical industry stays well below 0.4.
Generally speaking, stainless steel or other materials should have a low roughness value so that it is easy to clean and it better repels dirt and bacteria. Bacteria and microorganisms fare well on rough surfaces, porosity, micro cracks and other uneven surfaces. By clearing a surface from these organisms by sanding and grinding you can lay the foundation for a good surface roughness. After grinding it clean, you go through the various grits from coarse to fine, to reach a low surface roughness. A low roughness value has a smaller chance of corrosion: something you always want to have for all products and processes.
Sometimes a high roughness value is desired, for instance if you want to enhance adhesiveness when gluing two stainless steel panels together. These can be made rougher by roughly grinding the stainless steel. They will then attach to a base layer or facade. Another example for when a high roughness value is desired, is for the purpose of a non-slip surface. This can be done by grinding the material very roughly.
Surface roughness is often expressed in a roughness value, but this also has its drawbacks. Besides the roughness value (Ra), there are also the so-called Rt and Rz values. A professional surface roughness meter indicates all three of these values.
Fig. 1 The surface roughness is measured with a roughness meter. Here we see the Ra value, Rt value and Rz value.
The roughness values are measured by sliding a probe, a small sensitive needle, over the surface area you want to measure. It measures the high and low points on the surface. So if the material is ground, it measures the impression that the grinder left. This also means you can get different roughness values, depending on the type of sandpaper you use. A coarse grit makes a deep impression in the material and yields a high roughness value. Not desirable, if the goal is to reach a low Ra value. Often, the material should first be ground clean or get rid of its mill scale, and this is impossible with a fine grit.
The various surface roughness values
Ra value: As mentioned before, this is the most common one. It is an average value of all deviations of a straight line within the measured length.
Rt value: This indicates the biggest distance between the high and low point of the impression.
Rz value: This indicates the average of the sum of the five highest points and the five deepest impressions on the measured surface.
Fig. 2 A measurement report of the roughness values gives a good insight in the roughness of the material.
Surface area roughness and the Clean room
For many years, Van Geenen has been cleaning metals and plastic products for high-end industries such as high-tech & semiconductors, food, the medical and pharmaceutical industry and more as separate assignments, but also in combination with grinding and polishing activities. It goes without saying that products with a low surface roughness are easier to clean. Ideally, your products are first brought down to the right Ra value before they are cleaned, everything under one roof. Van Geenen offers various possibilities with no size restrictions.
- Large space for pre-cleaning and spray cleaning
- Large cleansing baths
- Large ultrasonic baths
- Large clean room space for assembling products under clean room conditions.
- Cleaning and packaging at Grade 2 and 4 level
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Fig. 3 After your product has the correct Ra value, we can immediately clean it in our ultrasonic baths, among other things.
Are you interested in our services?
Do you have questions regarding the surface roughness for your product? Are you interested in our services and products, but still have some pressing questions? Please feel free to contact us. You can also request a quotation for stainless steel grinding and polishing. We are happy to draft a voluntary offer for you. You can contact us by sending an email to: email@example.com or call us on: +31 (0) 548 – 543793.