For better categorisation, all abrasives are classified according to their grain size. The grain classification is based on the FEPA standard (Federation of European Producers of Abrasives, the non-commercial organisation of European abrasives manufacturers). The FEPA grain standards are divided into:
- 26 macro grain levels
- 11 micro grain levels
- Grain specification with regard to shape, size distribution and chemical properties
This applies to the abrasive materials:
- Boron nitride (cubic)
- Boron carbide
- Silicon carbide
Within the FEPA standard, a distinction is made between:
- Rigid abrasives such as grinding wheels, for example F180
- Flexible abrasives such as sandpaper and belts, for example P120
The above breakdown applies to coatings made of silicon carbide or corundum. In deviation from the letter additions F and P, abrasives with a diamond coating with letter identifier D and boron nitride coatings with letter identifier B are identified. Grain size classification and grain size designation are not only regulated according to FEPA, but also in ISO 6106.
The coating – also called scatter, coating density or surface density – provides information regarding the number of abrasive grains scattered on a surface per unit area. A distinction is made between:
Areas of application
Close coating: Substrate almost completely covered with abrasive grains, therefore high material removal.
Carbide (metal) working
Semi-open coating: Substrate is about 75 percent coated with abrasive grains. This leaves enough space between the abrasive grains so that enough abraded material can be removed from the work surface.
Schleifen von Kunststoffen, NE-Metallen, Hartholz oder Lacken
Open coating: Between 50 and 70 percent of the surface is coated with abrasive grains. This means that the chip clearance around the abrasive grains is larger, so that both grinding dust and residual material can be easily absorbed and removed. Another advantage is that less pressure is required for the same stock removal rate.
Bearbeitung von weichen, klebenden oder langspanenden Werkstoffen, wie zum Beispiel Nadelholz
- FEPA (European Federation of Producers of Abrasives) – determines the grain size for Europe by the mesh size of the sieves. The larger the number of meshes, the finer the grain.
- JIS (Japanese Industrial Standard) – commonly used in connection with increasingly widespread Japanese knives and cutting tools and the correspondingly growing number of Japanese grindstones for these. Also applies to water stones from Japanese and Chinese production.
- ANSI (American National Standards Institute) classification system of the American standards institute.
- MM – (micro-mesh grain), in English-speaking countries, the grain is determined according to the number of sieve openings per inch (unit: mesh).
- MICRON (μ) – grain size in μm.
Average grain sizes (μm)
Average grain sizes (μm)
The smaller the specified FEPA value of the grain, the coarser it is. A coarser grain means more stock removal, but the surface roughness of the workpiece increases. Grinding stones with a coarse grain, for example in the FEPA range of 300, remove a lot of material. It follows the principle of the harder the processing material and the smoother the surface result, the finer the grain size that must be selected.
There’s currently no globally standardised procedure for determining the grain size in the tool industry, but there is a procedure recommended by FEPA (sieve analysis).