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Abrasives
Abrasives and their applications are one of the largest single topics and can only be discussed in excerpts in the limited space provided by this encyclopedia. Natural abrasives were formed in primeval times by volcanic melting fusion. Even to this day, natural corundum and emery are used on occasion, although their structural composition was purely coincidental and the resulting abrasive performance leaves a lot to be desired. At the turn of the century artificially manufactured abrasives were made in electric furnaces. For this purpose silicon carbide crystals were produced by melting quartz sand and coke. Another process was discovered by which alumina was transformed into aluminous oxide and then into electric corundum, also using the electric fusion method. Special fused alumina is derived from very pure bauxite.
Thus, we have a number of basic abrasives which need to be attached to a backing material in order to make them technically useful.
Bond
Organic as well as anorganic bonding agents are used. Among the anorganic bonding agents are clay, feldspar, china clay and quartz sand which are molded, compressed and baked together with the abrasive grain. Generally, the bonding agents determine the approved maximum circumference speed for the abrasive tool
Organic bonding agents which can be found for instance in fiber-backed resin-bonded discs bond the abrasive grain with phenolic resin.
Grit size
The size of the abrasive grit is largely responsible for the quality of the sanding result. The unit of measurement mesh is used to describe the grit size. This unit of measurement is based on a screen which has a certain number of openings (mesh) for each inch of length. 8 openings per inch select abrasive grains with a diameter of approximately 2 mm. The mesh number is specified for all abrasives to describe the grit size, with 8 being the coarsest and 1200 the finest grade.
The following table shows which grit size you can use for which sanding application:
- coarse sanding 8, 10, 16, 20, 24, 30
- medium sanding 30, 36, 40, 46, 50, 60
- fine sanding 80, 100, 120
- super- fine sanding 220, 240, 300, 400
- lapping 400, 1200
Rough grinding and cutting discs have a grit size ranging between 16 and 30.
Hardness
Abrasive grains are extremely hard; if they were not, the grains would be unsuitable for sanding. When referring to abrasives, hardness usually means the hardness of the bond. A soft abrasive consists of extremely hard grains which can break easily from their bonding. The term bonding hardness would be more appropriate. It is indicated by a sequence of letters:
- E, F, G very soft
- H, I, J, K soft
- L, M, N, O medium
- P, Q, R, S hard
- T, U, V, W very hard
- X, Y, Z extremely hard
Structure
This term refers to the distribution of the abrasives on the carrier material. The more the structure is open, the better the heat dissipation and the cooler the sanding process.
The texture is indicated by a sequence of numbers:
1-4 dense texture (many
abrasive grains)
5-7 standard texture
8-10 open texture
11-14 very open texture
15-18 highly porous texture
All abrasive tools should include these specifications. The labelling of abrasive tools is governed by DIN 69120.
BOSCH cutting discs and rough grinding discs have coloured labels with the following meaning:
- green = for stone
- blue = for steel
An additional diagonal band indicates the approved maximum peripheral speed:
red = approved maximum circumference speed: 80 m/sec
blue = approved maximum circumference speed: 45 m/sec
Abrasives are available in many different shapes and for a wide variety of applications, such as rough grinding discs or cutting discs, conical or flat sanding discs, grinding points, grinding wheels, cup wheels, sanding sheets, emery paper, sanding linen, sanding belts, grinding rings, fan wheels and many more. Abrasive pastes can be applied like polishing agents and distributed with the sander to assist in the grinding process.
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