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The grit

The abrasive determines the quality and the time required for an abrasive processing of workpieces. This learning unit is part of a series of courses in which the abrasives are described in detail.
The selection of the abrasive grit in relation to the material is a decisive factor in the efficient and sensible use of abrasive tools. On the following pages you will find an introduction to the basics of the different grit types and their suitability for machining different materials.

Nicole Pinne-Wandmacher

Bauxite - a natural raw material

The raw material bauxite is composed of various clay minerals. It is mainly mined in opencast mines. Bauxite consists of aluminium oxides and aluminium hydroxides, which are often contaminated by other minerals.
The name bauxite comes from the first site "Les-Bauxed-Provence" where it was first discovered in 1821.

Production of corundum

Bauxite is melted in an electric arc furnace at about 2120 degrees Celsius. In this process the corundum is produced artificially. After melting, the material cools and is then broken in a further process. The individual abrasive grit is finally produced by increasingly fine crushing.

The "pure" corundum is only one of a total of four types of grit used on abrasives.
These grit types are:

  • Corundum
  • Zirconium corundum (ZK)
  • Ceramic Corundum (SG)
  • Silicon carbide (SIC)

Corundum (HEK)

Corundum is divided on the basis of its purity into:
Name Purity
Normal corundum 94 to 98%
Semi-precious corundum: 98 to 99%
Corundum approx. 99.5 to 100%
  • Normal corundum: purity approx. 94 to 98%.
  • Semi-precious corundum: purity approx. 98 to 99%.
  • Corundum: purity approx. 99.5 to 100%.

Slight contamination of the corundum increases its quality. Therefore, semi-precious corundum is usually used as abrasive. Corundum has a wedge-shaped appearance and grinds round under load.

Corundum (HEK)

Zirconium corundum (ZK)

Zircon corundum consists of corundum and zirconium oxide. Like corundum, it has a wedge-shaped form. However, it has a microcrystalline structure. Due to this structure, small parts of the grit break off again and again during grinding. Due to the new breaking edges that are created, the grit remains sharp and has a self-sharpening effect.

Zirconium corundum (ZK)

Ceramic Corundum (SG)

Like zirconia corundum, ceramic corundum has a self-sharpening effect due to its microcrystalline structure. However, it is more stable and pressure-resistant than zirconia alumina and wears microcrystalline. The microcrystalline wear ensures aggressive grinding over the entire service life of the abrasive belt. In order to ensure wear of the grit, correspondingly high contact forces are required, which can only be achieved with stationary machines.

Ceramic Corundum (SG)

Silicon carbide (SIC)

Silicon carbide has a strongly wedge-shaped form. Its surface is very hard and sharp-edged. These properties determine the range of application of silicon carbide. The sharpness is needed to work on "soft" materials that tend to avoid the cutting edge rather than being machinable (e.g. plastics, leather). The hardness is needed to machine hard materials (e.g. stone, glass).

Differentiation of grit types

The type of grit used depends mainly on the material to be processed. However, factors such as the grinding machine used, the contact elements and, last but not least, the price/performance ratio also determine the type of grit to be used. These factors to be taken into account are discussed in more detail on the following pages.

Hardness of the grit

  • If pressure is exerted on a material, it sooner or later gives way to the load.
  • The higher its resistance is, the harder the material is.
  • The harder the Grit, the more brittle and thus more fragile it becomes.
Abrasive Grain overview

Silicon carbide (SIC)
Corundum (HEK)
Zirconium corundum (ZK)
Ceramic Grit (SG)
Silicon carbide (SIC) is the hardest Grit

Grit toughness

In the context of abrasives, the toughness of the grit is the resistance of the grit to crack propagation. If the resistance is too low, the grit will wear out although there is still sufficient grit sharpness.

The toughness increases - from left to right - from silicon carbide (SIC) via corundum (HEK) to zirconium aluminium oxide (ZK) and ceramic grit (SG).

Example: wood processing

Using wood as an example, it can be shown that corundum is sufficient to achieve an ideal result. The corundum always has sufficient hardness and toughness to be able to process wood without wear of the Grit. Therefore, it is usually not necessary to use a higher quality, and therefore more expensive Grit.

Here you can find more information about wood sanding

Example: stainless steel processing

In the case of a sharp Grit, even with low pressure (and thus low load on the Grit), a high chip removal rate (removal volume per unit of time) is achieved.
If the Grit becomes dull, more and more energy goes into frictional heat. In order to still achieve the required chip removal rate, usually more pressure is exerted and resistance to crack formation is overcome. The application parameters must be set so that the loads on the abrasive Grit (the resistance of the Grit to cracking) overcome the toughness of the Grit as soon as the grinding pressure increases due to the abrasive Grit.

Here you can find more information about metall grinding