Grain types

The term grain types in the area of abrasives signifies the different types of minerals used during the manufacture of abrasive tools. Since the essential portion of the grinding process, namely the machining of the material to be processed, is performed by these minerals, the grain type used has a major impact on the suitability range and performance of a abrasive.

An overview of the different grain properties and the typical wear characteristics of the most commonly used grain types is given in the diagram below.

Grit

Grit is responsible for stock removal. When selecting the correct
grit two properties are particularly important:

Bond
Toughness

All Klingspor grit types are produced synthetically. This ensures a consistently high quality product. Four different grit types are used with coated abrasives:

Silicon carbide
Aluminium oxide
Zirconia alumina
Ceramic aluminium oxide

Grit classification in relation to bond, toughness and their primary applications

Grit Types	Bond/Toughness	Structure	Properties / wear properties
Silicon carbide	very hard / less tough	crystalline	sharp-edged, brittle, highly friable / micro-wear
Aluminium oxide	hard / tough	crystalline, irregular	wedge-shaped, block grain, even wear
Zirconia alumina	hard / very tough	crystalline, even	wedge-shaped, block, drop formed grit / micro-wear, self-sharpening
Ceramic aluminium oxide	hard / very tough	micro-crystalline	sharp-edged, pointed grit / micro-wear, self-sharpening

Grit type properties and their specific wear properties

In addition, there are also so-called grit agglomerates. This is not an independent grit type, rather it is a grit agglomeration, which is comprised of many individual aluminium-oxide or SIC-grits mixed with a resin bonding agent, which together form a large grit. Agglomerates are used almost exclusively on abrasive belts. The advantage of agglomerates is that they provide an even finish from the first use to the last. Dull grit breaks off from the agglomerate and creates space for new, sharp grit. The objective is to attain an even finish through continual stock removal and an extremely long service life.

Coating

Moreover, the coated abrasives are coated with different grit coverage densities. The term “grit density” describes the grit coverage density distributed on the backing. One differentiates between:

close coating
semi-open coating
open coating

Open coated grit has a large space between the individual grits, such that the stock removal and the grinding dust can be extracted
more easily from the grinding surface. This prevents premature
clogging when working on long-chipping materials, such as wood.
The decision as to which grit density to use is primarily dependent
upon how great the likelihood is that the grinding dust will clog the
space between the grits.

Historically, grain types fall into the two major groups of natural and synthetic grains. Subsumed in the category of natural grain types are, in particular, flint, garnet and emery; however, these grain types are barely used any more in the production of abrasives. Instead, the abrasives produced today are usually made with synthetic grain types such as aluminium oxide, zirconia alumina, silicon carbide and ceramic aluminium oxide. Compared to their natural siblings, synthetic grain types offer significant advantages in the areas of hardness and toughness; they, furthermore, offer greater uniformity across their properties, which is the defining attribute that qualifies them for demanding industrial applications in the first place.
In order to offer their customers a broad range of possible applications and perfect suitability for their specific purpose, Klingspor sells abrasives (belts, rolls, sheets, discs, strips, fibre discs, resinoid cutting-off wheels and grinding discs, abrasive mop discs, abrasive mop wheels and abrasives with an elastic bond) with different grain types. This approach ensures that customers will always achieve the perfect grinding result by selecting the grain type that is right for the material and work piece they need to process and the result they wish to accomplish.

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