Background

Abrasives are those materials used in operations such as grinding, polishing, lapping, honing, pressure blasting or other similar process. Abrasives come in different particle or grit sizes depending on how much material needs to be removed.

Materials used for abrasives are generally characterised by high hardness, and moderate to high fracture toughness.
How They Work

Each hard abrasive particle acts like a single point cutting tool. With hundreds if not thousands available in a small area, the effect they produce is quite significant.

Coarser grades/grits of abrasive are used where high volumes of material need to be removed, such as in coarse polishing, large scratch removal or operations requiring significant shape or dimensional change. Finer grades are generally used after coarser grades to produce a higher surface finish than are possible with coarse grades.
Materials Used for Abrasives

Some materials that are used as abrasives include:

· Silicon carbide, generally used for non-ferrous metals

· Aluminium oxide or alumina, the most widely used abrasive, generally used for ferrous alloys, high tensile materials and wood.

· Diamond, most often used in ceramic grinding or final polishing due to high hardness and cost

· Cubic boron nitride (CBN)

· Garnet, usually used for machining of wood.

· Zirconia/Alumina alloys, suited to carbon and stainless steels and welds

· Glass, usually used for pressure blasting operations.

· Colloidal silica, generally used for finishing operations as it is only available as a suspension of extremely fine particles.
Abrasive Sizes

The following table is a guide to the coarseness of various abrasive grit sizes.

Grit Size


Relative Coarseness

8-24


Coarse

30-60


Medium

70-180


Fine

220-1200


Very Fine
Types of Abrasives
Bonded Abrasives

These consist of abrasive grits that have been mixed with binders and formed in useful shapes, most often by pressing. They are fired to high temperatures to induce bonding via materials that forms a glassy matrix, hence the name vitrified bonded abrasives. Common shapes include wheels, cylinders, blocks and cones.
Coated Abrasives

Coated abrasives are those which are made up of abrasive grits bonded to flexible substrates such as paper, cloth, fibre or film. Bonding is via resins, glues of combinations of the two. Examples include belts, sheets and flap disks.
Non-Woven Nylon Abrasives

Random 3-dimensional networks of open weave nylon materials coated with abrasive. Versatile items that are suited to many applications, coming in forms such as pads, wheels and brushes.
Metal Bonded Abrasives

Some abrasives, most notably diamond are held together in a metal matrix in the form of a precision grinding tool. They may take the form of cutting wheels or hole cutting tools for machining of tiles, glass and other ceramic materials. In these items, the metal matrix is designed to wear away to expose the abrasive media, although “dressing” may be required to remove build up of abrided materials from the grinding surface of the tool.
Burs

Burs are generally small in size used for fine machining and precision operations. They are similar to metal bonded and usually have teeth on the cutting faces. Abrasives often employed include tungsten carbide and diamond. Typically used for enlarging of holes.
Lapping Compound

Sometimes referred to as lapping or polishing powder. Usually a very fine grit material used in fine polishing and finishing operations. It can be supplied in dry powder or paste forms.