Types of abrasives

Abrasives are used for grinding and polishing operations. It should have uniform physical properties of hardness, toughness and resistance to fracture. Abrasive may be classified into two principal groups. 

1. Natural abrasives 2. Artificial abrasives 

Natural abrasives

The natural abrasives are obtained from the Earth’s crust. They include sandstone, emery, corundum and diamond. Sandstone is used as abrasive to grind softer materials only. Emery is natural alumina. It contains aluminium oxide and iron oxide. Corundum is also a natural aluminium oxide. It contains greater percentage of aluminium oxide than emery. Both emery and corundum have a greater hardness and abrasive action than sandstone. Diamond is the hardest available natural abrasive. It is used in making grinding wheels to grind cemented carbide tools. 

Artificial abrasives 

Artificial abrasives are of two types. 1. Silicon carbide abrasives 2. Aluminium oxide abrasives 

Silicon carbide 

Silicon carbide is manufactured from 56 parts of silica, 34 parts of powdered coke, 2 parts of salt and 12 parts of sawdust in a long rectangular electric furnace of the resistance type that is built of loose brick work. There are two types of silicon carbide abrasives - green grit and black grit. Silicon carbide is next to diamond in the order of hardness. But it is not tough enough as aluminium oxide. It is used for grinding materials of low tensile strength such as cemented carbides, ceramic materials, grey brass, bronze, copper, aluminium, vulcanized rubber etc. This is manufactured under trade names of carborundum. It is denoted by the letter ‘S’. 

Aluminium oxide 

Aluminium oxide is manufactured by heating mineral bauxite, silica, iron oxide, titanium oxide, etc., mixed with ground coke and iron borings in arc type electric furnace. Aluminium oxide is tough and not easily fractured, so it is better adapted to grinding materials of high tensile strength such as most steels, carbon steels, high speed steels, and tough bronzes. This is denoted by the letter ‘A’.