GRINDING MACHINE: Types of ABRASIVES used in GRINDING WHEEL

GRINDING MACHINE

Grinding is a process of removing material by the abrasive action of a revolving wheel on the surface of a work piece, in order to bring it to the required shape and size.

Grinding is similar to other machining operations since the material is removed in the form of very small chips, similar to those obtained in other machining operations. The wheel used for performing the grinding operation is known as “Grinding Wheel”. It consists of sharp crystals, called abrasives, held together by a binding material or bond .It may be a single piece type or several segments joined together. In most of the cases, it is a finishing operation and a very small amount of material is removed from the surface during the operation.

ABRASIVES: 

It is the material of the grinding wheel, which does cutting action. These are extremely hard materials, consisting of very small particles, called grains, which carry a number of sharp cutting edges and corners. 

They are two types. 

1). Natural 

2). Artificial

Natural Abrasives: 

They are obtained directly from mines. The common natural abrasives are sand stone, emery, corundum, Quartz and diamond.

All the natural abrasives, except diamond are now obsolete. Sand stone is used only for sharpening wood working tools. All other natural abrasives are almost replaced by artificial abrasives. 

Diamond, still retains its place even in modern grinding processes. It is largely used for dressing grinding wheels and for grinding hard materials.

Artificial Abrasives: 

They are manufactured under controlled conditions in closed electric furnaces to avoid impurities and to achieve necessary temperature for the chemical reaction to take place. The main artificial abrasives are:

1. Silicon Carbide (Sic): 

It is made from Silicon dioxide, coke, sawdust and salt. These constituents are mixed together and piled up around carbon electrical conductor of a resistance type electric furnace. A heavy current is switched on and temperature of about 26000generated. The mass, under the action of intense heat, fuses. The outer shell is removed and the Silicon Carbide Crystals are broken in to grains.

2. Aluminum Oxide: 

This abrasive is very hard and tough grains having sharp cutting edges.

It is obtained by fusing impure Aluminum Oxide (Bauxite) in an electric arc furnace. Dry bauxite is mixed with ground coke and Iron chips. This mixture is heated in the furnace with a heavy current. It is then crushed and the powdered grains are formed are screened through standard meshes. These grains are not as hard as Silicon Carbide but less brittle preferred for grinding metals of high tensile strength like hardened tool steel components.

3. Artificial Diamonds: 

The diamonds produced through artificial means are quite comparable to the natural diamonds in their grinding characteristics and give normally better results than the natural diamonds.

Advantages and uses of artificial abrasives

The manufactured or artificial abrasives superseded the natural abrasives for the following reasons:

1. The controlled conditions in the electric furnace enable uniformity in the product.

2. The quantity of production and supply can easily be varied according to the demands.

3. They have largely abolished the dependence on natural means to meet the growing demand in the modern manufacturing processes.

The selection of a particular abrasive is governed by many factors, like hardness, toughness and other properties of work material.

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