GRINDING

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GRINDING:

GRINDING In grinding process an emery or corundum wheel is used as the cutting tool. Emery and corundum are naturally found abrasives and are impure form of aluminium oxide Al2O3. A grinding wheel is made up of thousands of tiny abrasive particles embedded in a matrix called the ‘bond’ An abrasive is an extremely hard material second in hardness only to diamond.

PRINCIPLE OF GRINDING:

PRINCIPLE OF GRINDING The edges of the abrasive particles project out of the periphery of the grinding wheel, and when grinding wheel rotates, each one of the particles acts like a tiny cutting tool machining away the material from the surface of the work piece. To the naked eye the cut material looks like metal dust mixed with powder from the grinding wheel. However, seen under magnifying lens, the metal dust shows all the characteristics of metal chips produced in other machining processes. The grinding process is truly a chip producing machining process.

PURPOSE OF GRINDING:

PURPOSE OF GRINDING The grinding process is capable of producing very accurate sizes, equally accurate geometry like flatness or circularity and extremely good surface finish. The grinding wheel is capable of machining hardened steel and even hardened high speed steel, which cannot be done by other machining processes. Machining materials which are too hard for other machining processes such as tool and die steels and hardened steel materials Close dimensional accuracy of the order of 0.3 to 0.5 m High degree of surface smoothness such as Ra = 0.15 to 1.25 m.

DEFECT IN GRINDING:

DEFECT IN GRINDING When a grinding wheel is applied to the workpiece, the sharp edges of the abrasive grains which are cutting, will eventually loose their cutting effect and become dull. At that stage, the abrasive grain should either split and form new edges or it should break away from the wheel exposing the next layer of grains to do their work. If the dulled grains stay in the wheel, they simply keep on rubbing on work without actually cutting. This defect is known as ‘glazing’

CLASSIFICATION OF WHEEL:

CLASSIFICATION OF WHEEL ABRASIVE GRIT BOND GRADE WHEEL STRUCTURE

CHOICE OF ABRASIVES :

CHOICE OF ABRASIVES Emery and corundum, are no longer used in modern grinding wheels. Instead artificially manufactured abrasives are used due to their high purity. These abrasives are Silicon Carbide, Aluminious oxide, Al2O3 Cubic Boron Nitride (CBN) Diamond

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Silicon carbide is greenish black in colour, whereas aluminous oxide is reddish brown. Silicon carbide is harder and more brittle than alumina. For this reason, it is used for grinding materials of low grinding resistance like cast iron, brass, copper etc. Aluminium oxide abrasive is more suitable for grinding most steels because of its greater toughness to cope with increased grinding resistance offered. The code for silicon carbide is C and for Al2O3 wheels it is A

GRIT:

GRIT Grit indicates, the size of abrasive grain. It is indicated by a number. Higher the number, smaller the size of grains. Abrasives finer than 200, are called ‘‘flours’’ designated as F , FF , and FFF . These and finer abrasive ‘flours’ are used by jewellers. For fine finish of ground surface, smaller grit size abrasive wheels are used. But their metal cutting capacity is limited. With larger size abrasive wheels, finish is rough but metal removal rate is higher.

Grain Size:

Grain Size

BOND AND GRADE:

BOND AND GRADE Bond refers to the substance of which the matrix of the grinding wheel is made. The degree of hardness possessed by the bond is called the grade of the wheel and indicates the strength of the grip with which the abrasive grains are held in the bond. The following bonds are generally employed in manufacture of grinding wheels: Vitrified Silicate Synthetic resin Rubber Shellac Metal

PowerPoint Presentation:

1. Vitrified bond: It is denoted by letter V and about 80% of the wheels used in the industry are of this bond. 2. Silicate bond: It is denoted by letter S and silicate of soda (commonly known as water glass) is the main constituent of this bond. 3. Shellac bond: It is denoted by letter E and shellac (a naturally available material) is the main constituent of the bond. 4. Rubber bond: Here the abrasive in kneaded in rubber and the wheels are moulded from this material. Denoted by letter R . 5. Resinoid bond: These wheels are made from bakelite and other resinous material. It is denoted by letter B .

GRADE:

GRADE The bond hardness or grade is usually represented by the letters of English alphabet. A represents very soft grade Z is very hard M and N represent medium grade hardness.

WHEEL STRUCTURE:

WHEEL STRUCTURE The proportion of bond material in a wheel varies from about 10% to 30% of its total volume. Structure of wheel depends upon this percentage. If abrasive grains are too tightly packed, the percentage of bond material will be on the lower side. This is called a closed structure. If the abrasive grains are less tightly packed in the same volume, the wheels are said to have an open structure. The structure is indicated by a number varying from 1 (very closed structure) to 15 (very open structure).

WHEEL SELECTION:

WHEEL SELECTION

MOUNTING OF WHEELS:

MOUNTING OF WHEELS

WHEET TRUING:

WHEET TRUING

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