BOND MATERIALS USED IN GRINDING WHEEL: Vitrified, Silicate, Oxychloride, Resinoid, Shellac, Rubber ~ MECHTECH GURU

BOND MATERIALS USED IN GRINDING WHEEL: Vitrified, Silicate, Oxychloride, Resinoid, Shellac, Rubber

BOND MATERIALS

In order to give an effective and continuous cutting action, it is necessary that the grains of abrasive material should be held firmly together to form a series of cutting edges.

The material used for holding them is known as bond. The principal bonds are:

1. Vitrified

2. Silicate

3. Oxychloride

4. Resinoid

5. Shellac

6. Rubber

1. Vitrified bond: 

It is a clay bond, reddish brown color. The base material is “Felspar” which is fusible clay. Proper proportions of Felspar, refractories and flux mixed thoroughly with abrasive grains to form a paste. The paste is placed is moulds to get the shape of a wheel and air dried. The wheels become enough hard are fed in to kiln at 12600and allowed to remain there for few days. This process is known as fusing and it provides uniform distribution of bond through and the wheel. After this, the wheels are trimmed to the required size. For obtaining very hard and close grained wheels, the paste after being place in the moulds, is pressed under hydraulic pressure.

Advantages of Vitrified bond

1. It is made porous and enables quicker metal removes.

2. It is not affected by water, oil, acids, temperature or climatic conditions.

3. The bond itself is very hard and acts as an abrasive.

4. On account of excessive heat in the kiln the impurities are burnt and only bond and abrasive left.

5. The structure of the wheel is uniform due to wet mixing of different constituents.

Disadvantages of Vitrified bond

1. The process of manufacture is very slow.

2. Cracks may develop in large size wheels during fusing.

3. Wheels over 750 mm dia can’t be easily produced.

4. Proper control during fusing becomes difficult.

5. High temperature in kiln tends to make the abrasive grains weak.

2. Silicate bond: 

It s base material is Silicate of soda. The process of mixing, moulding, packing or ramming, drying etc are done in the same way as vitrified bond, but the oven carries a temperature of about 2600only. The application of lower temperature results in high tensile strength. As usual, the paste mixture after moulding is subjected to hydraulic pressure if hard and close grained wheels are needed. They are light grey in color. 

These wheels are used where a cool cutting action with less wear is needed as in grinding the edges of the heat treated steel cutting tools. The cool cutting action is due to the bond releases the abrasive grains more quickly than vitrified bond.

Advantages of Silicate bond

1. It is more rapid process than vitrified bond.

2. Because of the moderate temperature in kiln, there is no tendency to weaken the grains.

3. Fusing is better controlled, results in more reliable bond.

4. When wet grinding is performed, the soda acts as a lubricant.

5. Large wheels up to 1500 mm dia can be easily produced.

6. The cutting action of the wheel is smoother and cooler.

7. Because of low fusing temperature the wheel can be moulded on Iron backs, which is not possible in case of vitrified bond.

Disadvantages of Silicate bond

1. Extra hard wheels cannot be produced with this bond.

2. Harder grades of this bond do not provide a free cutting action.

3. Oxychloride bond: 

It is a mixture of Oxide and Chloride of Magnesium and setting takes place in cold state. The process of wheel manufacture is similar to the above two, but no heating and subsequent cooling is required on account of the cold setting property. Ageing is necessary so that the bonded wheel gets adequate hardness. 

This bond provided a cool cutting action, but grinding is usually done dry as it is very susceptible to the action of conventional coolants and therefore, the full use of the cutting capability of the wheel cannot be taken.

4. Resinoid bond: 

It is a synthetic organic compound, which is enough strong and flexible. It provides a sharp cutting action and enables a high rate of stock removal at high speeds.

Mainly used for cutting bar stocks, fine grinding of cams, precision grinding of rolls etc.

These wheels are manufactured from a mixture of abrasive grains, synthetic resins and some compounds. This mixture is filled in moulds and then fed in to the furnace for heating. A constant temperature of about 20000is maintained in the furnace.

Due to heat, the resin sets and binds the abrasive grains together. The shape and size of the bonded wheels will depend upon the shape and size of the mould.

5. Shellac bond: 

These wheels are produced by mixing the bond and the abrasive grain in a steam heated mixer, followed by moulding under pressure in steam heated moulds. These wheels are cool cutting and are vastly used on hardened tool steel and thin sections. They also help in producing high surface finish, as is normally required on components like cam shafts.

They can run safely in water but use of oil or caustic soda should be avoided.

6. Rubber bond: 

It is composed of fairly hard vulcanized rubber. The common manufacturing process consists of passing of rubber and sulphur through the mixing rolls and adding the abrasive grains slowly as the above two constituents pass through the rolls.

Adding of abrasive grains continues till the required proportion is achieved. The mixture is then passed through another set of rolls to obtain the required thickness. The wheels are then cut and placed in preheated moulds and vulcanized under pressure. These wheels are quite strong, close grained and can be made in very thin sections. They are mainly used where a very high class surface finish with close dimensional accuracy is required. During the operation water can be safely used as a coolant, but caustic soda and oil should not be used as caustic soda will disintegrates the bond and the oil will softens it.

Symbols used for representing the type of bond for grinding wheel.

Vitrified- V

Silicate -S

Oxychloride- O

Resinoid -B

Shellac- E

Rubber -R


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