1. Grinding and Grinding Machines
Unit 5

2. Grinding Wheel
A grinding wheel is a self-sharpening tool composed of discrete abrasive grains held together by a bonding agent with composite structure of many clearance allowance for the cutting edges. The characteristics of a grinding wheel depends upon the combined elements of abrasive, grit size, grade, structure and bond.
The main components of grinding wheel are:
1.Abrasive
2. Bond
3. Pore

3. 1.Abrasive
The abrasive grain is the element that actually performs the cutting activity in the grinding process. And the choice of abrasive grain depends on the material to be ground.
2.Bond
The role of bond is to hold the individual grains together. The type of bond depends on the operating speed of wheel, the type of operation and the surface finish required.
3.Pore
The exists between grains and bond. In order to provide chip clearance, air space(pore) must be existed between grains and bond. Dense spacing is denoted by low numbers and open spacing by high numbers.0

4. Abrasives: The abrasives are of two types: Natural Abrasives
Artificial or manufactured abrasives
Natural Abrasives:
Natural abrasives have been in use right from the time of the ancient Greeks and Romans, who used hard, rough stones and emery to polish precious gems, marble and metals. Some of the important abrasives used through the ages and in modern times as well are corundum, diamond, garnet, pumice, talc, quartz and sandstone. Sandpaper, a very inexpensive abrasive, uses granulated sand stuck on a paper base. Diamond is the most expensive abrasive. It was in the early 20th century that research on synthetic abrasives began to gather pace. The importance of synthetic abrasives as a better alternative to natural materials came to the fore after the invention of synthetic diamonds.

5. Artificial Abrasives:
They are manufactured under controlled conditions in closed electric furnace in order to avoid the introduction of impurities and to achieve the necessary temperature for the chemical reaction to take place. The main artificial abrasives are :
Silicon Carbide (SiC) : It is made from silicon dioxide (or pure white quartz), coke, sawdust and salt. These constituents are mixed together and piled up around the carbon electrical conductor of a resistance type electrical furnace. A heavy current is switched on and a temperature of around 2600 deg Celsius is generated. The mass, under the action of intense heat, fuses and the following chemical reaction takes place :
SiO2 + 3C = SiC + 2CO
The silicon of the quartz combines with the carbon of coke to form silicon carbide. The salt vaporizes to form chlorides with metallic impurities present and thus help in removing them. The saw dust burns and provides porosity to the mass to allow the escape of gases. The silicon carbide crystals thus forms.

6. 2. Aluminum Oxide (Al2O3) : This abrasive carries very hard and tough grains having sharp cutting edges. It is obtained by fusing the pure aluminum oxide, called Bauxite, in an electric arc furnace. For this, the dry bauxite is mixed with ground coke and iron chips. This mixtures is charged into the furnace and a heavy current is passed through it. The mass fuses on account of excessive heat generation and a block of aluminum oxide is formed.
The iron scrap acts as a flux, collects up the impurities and settles at the bottom of the furnace. Thus the aluminum oxide crystal form after cooling. They are preferred for grinding metals of high tensile strength.
3. Artificial Diamond : The diamond produced through artificial means are quite comparable to the natural diamonds in their grinding characteristic and gives normally better results than the latter.

7. Silicon Oxide & Aluminum Oxide Abrasive Powders

8. Bonds: There are several types of bonds as explained below :
Vitrified bonds
Resin bonds
Silicate bonds
Shellac bonds
1. Vitrified Bonds: Vitrified bonds are used on more than 75 percent of all grinding wheels. Vitrified bond material is comprised of finely ground clay and fluxes with which the abrasive is thoroughly mixed. The mixture of bonding agent and abrasive in the form of a wheel is then heated to 2,400°F to fuse the materials.
Vitrified wheels are strong and rigid. They retain high strength at elevated temperatures and are practically unaffected by water, oils or acids. One disadvantage is that they exhibit poor shock resistance. Therefore, their application is limited where impact and large temperature differentials occur.

9. 2.Resinoid bonds: Resinoid bonded grinding wheels are second in popularity to vitrified wheels. Phenolic resin in powdered or liquid form is mixed with the abrasive grains in a form and cured at about 360F. Resinoid wheels are used for grinding speeds up to 16,500 SFPM. Their main use is in rough grinding and cut-off operations.
3.Silicate bond: This bonding material is used when heat generated by grinding must be kept to a minimum. Silicate bonding material releases the abrasive grains more readily than other types of bonding agents. Speed is limited to below 4,500 SFPM.
4. Shellac bond: It's an organic bond used for grinding wheels that produce very smooth finishes on parts such as rolls, cutlery, camshafts and crankpins. Generally, they are not used on heavy-duty grinding operation.
5. Rubber bond: Rubber-bonded wheels are extremely tough and strong. Their principal uses are as thin cut-off wheels and driving wheels in centerless grinding machines. They are used also when extremely fine finishes are required on bearing surfaces.

10. Grain or Grit
The term ‘Grain’ or ‘Grit’ denotes the approximate size of the abrasive particles and gives an idea of the coarseness or fineness of the wheel. A grinding wheel may have the abrasive particles of same or different sizes. The choice of grain depends upon many factors such as quality of finish, amount of stock material to be removed. Coarse wheels are suitable for grinding soft and ductile materials whereas hard and brittle materials are best ground with finer grit wheels.

11. Grade
The term ‘Grade’ indicate the strength of bond in a wheel, i.e. , the power of the abrasive particle to hold together and resist disintegration under the cutting pressure. Higher the proportion of bond for a specified quantity of abrasive particles the harder will be the wheel. The abrasive grains of a soft wheel will be easily broken away from the bond whereas the hard wheel will retain these particles for a longer period. The selection of a particular grade of wheel is largely governed by nature of work, its composition, its hardness, etc.
The machine conditions also plays an important role in this selection. Where vibrations are likely to occur harder wheels are employed.

12. Structure
The structure of a grinding wheel refers to the relative spacing of the abrasive grains; it is the wheel's density. There are fewer abrasive grains in an open-structure wheel than in a closed-structure wheel. A number from 1 to 15 designates the structure of a wheel. The higher the number, the more open the structure will be; and the lower the number, the more dense the structure will be.

14. Selection of Grinding Wheel:
Selection of proper grinding wheel is a vital necessity to obtain the best results in grinding work. A wheel may be required to perform various different functions like quick removal of stock material, give a high class of surface finish, maintain close dimensional tolerance and a single wheel will fail to meet all the requirements. It is therefore, necessary, that proper grain size, bond, grade, strength, shape and size of the wheel should be selected to meet the specific requirements of a job.
The factors upon which the above selections will depend are as follows :
Properties of the material to be machined, i.e. , its hardness, toughness, strength, etc.
Quality of surface finish required.

15. 3. Grinding allowance provided on the workpiece, i. e
3. Grinding allowance provided on the workpiece, i.e. , the amount of stock material to be removed.
4. Dimensional accuracy required.
5. Method of grinding, i.e. , wet or dry.
6. Rigidity, size and type of machine.
7. Relative sizes of wheel and job.
8. Type of grinding to be done.
9. Speed and feed of the wheel.

16. Work Holding Device : Magnetic Chuck
(a)Round or Circular type
(b)Rectangular Type

17. Introduction: The first type is a electromagnetic chuck.
1. It implies that the use of electric current for developing a strong electromagnet which hold the job centrally in the chuck. Although, many design of these chucks are prevalent, still the rotary type is in common use.
2. One notable feature of this type of chuck is that it is not capable of withstanding heavy cuts on the job it grips and also that the job to be held in this chuck should be of iron or steel. They are also called as electromagnetic chuck.
The other type of magnetic chuck is the permanent magnet type in which very powerful magnets are incorporated.
1.For this, no electric current is needed.
2.It is important to note that the job held in a magnetic chuck will also be magnetizes and to neutralize this effect, some demagnetizer should be employed otherwise the part may create difficulty when assembled.
Two popular designs are depicted in the previous page.

18. Principle:
The principle involved in the working of these chucks is that the magnetic flux is produced either by electro-magnets or by permanent magnets, due to which the work is held. When the operating lever brings the magnet to ‘ON’ position the flux created inside the chuck passes through the work and hence, holds it.
While being in ‘off’ position, the magnets are shifted to such locations that they come in contact with keepers, which are highly permeable. Thus, the passage of the flux is short circuited and, instead of passing through the workpiece, the flux now passes through the keepers. The workpiece, therefore, is no more in the grip of the chuck and is released.

19. Centerless Grinding Operation:

20. Principle:
Centerless grinding is a machining process that uses abrasive cutting to remove material from a workpiece. The workpiece is secured between two rotary grinding wheels, and the speed of their rotation relative to each other determines the rate at which material is removed from the workpiece.

21. Principle:
In centerless grinding, the workpiece is held between two grinding wheels, rotating in the same direction at different speeds, and a work holding platform. One wheel, known as the grinding wheel (stationary wheel in the diagram), is on a fixed axis and rotates such that the force applied to the workpiece is directed downward, against the work holding platform. This wheel usually performs the grinding action by having a higher tangential speed than the workpiece at the point of contact. The other wheel, known as the regulating wheel (moving wheel in the diagram), is movable. This wheel is positioned to apply lateral pressure to the workpiece, and usually has either a very rough or rubber-bonded abrasive to trap the workpiece.
The speed of the two wheels relative to each other provides the grinding action and determines the rate at which material is removed from the workpiece. During operation the workpiece turns with the regulating wheel, with the same linear velocity at the point of contact and (ideally) no slipping. The grinding wheel turns faster, slipping past the surface of the workpiece at the point of contact and removing chips of material as it passes.

22. Advantages of centerless grinding:
The need of centering and use of fixture, etc, is totally avoided.
It can be applied to both external and internal grinding.
The workpiece is supported rigidly during the operation and can be subjected to heavy cuts, resulting in a rapid and more economical grinding.
Once the setup has been made, it is faster method than centre type grinding.
A very little maintenance is needed for the machine.
Very high skilled operators are not needed for operating centerless grinders.

23. Loading & Glazing of Grinding wheel:
Glazing: After continuous use of the grinding wheel the sharp points (cutting points) of the abrasive grains become dull. They lose their cutting ability and sharpness and are severely worn out. Consequently, the wheel face becomes smooth and it, instead of biting into work material, provides a sort of rubbing action only. The above phenomenon which renders the wheel unuseful, is called Glazing of grinding wheel, and is more predominant in hard wheels at higher speeds. With softer wheels and relatively lower speeds, this effect is less prominent. The grinding wheel so effected is called a glazed wheel.
Loading: Another problem associated with the grinding wheels in operation is the adherence of the cut particles of the work material to the face of grinding wheel. These particle occupy the open space between the cutting points. Due to this, the sharpness of the cutting point is lost and the face becomes smooth, depriving the wheel of its cutting ability. This phenomenon is known as Loading of wheel and the affected wheel is loaded wheel. This effect is seen more prominently with those wheels which carry a hard bond, when softer materials are ground and while slower cutting speeds are used.

24. Trueing and dressing of grinding wheel :
Trueing: When the grinding wheel is mounted to the grinding wheel spindle, the run-out on wheel operating surface is removed, the wheel during contour grinding is trued or worn grinding wheel is corrected. Thus, these operations are for the trueing.
Dressing: When the sharpness of grinding wheel becomes dull because of glazing and loading, dulled grains and chips are removed (crushed or fallen) with a proper dressing tool to make sharp cutting edges and simultaneously, make recesses for chips by properly extruding to grain cutting edges. Thus, these operations are for the dressing.

25. Trueing & Dressing

26. Use of coolants:
The cutting fluid or coolants perform the same function in grinding as in other operations. Mostly use coolants include the water solutions and emulsions. Oils are also used. The use of a suitable coolant is needed in grinding for the following reasons:
To reduce the excessive heat generated during the operation and avoid its extreme localization.
To maintain uniform temperature, in order to prevent distortion of the jobs and breakage of the wheel.
To prevent the metal chips from clogging into the grain spaces and, thus, avoid loading the wheel face.
To drive away the local abrasive grains and chips, so that they may not scratch against the finished surface and spoil it.

27. Thank You……