1. What is Grinding process?
- In the grinding process, the cutting tool used is of emery or corundum.
- Emery and corundum are naturally found abrasives and are impure forms of Aluminium oxide Al2O3.
- A grinding wheel consists of tiny abrasive particles embedded in a matrix called the ‘bond’.
- An abrasive is an extremely hard material.
- The cut material looks like metal dust mixed with powder from the grinding wheel to the naked eye.
- The metal dust shows all the characteristics of metal chips produced in other machining processes if seen under a magnifying lens.
- The grinding process is capable of producing accurate sizes, flatness or circularity, and extremely good surface finish.
- The grinding wheel is capable of machining hardened steel and even hardened high speed steel.
2. What is glazing?
- The sharp edges of the abrasive grains which are cutting, will eventually lose their cutting effect and become dull when a grinding wheel is applied to the workpiece.
- At that stage, the abrasive grain should either split or form new edges or it should break away from the wheel exposing the next layer of grains to do their work.
- If the grains have become dull due to excess use they lose the ability to cut the material.
- This defect is known as ‘glazing’.
3. Which are the most widely used abrasives nowadays?
- The modern grinding wheels do not contain Emery and Corundum.
- Instead, artificially manufactured abrasives are used due to their high purity.
These abrasives are (a) Silicon carbide, and (b) Aluminium oxide, Al2O3
- Silicon carbide is greenish-black in colour, whereas aluminium oxide is reddish-brown.
- Silicon carbide is harder and more brittle than alumina.
- For this reason, it is used for grinding materials like cast iron, brass, copper, etc.
- Most steels are grinded with the help of aluminium oxide 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.
For more details on DRILLING PROCESS
4. How are wheels classified?
The classification of the wheels is as follows:
- The size of abrasive grain is indicated by the grit in the form of the number.
- If the number of grit is higher, the size of the grains is smaller.
- Flours are the abrasives which are finer than 200 and designated as F, FF, and FFF.
- These and finer abrasive ‘flours’ are used by jewellers.
- Smaller grit size abrasive wheels give fine finish but the metal cutting capacity is limited.
- With larger size abrasive wheels, the finish is rough but the metal removal rate is higher.
b. 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 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 bond
- Silicate bond
- Shellac bond
- Rubber bond
- Resinoid bond
c. WHEEL STRUCTURE
- The bond material amount in a wheel varies from about 10% to 30% of its total volume.
- If abrasive grains are too tightly packed, the percentage of bond material will be on the lower side which 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).
d. WHEEL SHAPES
Grinding wheels are made in a wide variety of shapes to suit the immense range of work and special features of machine tools on which the wheels shall be put to use. Many common shapes are shown in Fig. 1.
Fig. 1 Grinding wheel shapes
5. What are the different types of grinding process?
- Surface grinding process.
- Cylindrical grinding process.
- Internal grinding process.
- Centerless grinding process.
- Contour grinding process.
- Gear grinding process.
- Thread grinding process.
6. What is cylindrical grinding process?
Fig. 2 Plain cylindrical grinder
- Cylindrical grinding process takes place on a machine which is made in two varieties ‘‘plain’’ and the ‘‘universal’’ type.
- The basic design is the same in both cases, but the universal machine can be adopted for internal grinding operations as well.
- In cylindrical grinding process, the work is mounted between two centers and is rotated.
- The spindle consist of the grinding wheel and the wheel revolves at much higher r.p.m. than the work.
- The work centers are mounted on a table that can traverse at various feeds so that the entire length of the work passes to and fro in front of the wheel.
- The depth of the cut is very small, about 0.015 mm.
- When the entire length of work has passed in front of the wheel, the wheel advances forward by another 0.015 mm at the end of the traverse and so the cycle of machining goes on until the desired diameter of the workpiece is reached.
- The result is a long cylinder of a perfectly circular profile with a very fine surface finish.
7. What is internal grinding?
Fig. 3 Internal grinding
- Internal grinding operation basically is the grinding of internal holes or bores.
- The principle of internal grinding is shown in Fig. 3.
- Internal grinding helps to grind the surface of bores whether they are plain or tapered with the help of a small grinding wheel.
- This process improves the geometry of the hole as well as the surface finish.
- A softer wheel is used for internal grinding.
8. What is surface grinding process?
Fig. 4 Methods of surface grinding process
- A flat surface can be ground in many ways with a grinding wheel.
- Some possible configurations are illustrated in Fig. 4.
- Flat surfaces may be ground either by using the periphery of a disc wheel or by grinding with the end of a cup-shaped wheel.
- These methods can be further sub classified according to the method of feeding the work to the wheel.
- The method of using disc wheels entails the use of a horizontal spindle grinding machine.
- The cup wheels may be used in conjunction with either a horizontal or vertical spindle machine.
9. What is the recommended wheel speed for different grinding operations?
The recommended wheel speed for different grinding operations is as follows:
a. Cylindrical grinding process: 2000 metres/minutes
b. Internal grinding process: 700–1000 metres/minutes
c. Surface grinding process: 1200–1600 metres/minutes
d. Cutting off with rubber, shellac and bakelite wheels: 3000–4000 metres/minutes
10. Which coolant is used in grinding process?
- In the grinding operation, a lot of heat is generated. Hence an effective coolant is used.
- The most common coolant for grinding process is water in which the same soda ash has been dissolved.
- There should be a copious flow of coolant at the work-wheel interface.
- The coolant also washes away the ground chips and swarf.
- The coolant must not have lubricant in it; otherwise, it may lead to the glazing of wheels.
11. Mention the advantages of grinding process.
- Lesser investment for setup.
- Simple to use.
- Skilled labour is not required.
- Surface finish would be approximately 10 times better than the other conventional processes like milling and turning.
- Better dimensional accuracy.
- This process can be applied to any surface of the material whether it is hardened or non hardened.
12. Mention the disadvantages of grinding process.
- Tool cost is higher.
- Costly process.
- High amount of material removal cannot be done.
- Time required is more.
- Cautiously the part must be grinded to avoid any damages.