Grinding MCQ Quiz - Objective Question with Answer for Grinding - Download Free PDF

Explanation:

Planetary Internal Grinder Machine

• A planetary internal grinder machine is a specialized type of grinding machine used for grinding the internal surfaces of a workpiece. It employs a planetary motion, where the grinding wheel rotates around its own axis while simultaneously moving around the axis of the bore being ground. This dual motion allows for precise and efficient grinding of complex internal geometries.
• The planetary internal grinder machine operates by having the grinding wheel mounted on a spindle that rotates around its axis. This spindle is also part of a larger mechanism that rotates around the axis of the workpiece bore. This planetary motion enables the grinding wheel to cover the entire internal surface of the bore, ensuring uniform material removal and high precision.

Advantages:

• High precision and accuracy in grinding internal surfaces.
• Capability to grind complex and irregular shapes.
• Efficient material removal due to the dual motion of the grinding wheel.

Disadvantages:

• Complexity in design and operation, requiring skilled operators.
• Higher initial cost compared to simpler grinding machines.

Applications: Planetary internal grinder machines are commonly used in industries where precision grinding of internal surfaces is critical, such as in the manufacturing of bearings, gears, and hydraulic components.

Explanation:

Cubic Boron Nitride (CBN)

Definition: Cubic Boron Nitride (CBN) is a synthetically produced material that is used as an abrasive in grinding processes. It is known for its exceptional hardness, second only to diamond, and is primarily used in applications that require high precision and durability.

Properties: CBN is characterized by its high thermal stability, chemical resistance, and hardness. These properties make it an excellent material for cutting and grinding tools, particularly when working with hard and ferrous materials.

Manufacturing Process: CBN is produced by transforming hexagonal boron nitride (hBN) into its cubic form under high-pressure and high-temperature conditions. The process involves placing hBN in a press, subjecting it to pressures of around 5-10 GPa and temperatures of about 1500-2000°C. This transformation alters the crystal structure, resulting in the formation of CBN.

Advantages:

• Hardness: CBN is one of the hardest known materials, making it highly effective for grinding and cutting applications.
• Thermal Stability: It retains its hardness at high temperatures, which is crucial for maintaining performance during high-speed grinding processes.
• Chemical Resistance: CBN is chemically inert with ferrous materials, preventing reactions that could degrade the tool or the workpiece.
• Longevity: Tools made with CBN have a longer lifespan compared to those made with conventional abrasives, reducing the frequency of tool replacement and downtime.

Applications: CBN is widely used in various industrial applications, including:

• Grinding: CBN grinding wheels are used for precision grinding of hardened steels and superalloys.
• Cutting Tools: It is used in the production of cutting tools for machining hard metals.
• Finishing: CBN is used for finishing operations where high precision and surface quality are required.

Correct Option Analysis:

The correct option is:

Option 4: Cubic boron nitride

This option correctly identifies the full form of "CBN," which is Cubic Boron Nitride. CBN is an abrasive material used in grinding processes due to its exceptional hardness and thermal stability.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Calcium bi nitrous

This option is incorrect as "Calcium bi nitrous" does not refer to any known abrasive material used in grinding processes. It is not related to the term "CBN."

Option 2: Carbon boron nitrate

This option is incorrect because "Carbon boron nitrate" is not the correct full form of CBN. Additionally, it is not a material used in grinding processes. The correct term is Cubic Boron Nitride.

Option 3: Copper boron nitride

This option is incorrect as "Copper boron nitride" does not exist as a commonly known material in grinding processes. The term CBN specifically refers to Cubic Boron Nitride, not a compound involving copper.

Conclusion:

Understanding the correct full form of CBN is essential for recognizing its significance in industrial applications. Cubic Boron Nitride (CBN) is a critical material used in grinding and cutting tools due to its exceptional hardness, thermal stability, and chemical resistance. It is important to distinguish CBN from other materials and understand its specific properties and applications to appreciate its role in manufacturing and precision engineering.

Explanation:

Centerless Grinding:

• Centerless grinding is a machining process that uses abrasive cutting to remove material from a workpiece. Unlike traditional grinding processes that require the use of centers to hold the workpiece, centerless grinding holds the workpiece between two wheels – a grinding wheel and a regulating wheel – and a work rest blade. The process is typically used to create cylindrical parts with precise dimensions and smooth surface finishes.

Working Principle: In centerless grinding, the workpiece is supported by a work rest blade and is positioned between a high-speed grinding wheel and a slower-speed regulating wheel. The grinding wheel performs the cutting action, while the regulating wheel controls the rotational speed and the feed rate of the workpiece. The workpiece is rotated and fed axially through the wheels, allowing for continuous and efficient grinding.

Role of the Regulating Wheel:

• The regulating wheel is a key component in the centerless grinding process. It provides the necessary horizontal force to the workpiece, ensuring that it is held securely and fed through the grinding wheel at the correct speed and orientation. The regulating wheel's speed and angle of inclination can be adjusted to control the feed rate and the amount of material removed during the grinding process.

Advantages of the Regulating Wheel:

• Ensures consistent feed rate and rotational speed of the workpiece, leading to precise and uniform grinding results.
• Allows for continuous processing of workpieces without the need for manual intervention or repositioning.
• Enables the grinding of complex shapes and profiles with high accuracy and repeatability.

Explanation:

Average Chip Length in Grinding

• Grinding is a machining process that uses an abrasive wheel as the cutting tool. The average chip length in grinding is a significant factor as it impacts the surface finish, tool life, and material removal rate. The chip formation mechanism in grinding involves the interaction between the abrasive grains on the grinding wheel and the workpiece material. To understand the dependency of average chip length on the diameter, we need to delve into the mechanics of grinding and the role of different parameters.
• In grinding, the chip length is influenced by several factors, including the wheel speed, workpiece speed, depth of cut, and the diameter of the workpiece. The relationship between the chip length and the diameter can be derived from the fundamental principles of grinding mechanics.
• When considering the grinding process, the average chip length L can be related to the diameter D of the workpiece through a power law relationship.
• Empirical studies and theoretical models have shown that the average chip length in grinding is proportional to the square root of the workpiece diameter. This relationship can be mathematically expressed as:

Average Chip Length (L) ∝ √Diameter (D)

This means that if we increase the diameter of the workpiece, the average chip length will increase, but not linearly. Instead, it increases at a rate proportional to the square root of the diameter. This relationship arises due to the complex interaction between the abrasive grains and the workpiece material, as well as the distribution of cutting forces and heat generation during the grinding process.

Explanation:

The marking "S14 K 14S" on a grinding wheel follows the standard ISO marking system, which represents the wheel's specifications in a specific order. Here's the breakdown:

S – Abrasive type (S = Silicon Carbide).

14 – Grain size (grit number, 14 = coarse grain).

K – Grade (hardness, K = medium-hard bond strength).

14 – Structure (spacing between abrasive grains, 14 = open structure).

S – Bond type (S = Silicate/Silicate bond).

Explanation:

Glazing: When a surface of the wheel develops a smooth and shining appearance, it is said to be glazed. This indicates that the wheel is blunt, i.e. the abrasive grains are not sharp.

• Glazing is caused by grinding hard materials on a wheel that has too hard a grade of bond. The abrasive particles become dull owing to cutting the hard material. The bond is too firm to allow them to break out. The wheel loses its cutting efficiency.
• Glazing of grinding wheel is more predominant in hard wheels with higher speeds. With softer wheels and relatively lower speeds, this effect is less prominent.

Concept:

Abrasive grains are held together in a grinding wheel by a bonding material. The bonding material does not cut during the grinding operation. Its main function is to hold the grains together with varying degrees of strength. Standard grinding wheel bonds are silicate, vitrified, resinoid, shellac, rubber and metal.

Rubber bond (R): 

• 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.

Silicate bond (S): 

• This bonding material is used when the 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. 
• This is the softest bond in grinding wheel.

Vitrified bond (V): 

• Vitrified bonds are used on more than 75 per cent of all grinding wheels.
• Vitrified bond material is comprised of finely ground clay and fluxes with which the abrasive is thoroughly mixed.

Resinoid bond (B): 

• Resinoid bonded grinding wheels are second in popularity to vitrified wheels.
• The phenolic resin in powdered or liquid form is mixed with the abrasive grains in a form and cured at about 360F.

Shellac bond (E): 

• 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 operations.

Metal bond (M): 

• Metal bonds are used primarily as binding agents for diamond abrasives.
• They are also used in electrolytic grinding where the bond must be electrically conductive.

Explanation:

• Grinding involves an Abrasive action and while removing material abrasive also wears out and when the rubbing force reaches the threshold, the worn-out abrasives are pulled out of the wheel.
• Thereby giving chance to a fresh layer of abrasives for removing material. This is known as the self-sharpening behavior of the grinding wheel.
• The ratio of the volume of material removed to the volume of wheel wear is known as grinding ratio.

\(Grinding\;ratio = \frac{{{V_m}}}{{{V_w}}} = \frac{{l \times b \times d}}{{\frac{\pi }{4} \times w \times \left( {D_i^2 - D_f^2} \right)}},\;where\;w = width\;of\;wheel\)

• The grinding ratio varies from 1.0 - 5.0 in very rough grinding.

Explanation:

Abrasives are classified into two categories:

Natural Abrasives:

• Garnet, Corundum, Emery (impure corundum), Calcite (calcium carbonate), Diamond dust, Novaculite, Pumice, Rouge, Sand, Sandstone, Tripoli, Powdered feldspar, Staurolite

Synthetic Abrasives:

• Boron carbide, Borazon (cubic boron nitride or CBN), Ceramic, Ceramic aluminium oxide, Ceramic iron oxide, Dry ice, Glass powder, Steel abrasive, Zirconia alumina, Slags

Concept:

A grinding wheel consists of the abrasive that does the cutting, and the bond that holds the abrasive particles together.

A standard marking system is used to specify and identify grinding wheels.

The following is the sequence of arrangement:

Abrasive type – Grain size – Grade of bond – Structure – Bond type

The number ‘46’ specifies the average grit size in inch mesh. For a very large size grit, this number may be as small as 6 whereas for a very fine grit the designated number may be as high as 600. 

• Abrasive type: ‘A’ for aluminium oxide, ‘C’ for silicon carbide
  • A =  Aluminium oxide
  • B = Cubic boron nitride
  • C = Silicon carbide
  • D = Diamond
• Grain size: They are indicated by a number ranging from 10 (coarse) up to 600 (very fine)
• Grade of bond: The grades range from ‘A’ indicating light or ‘soft’ bond to ‘Z’ indicating a firm or ‘hard’ bond
• Structure: This structure is indicated by a number from 1 to 12. The higher numbers indicate a progressively more open structure
• Bond type: V – Vitrified, S – Silicate, B – Resinoid, R – Rubber, E – Shellac, O – Oxychloride

Explanation:

Centre Lathe → Knurling

Milling → Slotting

Grinding → Dressing

Drilling → Counter-boring

Knurling

Knurling is the operation of producing a straight-lined, diamond-shaped pattern or cross lined pattern on a cylindrical external surface by pressing a tool called knurling tool. Knurling is not a cutting operation but it is a forming operation.

A lathe is used for many operations such as turning, threading, facing, grooving, Knurling, Chamfering, centre drilling

Counter - boring

Counter - boring is an operation of enlarging a hole to a given depth, to house heads of socket heads or cap screws with the help of a counterbore tool.

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.

The dressing is the operation of cleaning and restoring the sharpness of the wheel face that has become dull or has lost some of its cutting ability because of loading and glazing.

Slot Milling:

Slot milling is an operation of producing slots like T - slots, plain slots, dovetail slots etc.

Explanation:

Grinding:

• Grinding is the process of removing metal by the application of abrasives which are bonded to form a rotating wheel. When the moving abrasive particles contact the workpiece, they act as tiny cutting tools, each particle cutting a tiny chip from the workpiece.
• It is a common error to believe that grinding abrasive wheels remove material by a rubbing action; actually, the process is as much a cutting action as drilling, milling, and lathe turning.

Grain size:

• The grain or grit size of your grinding wheel influences the material removal rate and the surface finish and the grain size varies from 8 to 600 (8 is coarse and 600 is very fine).
• The grinding wheel grain size controls the possible amount of depth of cut. Bigger grain size protrudes more on the grinding wheel periphery or face resulting in a higher depth of cut and smaller grains protrude less resulting in a lower depth of cut. Hence the size of the chip is fine in the case of smaller grain size wheels.
• Large grain size grinding wheel is used for ductile materials.

Explanation:

1. Grinding: Grinding is an abrasive machining process that uses a grinding wheel or grinder as the cutting tool. Grinding is a subset of cutting, as grinding is a true metal-cutting process. Grinding is very common in mineral processing plants and the cement industry. Grinding is used to finish workpieces that must show high surface quality and high accuracy of shape and dimension.
2. Honing: Honing machines are metal abrading tools and process utilizing hard tooling and perishable abrasives stones. The hone process was developed to allow for perfection of bore geometry, size control, final surface finish and surface structuring. The honing process provides the final sizing and creates the desired finish pattern on the interior of tubing or cylinder bores. Finishing is accomplished by expanding abrasive stones of suitable grit and grade against the work surface. 
3. Burnishing: Burnishing is the process of rubbing metal with a small hard tool, which can be either a ball type or roller type, to compact the surface. It is a very useful finishing technique that can increase the workpiece surface finish as well as add microhardness.
4. Buffing: Buffing is a rotating cloth wheel that is impregnated with fine abrasive compounds, and it produces a bright-luster finish on metal and composites. Buff wheels are impregnated with liquid rouge or a greaseless compound-based matrix of specialized fine abrasive called compound. The compound is sprayed or pressured into the rotating buffing wheel. The buff wheel acts as the carrier of the compound, which ultimately does the surface finishing.

Explanation:

Grinding-

• Grinding is an abrasive machining process is performed by means of a rotating abrasive wheel, vaguely similar to a milling cutter.
• Grinding wheels are composed of many small grains of abrasive particles bonded together, each acting as a miniature cutting point.
• Cutting tools constituted by projected abrasive particles.

The bulk grinding wheel – workpiece interaction can be divided into the following:

• Grit – Workpiece interaction (Forming chip)
• Chip – bond interaction
• Chip – workpiece interaction
• Bond – workpiece interaction

Except for the grit workpiece interaction which is expected to produce a chip, the remaining three undesirably increases the total grinding force and power requirement. Therefore, efforts should always be made to maximize grit-workpiece interaction leading to chip formation and to minimize the rest for the best utilization of the available power.

Generally abrasive properties like hardness, toughness and resistance to fracture uniformly abrasives are classified into two principal groups:

Natural abrasives: The natural abrasives are emery and corundum. These are impure forms of aluminium oxide.

Artificial abrasives: Artificial abrasives are silicon carbide and aluminium oxide.

• Silicon Carbide: It is less hard than diamond and less tough than aluminium oxide; It is used for grinding of the material of low tensile strength like cemented carbide, stone and ceramic, grey cast iron, copper, brass, bronze, aluminium, vulcanized rubber, etc
• Aluminium Oxide: Aluminium oxide is tough and fracture-resistant; It is preferred for grinding of materials of higher tensile strengths like steel; high carbon and high-speed steel and tough bronze.

Applications of abrasive grains:

Explanation:

The code ‘V’ represents the bond.

Designation of Grinding Wheel:

Prefix / Suffix: These are the secret codes used by the manufacturers to represent the wheel by its size and shapes respectively.

Type of Abrasives / Grain type:

• It indicates materials used for the manufacturing of abrasive particles.
• Out of the abrasives B₄C is giving the poor performance during machining and diamond is very costly, therefore Al₂O₃ or SiC is the most commonly bused abrasives in the grinding wheel.
• Al₂O₃ soft and tougher than the SiC whereas SiC will be hard and brittle than Al₂O₃
• The type of abrasive is selected based on the mechanical properties of workpiece material I.e. for machining of soft and ductile workpieces, Al₂O_3, and machining of hard and brittle workpiece SiC will be used.   
• A- Al₂O₃, B – B₄C, C – SiC, D - Diamond

Grain size or Grit size:

• It indicates the size of abrasive particles.
• i.e. Side if abrasives = 1/ Grain Size Number(GSN)
• when the GSN > 600, the size of the abrasive particles becomes very very small and it cannot act like a cutting tool, therefore MRR is less.
• When GSN < 600, the actual size of abrasive is increasing, the chip size is increasing and MRR is increasing.
• As the GSN is reducing or the size of abrasive is increasing, the MRR is increasing first and then reducing.
• The grain size is selected based on the surface finish required on the workpiece i.e. for a rough grinding, course or medium grain size is selected and for finished grinding fine or very fine grain size will be selected.
• 10-24= Coarse, 30-60 = Medium, 80 -180 = Fine, 220 – 600 = Very fine

Grades of Grinding Wheel:

• It indicates the hardness of the grinding wheel.
• The grade of the grinding wheel is selected based on the mechanical properties of the workpiece material.
• Soft wheels are used for grinding of hard workpiece because the rubbing forces induced by the blunt abrasive particle i.e. the self-sharpening is taking place and no dressing is required.
• Hard wheels are used for grinding of the soft workpiece, the abrasive particle will be effectively utilized so that at the end of effective utilization the dressing will be carried for resharpening of grinding wheel.
• A –H = Soft, I – P = Medium, Q – Z = Hard

Structure:

• The structure is indicating the average gap between the two consecutive abrasive particles.
• As the average gap is large, the number of abrasive particle presents per unit area will be small hence it is called the open structure.
• The structure of a grinding wheel can be varied by varying the % of abrasive particles and bonding material in the manufacturing of a grinding wheel. i.e. when higher % of abrasives and lower % of bonding material is used in manufacturing it produces the dense structure and vice-versa.
• 0 – 7 = Dense, 8 – 16 = Open

Bonds:

• Bond indicates the bonding material used for the manufacturing of the grinding wheel.
• Out of the different bonding materials, vitrified is the most commonly used bonding material because it gives higher bonding strength, high temperature withstanding capability, and high thermal conductivity.
• For the manufacturing of flexible grinding wheels also called buffing wheels, shellac or rubber can be used as the bonding material.
• V – Vitrified, B – bakelite, S – Silicate, E – Shellac, R - Rubber