Machining Processes and Machine Tools MCQ Quiz - Objective Question with Answer for Machining Processes and Machine Tools - Download Free PDF

Explanation:

Function of Coolant in Drilling Operation

A coolant is an essential component in drilling operations, playing a pivotal role in ensuring the efficiency, accuracy, and longevity of both the tool and the workpiece. In the context of machining processes, including drilling, coolants are fluid substances applied to the cutting zone to achieve multiple objectives, primarily focusing on thermal regulation and lubrication.

When a drill bit penetrates a workpiece, the friction between the cutting edges of the tool and the material generates significant heat. This heat, if not managed, can lead to various issues such as tool wear, dimensional inaccuracies, and even damage to the workpiece. Additionally, the cutting edges of the drill bit endure substantial mechanical stress, which can further exacerbate wear and reduce tool life. To mitigate these challenges, a coolant is employed.

Key Functions of Coolant in Drilling:

• 1. Heat Reduction: The primary function of a coolant is to dissipate the heat generated during the drilling process. As the drill bit rotates and cuts through the material, friction and plastic deformation produce heat. The coolant absorbs this heat and carries it away from the cutting zone, maintaining a stable temperature. This prevents thermal damage to both the tool and the workpiece, ensuring dimensional accuracy and surface finish.
• 2. Improved Tool Life: By reducing the temperature at the cutting edges, the coolant minimizes thermal wear and prolongs the life of the drill bit. Excessive heat can cause the tool material to lose its hardness and cutting efficiency, leading to premature failure. Coolants help maintain optimal tool performance over an extended period.
• 3. Lubrication: In addition to cooling, many coolants also serve as lubricants. They reduce friction between the tool and the workpiece, lowering cutting forces and minimizing mechanical wear. This dual action of cooling and lubrication ensures smoother drilling operations and better tool performance.
• 4. Chip Removal: During drilling, chips are produced as the material is removed from the workpiece. These chips can obstruct the cutting zone, leading to poor surface finish and increased tool wear. Coolants help flush away these chips, keeping the cutting area clean and ensuring uninterrupted drilling.
• 5. Prevention of Work Hardening: In some materials, excessive heat can cause the surface to harden, making subsequent machining operations more difficult. By maintaining lower temperatures, the coolant prevents work hardening and facilitates smoother machining processes.

Correct Option Analysis:

The correct option is:

Option 2: To reduce heat and improve tool life.

This option accurately describes the primary function of coolants in drilling operations. Coolants play a critical role in reducing the heat generated during the cutting process, thereby protecting the tool from thermal wear and extending its lifespan. By maintaining a stable temperature and providing lubrication, coolants enhance the efficiency and effectiveness of the drilling operation.

Important Information

Analysis of Other Options:

• Option 1: To remove burrs from the hole: While coolants can assist in flushing out chips and debris, their primary function is not to remove burrs. Burr removal is generally achieved through secondary processes like deburring or finishing operations.
• Option 3: To increase spindle speed: Coolants do not directly influence spindle speed. Spindle speed is a parameter set based on the material, tool, and desired cutting conditions. Coolants primarily aid in thermal management and lubrication.
• Option 4: To clean the machine table: Cleaning the machine table is not a function of the coolant used in drilling operations. While some residual coolant may spill over and clean adjacent areas, this is incidental and not its intended purpose.

Explanation:

Cross slide:

• The cross slide is a critical component of a lathe machine, allowing precise lateral movement of the cutting tool. This movement is essential for operations such as facing, parting, and taper turning. The cross slide is mounted on the saddle and moves in a direction perpendicular to the axis of the workpiece. The mechanism that enables this movement is the feed screw, which is turned by the handwheel.
• The handwheel is a manually operated wheel fixed to one end of the feed screw. When the handwheel is rotated, it causes the feed screw to turn. The feed screw, in turn, moves the cross slide along its guideways. The handwheel provides the operator with precise control over the position of the cross slide, enabling accurate machining operations. This is especially important for tasks that require high levels of precision, such as machining intricate parts or achieving tight tolerances.
• The feed screw is typically a square-threaded screw designed to handle high loads and provide smooth motion. Square threads are commonly used in machine tools because they offer low friction and high strength, making them ideal for transmitting motion and force.
• In summary, the handwheel is a crucial component for operating the cross slide, allowing the operator to move the slide accurately and perform various machining tasks efficiently.

Additional Information

Mains Parts of lathe machine:

• Headstock: The headstock is usually located on the left side of the lathe and is equipped with gears, spindles, chucks, gear speed control levers, and feed controllers.
• Tailstock: Usually located on the right side of the lathe, the workpiece is supported at the end.
• Bed: The main parts of the lathe, all parts are bolted to the bed. It includes the headstock, tailstock, carriage rails, and other parts.
• Carriage: The carriage is located between the headstock and the tailstock and contains an apron, saddle, compound rest, cross slide, and tool post.
• Lead Screw: Lead screw is used to move the carriage automatically during threading.
• Feed Rod: It is used to move the carriage from left to right and vice versa.
• Chip Pan: It is present at the bottom of the lathe. A chip pan is used to collect the chips that are produced during the lathe operation.
• Hand Wheel: It is the wheel that is operated by hand to move the cross slide, carriage, tailstock, and other parts which have a handwheel.

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Explanation:

Direct Cost Analysis:

• The wages of a welder fabricating a structure is an example of a direct cost. Direct costs are those expenses that can be directly traced to a specific product, service, or activity. In this case, the welder is actively involved in fabricating a specific structure, meaning their wages can be directly attributed to that particular task. Direct costs are typically variable costs, as they vary depending on the level of activity or production.

Characteristics of Direct Costs:

• Traceability: These costs can be easily and directly traced to a specific cost object, such as a product, project, or department.
• Variability: Direct costs often vary with the level of production or activity. For example, if the number of fabricated structures increases, the total wages paid to welders will also increase.
• Examples: Direct costs typically include wages of workers directly involved in production, raw material costs, and specific operational expenses tied to a project.

Explanation:

Soluble oil:

• Soluble oil is a type of cutting fluid commonly used in machining processes. It is essentially a concentrate that, when mixed with water, forms a stable emulsion. This emulsion serves several critical purposes, including cooling, lubrication, and corrosion protection. A key component of soluble oil is the 'soap' or emulsifier, which plays a pivotal role in enabling the oil to mix uniformly with water and form a stable emulsion.

Emulsifying agents:

• Emulsifying agents are substances that help disperse one liquid into another immiscible liquid to form a stable mixture. In the case of soluble oil, the emulsifying agent (soap) ensures that the oil particles are evenly distributed throughout the water, creating a stable emulsion. This stable emulsion is crucial for the effective functioning of soluble oil as a cutting fluid.

The 'soap' in soluble oil serves as an emulsifying agent for the following reasons:

• Formation of Stable Emulsions: The soap molecules have hydrophilic (water-attracting) and hydrophobic (oil-attracting) ends. This dual nature allows the soap to bind oil particles with water, forming a stable emulsion. Without the emulsifying agent, the oil and water would separate, rendering the cutting fluid ineffective.
• Uniform Distribution: The emulsifying agent ensures that the oil particles are uniformly dispersed throughout the water. This uniform distribution is critical for maintaining consistent cooling and lubrication during machining operations.
• Enhanced Cooling and Lubrication: While the soap itself does not directly contribute to cooling or lubrication, the stable emulsion it creates allows the soluble oil to perform these functions effectively. The oil particles in the emulsion provide lubrication, while the water component aids in cooling.
• Corrosion Protection: The stable emulsion also helps protect the workpiece and machine components from corrosion, as the oil forms a protective layer on the metal surfaces.

Explanation:

Machinability and Hardness of the Material

• Machinability and hardness of the material are critical characteristics of a workpiece that significantly influence process engineering decisions. These factors directly impact the choice of machining processes, cutting tools, and parameters that are used to manufacture a component. Understanding these characteristics ensures efficient production, optimal tool life, and high-quality output. Let's delve deeper into why machinability and hardness are vital considerations:

Machinability:

• Machinability refers to the ease with which a material can be machined to achieve the desired surface finish, dimensional accuracy, and tolerances. It is influenced by several factors, including the material's composition, microstructure, and thermal properties. Materials with better machinability require less cutting force, generate less heat, and result in longer tool life. Process engineers prioritize machinability to ensure efficient and cost-effective manufacturing.

Factors Affecting Machinability:

• Material Composition: The presence of alloying elements, such as carbon, chromium, and nickel, affects machinability.
• Hardness: Softer materials are generally easier to machine, while harder materials may pose challenges.
• Thermal Conductivity: Materials with high thermal conductivity dissipate heat better, reducing tool wear.
• Microstructure: Fine-grained structures are easier to machine compared to coarse-grained ones.

Hardness:

• Hardness is a measure of a material's resistance to deformation, wear, and scratching. It plays a crucial role in determining the cutting forces required, tool wear rates, and the achievable surface finish. Harder materials often require specialized tools and machining strategies, such as the use of carbide or diamond-coated tools, to ensure successful machining.

Impact of Hardness on Process Engineering Decisions:

• Tool Selection: Harder materials necessitate the use of tools made from advanced materials like tungsten carbide or polycrystalline diamond.
• Cutting Parameters: High hardness often requires lower cutting speeds and feeds to minimize tool wear and ensure precision.
• Surface Finish: Achieving a smooth surface finish on hard materials may require additional finishing operations.
• Machine Capability: Machines with higher rigidity and power are needed to handle harder materials.

Applications: Materials with specific machinability and hardness characteristics are chosen based on the application's requirements. For instance, aerospace components often require materials with high hardness and strength, while automotive parts may prioritize machinability for mass production.

Importance in Process Engineering:

Understanding and analyzing machinability and hardness enable process engineers to:

• Select appropriate machining processes, such as turning, milling, or grinding.
• Determine suitable cutting tools and tool materials.
• Optimize cutting parameters to balance productivity and tool life.
• Ensure cost-effective and high-quality manufacturing.

Explanation:

Electrochemical Machining: 

In electrochemical machining, the metal is removed due to electrochemical action i.e. Ion displacement where the workpiece is made anode and the tool is made the cathode. A high current is passed between the tool and workpiece through the electrolyte. Metal is removed by the anodic dissolution and is carried away by the electrolyte.

The tool material used in ECM should have the following property

• It should have high electrical conductivity
• It should be easily machinable and it should have high stiffness
• Its corrosion resistance should be high.

The advantages of ECM include

• Complex shapes can be made accurately
• The surface finish is good due to atomic level dissolution
• Tool wear practically absent
• Its material removal rate is the highest.

The limitation of the Electro-Chemical Machining (ECM) process is the use of corrosive media as electrolytes makes it difficult to handle.

Concept:

Table speed in mm/minute = ft × Z × N

where, N = RPM, Z = no. of teeth, ft = Feed per tooth

Calculation:

Given:

Z = 10, N = 150 rpm, ft = ?, f_m = 400 mm/min

Table speed in mm/minute, 400 = 150 × 10 × ft

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.

Concept:

Time for machining $=\frac{L}{f\times N}$

where L is job length (mm), f is feed (mm/rev), N is job speed (rpm)

Calculation:

Given:

f = 3 mm/rev, N = 600 rpm, L = 300 mm

Time for machining $=\frac{L}{f.N}=\frac{300}{3\times 600}$ = 0.1666 minutes = 0.1666 × 60 = 10 sec

The total machining time to turn the cylindrical surface is 10 sec.

Concept:

Chip velocity ‘VC’:

• The velocity with which the chip moves over the rake face of the cutting tool. Also represented as V_f
• The chip velocity V_c is the velocity of the chip relative to the tool and directed along the tool face.
• Since chip velocity is the relative velocity between tool and chip hence, If we assume the chip to be stationary then this chip velocity can be considered as the velocity of the tool along the tool rake face.

Shear velocity ‘VS’: The velocity with which metal of the work-piece shears along the shear plane. It is also called the velocity of the chip relative to work-piece.

Cutting velocity ‘V’: The velocity with which the tool moves relative to the work-piece.

The relationship b/w these velocities are:

From velocity triangle-

V = VC + VS

and from sine rule

The velocity relationship is given by the equation:

$\frac{v}{\mathrm{c}\mathrm{o}\mathrm{s}\left(\varphi -\alpha \right)}=\frac{{\mathrm{v}}_{\mathrm{c}}}{\sin \varphi }=\frac{v_s}{\cos \alpha }$

Here is v_s the velocity of chip sliding along shear plane.

$v_s=\frac{v}{\mathrm{c}\mathrm{o}\mathrm{s}\left(\varphi -\alpha \right)}\times cos\alpha =\frac{v\cos \alpha }{\cos \left(\varphi -\alpha \right)}$

A lubricant is a substance, which reduces friction between mating parts. Lubricants are grouped into three categories.

1. Liquid lubricants: Some of the most commonly used liquid lubricants are mineral oil, fatty or vegetable oils, synthetic oils.
2. Semi-liquid lubricants: Greases are most commonly used lubricants with a higher viscosity than oils. These are employed for slow speed and heavy pressure operations like drawing, rolling and extrusion processes.
3. Solid lubricants: Graphite is the commonly used solid lubricant. Other types of solid lubricants are soapstone, talc, French chalk etc.

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.

$Grindingratio=\frac{V_m}{V_w}=\frac{l\times b\times d}{\frac{\pi }{4}\times w\times \left(D_i^2-D_f^2\right)},wherew=widthofwheel$

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

Explanation:

• Gear shaping is a generating process. The cutter used is virtually a gear provided with cutting edges. The tool is rotated at the required velocity ratio relative to the gear to be manufactured and anyone manufactured gear tooth space is formed by one complete cutter tooth. This method can be used to produce cluster gears, internal gears, racks, etc with ease, which may not have the possibility to be manufactured in gear hobbing.
• Gear Hobbing is a continuous generating process in which the tooth flanks of the constantly moving workpiece are formed by equally spaced cutting edges of the hob. The main advantage of this process is its versatility to produce a variety of gears including Spur, Helical, Worm Wheels, Serrations, Splines, etc. The main advantage of the method is the higher production rate of the gears due to continuously indexing.
• Gear Milling is one of the initial and best known and metal removal process for making gears. This method requires the usage of a milling machine. This method is right now used only for the manufacture of gears requiring very less dimensional accuracy.
• Gear forming: In gear form cutting, the cutting edge of the cutting tool has a shape identical with the shape of the space between the gear teeth. Two machining operations, milling and broaching can be employed to form cut gear teeth.

Points to remember:

• Internal gears are manufactured by shaping process with a pinion cutter.
• Hobbing, milling and shaping with rack cutter is mainly used for external gears.

Explanation:

Three Jaw Chuck:

• It is also known as three jaws universal chuck, self-centering chuck, and concentric chuck having three jaws that work at the same time.
• The 3 jaws, which are generally made of high-quality steel, are arrogated at an angle of 120° to each other. During the operation, the jaw teeth are made to mesh with scrawl spiral teeth (Bevel’s teeth).
• The meshing causes a moment of all 3 jaws either towards or away from the chuck center, depending upon the direction of rotation of the bevel pinion.
• Three jaw chucks are used to hold only perfect round and regular jobs, workpieces of circular and hexagonal shapes.

Four Jaw Chuck:

• The four-jaw chuck is also called an independent chuck since each jaw can be adjusted independently.
• Four jaw chucks are used for a wide range of regular and irregular shapes.