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

Explanation:

Gear Hobbing

• Gear hobbing is a machining process used for cutting gears, splines, and sprockets. It is one of the most widely used methods in gear manufacturing and is especially suitable for producing high-quality gears with precision. The process involves the use of a specialized cutting tool called a hob, which is a cylindrical cutting tool with helical teeth. The hob and the workpiece (gear blank) rotate in a synchronized manner, allowing the hob to cut teeth into the blank progressively. The process is efficient, versatile, and suitable for mass production of gears.

Working Principle:

• In gear hobbing, the hob rotates at a high speed while the gear blank is mounted on a spindle and rotates at a specific speed ratio relative to the hob. The hob's teeth cut into the blank as the two rotate together, shaping the gear teeth. The synchronization between the hob and the blank ensures the correct tooth profile and spacing.
• The cutting operation in gear hobbing is continuous, which makes it faster and more economical compared to other gear manufacturing processes, such as gear shaping. The hob's design and rotational speed determine the gear's characteristics, such as the number of teeth, module, and pressure angle.

Applications of Gear Hobbing:

• Manufacturing spur gears, helical gears, and worm gears.
• Production of splines and sprockets.
• Used in the automotive, aerospace, and industrial machinery sectors for producing precision gears.
• Ideal for high-volume production due to its efficiency and speed.

Explanation:

Broaching Process:

• Broaching is a machining process that involves the use of a multi-toothed cutting tool called a broach to remove material from a workpiece. The process is designed to perform precision machining operations, such as shaping, sizing, or finishing, in a single pass. Broaching is particularly well-suited for applications where high accuracy, good surface finish, and complex profiles are required.
• The broaching process is performed by moving the broach either linearly (linear broaching) or rotationally (rotary broaching) relative to the workpiece. The broach consists of a series of cutting teeth arranged in a specific sequence, with each tooth progressively increasing in size. This arrangement allows the broach to remove material gradually, resulting in a smooth and accurate cut.

Key Features of Broaching:

• High Precision: Broaching provides excellent dimensional accuracy and surface finish, making it ideal for applications where tight tolerances are required.
• Complex Shapes: The process can produce intricate and non-circular shapes, such as splines, keyways, and gears, that are difficult to achieve with other machining methods.
• Efficiency: Broaching is a fast and efficient process, as it completes the machining operation in a single pass.
• Versatility: It can be used on various materials, including metals, plastics, and composites.
• Tool Design: The broach is designed with a gradual increase in tooth height, which minimizes cutting forces and reduces the risk of tool wear and breakage.

Types of Broaching:

• Internal Broaching: Used to create internal features, such as holes, keyways, and splines, within a workpiece.
• External Broaching: Used to machine external surfaces, such as flat, contoured, or cylindrical features.
• Linear Broaching: The broach moves linearly relative to the workpiece.
• Rotary Broaching: The broach rotates relative to the workpiece, often used for machining hexagonal or other non-circular holes.

Applications of Broaching:

• Manufacturing of gears, splines, and keyways.
• Creating complex profiles in automotive, aerospace, and industrial components.
• Producing high-precision parts for medical devices and other specialized industries.

Explanation:

Grinding Process:

• Grinding is a machining process that uses an abrasive wheel as the cutting tool to remove material from a workpiece surface. The abrasive grains on the grinding wheel's surface act as cutting edges, interacting with the workpiece to produce chips. This process is widely used in manufacturing for precision machining, finishing surfaces, and achieving tight tolerances. The interaction that primarily produces chips during grinding involves the abrasive grit and the workpiece material.

Grit-workpiece

• The interaction between the grit and the workpiece is responsible for the removal of material in the form of chips during the grinding process. The abrasive grits embedded in the grinding wheel's surface serve as cutting tools. When the grinding wheel rotates and comes into contact with the workpiece, the abrasive grits cut into the material, shearing off tiny chips. This cutting mechanism is similar to conventional machining processes, where a cutting tool removes material to shape the workpiece. The following steps outline how this interaction produces chips:
  • Contact between Grit and Workpiece: As the grinding wheel rotates, individual abrasive grits come into contact with the workpiece surface. The pressure exerted by the wheel forces the grits into the material.
  • Material Penetration: The sharp edges of the abrasive grits penetrate the workpiece surface, removing material in the form of chips. The depth of penetration depends on factors such as grit size, wheel speed, feed rate, and the material properties of the workpiece.
  • Chip Formation: The material undergoes plastic deformation and shearing due to the high-pressure and high-speed interaction between the grit and the workpiece. This results in the formation of small chips, which are carried away by the grinding wheel or coolant.
  • Surface Finish: The continuous engagement of abrasive grits with the workpiece surface generates a smooth and precise finish. The size and distribution of the grits significantly influence the surface quality and material removal rate.

Explanation:

Special-Purpose Lathe:

• A special-purpose lathe is designed for specific machining applications that cannot be efficiently performed using standard lathes. These machines are tailored for specialized tasks, offering enhanced precision, productivity, and functionality in handling unique or heavy-duty machining requirements. Among the various types of special-purpose lathes, a wheel lathe stands out for heavy-duty applications, particularly in machining railway components like wheels, journals, and treads.

Key Features of Wheel Lathes:

• Heavy-Duty Design: Wheel lathes are robust and designed to machine large, heavy components, ensuring stability and precision during operation.
• Specialized Machining: They are equipped with tools and features specifically for machining railway wheels, including their journals and treads.
• Precision and Efficiency: These lathes ensure accurate machining of components, which is critical for the safe operation of railway systems.
• Automated Functions: Many modern wheel lathes come with automation features for enhanced productivity and reduced manual intervention.

Applications:

• Machining railway wheels to maintain dimensional accuracy and surface finish.
• Repairing worn-out journals and treads of railway wheels.
• Ensuring the safety and reliability of railway components by maintaining strict tolerances.

Explanation:

Parting-Off Operation

• Parting-off is a machining operation performed on a lathe where a thin cutting tool, known as a parting tool, is used to cut off a portion of the workpiece. This operation is commonly used to separate finished components from the raw material or to create grooves or recesses in the workpiece. The parting tool is fed perpendicular to the axis of the workpiece, slicing through it as the lathe spindle rotates.
• In a parting-off operation, the cutting tool is mounted on the tool post, and the tool post is attached to the cross-slide of the lathe. The cross-slide allows the tool to move in a direction perpendicular to the axis of the workpiece. During the operation, the operator manually rotates the cross-slide screw to feed the parting tool into the workpiece. This manual control ensures precision and allows the operator to adjust the feed rate based on the material type, tool geometry, and cutting conditions.

Why Option 2 is Correct:

1. Mechanism of Cross-Slide Feed: The cross-slide screw is a threaded component connected to the cross-slide of the lathe. When the operator rotates the screw, the cross-slide moves along its dovetail ways, carrying the tool closer to or away from the workpiece. This movement is precisely controlled, which is essential for delicate operations like parting-off.

2. Perpendicular Feed: Parting-off requires the tool to move perpendicular to the workpiece's axis to achieve a clean separation. The cross-slide screw facilitates this perpendicular feed, making it the most appropriate choice for the operation.

3. Manual Control: Unlike automated feeds, manual control allows the operator to feel the resistance encountered during cutting. This tactile feedback helps the operator avoid excessive tool pressure, which can lead to tool breakage or deformation of the workpiece.

4. Practical Application: In most lathe operations, especially in small workshops or for custom jobs, manual control of the cross-slide is preferred for parting-off. It provides the flexibility to accommodate variations in material properties and tool wear.

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 \(= \frac{L}{{f.N}} = \frac{ 300}{3×600} \) = 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:

\(\frac{v}{{{\rm{cos}}\left( {\phi - \alpha } \right)}} = \frac{{{{\rm{v}}_{\rm{c}}}}}{{\sin \phi }} = \frac{{{v_s}}}{{\cos \alpha }}\)

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

\({v_s} = \frac{v}{{{\rm{cos}}\left( {\phi - \alpha } \right)}} \times cos\;\alpha =\frac{{v\cos \alpha }}{{\cos \left( {\phi - \alpha } \right)}} \)

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.

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

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

Explanation:

• 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.