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

Explanation:

Surface Roughness:

• Surface roughness refers to the irregularities and deviations found on the surface of a workpiece. It is a critical factor in manufacturing as it affects the performance, appearance, and longevity of the final product. The measurement of surface roughness is typically quantified by parameters such as Ra (average roughness), Rz (maximum height of the profile), and others.
• Achieving minimal surface roughness is crucial for applications requiring high precision and smooth finishes. It is important for improving the performance of mechanical components, reducing wear and tear, enhancing aesthetic appeal, and ensuring proper functioning of parts in assemblies.

Processes to Achieve Minimal Surface Roughness:

• Honing
• Superfinishing
• Grinding
• Lapping

Superfinishing

• Superfinishing is a process designed specifically to produce very high-quality surface finishes with extremely low roughness values. It involves the use of fine abrasive stones or tapes that are pressed against the workpiece in a controlled manner. The main goal of superfinishing is to remove the final traces of irregularities left by previous machining processes and achieve a mirror-like finish.
• In superfinishing, the workpiece is rotated and simultaneously subjected to oscillatory motion by abrasive stones or tapes. The abrasive particles gently remove the microscopic peaks and valleys on the surface, resulting in a highly smooth finish. The pressure and motion are precisely controlled to avoid excessive material removal and ensure uniformity.

Advantages:

• Produces extremely low surface roughness values (often in the range of 0.01 to 0.05 micrometers Ra).
• Improves the wear resistance and fatigue life of components.
• Enhances the aesthetic appeal of the finished product.
• Reduces friction and improves the performance of mechanical parts.

Additional InformationOption 1: Honing

Honing is a machining process that involves the use of abrasive stones to improve the surface finish and dimensional accuracy of cylindrical parts. While honing can achieve good surface finishes (typically around 0.1 to 0.4 micrometers Ra), it does not usually reach the ultra-fine finishes produced by superfinishing.

Option 3: Grinding

Grinding is a widely used abrasive machining process where a grinding wheel removes material from the workpiece surface to achieve the desired finish. Grinding can produce relatively low surface roughness values (around 0.2 to 0.8 micrometers Ra) but generally does not achieve the ultra-smooth finishes of superfinishing.

Option 4: Lapping

Lapping is a precision surface finishing process that uses abrasive slurry applied between the workpiece and a lapping plate. Lapping can achieve very smooth finishes (around 0.05 to 0.2 micrometers Ra) and is particularly effective for flat surfaces. However, superfinishing often surpasses lapping in terms of achieving the absolute lowest roughness values.

Explanation:

Electron Beam Welding

• Electron Beam Welding is a fusion welding in which coalescence is produced by heating the workpiece due to impingement of the concentrated electron beam of high kinetic energy on the workpiece.
• As the electron beam impinges the workpiece the kinetic energy of the electron beam converts into thermal energy resulting in melting and even evaporation of the work material. 
• Electron beam welding joins metals by bombarding a specific confined area of the base metal with highvelocity electrons.
• It is performed in a vacuum without shielding gas to prevent the reduction of electron velocity. 
• Neither an electrode nor a filler rod is used.
• Electron beam welding is focused on the weld spot using the magnetic lens.

• It allows fusion welds of great depth with a minimum width because the beam can be focused and magnified.

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:

Taper Turning by Swiveling the Compound Rest:

• Taper turning by swiveling the compound rest is a machining process used to create tapered surfaces on a workpiece. The compound rest, which is part of the lathe, is swiveled to a specific angle relative to the lathe axis to achieve the desired taper.
• In this method, the compound rest is set at an angle corresponding to the taper angle to be produced. As the tool moves along the compound rest, it cuts the workpiece to create a taper. The angle of the compound rest determines the taper angle of the workpiece.
• Due to the limited travel of the compound rest, this method is only suitable for producing short tapers. The compound rest can only move a limited distance, restricting the length of the taper that can be produced. For longer tapers, other methods such as taper turning attachments or tailstock offset methods are more appropriate.

Limitations:

• Limited to short tapers due to the constraints of compound rest travel.
• Not suitable for high production efficiency as the setup and operation are relatively slow.
• Surface finish may not be optimal compared to other taper turning methods.

Explanation:

Broaching Operation

• Broaching is a machining process that uses a toothed tool, called a broach, to remove material. There are two main types of broaching: linear and rotary. In both processes, the broach is used to machine internal holes, splines, keyways, or other shapes in a workpiece.
• In a broaching operation, the broach is drawn or pushed through the workpiece to shape or enlarge a hole. The broach has a series of progressively larger teeth that cut the material in a single pass, producing a precise and smooth finish. The operation can be performed on a broaching machine, which is specifically designed for this purpose.
• In internal broaching operations, the broach is typically pulled through the workpiece using a puller. The pilot is a cylindrical portion at the front of the broach that helps in aligning and guiding the tool through the hole before the cutting teeth engage. The puller attaches to the pilot to draw the broach through the component, ensuring proper orientation and control during the cutting process.

Applications: Broaching is widely used in industries such as automotive, aerospace, and manufacturing to produce internal gears, keyways, splines, and other precise shapes.

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.

Number of Jaws required for self centred chuck is three, thus it is used for centrering symmetrical workpieces. For unsymmetrical workpiece we require Four-Jaw chuck.

A lathe chuck is a holding device which is used for holding the job firmly against the cutting tool.

There are two types of chuck:

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

Three Jaw Chuck:

It is also known as three jaws universal chuck, self-centring chuck and concentric chuck having three jaws which work at the same time. Three jaw chucks are used to hold only perfect round and regular jobs