Category: Manufacturing Engineering Objective Questions

Manufacturing Processes Multiple Choice Questions on “Laser Beam Machining – 3”.

1. The flash tube is operated in _____ mode.
a) pulsed
b) continuous
c) reversed
d) synchronous
Answer: a
Clarification: The flash tube is operated in pulsed mode by charging and discharging of the capacitor. Thus the pulse on time is decided by the resistance on the flash tube side and pulse off time is decided by the charging resistance. There is also a high voltage switching supply for initiation of pulses.

2. How many types of flows are possible in gas lasers?
a) 2
b) 3
c) 4
d) 5
Answer: b
Clarification: Gas lasers can be:
• Axial flow
• Transverse flow
• Folded flow.

3. The power of CO₂ laser is around______
a) 15 Watt per meter of tube length
b) 55 Watt per meter of tube length
c) 100 Watt per meter of tube length
d) 1 MW per meter of tube length
Answer: c
Clarification: The power of a CO₂ laser is typically around 100 Watt per metre of tube length. Thus to make a high power laser, a rather long tube is required which is quite inconvenient. For optimal use of floor space, high-powered CO₂ lasers are made of folded design.

4. In a CO₂ laser, a mixture of _____ is circulated through the gas tube.
a) CO₂, N₂ and He
b) CO₂, N₂ and Ar
c) CO₂, H₂ and N₂
d) CO₂, I₂ and O₂
Answer: a
Clarification: In a CO₂ laser, a mixture of CO₂, N₂ and He continuously circulate through the gas tube. Such continuous recirculation of gas is done to minimize consumption of gases.

5. In CO₂ laser, ‘He’ gas is used for cooling purpose.
a) True
b) False
Answer: a
Clarification: CO₂ acts as the main lasing medium whereas nitrogen helps in sustaining the gas plasma. Helium on the other hand helps in cooling the gases. High voltage is applied at the two ends of the tube leading to discharge and formation of gas plasma.

6. CO₂ lasers are folded to achieve _____________
a) high power
b) high depth of cuts
c) high material removal rate
d) avoid over heating
Answer: a
Clarification: CO₂ lasers are folded to achieve high power. In folded laser, there would be a few 100% reflective turning mirrors for manoeuvring the laser beam from the gas supply as well as a high voltage supply.

7. Nd-YAG laser can be used for drilling holes in the range of _____ diameter.
a) 0.25 mm – 1.5 mm
b) 1 mm – 1.5 mm
c) 1.5 mm – 2 mm
d) 2 mm – 2.5 mm
Answer: a
Clarification: For drilling holes as small as 0.25mm and as large as 1.5mm diameter, following lasers are used;
• Nd-YAG,
• Nd-glass, and
• Ruby.

8. For which of the following materials CO₂ laser is not used?
a) Plastics
b) Metals
c) Organic materials
d) Ceramics
Answer: b
Clarification: For,
• Plastics— CO₂ laser is used
• Metals—Nd-YAG, Ruby, Nd-glass lasers are used
• Organic materials and non-metals—Pulsed CO₂ laser is used
• Ceramics—Pulsed CO₂, Nd-YAG lasers are used.

9. Which of the following does not hold true about laser beam machining?
a) High initial cost
b) High running cost
c) No heat affected zone
d) It is not suitable for heat sensitive materials
Answer: c
Clarification: Following are the limitations of laser beam machining;
• High initial capital cost
• High maintenance cost
• Not very efficient process
• Presence of Heat Affected Zone – specially in gas assist CO₂ laser cutting
• Thermal process – not suitable for heat sensitive materials like aluminium glass fibre laminate.

10. Using lasers, large aspect ratio in drilling can be achieved.
a) True
b) False
Answer: a
Clarification: Following are the advantages of laser beam machining:
• In laser machining, there is no physical tool. Thus no machining force or wear of the tool takes place.
• Large aspect ratio in laser drilling can be achieved along with acceptable accuracy or dimension, form or location
• Micro-holes can be drilled in difficult – to – machine materials
• Though laser processing is a thermal processing but heat affected zone especially in pulse laser processing is not very significant due to the shorter pulse duration.

Manufacturing Processes Multiple Choice Questions on “Annealing – 1”.

1. The metal is said to be cold worked if it is plastically deformed at temperatures lower than _____
a) sublimation point
b) critical point
c) melting point
d) saturation point
Answer: c
Clarification: When a metal is plastically deformed at temperatures that are low relative to its melting point, it is said to be cold worked. A rough rule-of-thumb is to assume that plastic deformation corresponds to cold working if it is carried out at temperatures lower than one-half of the melting point measured on an absolute scale.

2. The fraction of total energy stored in the material _____ with increasing deformation.
a) increases
b) decreases
c) first increases and then decreases
d) remains constant
Answer: b
Clarification: Most of the energy expended in cold work appears in the form of heat, but a finite fraction is stored in the metal as strain energy associated with various lattice defects created by the deformation. The stored energy increases with increasing deformation, but at a decreasing rate, so that the fraction of the total energy stored decreases with increasing deformation.

3. The amount of stored energy can be greatly increased by _____
a) lowering the deformation temperature
b) increasing the deformation temperature
c) increasing the purity of the metal
d) reducing the net volume
Answer: a
Clarification: The amount of stored energy can be greatly increased by,
• increasing the severity of the deformation
• lowering the deformation temperature
• changing the pure metal to an alloy.

4. A soft annealed metal can have dislocation densities of the order of _____
a) 10⁵ to 10⁶ m^-2
b) 10⁸ to 10¹⁰ m^-2
c) 10¹⁰ to 10¹² m^-2
d) 10¹⁰ to 10¹⁵ m^-2
Answer: c
Clarification: Cold working is known to increase greatly the number of dislocations in a metal. A soft annealed metal can have dislocation densities of the order of 10¹⁰ to 10¹² m^-2, and heavily cold-worked metals can have approximately 10¹⁶.

5. Strain energy of the metal _____ with increasing the dislocation densities.
a) increases
b) decreases
c) does not change
d) first decreases and then remains constant
Answer: a
Clarification: Since each dislocation represents a crystal defect with an associated lattice strain, increasing the dislocation density increases the strain energy of the metal. Cold working is able to increase the number of dislocations in metal by a factor as large as 10⁴to 10⁶.

6. Vacancies in the metal lattice will be formed in greater numbers than interstitial atoms during _____
a) plastic deformation
b) brittle deformation
c) heating over melting temperature
d) non-linear deformation
Answer: a
Clarification: Since the strain energy associated with a vacancy is much smaller than that associated with an interstitial atom, it can be assumed that vacancies will be formed in greater numbers than interstitial atoms during plastic deformation.

7. Which of the following relations is true?
a) ΔG = ΔH – TΔS
b) ΔH = ΔG – TΔS
c) ΔT = ΔH – GΔS
d) ΔG = TΔH – ΔS
Answer: a
Clarification: ΔG = ΔH – T ΔS, this equation is known as Gibb’s free energy equation. While plastic deformation certainly increases the entropy of a metal, the effect is small compared to the increase in internal energy (the retained strain energy). The term T∆S in the free energy equation may be neglected and the free-energy increase equated directly to the stored energy. Where ∆G is the free energy associated with the cold work, ∆H is the enthalpy or stored strain energy, ∆S is the entropy increase due to the cold work, and T is the absolute temperature.

8. Free energy of cold-worked metals is _____ than/as that of annealed metals.
a) greater
b) lower
c) same
d) greater of few metals
Answer: a
Clarification: Since the free energy of cold-worked metals is greater than that of annealed metals, they may soften spontaneously. Heating a deformed metal greatly speeds up its return to the softened state.

9. The anisothermal anneal method, the energy release is determined as a function of temperature.
a) True
b) False
Answer: a
Clarification: The anisothermal anneal method, the cold worked metal is heated continuously from a lower to a higher temperature and the energy release is determined as a function of temperature. One form of the anisothermal anneal measures the difference in the power required to heat two similar specimens at the same rate.

10. The one specimen form above mentioned specimen is cold worked.
a) True
b) False
Answer: a
Clarification: One specimen of the two is cold worked before the heating cycle, while the other serves as a standard and is not deformed. During the heating cycle, the cold-worked specimen undergoes reactions that release heat and lower the power required to heat it in comparison with that required to heat the standard specimen. Measurements of the difference in power give direct evidence of the rate at which heat is released in the cold-worked specimen.

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Manufacturing Processes Multiple Choice Questions on “CNC Machining”.

1. In machining of a workpiece, the material is removed by_____
a) drilling action
b) melting action
c) shearing acting
d) using brittleness of the material
Answer: c
Clarification: There are different machining processes, such as turning, milling, boring etc. In all these cases metal is removed by a shearing process, which occurs due to the relative motion between the workpiece and the tool. Generally, one of the two rotates at designated and generally high speed, causing the shearing of material (known as chips), from the workpiece. The other moves relatively slowly to effect removal of metal throughout the workpiece.

2. The depth that the tool is plunged into the surface is called as ______
a) feed
b) depth of cut
c) depth of tool
d) working depth
Answer: b
Clarification: The depth of cut, DOC is the depth that the tool is plunged into the surface. Feed defines the relative lateral movement between the cutting tool and the workpiece. Thus, together with the depth of cut, feed decides the cross section of the material removed for every rotation of the job or the tool.

3. Feed is measured in units of _____
a) length/revolution
b) degree/revolution
c) length
d) velocity
Answer: a
Clarification: Feed is the amount of material removed for each revolution or per pass of the tool over the workpiece and is measured in units of length/revolution, length/pass or other appropriate units for the particular process.

4. CNC machining centres do not include operations like ______
a) milling
b) boring
c) welding
d) tapping
Answer: c
Clarification: CNC machining centres are developed for machining prismatic components combining operations like milling, drilling, boring and tapping. Gradually machines for manufacturing cylindrical components, called turning centres are also developed.

5. In CNC systems multiple microprocessors and programmable logic controllers work ______
a) in parallel
b) in series
c) one after the other
d) for 80% of the total machining time
Answer: a
Clarification: In CNC systems multiple microprocessors and programmable logic controllers work in parallel for simultaneous servo position and velocity control of several axes of a machine for contour cutting as well as monitoring of the cutting process and the machine tool.

6. Which of the following is not the advantage of CNC machines?
a) Higher flexibility
b) Improved quality
c) Reduced scrap rate
d) Improved strength of the components
Answer: d
Clarification: CNC machines offer the following advantages in manufacturing:
• Higher flexibility
• Increased productivity
• Improved quality
• Reduced scrap rate
• Reliable and Safe operation
• Smaller footprint.

7. In how many ways CNC machine tool systems can be classified?
a) 2
b) 3
c) 4
d) 5
Answer: b
Clarification: CNC machine tool systems can be classified in various ways such as:
• Point-to-point or contouring: depending on whether the machine cuts metal while the workpiece moves relative to the tool
• Incremental or absolute: depending on the type of coordinate system adopted to parameterise the motion commands
• Open-loop or closed-loop: depending on the control system adopted for axis motion control.

8. Point-to-point systems are used for _____
a) reaming
b) parting
c) grooving
d) facing
Answer: a
Clarification: Such systems are used, typically, to perform hole operations such as drilling, boring, reaming, tapping and punching. In a PTP system, the path of the cutting tool and its feed rate while traveling from one point to the next are not significant, since, the tool is not cutting while there is motion.

9. In part programming, interpolation is used for obtaining _______ trajectory.
a) helicoidal
b) pentagonal
c) triangular
d) zig-zag
Answer: a
Clarification: Interpolation consists of the calculation of the coordinated movement of several axes using the programmed parameters, in order to obtain a resulting trajectory, which can be of various types, such as:
– Straight line
– Circular
– Helicoidal.

10. For CNC machining skilled part programmers are needed.
a) True
b) False
Answer: a
Clarification: The main disadvantages of NC systems are:
• Relatively higher cost compared to manual versions
• More complicated maintenance due to the complex nature of the technologies
• Need for skilled part programmers.

11. An absolute NC system is one in which all position coordinates are referred to one fixed origin called the zero point.
a) True
b) False
Answer: a
Clarification: An absolute NC system is one in which all position coordinates are referred to one fixed origin called the zero point. The zero point may be defined at any suitable point within the limits of the machine tool table and can be redefined from time to time.