ISRO Mechanical Engineering Manufacturing Technology Questions, Answers and Explanation
Welding
2018.10. In a linear arc welding process, the heat input per unit length is inversely proportional to
a) welding current
b) welding speed
c) welding voltage
d) duty cycle of power source
Answer
b) welding speed
Explanation
Welding power gives the heat input per unit length. Welding current and welding voltage is directly proportional to welding power and hence directly proportional to the heat input per unit length.
Duty cycle is the ratio of arcing time to the weld cycle time multiplied by 100. Welding cycle time is either 5 minutes as per European standards or 10 minutes as per American standard and accordingly power sources are designed. If arcing time is continuously 5 minutes then as per European standard it is 100% duty cycle and 50% as per American standard. At 100% duty cycle minimum current is to be drawn i.e. with the reduction of duty cycle current drawn can be of higher level. Duty cycle and associated currents are important as it ensures that power source remains safe and its windings are not getting damaged due to increase in temperature beyond specified limit. The maximum current which can be drawn from a power source depends upon its size of winding wire, type of insulation and cooling system of the power source. Hence with other parameters constant duty cycle is directly proportional to welding power or heat input per unit length
With other parameters constant higher the welding speed lesser the heat input per unit length. So it is inversely proportional
Machining Analysis
2017.2.1. The specific metal cutting energy is expressed as
a) τ cos(β - α)⁄sin φ cos(φ+β-α)
b) τ sin(β - α)⁄sin φ cos(φ-β+α)
c) τ cos(α - β)⁄sin φ cos(φ-β+α)
d) τ sin(α - β)⁄sin φ cos(φ+β-α)
where α is rake angle, β is friction angle, φ is shear angle and τ is the shear stress.
Answer
a) τ cos(β - α)⁄sin φ cos(φ+β-α)
Explanation
Specific metal cutting energy is defined as the energy expended in removing a unit volume of work-piece material.
= Energy/Volume Removed
Energy = Force of cut x velocity of tool
Volume Removed = Velocity of tool x Width of cut x Depth of cut
Specific metal cutting energy = Force of cut/(Width of cut x Depth of cut)
Shear force = shear stress x Area of shear
= τ x width of cut x depth of cut /sin φ
Shear force/Force of cut = cos(φ+β-α)/cos(β - α)
Force of cut = Shear force x cos(β - α)/cos(φ+β-α)
Specific metal cutting energy = Shear force x cos(β - α)/(cos(φ+β-α)x width of cut x depth of cut x sin φ)
= Shear stress x cos(β - α)/(cos(φ+β-α) x sin φ)
= τ cos(β - α)⁄sin φ cos(φ+β-α)
Specific metal cutting energy = Force of cut/(Width of cut x Depth of cut)
Shear force = shear stress x Area of shear
= τ x width of cut x depth of cut /sin φ
Shear force/Force of cut = cos(φ+β-α)/cos(β - α)
Force of cut = Shear force x cos(β - α)/cos(φ+β-α)
Specific metal cutting energy = Shear force x cos(β - α)/(cos(φ+β-α)x width of cut x depth of cut x sin φ)
= Shear stress x cos(β - α)/(cos(φ+β-α) x sin φ)
= τ cos(β - α)⁄sin φ cos(φ+β-α)
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= Energy/Volume Removed
Energy = Force of cut x velocity of tool
Volume Removed = Velocity of tool x Width of cut x Depth of cut
Specific metal cutting energy = Force of cut/(Width of cut x Depth of cut)
Shear force = shear stress x Area of shear
= τ x width of cut x depth of cut /sin φ
Shear force/Force of cut = cos(φ+β-α)/cos(β - α)
Force of cut = Shear force x cos(β - α)/cos(φ+β-α)
Specific metal cutting energy = Shear force x cos(β - α)/(cos(φ+β-α)x width of cut x depth of cut x sin φ)
= Shear stress x cos(β - α)/(cos(φ+β-α) x sin φ)
= τ cos(β - α)⁄sin φ cos(φ+β-α)
Specific metal cutting energy = Force of cut/(Width of cut x Depth of cut)
Shear force = shear stress x Area of shear
= τ x width of cut x depth of cut /sin φ
Shear force/Force of cut = cos(φ+β-α)/cos(β - α)
Force of cut = Shear force x cos(β - α)/cos(φ+β-α)
Specific metal cutting energy = Shear force x cos(β - α)/(cos(φ+β-α)x width of cut x depth of cut x sin φ)
= Shear stress x cos(β - α)/(cos(φ+β-α) x sin φ)
= τ cos(β - α)⁄sin φ cos(φ+β-α)
Tool life and wear
2018.9. In a machining operation cutting speed is reduced by 50%. Assuming n = 0.5, C = 300 in Taylor's equation. The increase in tool life is
a) 2
b) 4
c) 8
d) 16
Answer
b) 4
Explanation
When both V and T are plotted in log-scale, linear relationship appears With the slope, n and intercept, c, the equation is VTn = C
where, n is called, Taylor’s tool life exponent. The values of both ‘n’ and ‘c’ depend mainly upon the tool-work materials and the cutting environment (cutting fluid application).
V2/V1 = 50%, T2/T1 = ?
V2/V1 . (T2/T1)n = 1
0.5 . (T2/T1)0.5 = 1
(T2/T1)0.5 = 2
T2/T1 = 4
Answer
b) f = 0.5r
Explanation
Rough turning is the removal of excess stock from a work piece as rapidly and efficiently as possible.
Small nose radius
Large nose radius
2017.2.12. Margin wear in drill is due to
a) abrasion
b) vibration
c) thermal softening
d) diffusion
Answer
c) thermal softening
Explanation
The margin edge of a drill contacts the drilled hole surface, therefore its wear has a large impact on the hole quality. The only clear sign of margin wear is revealed by lateral vibration at the final stage of the drilling process.
Thermal softening is a heat treatment process; softening processes remove the lattice defects introduced into the aluminium structure during cold working (such as rolling or cold impact extrusion). This also leads to an increase in ductility. In contrast to soft annealing, thermal softening occurs as a temperature below the recrystallisation temperature.
Work-piece material is subjected to work hardening and thermal softening effect during machining, especially at high cutting temperature and pressure. When machining titanium alloys, the hardness just beneath the machined surface was found to be softer than the bulk material hardness due to the thermal softening effect. However, when the depth below the machined surface increases, the hardness value starts to increase before reaching its peak value and finally drops gradually to the bulk material hardness. The increase in hardness value is directly associated with the effect of work hardening. This effect depends on the temperature, cutting time and the mechanism of internal stress relaxation.
Attrition and diffusion are the dominant tool wear mechanisms, especially in the helical flute of drill. With prolonged drilling, these tool wear mechanisms lead to the catastrophic failure of the drill.
Attrition wear is a removal of grains or agglomerates of tool material by the adherent chip or work-piece. This could be due to intermittent adhesion between the tool and the work-piece as a result of the irregular chip flow and the breaking of a partially stable built-up edge. When seizure between the tool and the work-piece is broken, small fragments of the tool can be plucked out due to weakening of the binder and transported material via the underside of the chip or by the work-piece. The presence of fatigue during machining operation can initiate cracks and also encourage cracks propagation on the tool.
Diffusion wear is associated with the chemical affinity between the tool and work-piece materials under high temperature and pressure during machining of titanium alloys. An intimate contact between the tool work piece interface at temperature above 800º C provides an ideal environment for diffusion of tool material across the tool-work-piece interface. Due to high chemical reactivity of titanium alloys, carbon reacts readily with titanium. Therefore, the formation of titanium carbide occurs at the interface between the tool and work material.
Answer
c) thermal softening
Explanation
The margin edge of a drill contacts the drilled hole surface, therefore its wear has a large impact on the hole quality. The only clear sign of margin wear is revealed by lateral vibration at the final stage of the drilling process.
Thermal softening is a heat treatment process; softening processes remove the lattice defects introduced into the aluminium structure during cold working (such as rolling or cold impact extrusion). This also leads to an increase in ductility. In contrast to soft annealing, thermal softening occurs as a temperature below the recrystallisation temperature.
Work-piece material is subjected to work hardening and thermal softening effect during machining, especially at high cutting temperature and pressure. When machining titanium alloys, the hardness just beneath the machined surface was found to be softer than the bulk material hardness due to the thermal softening effect. However, when the depth below the machined surface increases, the hardness value starts to increase before reaching its peak value and finally drops gradually to the bulk material hardness. The increase in hardness value is directly associated with the effect of work hardening. This effect depends on the temperature, cutting time and the mechanism of internal stress relaxation.
Attrition and diffusion are the dominant tool wear mechanisms, especially in the helical flute of drill. With prolonged drilling, these tool wear mechanisms lead to the catastrophic failure of the drill.
Attrition wear is a removal of grains or agglomerates of tool material by the adherent chip or work-piece. This could be due to intermittent adhesion between the tool and the work-piece as a result of the irregular chip flow and the breaking of a partially stable built-up edge. When seizure between the tool and the work-piece is broken, small fragments of the tool can be plucked out due to weakening of the binder and transported material via the underside of the chip or by the work-piece. The presence of fatigue during machining operation can initiate cracks and also encourage cracks propagation on the tool.
Diffusion wear is associated with the chemical affinity between the tool and work-piece materials under high temperature and pressure during machining of titanium alloys. An intimate contact between the tool work piece interface at temperature above 800º C provides an ideal environment for diffusion of tool material across the tool-work-piece interface. Due to high chemical reactivity of titanium alloys, carbon reacts readily with titanium. Therefore, the formation of titanium carbide occurs at the interface between the tool and work material.
Machining Processes
2017.2.7. Thumb rule between feed and nose radius in rough turning is
a) f = 0.3r
b) f = 0.5r
c) f = 0.7r
d) f = 0.9r
Answer
b) f = 0.5r
Explanation
Rough turning is the removal of excess stock from a work piece as rapidly and efficiently as possible.
Small nose radius
- Ideal for small cutting depth
- Reduces vibration
- Less insert strength
Large nose radius
- Heavy feed rates
- Large depths of cut
- Strong edge
- Increased radial forces
The radial forces that push the insert away from the cutting surface becomes more axial as the depth of cut increases. It is preferable to have more axial forces instead of radial, which have a negative effects on the cutting action e.g. with more tendencies to vibrate and bad surface finish with increased radial forces.
The nose radius also affects the chip formation. Generally, chip breaking improves with a smaller radius.As a general rule of thumb, the depth of cut should be greater than or equal to 2/3 of the nose radius, or 1/2 of the nose radius in the feed direction.
Various Gauges
2017_2.5. Smallest thickness which can be measured by a slip gauge is
a) 1.001 mm
b) 0.01 mm
c) 0.001 mm
d) none of these
Answer
a) 1.001 mm
Explanation
Slip gauge set of 56 slips is made up as shown below :
9 slips 1.001 to 1.009 in steps of 0.001 mm
9 slips 1.01 to 1.09 in steps of 0.01 mm
9 slips 1.0 to 1.9 in steps of 0.1 mm
25 slips 1 to 25 in steps of 1.0 mm
3 slips 25 to 75 in steps of 25 mm.
From the given choices the smallest thickness that can be measured is 1.001mm and not 0.01mm or 0.001mm.
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a) 1.001 mm
Explanation
Slip gauge set of 56 slips is made up as shown below :
9 slips 1.001 to 1.009 in steps of 0.001 mm
9 slips 1.01 to 1.09 in steps of 0.01 mm
9 slips 1.0 to 1.9 in steps of 0.1 mm
25 slips 1 to 25 in steps of 1.0 mm
3 slips 25 to 75 in steps of 25 mm.
From the given choices the smallest thickness that can be measured is 1.001mm and not 0.01mm or 0.001mm.
Operations Research
2017.2.10. Transportation method is concerned with
a) Value analysis
b) Linear programming
c) Queuing theory
d) Break-even analysis
Answer
b) Linear programming
Explanation
Answer
b) Linear programming
Explanation
Value analysis - the systematic and critical assessment by an organization of every feature of a product to ensure that its cost is no greater than is necessary to carry out its functions.
Linear programming - Linear programming is a method to achieve the best outcome in a mathematical model whose requirements are represented by linear relationships.
Queuing theory - Queuing theory is the mathematical study of waiting lines, or queues. A queuing model is constructed so that queue lengths and waiting time can be predicted.
Break-even analysis - Break-even analysis is a technique widely used by production management and management accountants. ... Total variable and fixed costs are compared with sales revenue in order to determine the level of sales volume, sales value or production at which the business makes neither a profit nor a loss
Transportation method - The Transportation Method of linear programming is applied to the problems related to the study of the efficient transportation routes i.e. how efficiently the product from different sources of production is transported to the different destinations, such as the total transportation cost is minimum.
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