← MES Jr. Grade
Past Questions
Use of more acetylene with less volume of oxygen in an oxyacetylene torch produces a ___________ flame.
A)
Neutral
B)
Oxidizing
C)
Carburizing
D)
None of these
In oxy-acetylene welding, the type of flame produced depends on the ratio of oxygen to acetylene. There are three primary types of flames:
1. **Neutral Flame**: Produced when equal volumes of oxygen and acetylene are used. It is used for most general welding purposes.
2. **Oxidizing Flame**: Produced when there is an excess of oxygen (more oxygen than acetylene). This flame is hotter and is typically used for welding brass or bronze.
3. **Carburizing Flame**: Produced when there is an excess of acetylene (more acetylene than oxygen). This flame contains unburnt carbon (acetylene), which creates a 'reducing' atmosphere. It is characterized by three distinct zones (inner cone, acetylene feather, and outer envelope) and is used for welding high-carbon steels, hard-facing, or aluminum.
Since the question specifies the use of 'more acetylene with less volume of oxygen', the resulting flame is a carburizing flame.
Economic Order Quantity is the quantity at which the cost of carrying is:
A)
Minimum
B)
Equal to the cost of ordering
C)
Less than the cost of ordering
D)
Cost of overstocking
Economic Order Quantity (EOQ) is the optimal order size that minimizes the total inventory cost, which is the sum of the annual ordering cost and the annual carrying (holding) cost.
1. Annual Ordering Cost: This cost decreases as the order quantity increases because fewer orders are placed per year. It is calculated as (Annual Demand / Order Quantity) * Cost per Order.
2. Annual Carrying Cost: This cost increases as the order quantity increases because more inventory is held on average. It is calculated as (Order Quantity / 2) * Holding Cost per unit.
Mathematically, the total cost curve is U-shaped. The minimum point of this total cost curve occurs exactly where the Ordering Cost curve and the Carrying Cost curve intersect. Therefore, at the EOQ, the annual cost of carrying inventory is exactly equal to the annual cost of ordering inventory.
Why other options are incorrect:
- Option (a): The carrying cost itself is not minimum at EOQ; it is minimum when the order quantity is zero. It is the TOTAL cost that is minimum at EOQ.
- Option (c): If carrying cost is less than ordering cost, the order quantity is below the EOQ.
- Option (d): Overstocking is a result of poor inventory management, not a definition of the EOQ point.
What is the iron-carbon diagram? Draw a neat and labelled schematic iron-carbon diagram (need not be in scale).
For a ductile material, toughness is the measure of:
A)
resistance to scratching
B)
ability to absorb energy up to fracture
C)
ability to absorb energy till elastic limit
D)
resistance to indentation
Toughness is a fundamental material property defined as the ability of a material to absorb energy and deform plastically before it eventually fractures. In a stress-strain curve, toughness is represented by the total area under the curve from the origin to the point of fracture.
Analysis of options:
- Option (a) 'resistance to scratching' and Option (d) 'resistance to indentation' both describe 'Hardness', not toughness.
- Option (c) 'ability to absorb energy till elastic limit' describes 'Resilience' (the capacity of a material to absorb energy when it is deformed elastically and then release that energy upon unloading).
- Option (b) correctly identifies toughness as the total energy absorption capacity up to the point of failure (fracture), which is especially critical for materials subjected to impact loading.
How does the distribution of heat between the tool, chip, and workpiece affect machining performance?
Which of the following method is NOT used for obtaining the initial basic feasible solution in transportation problems?
A)
North west corner method
B)
Least cost entry method
C)
Vogel’s approximation method
D)
MODI method
In Transportation Problems (a specialized type of Linear Programming Problem), the solution process is divided into two main phases: finding an Initial Basic Feasible Solution (IBFS) and then testing that solution for optimality.
1. Initial Basic Feasible Solution (IBFS) methods are used to get a starting point. Common methods include:
- North West Corner Method (NWCM): A simple method that ignores costs and starts from the top-left cell.
- Least Cost Method (LCM): A method that prioritizes cells with the lowest unit transportation cost.
- Vogel's Approximation Method (VAM): A more sophisticated method using 'penalties' to find a solution closer to the optimum.
2. Optimality Testing methods are used to check if the IBFS can be improved to reduce the total cost. The most common methods for this are:
- MODI Method (Modified Distribution Method), also known as the u-v method.
- Stepping Stone Method.
Since the MODI method is used for optimality testing (improving an existing solution) and NOT for obtaining the initial basic feasible solution, it is the correct answer.
Explain the different types of crystal imperfections.
Which of the following casting is least likely to have directional solidification?
A)
Sand casting
B)
Investment casting
C)
Centrifugal casting
D)
Shell mould casting
Directional solidification occurs when the solidification front moves steadily from one end of the casting to the other, typically from the coldest part (mold wall) toward the hottest part (riser). This is a key goal in sand, investment, and shell molding to prevent shrinkage cavities.
1. Sand Casting (a), Investment Casting (b), and Shell Mould Casting (d) are all static casting processes. In these processes, heat is extracted primarily through the mold walls, and engineers use chills or risers to intentionally create a directional solidification gradient.
2. Centrifugal Casting (c) involves rotating the mold at high speeds. The centrifugal force pushes the molten metal against the outer wall of the mold. Because the mold wall is the primary cooling surface and the metal is being continuously forced outward and compressed by centrifugal pressure, the solidification happens rapidly and uniformly from the outer periphery inward toward the center. However, the dynamic nature of the process (rotation and centrifugal pressure) suppresses the traditional 'directional' flow from a specific gate/riser system seen in static castings. More importantly, in centrifugal casting, the impurities and gases are pushed toward the center (the axis of rotation), and the solidification is characterized by a radial pattern rather than a linear directional gradient from a feeder to a casting. Among the given options, the centrifugal process is the least dependent on and least likely to exhibit the traditional directional solidification used to eliminate shrinkage in static molds.
The crystal structure of austenite is:
A)
body centered cubic
B)
face centered cubic
C)
hexagonal closed packed
D)
body centered tetragonal
In the study of the Iron-Carbon equilibrium diagram, iron exists in different allotropic forms depending on the temperature.
1. **Austenite (γ-iron)**: This is the stable phase of iron between 912°C and 1394°C. It possesses a **Face Centered Cubic (FCC)** crystal structure. Because of the FCC arrangement, austenite has larger interstitial sites, allowing it to dissolve more carbon compared to ferrite.
2. **Ferrite (α-iron)**: This is the stable phase at room temperature (up to 912°C) and possesses a **Body Centered Cubic (BCC)** structure. This corresponds to option (a).
3. **Delta Ferrite (δ-iron)**: This is the stable phase at very high temperatures (above 1394°C) and also possesses a **Body Centered Cubic (BCC)** structure.
4. **Martensite**: When austenite is quenched rapidly, it transforms into a non-equilibrium phase called Martensite, which has a **Body Centered Tetragonal (BCT)** structure. This corresponds to option (d).
5. **Hexagonal Closed Packed (HCP)**: While some metals exhibit HCP structures, the standard phases of pure iron/steel (Austenite, Ferrite) do not. This corresponds to option (c).
Therefore, the crystal structure of austenite is specifically Face Centered Cubic (FCC).
Wrought iron can be manufactured by____________.
A)
Cementation processes
B)
Puddling processes
C)
Duplex processes
D)
None of these
Wrought iron is a nearly pure form of iron with a very low carbon content and contains small inclusions of slag. It is manufactured by removing carbon from pig iron. The 'Puddling process' is the traditional method used to produce wrought iron; it involves stirring (puddling) molten pig iron in a furnace to oxidize the carbon and impurities.
Analysis of other options:
- a. Cementation process: This is a method used to add carbon to the surface of low-carbon steel or wrought iron to create a hard, wear-resistant surface (case hardening), not to manufacture wrought iron itself.
- c. Duplex process: This is a combined heat treatment process (usually involving quenching and tempering) used to improve the mechanical properties of steel, not a manufacturing process for wrought iron.
- d. None of these: Incorrect, as option b is the standard manufacturing method.
Write the characteristics of a grinding wheel. Also, write the factors that affect the performance of the grinding wheel.
A spherical drop of molten metal of radius 5 mm was found to solidify in 12 s. In how much time will a similar drop of radius 10 mm would solidify?
What is strain hardening? Explain its cause.
The primary advantage of investment casting over sand casting is:
A)
Higher tooling rate
B)
Lower tooling cost
C)
Ability to cast intricate shapes with excellent surface finish
D)
Reduced melting temperature
Investment casting (also known as the lost-wax process) is specifically designed for high-precision components.
1. Why Option (c) is correct: In investment casting, a wax pattern is coated with a ceramic slurry to create a mold. Since the wax is melted out, there is no need for a 'parting line' or 'draft angles' required in sand casting. This allows for the creation of extremely complex, intricate geometries and provides a superior surface finish (low roughness) and tight dimensional tolerances.
2. Why Option (a) is incorrect: Tooling rate is not a standard primary advantage of investment casting; in fact, the process is generally slower due to the multiple steps involved (wax injection, shelling, dewaxing).
3. Why Option (b) is incorrect: Investment casting typically has higher tooling and operational costs compared to sand casting because of the cost of wax patterns and ceramic materials.
4. Why Option (d) is incorrect: The melting temperature of the metal is a property of the material being cast, not the casting process itself. Both sand casting and investment casting can be used for the same metals at the same melting temperatures.
Discuss in detail the various mechanisms of tool wear, such as flank wear, crater wear, notch wear, and built-up edge.
In cold working:
A)
strength, hardness and ductility increase
B)
strength and hardness increase, but ductility decreases
C)
strength, hardness and ductility decrease
D)
strength and hardness decrease, but ductility increases
Cold working (also known as strain hardening or work hardening) is the process of plastically deforming a metal below its recrystallization temperature. During this process, the dislocation density within the crystal lattice increases significantly. As these dislocations move and interact, they become entangled and impede each other's motion, making it more difficult for further deformation to occur. This results in an increase in the material's yield strength and hardness. However, because the capacity for further plastic deformation is reduced as the material becomes 'saturated' with dislocations, the ductility (the ability of the material to deform plastically before fracture) decreases. Therefore, strength and hardness increase while ductility decreases.
What is a surface imperfection, which separates crystals of different orientations a polycrystalline aggregate, called?
A)
Edge dislocation
B)
Stacking fault
C)
Grain boundary
D)
Screw dislocation
In material science, most engineering materials are polycrystalline, meaning they consist of many small crystals called 'grains'. Each grain has a specific crystallographic orientation. A grain boundary is the interface or surface imperfection that separates two grains of different orientations. These boundaries are considered 2D (surface) defects.
Why other options are incorrect:
- a. Edge dislocation: This is a 1D (line) defect involving an extra half-plane of atoms inserted into the crystal lattice.
- b. Stacking fault: This is a 2D (surface) defect, but it occurs within a single crystal due to an error in the sequence of stacking atomic planes (e.g., ABCABC becoming ABCBCA), not as a boundary between crystals of different orientations.
- d. Screw dislocation: This is a 1D (line) defect where the lattice is distorted in a helical path around the dislocation line.
In investment casting, the primary reason for preheating the ceramic mould before pouring is:
A)
To reduce thermal shock and prevent mould cracking
B)
To improve surface roughness of the casting
C)
To accelerate solidification
D)
To remove trapped gases in molten metal
In investment casting (also known as lost-wax casting), the ceramic shell is relatively brittle. When molten metal is poured into a cold ceramic mould, the extreme temperature difference between the liquid metal and the mould material creates high internal stresses due to rapid thermal expansion. This phenomenon is known as thermal shock, which can lead to the cracking or shattering of the ceramic mould, resulting in casting defects like fins or leakage. Preheating the mould reduces this temperature gradient, thereby minimizing thermal shock and ensuring the structural integrity of the mould.
Analysis of other options:
- Option (b) is incorrect because preheating generally improves surface finish by allowing the metal to flow more smoothly before freezing, but it is not the 'primary reason' for the preheating process itself.
- Option (c) is incorrect because preheating actually slows down the solidification rate (by reducing the cooling rate), which is often desired to prevent premature freezing in thin sections.
- Option (d) is incorrect because removing trapped gases is typically achieved through the permeability of the mould or vacuum assistance, not by preheating the mould shell.
In thread cutting operation the saddle is moved by:
A)
Head stoke
B)
Tail stoke
C)
Tool post
D)
Lead screw
In a lathe machine, thread cutting requires a precise relationship between the rotation of the spindle (workpiece) and the longitudinal movement of the cutting tool. This is achieved using the lead screw. The lead screw is a long, threaded shaft that is engaged with the saddle via a half-nut mechanism. As the lead screw rotates, it translates the rotary motion into a precise linear motion of the saddle, ensuring that the tool moves a specific distance (the lead) for every single revolution of the spindle, thereby creating a consistent thread pitch.
Why other options are incorrect:
- a. Headstock: This is the stationary part of the lathe that houses the main spindle and gear mechanism; it does not move the saddle.
- b. Tailstock: This is used to support long workpieces or hold drilling/reaming tools; it does not drive the saddle for threading.
- c. Tool post: This is the component that holds the cutting tool securely in place on top of the compound rest; it is a passive holder and not a driving mechanism.
The properties of material to resist penetration by another material is known as:
A)
Impact strength
B)
Hardness
C)
Fatigue
D)
Resilience
Step-by-step reasoning:
1. Definition of Hardness: In material science, hardness is defined as the resistance of a material to localized plastic deformation, such as indentation, scratching, or penetration by a harder object. Common tests for hardness include the Brinell, Rockwell, and Vickers tests, which all measure the depth or area of penetration by a standardized indenter.
2. Analysis of other options:
- Impact strength (a): This refers to the ability of a material to absorb energy and plastically deform before fracturing under a high-rate load (sudden impact). It is measured by tests like the Charpy or Izod tests.
- Fatigue (c): This is the tendency of a material to break or fail under repeated or cyclic loading, even if the stress is below the yield strength.
- Resilience (d): This is the capacity of a material to absorb energy when it is deformed elastically and then release that energy upon unloading.
3. Conclusion: Since the question specifically asks for the property to resist 'penetration', the correct term is Hardness.
What is directional solidification in casting, and why is it important in achieving defect-free castings?
Extrusion force does not depend upon the:
A)
extrusion ratio
B)
type of extrusion process
C)
material of the die
D)
working temperature
Extrusion force is the force required to push a metal billet through a die to create a desired cross-section. It is governed by the formula: F = A₀ * σ_f * ln(R), where A₀ is the initial area, σ_f is the flow stress of the material, and R is the extrusion ratio.
1. Extrusion Ratio (a): The force is directly proportional to the natural log of the extrusion ratio (R = A₀/A_final). A higher ratio requires more force.
2. Type of Extrusion Process (b): Direct extrusion requires more force than indirect extrusion because in direct extrusion, there is additional friction between the billet and the container wall, whereas in indirect extrusion, there is no relative motion between the billet and the container.
3. Working Temperature (d): The flow stress (σ_f) of the metal is highly dependent on temperature. Increasing the temperature reduces the yield strength of the material, thereby reducing the required extrusion force.
4. Material of the Die (c): While the die material must be strong and wear-resistant enough to withstand the process, the *force required to deform the workpiece* depends on the workpiece's properties and the geometry of the process, not the chemical composition or material of the die itself (assuming the die does not deform).
The maximum heat in resistance welding is at the:
A)
tip of the positive electrode
B)
tip of the negative electrode
C)
top surface of the plate at the time of electric contact with the electrode
D)
interface between the two plates being joined
In resistance welding (such as spot welding), heat is generated based on Joule's Law: H = I²Rt, where 'H' is the heat generated, 'I' is the current, 'R' is the electrical resistance, and 't' is the time. The heat generated is directly proportional to the electrical resistance of the material. In a resistance welding setup, the electrodes are typically made of high-conductivity materials (like copper alloys) to minimize their own resistance and prevent them from overheating. The highest electrical resistance in the entire circuit occurs at the interface between the two workpieces (the plates) due to the presence of surface oxides, contaminants, and the microscopic gaps between the two surfaces before they fuse. Because the resistance (R) is maximum at this interface, the maximum heat is generated there, allowing the metal to melt and form a weld nugget while the electrodes remain relatively cool.
What is gas welding? Describe the types of flames used in oxy-acetylene welding and their applications.
The process of producing film or thin sheets by squeezing a thermoplastics material between the revolving cylinders is known as:
A)
Lamination
B)
Contact moulding
C)
Calendaring
D)
Pressure bag moulding
Step-by-step reasoning:
1. **Analysis of the Process**: The question describes a process where thermoplastic material is squeezed between revolving cylinders to produce thin sheets or films. This is the fundamental definition of 'Calendaring'.
2. **How Calendaring Works**: In a calender machine, a softened plastic material is passed through a series of heated rollers (cylinders). The gap between the rollers determines the thickness of the final sheet, and the pressure applied ensures a uniform, smooth finish.
3. **Evaluating Other Options**:
- **Lamination (a)**: This is the process of bonding two or more layers of material together to create a composite. While it may involve rollers, its primary purpose is bonding, not the initial formation of a sheet from bulk thermoplastic.
- **Contact Moulding (b)**: This is a simple compression moulding technique where a material is placed in a heated mold and pressed. It does not involve revolving cylinders.
- **Pressure Bag Moulding (d)**: This is a variation of compression moulding where a flexible bag is used to apply uniform pressure to a part. It does not involve revolving cylinders.
4. **Conclusion**: Based on the mechanical action (squeezing between revolving cylinders) and the output (films/thin sheets), Calendaring is the only correct process.