Question 1

Three-high rolling mill — relative roll rotation directions In a three-high mill (three work rolls placed one above another), which statement correctly describes the rotation directions so that the strip can pass alternately up and down without reve…

Accepted Answer

Introduction / Context: Three-high mills permit consecutive passes without stopping to reverse roll rotation. By arranging the rotation directions properly, material can be threaded back and forth between gaps to increase reduction efficiency. Given Data / Assumptions: Three stacked rolls: top, middle, bottom. Strip is alternately fed through top–middle and middle–bottom gaps. No electrical reversing between passes is desired. Concept / Approach: To draw the strip into each gap, the rolls that form that gap must present opposing surface velocities at the nip. Therefore, the middle roll must rotate opposite to both top and bottom rolls, while the top and bottom rotate the same way. This allows the strip to pass upward through one gap and downward through the other simply by changing the vertical position. Step-by-Step Solution: Require opposite surface motion at each nip for biting.Set middle roll opposite to the other two.Conclude: top and bottom same direction; middle opposite. Verification / Alternative check: Milling schematics of three-high stands depict exactly this arrangement to avoid motor reversal while enabling multi-pass reductions. Why Other Options Are Wrong: Top–middle same direction: one of the nips would not bite simultaneously. Bottom–middle same direction: same issue. “Any one of the above” or “all same direction”: physically incorrect for feeding. Common Pitfalls: Assuming speed differences can substitute for correct direction; the nip requires opposing tangential velocities. Final Answer: The upper and bottom rolls rotate in the same direction whereas the middle roll rotate in opposite direction.

Question 2

Oxy–acetylene technique — travel direction in forehand (leftward) welding In gas welding practice, forehand welding (also known as leftward welding) is characterized by the torch leading the filler. In which travel direction is the weld typically ma…

Accepted Answer

Introduction / Context: Oxy–acetylene welding uses distinct techniques: forehand (leftward) and backhand (rightward). Knowing the correct travel direction improves heat distribution, puddle control, and joint quality, especially on thin sheets. Given Data / Assumptions: Forehand (leftward) technique is specified. Operator maintains a neutral flame unless otherwise required. Filler rod is fed ahead of the molten pool with the torch leading. Concept / Approach: In forehand welding, the torch points in the direction of travel, preheating the joint ahead of the puddle. The conventional description is left-to-right travel (hence leftward), ideal for thin sections due to broader preheating and lower risk of burn-through. Backhand welding is the reverse, commonly used for thicker sections. Step-by-Step Solution: Identify named method: forehand (leftward).Apply definition: torch leads and weld progresses left to right.Confirm that backhand is the right-to-left counterpart. Verification / Alternative check: Welding training manuals define forehand as leftward travel with specified torch and rod angles conducive to sheet work. Why Other Options Are Wrong: Right to left: that is backhand (rightward) welding. Both directions or circular weave only: not specific to forehand definition. Common Pitfalls: Confusing torch angle and rod angle; mixing up forehand/backhand names versus the actual movement on the workpiece. Final Answer: from left to right

Question 3

Bulk deformation processes — what is rotary swaging used for? Rotary swaging (also called rotary forging or radial forging) is a hammering process using rapidly opening-and-closing dies around a rotating work. Which of the following best describes i…

Accepted Answer

Introduction / Context: Rotary swaging is a high-speed, incremental deformation process that reduces diameter, forms tapers, and compacts material, particularly for rods and tubes. It creates surface work hardening and improved dimensional control without continuous dies. Given Data / Assumptions: Process involves segmented dies that open/close rapidly. Workpiece is typically round bar or tube. Cold or warm operation is common, depending on alloy and reduction. Concept / Approach: In swaging, hammers driven mechanically or pneumatically strike the dies to shrink the work in small steps as it rotates/feeds axially. The method is well suited to forming tapered shafts, reducing tube ends for fittings, and consolidating powder metal preforms. It is not the same as cold heading (bolt making), shot peening (surface compressive stresses), or extrusion (forcing metal through a die orifice under high hydrostatic pressure). Step-by-Step Solution: Match the described die action (opening/closing) to rotary swaging machines.Identify the typical product geometry: round bars and tubes.Select the option that states diameter reduction using oscillating dies. Verification / Alternative check: Process capability charts list swaging for reductions of up to moderate percentages per pass, with good surface finish and minimal material loss. Why Other Options Are Wrong: Bolt and rivet manufacture: primarily cold heading. Improving fatigue by surface compression: that is shot peening. Pressing metal through an orifice: that is extrusion. Scale removal: that is descaling or pickling, not swaging. Common Pitfalls: Confusing swaging with drawing or extrusion; overlooking that swaging can process tubes with internal mandrels to control wall thickness. Final Answer: is used for reducing the diameters of round bars and tubes by rotating dies which open and close rapidly on the work

Question 4

Cold forming for fasteners — most widely used process for bolts and nuts Which manufacturing process is most extensively used to produce bolt and nut heads efficiently with high material utilization and good mechanical properties?

Accepted Answer

Introduction / Context: Fastener production hinges on speed, consistency, and strength. The chosen process should minimize waste, use inexpensive wire stock, and increase strength via work hardening and favorable grain flow around the head. Given Data / Assumptions: Products: bolts and nuts (especially bolt heads and upset features). Material: low-to-medium carbon steels or alloy steels, often from coil. Requirement: high-volume, repeatable production. Concept / Approach: Cold heading upsets the wire end in a die cavity at room temperature to form heads and other features. This process offers high material utilization, excellent surface finish, and enhanced mechanical properties through strain hardening and oriented grain flow. Subsequent thread rolling further improves fatigue resistance. Alternative processes exist but are less optimal for high-volume fasteners. Step-by-Step Solution: Feed wire → cut to length (blank) → cold head to form head.Optionally perform multi-die progressive heading for complex geometries.Follow with thread rolling and heat treatment as required. Verification / Alternative check: Industry practice and standards identify cold heading as the dominant process for bolt heads and many nut bodies, with extrusion reserved for special forms. Why Other Options Are Wrong: Hot piercing: used for making holes in hot workpieces, not heads. Extrusion: viable for some shapes but not the mainstream choice for bolts/nuts. Cold peening: surface treatment for residual compressive stresses, not head formation. Electroforming: additive process, not applicable to fastener mass production. Common Pitfalls: Confusing heading with machining; cold heading greatly reduces waste compared to turning from bar. Final Answer: cold heading

Question 5

Hand sawing — preventing the blade body from binding in the kerf When using a hand or power saw, what practice is applied to the teeth so that the blade's body does not jam in the saw cut (kerf) and friction is reduced?

Accepted Answer

Introduction / Context: Effective sawing depends on managing kerf width. If the kerf is the same as or smaller than the blade body, friction skyrockets and the blade binds. Toolmakers therefore modify tooth geometry to create a kerf slightly wider than the blade thickness. Given Data / Assumptions: Sawing of wood or metal with toothed blades (hand hacksaw, bandsaw). Goal: avoid binding and reduce heating. Tooth set options include alternate, raker, or wavy set depending on blade type. Concept / Approach: “Setting” the teeth means bending them alternately to left and right (or in a wavy pattern) so the cutting edges produce a kerf wider than the blade body. Sharpening restores cutting efficiency but does not, by itself, widen the kerf. Strengthening relates to material or heat treatment and is not a direct operational adjustment. Step-by-Step Solution: Ensure the tooth set exceeds blade thickness slightly.Select the appropriate set pattern for material and blade style.Check that teeth are sharp to take advantage of the set without rubbing. Verification / Alternative check: Blade manufacturers specify tooth set values and patterns to match the intended cut and minimize binding. Why Other Options Are Wrong: Sharpened: necessary but insufficient; kerf may still match blade thickness. Strengthened: unrelated to kerf width. “All of these”: only tooth set directly prevents jamming. Staggered gullets alone do not guarantee a wider kerf. Common Pitfalls: Using a blade with worn-away set; excessive set causing rough cut; insufficient lubrication on metal cutting. Final Answer: set

Question 6

Forehand gas welding — recommended filler rod angle In oxy–acetylene forehand (leftward) welding, what approximate angle should the filler rod make with the work surface for effective deposition and puddle control?

Accepted Answer

Introduction / Context: Torch and filler rod angles strongly influence heat distribution, puddle protection, and bead shape in oxy–acetylene welding. Forehand technique favors broader preheat of the joint ahead of the puddle, especially useful on thin materials. Given Data / Assumptions: Technique: forehand (leftward) welding. Neutral flame unless otherwise specified. Objective: choose a practical filler rod angle for steady feeding. Concept / Approach: Typical practice sets the torch at approximately 40°–50° to the work and the filler rod at a slightly lower angle to allow easy dipping and shielding of the molten pool. Keeping the rod at about 30°–40° helps preheat the rod tip, minimizes disturbance of the puddle, and ensures proper wetting of the edges. Step-by-Step Solution: Position the torch to lead the direction of travel (forehand).Hold the filler rod at about 30°–40° to the work surface, pointing into the leading edge of the puddle.Maintain steady addition while adjusting torch distance to control bead width. Verification / Alternative check: Training guides for gas welding specify rod angles near 30°–40° for forehand, with slight variations by joint type and thickness. Why Other Options Are Wrong: Angles above 50° obstruct the view and disturb the pool. Very small angles (10°–20°) starve the puddle and lead to dragging the rod. 40°–50° is more typical of the torch angle, not the rod angle in many curricula. Common Pitfalls: Confusing torch and rod angles; pushing the rod too vertically, which splashes metal and oxidizes the tip. Final Answer: 30°–40°

Question 7

Foundry defects — identification by visual symptom In sand casting practice, which named defect describes a general enlargement or bulging of the entire casting shape due to pressure of molten metal acting on a weak mould face (i.e., the casting com…

Accepted Answer

Introduction / Context: Recognising common casting defects is essential for process troubleshooting in foundry engineering. “Swell” is a defect that causes the whole casting or a region of it to appear enlarged compared to the pattern. This question checks your ability to match the correct technical term to the observed symptom and link it to its root causes in moulding practice. Given Data / Assumptions: Sand moulding with metallic pattern or wooden pattern is assumed. The final casting is consistently larger in some area than intended, without a simple relative displacement between cope and drag. Molten metal exerts hydrostatic and ferrostatic pressure on the mould wall. Concept / Approach: A “swell” occurs when weakly rammed or low-strength sand yields under metal pressure, allowing the mould face to move outward. The result is a uniformly “fatter” profile in the affected zone. This is different from “shift”, which is a mismatch caused by misalignment of the cope and drag; “scab”, which is a raised, scaly patch due to sand lifting; and “sand wash”, which is erosion or washing of sand into the cavity causing rough surfaces. Correct identification suggests corrective actions such as improving ramming uniformity, increasing mould hardness, or adding reinforcements like gaggers. Step-by-Step Solution: Observe the symptom: overall enlargement/bulging rather than a discrete patch or a clean mismatch line.Relate symptom to cause: inadequate mould hardness/strength allows wall movement under metal head.Map to terminology: this behaviour matches the defect known as “swell”.Confirm by prevention strategy: better ramming, correct moisture, and stronger facing sand reduce swell. Verification / Alternative check: If the defect were a “shift”, measurement would show two halves of the casting offset across the parting plane; if it were a “scab”, you would see a flaky, sand-laden patch. Absence of these confirms “swell”. Why Other Options Are Wrong: Shift: misalignment at the parting line, not uniform enlargement. Sand wash: erosion produces rough, sand-embedded surfaces, not bulging. Scab: localized raised blister with sand inclusion, not overall growth. Misrun: incomplete filling, giving undersize/rounded edges, opposite of enlargement. Common Pitfalls: Confusing “swell” with machining allowance; swell is unplanned and irregular, machining allowance is deliberate and uniform. Final Answer: swell

Question 8

Fits and tolerances — count of standard tolerance grades (per Indian/ISO practice) According to Indian Standards harmonised with ISO (e.g., IS 919), how many standard tolerance grades (IT grades) are recognised across the full accuracy range?

Accepted Answer

Introduction / Context: Manufacturing drawings specify sizes as well as acceptable variation. Standard systems, such as ISO/Indian Standard tolerance grades (IT grades), define bands of tolerance quality from very precise gauge work to coarse commercial fits. Knowing how many grades exist helps you choose appropriate quality levels for cost-effective production. Given Data / Assumptions: Indian Standards align with ISO tolerance system nomenclature. IT grades span from very fine metrology (e.g., IT01) to coarse (e.g., IT16). Count should include the special fine grades along with the numbered sequence. Concept / Approach: The well-known sequence of standard tolerance grades includes IT01, IT0, and IT1 through IT16. Counting them yields a total of 18 grades. These are applied with fundamental deviations (letters) to generate holes and shafts fits for the hole-basis or shaft-basis systems. Selecting a higher IT number increases the tolerance band, reducing machining cost but also reducing accuracy. Step-by-Step Solution: Enumerate: IT01, IT0, IT1, IT2, …, IT16.Compute count: 2 special + 16 numbered = 18 total grades.Relate to practice: fine gauges or precision bearings use very low IT numbers; structural parts may use higher IT numbers. Verification / Alternative check: Standards tables list tolerances for each grade versus basic size; cross-check any machining handbook to see the 18-grade set confirmed. Why Other Options Are Wrong: 8, 12, 20, 24: do not match the complete recognised set including special fine grades. Common Pitfalls: Forgetting to count IT01 and IT0; learners often start at IT1 and arrive at 16, which is incomplete. Final Answer: 18

Question 9

Welding processes — operating environment for electron beam welding (EBW) EBW achieves deep-penetration fusion by a focused high-velocity electron beam. In standard industrial practice, EBW is carried out in which environment to prevent beam scatter…

Accepted Answer

Introduction / Context: Electron beam welding (EBW) is a high-energy-density process used for deep, narrow welds with minimal distortion. The focused electron beam must travel from the gun to the work without being scattered. Understanding the required environment is crucial to process capability and equipment design. Given Data / Assumptions: Electron beam quality is sensitive to gas molecules in the path. High vacuum eliminates scattering and prevents oxidation of the hot metal. Low-voltage, non-vacuum “EBW-like” guns exist in niche applications, but the standard high-quality process is vacuum based. Concept / Approach: The momentum of electrons is sufficient for deep penetration, but the mean free path must be long to maintain a sharp focus. Air or even shielding gas causes collisions that spread the beam, reduce energy density, and introduce oxygen/nitrogen contamination. Therefore, conventional EBW chambers are evacuated to high vacuum, often below 10^-4 mbar, before firing the beam. Step-by-Step Solution: Identify the performance requirement: long mean free path and oxidation control.Match to environment: only vacuum delivers both sharp focus and clean atmosphere.Select the correct option: “vacuum”. Verification / Alternative check: Process specifications for aerospace, automotive gear welding, and power devices consistently cite vacuum chambers and pump-down cycles as part of EBW setup. Why Other Options Are Wrong: Open air / shielded gas: excessive scattering; severe oxidation risk. Pressurised inert chamber: pressure worsens scattering; not used for standard EBW. Underwater: impractical and would extinguish the beam immediately. Common Pitfalls: Confusing EBW with laser beam welding; lasers can operate in air with shielding, but EBW requires vacuum for focus and cleanliness. Final Answer: vacuum

Question 10

Moulding sands — binder type used in oil sand mixes In foundry oil sands (e.g., linseed-oil bonded sands) used for core making or special moulds, the binder type is classified as which of the following?

Accepted Answer

Introduction / Context: Binders provide strength and cohesiveness to moulding sand. Oil-bonded sands are a traditional route for producing smooth surfaces and strong cores without moisture-related defects. Knowing the binder category helps in selecting drying/curing cycles and predicting gas evolution. Given Data / Assumptions: “Oil sand” refers to mixes with vegetable oils (e.g., linseed, tung) or synthetic organic resins. They cure by oxidation/polymerisation when baked. Clay binders (e.g., bentonite) are mineral and activated by water, not by baking oil films. Concept / Approach: Oil-bonded sands use organic binders. During baking, the oil oxidises and forms a strong film around sand grains, developing dry strength and excellent finish. In contrast, clay-type binders are inorganic phyllosilicates needing moisture; inorganic chemical binders include sodium silicate (CO2 sand) which hardens by chemical reaction, not by oil polymerisation. Step-by-Step Solution: Identify the system: oil-bonded sand for cores/moulds.Classify binder: organic.Select option “organic-type binder”. Verification / Alternative check: Foundry manuals list linseed oil, cereal flours, or resins under organic binder families for core sands. Why Other Options Are Wrong: Clay-type: water-activated, not baked oil systems. Inorganic-type: e.g., sodium silicate; different curing mechanism. Any one / no binder: incorrect—oil sands rely on organic binders. Common Pitfalls: Assuming “oil” implies lubrication only; here it is a true binding agent that polymerises to provide strength. Final Answer: organic-type binder

Question 11

Forging fundamentals — definition of upsetting In hot or cold forging terminology, “upsetting” refers to which primary deformation outcome for a bar or rod?

Accepted Answer

Introduction / Context: Forging processes are categorised by the way metal flows. Upsetting is a basic operation used to form heads on bolts, valves, and other components by increasing cross-sectional area at a local region while shortening the length. Understanding the definition helps distinguish it from drawing down, bending, welding, and extrusion. Given Data / Assumptions: A cylindrical or prismatic bar is subjected to axial compression. Material is within the plastic range (hot forging or sometimes cold heading). Friction at dies and temperature distribution are such that diameter increases as length decreases locally. Concept / Approach: Upsetting applies compressive forces along the axis to thicken the workpiece, causing a rise in cross-section and a reduction in length where deformation occurs. This is the opposite of drawing (which elongates and reduces section). It is central to operations like bolt heading, rivet making, and flange formation prior to piercing or trimming. Step-by-Step Solution: Identify deformation mode: axial compression on the bar end or mid-length.Predict metal flow: outward (radial) flow increases diameter; axial length diminishes.Relate to term: this outcome is “upsetting”. Verification / Alternative check: Shop examples include cold heading of fasteners where wire is upset in a die cavity to form a head before shank forming. Why Other Options Are Wrong: Reducing cross-section: describes drawing or swaging. Joining under pressure: that is forge welding. Bending: changes direction but not necessarily cross-section. Extruding: forces metal through a die to reduce section/reshape continuously. Common Pitfalls: Assuming any compression equals upsetting; true upsetting specifically increases cross-section noticeably. Final Answer: increasing the cross-section of a bar

Question 12

Press tool theory — identify the incorrect statement Which of the following statements is WRONG with respect to sheet-metal press operations and die geometry?

Accepted Answer

Introduction / Context: Accurate pressworking requires correct die clearance, proper punch and die sizing, and understanding of material flow during operations such as blanking and deep drawing. This question probes common misconceptions that lead to dimensional errors or poor edge quality. Given Data / Assumptions: Conventional cold press operations on sheet steel or non-ferrous alloys. Standard practice: punch size controls pierced hole size; die opening controls blank size. Typical die angular clearance is small to facilitate stripping and reduce friction. Concept / Approach: In piercing, the punch shears through the sheet and the hole left in the work matches the punch dimension (with allowance for elastic recovery and edge break). Therefore, the hole size depends directly on the punch size. Drawing is a plastic flow process where the blank is pulled into the die by the punch while material thins or thickens slightly, so statement (b) is correct. Small angular clearance on die walls (0.5° to 1°) aids part ejection and reduces scuffing, so (a) is also correct. Step-by-Step Solution: Recall sizing rule: pierced hole size ≈ punch size; blank size ≈ die opening.Evaluate statement (c): says “independent of punch size” which contradicts the rule.Conclude: (c) is the wrong statement. Verification / Alternative check: Tolerance tables in die design handbooks specify punch undersize/oversize based on desired hole/blank size and material springback, reaffirming dependence on punch dimensions. Why Other Options Are Wrong: (a) True: small angular relief is standard. (b) True: drawing relies on plastic flow controlled by blank-holder pressure and friction. (d) “none of the above” is false because (c) is wrong. (e) True statement about clearance influence. Common Pitfalls: Mixing the rules for blanking and piercing; remember: punch controls hole, die controls blank. Final Answer: In press operations, the final size of a pierced hole is independent of the punch size.

Question 13

Resistance spot welding — recommended edge distance For a satisfactory spot weld near an edge, the minimum distance from the nearest sheet edge to the centre of the weld nugget should be approximately:

Accepted Answer

Introduction / Context: In resistance spot welding, electrode indentation, current density, and heat flow must be balanced to create a strong nugget without edge burn-out or expulsion. Proper spacing from edges prevents molten metal from escaping and ensures adequate current paths. Given Data / Assumptions: d denotes the weld nugget diameter or, in practice, the electrode face diameter used for sizing. Sheets of comparable thickness are being welded. Standard shop guidelines are applied for mild steel. Concept / Approach: The minimum edge distance is set to keep sufficient surrounding material to contain the nugget and provide adequate current and force distribution. A commonly used rule is edge distance ≈ 1.5 d to 2 d for steels. At this spacing, the risk of edge expulsion, undersized nuggets, or severe indentation is reduced, while still allowing welds close to flanges or corners. Step-by-Step Solution: Relate edge distance to nugget size: choose a factor that ensures containment of the fusion zone.Select the lower standard bound: 1.5 d is widely accepted as a minimum.Hence, the recommended value is 1.5 d. Verification / Alternative check: Automotive and appliance welding schedules typically specify 1.5 d to 2 d; destructive peel tests near edges confirm sufficient strength at these spacings. Why Other Options Are Wrong: d or 0.75 d: too close; causes edge burning, expulsion, and weak nuggets. 3 d or 6 d: acceptable but unnecessarily conservative for most designs, reducing layout efficiency. Common Pitfalls: Confusing pitch (spacing between adjacent welds) with edge distance; the pitch is often 3 d to 6 d, not the edge distance. Final Answer: 1.5 d

Question 14

Projection welding — placing projections for unequal thickness joints When projection welding two sheets of unequal thickness, on which sheet should the projections be formed to obtain consistent heat concentration and collapse?

Accepted Answer

Introduction / Context: Projection welding localises current and force at small raised points, creating repeatable nuggets while allowing multiple welds in one squeeze. For dissimilar sheet thicknesses, projection placement determines current density, heat balance, and mechanical collapse of the projection. Given Data / Assumptions: Two low-carbon steel sheets with different thicknesses are to be joined. Standard dome or truncated cone projections are available. Objective is reliable fusion at the interface without burn-through of the thin sheet. Concept / Approach: Projections are generally formed on the thicker sheet. The thicker sheet provides structural support for the projection during squeeze, maintains geometry, and reduces the risk of overheating or expulsion in the thin sheet. As current concentrates at the small contact area of the projection tip, heat is generated at the interface; the projection collapses plastically, forging the joint. Putting the projection on the thin sheet raises the risk of local overheating and burn-through before adequate forging pressure is attained. Step-by-Step Solution: Identify goal: concentrate heat at interface while preventing thin-sheet burn-through.Choose projection location to maximise robustness: on the thicker sheet.Result: stable collapse and balanced heat input yield sound nuggets. Verification / Alternative check: OEM welding practice standards and projection nut applications consistently place projections on the thicker member or on the fastener (nut), which is thicker than the sheet. Why Other Options Are Wrong: Thinner sheet: prone to overheating and expulsion. Either/both: not optimal for heat balance and tooling economy. Interlayer only: used for special fasteners, not typical sheet-to-sheet joints. Common Pitfalls: Assuming electrical resistance of the thin sheet automatically places heat there; projection geometry intentionally controls resistance and pressure at the interface. Final Answer: thicker

Question 15

Oxy-fuel cutting — material best suited to oxygen cutting Which material can be most effectively and cleanly cut using the oxy-acetylene (oxygen cutting) process?

Accepted Answer

Introduction / Context: Oxy-fuel cutting relies on a chemical reaction: rapid oxidation of hot iron releases heat that sustains the cut and blows away iron oxides with an oxygen jet. The process favours materials that form oxides with low melting points and support exothermic oxidation. Given Data / Assumptions: Cutting torch with preheat flames and oxygen jet. Base metals considered: mild steel versus non-ferrous alloys. Goal: steady cut with narrow kerf and reasonable speed. Concept / Approach: Mild steel (low-carbon steel) is the ideal candidate because iron oxidises exothermically and the iron oxide (FeO/Fe2O3/Fe3O4) melts at a temperature lower than the steel, allowing the jet to eject slag. Non-ferrous metals like copper, aluminium, and brass form refractory oxides or conduct heat away too rapidly; their oxides either melt at very high temperatures or do not support the exothermic reaction well, causing stalling and a ragged kerf. Step-by-Step Solution: Preheat the steel edge to ignition temperature with the torch.Press the oxygen lever; the iron rapidly oxidises and the jet expels molten oxides.The reaction sustains itself along the cut path in mild steel: clean, continuous cutting. Verification / Alternative check: Shop practice confirms that stainless steels and non-ferrous metals require plasma or mechanical cutting, while carbon steels are routinely oxy-fuel cut on site and in fabrication shops. Why Other Options Are Wrong: Brass/copper/aluminium: high thermal conductivity and oxide behaviour impede oxygen cutting. Austenitic stainless steel: chromium oxide layer resists rapid oxidation; oxy-fuel is ineffective. Common Pitfalls: Confusing oxy-fuel cutting with oxy-fuel heating or brazing; the cutting mechanism is oxidation, not melting alone. Final Answer: mild steel

Question 16

Arc welding polarity — definition of straight polarity (DCEN) In manual metal arc (SMAW) with direct current, the setup is called “straight polarity” when which connection is used?

Accepted Answer

Introduction / Context: Polarity in DC arc welding affects heat distribution between the electrode and the work. Correct selection influences penetration, spatter, and electrode performance. The two basic options are straight polarity (DCEN) and reverse polarity (DCEP). Given Data / Assumptions: SMAW setup with a DC power source. Electrode holder carries the consumable electrode; the work clamp connects to the workpiece. Names: DCEN = direct current electrode negative (straight); DCEP = direct current electrode positive (reverse). Concept / Approach: In DCEN, electrons flow from the electrode (negative) to the work (positive). About two-thirds of the heat is generated at the positive side due to electron impact. Therefore, DCEN concentrates more heat in the workpiece, yielding deeper penetration and cooler electrode tip. This configuration is traditionally called straight polarity. Conversely, DCEP (reverse polarity) focuses more heat at the electrode and is beneficial for certain coatings and cleaning actions in processes like GTAW on aluminium. Step-by-Step Solution: Translate terminology: “straight polarity” = “electrode negative”.Apply to leads: electrode holder to negative; work to positive.Select the matching option. Verification / Alternative check: Welding textbooks and power source panels often label terminals EN (electrode negative) and EP (electrode positive), confirming the definition. Why Other Options Are Wrong: Option (b) describes reverse polarity (DCEP). Options (c) and (d) misuse “earth” and do not define DCEN correctly. Option (e) describes AC, not DC straight polarity. Common Pitfalls: Memorise: Straight = Electrode Negative (S → EN). Reverse = Electrode Positive. Final Answer: work connected to positive and electrode holder to negative

Question 17

Hot working of metals — identify the combined effects: During hot working (deformation above the material’s recrystallisation temperature), which of the following outcomes are achieved in the workpiece?

Accepted Answer

Introduction / Context: Hot working is a fundamental metal forming route where deformation is carried out above the recrystallisation temperature. This regime enables concurrent softening by dynamic recovery and recrystallisation, profoundly improving the internal quality and properties of the metal. Understanding the collective benefits of hot working is essential for selecting forming processes and setting parameters for forging, rolling, and extrusion. Given Data / Assumptions: Deformation is performed above the recrystallisation temperature. Material exhibits dynamic recovery/recrystallisation during deformation. Sufficient strain and temperature are achieved to heal defects and refine grains. Concept / Approach: At elevated temperature, dislocations rearrange and new, strain-free grains form, breaking up prior coarse grains and eliminating work hardening. Pores and microvoids are reduced by plastic flow and diffusion-assisted healing. The net effect is a refined microstructure and improved mechanical properties, especially toughness and ductility. Directional flow may also align inclusions more favourably for service loading. Step-by-Step Solution: During hot deformation, recovery reduces dislocation density and lowers flow stress.Dynamic recrystallisation nucleates and grows new fine grains, refining the structure.Sustained pressure and temperature allow closure of porosity and weld internal voids.The refined, cleaner microstructure improves tensile ductility, impact toughness, and often fatigue resistance. Verification / Alternative check: Metallographic evidence after hot forging shows equiaxed fine grains and reduced porosity compared to as-cast material. Why Other Options Are Wrong: Each of options A, B, and C states a true partial effect; the most complete and therefore correct choice is the combination “all of the above”. Common Pitfalls: Assuming hot working always increases strength; strength at room temperature depends on final grain size and any post-process heat treatment. Confusing hot working with hot isostatic pressing (HIP); while both can reduce porosity, mechanisms and outcomes differ. Ignoring oxidation and scale formation, which must be managed by protective atmospheres or machining allowances. Final Answer: all of the above

Question 18

Die casting finish and allowances: In pressure die casting of non-ferrous alloys (e.g., aluminium, zinc), what level of machining allowance is typically provided on the casting?

Accepted Answer

Introduction / Context: Die casting is a near-net-shape process that uses precision steel dies and high-pressure injection to produce components with excellent dimensional accuracy and surface finish. Correctly choosing machining allowance affects cost, cycle time, and tool life. This question probes understanding of process capability compared with sand or investment casting. Given Data / Assumptions: High-pressure die casting (hot- or cold-chamber). Dimensional repeatability is high; surfaces are smooth. Only minor post-processing may be required (e.g., flash trimming, drilling). Concept / Approach: Because steel dies provide stable cavity dimensions and controlled cooling, die castings achieve tight tolerances and smooth surfaces. Therefore, only a small machining allowance is needed, typically for critical fits, sealing faces, or to remove minor die wear effects. Large allowances would negate the economic advantages and increase machining cost unnecessarily, while “no allowance” is risky for precision fits or after surface treatments that slightly alter dimensions. Step-by-Step Solution: Compare process capabilities: die casting vs. sand casting.Recognise that die casting yields near-net shape with good finish.Provide only minimal stock on critical features for finishing.Thus, machining allowance is small. Verification / Alternative check: Tolerancing guides show typical die casting dimensional tolerances tighter than sand-cast parts, confirming minimal finishing stock is adequate. Why Other Options Are Wrong: Large or very large allowances waste material and time. “Not provided” can lead to undersize after finishing steps; some stock is prudent. Sand casting needs larger allowances; equating the two ignores different accuracy levels. Common Pitfalls: Ignoring die thermal expansion and shrink factors; not accounting for surface treatments (anodising, plating) that can change dimensions slightly; assuming all faces need machining when as-cast surfaces often suffice. Final Answer: small

Question 19

Thermit welding chemistry: In the thermit process, which mixture reacts exothermically to produce molten metal for joining?

Accepted Answer

Introduction / Context: Thermit welding relies on an aluminothermic reaction that generates intense heat and molten metal without external power sources. It is widely used for joining railway rails and heavy sections where conventional arc access is difficult. Given Data / Assumptions: Thermit mixture is reacted in a refractory crucible above the joint. The reaction must be highly exothermic to melt steel and filler. Fluxes and ignition powder (e.g., barium peroxide/magnesium) may be used to initiate. Concept / Approach: The classic aluminothermic reaction uses finely divided aluminium as a strong reducing agent to reduce iron oxide to metallic iron while forming aluminium oxide. The reaction releases large quantities of heat, producing molten iron/steel which is directed into a prepared mould around the joint gap. Step-by-Step Solution: Choose reagents: Fe2O3 (or Fe3O4) + Al as fuel/reductant.Reaction (plain text): iron oxide + aluminium → aluminium oxide + iron + heat.The molten iron fills the mould and fuses the components after solidification.Therefore, the correct mixture is iron oxide and aluminium. Verification / Alternative check: Field practice and handbooks specify thermit as iron oxide and aluminium; charcoal is not part of the classical mixture. Why Other Options Are Wrong: Charcoal (carbon) does not deliver the required reaction rate/temperature in this setup. Adding charcoal to the standard mix is unnecessary and detrimental. Magnesium is used sometimes as an igniter, not the main reductant. Common Pitfalls: Incorrect proportions leading to incomplete reduction; poor preheat of the joint mould; inadequate gating causing shrinkage cavities. Final Answer: iron oxide and aluminium

Question 20

Pattern materials — selection breadth: Which of the following materials can be (and are) used for making foundry patterns, depending on quantity, accuracy, and process?

Accepted Answer

Introduction / Context: Patterns create mould cavities that define the external shape of castings. The choice of pattern material affects dimensional accuracy, surface finish, tool life, and cost. Recognising the diversity of viable pattern materials is critical for process planning across sand and investment casting. Given Data / Assumptions: Different casting methods: green sand, shell, and investment casting. Production volumes may range from prototype to mass production. Material selection balances cost, durability, and dimensional stability. Concept / Approach: Aluminium is a common metal pattern material: it is light, machinable, and dimensionally stable. Wax is universally used as expendable patterns in investment casting (lost-wax), where patterns are sacrificed during dewaxing. Low-melting metals like lead have been used historically for special patterns, gauges, or short-run work; modern practice may prefer aluminium or plastics for health and handling reasons, but lead remains a feasible material in controlled environments. Step-by-Step Solution: Identify each option’s role: aluminium for durable, precise metal patterns.Wax for expendable patterns in investment casting trees.Lead as a low-melt option historically/for limited cases.Hence, all listed materials can serve as pattern materials. Verification / Alternative check: Process manuals show aluminium patterns for high-volume sand casting, wax for investment casting, and various metals (including low-melting) used where appropriate. Why Other Options Are Wrong: Rejecting any single material ignores established processes (e.g., lost-wax for wax patterns). “None of these” contradicts standard practice. Common Pitfalls: Overlooking safety and environmental protocols for lead; not accounting for thermal expansion differences among pattern materials; ignoring draft and shrinkage allowances specific to each process. Final Answer: all of these

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