Tamil Nadu HSE Second Year Basic Mechanical Engineering common mistakes
Tamil Nadu HSE Second Year Basic Mechanical Engineering · Understand the traps and how to avoid them.
In engineering drawing, when converting a pictorial view into orthographic views, they place the top view above the front view and still label the sheet as first-angle projection.
They memorise the first-angle rule loosely but copy a third-angle layout from a reference figure without checking the projection symbol.
What to do instead
Draw the standard first-angle projection symbol first, then write the placement rule on the sheet: in first-angle, the top view is placed below the front view and the left-side view is placed on the right. Check the symbol before labelling any view.
In materials questions, they describe grey cast iron as ductile and forgeable because it contains carbon, and they state that steel contains more carbon than cast iron.
They remember that carbon affects strength but not the actual carbon ranges, and they confuse graphite flakes in cast iron with the ability of the metal to deform.
What to do instead
Build a carbon-range table: wrought iron below about 0.15% C, mild steel about 0.15–0.3% C, cast iron about 2–4% C. Link graphite flakes to brittleness and the inability to forge, then practise separating the three by carbon content and typical use.
In equilibrium numericals on a uniform ladder leaning against a wall, they take the ladder’s weight to act at the top end or at the point of contact with the wall.
They draw the sketch quickly and put all forces at the visible ends, forgetting that a uniform body’s centre of gravity is at its mid-point.
What to do instead
Always draw a free-body diagram first: weight at the midpoint, normal reaction at the floor, horizontal wall reaction, and friction at the floor. Then take moments about the floor contact point so two unknown reactions are eliminated before solving.
In casting questions, they say the riser is provided at the bottom of the mould to let gases escape and that the sprue feeds shrinkage.
They memorise riser, sprue, runner and vent as a list of names without linking each part to its function in the flow path of molten metal.
What to do instead
Draw and label the sequence pouring basin → sprue → runner → ingate → mould cavity → riser → vent. State that the riser is a liquid-metal reservoir placed above a hot spot to feed solidification shrinkage, while the vent escapes gases and the sprue is the vertical pouring passage.
In machine-tool questions, they define boring as making a hole in a solid workpiece and describe reaming as enlarging a drilled hole by a large amount.
Drilling, boring, reaming and tapping are remembered as four hole operations without distinguishing whether the hole already exists and how much material is removed.
What to do instead
Pair each operation with its starting condition: drilling starts from solid; boring enlarges an existing hole and corrects location; reaming removes a small allowance for finish and size; tapping cuts internal threads. Write one sentence for each with a simple sketch.
In gas-law and steam numericals, they substitute 27°C or 100°C directly into PV = mRT and use gauge pressure in place of absolute pressure.
They focus on choosing the formula and treat temperature and pressure as plain numbers, so the gauge/absolute and Celsius/kelvin conversions are skipped.
What to do instead
Before substitution, write a unit line: T(K) = t(°C) + 273 and p_abs = p_gauge + p_atm. Convert every pressure to Pa absolute and every temperature to kelvin, then check the units of the final answer.
In IC engine and steam plant questions, they calculate mechanical efficiency as IP/BP and indicated thermal efficiency as BP/(mf × CV).
They memorise “efficiency is output divided by input” but do not identify which power is the engine output and which is the fuel-energy input.
What to do instead
Define the boundaries in words before using symbols: mechanical efficiency = BP/IP; indicated thermal efficiency = IP/(mf × CV); brake thermal efficiency = BP/(mf × CV). Write the numerator and denominator words first, then substitute values.
In continuity and Bernoulli problems, they substitute pipe diameters directly into A1V1 = A2V2, or they apply Bernoulli across a pump without adding the pump head.
They see a diameter in the question and use it as area, and they treat Bernoulli’s equation as valid across machines as well as across pipe sections.
What to do instead
Convert every diameter to area with A = πd²/4 before using continuity. For a pump, use Bernoulli between two sections with pump head added on the downstream side and turbine head subtracted, and name the two sections chosen.
In vernier calliper readings, they record only the main-scale reading and ignore the vernier coincidence and zero error.
They read the main scale quickly and treat the vernier as confirmation, while zero error is seen as a practical detail rather than part of the calculation.
What to do instead
Use a fixed routine: LC = 1 MSD − 1 VSD; observed reading = MSR + (VSD × LC); corrected reading = observed reading − zero error. Practise ten readings with both positive and negative zero error before the exam.
In workshop-practice questions, they mount a hacksaw blade with the teeth pointing toward the handle and then press hard on the return stroke.
They copy the blade orientation from memory without checking the cutting direction, and they think extra pressure on both strokes will speed up the cut.
What to do instead
Mount the blade with teeth pointing forward, away from the handle; cut on the forward stroke and release pressure on the return stroke. Also match PPE to the operation: goggles for grinding, face shield and gloves for welding.