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BT-203 · Basic Mechanical Engineering/Quick Revision Short Notes

Basic Mechanical Engineering (BT-203) - Unit 2 Short Notes

How unit 2 is examined

This unit covers measurement basics, instruments, and workshop processes; the marks lie in measuring instruments (vernier, sine bar, slip gauge), temperature/pressure/velocity measurement, and lathe and drilling machines.

Concept of measurements

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. <mark>Measurement is the process of comparing an unknown quantity with a standard of the same kind to find its numerical value.</mark>

Key points.

  1. Every measurement has a measurand (quantity measured), a standard (unit) and a measuring instrument.
  2. A generalised instrument has a sensor (detector), a transducer or signal conditioner, and a display (indicator) stage.
  3. Accuracy is closeness to the true value, while precision is closeness of repeated readings to each other.
  4. Least count is the smallest value an instrument can read, and range is the span between its minimum and maximum readings.
  5. Sensitivity is the change in output per unit change in input.

Errors in measurement

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. ==Error is the difference between the measured value and the true value of the quantity: $e = V_{measured} - V_{true}$.==

Key points.

  1. Systematic errors follow a fixed pattern (zero error, calibration or wear) and can be corrected.
  2. Random errors occur unpredictably from small changes in conditions and are reduced by averaging many readings.
  3. Gross errors come from human mistakes such as wrong reading or recording and are removed by care.
  4. Parallax error arises from viewing the scale at an angle, so the eye must be kept perpendicular to it.

Temperature, pressure, velocity, flow, strain, force and torque measurement

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>

Temperature methods. Liquid-in-glass thermometer, bimetallic thermometer, thermocouple, resistance temperature detector (RTD), thermistor, and radiation (optical) pyrometer.

Thermocouple. <mark>A thermocouple works on the Seebeck effect: two dissimilar metals joined at two junctions at different temperatures generate an emf proportional to the temperature difference.</mark>

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Radiation pyrometer. It is a non-contact instrument: a lens focuses the radiation emitted by a hot body ($E=\sigma T^4$) on a thermopile or receiver, whose output emf is calibrated in temperature. It measures very high temperatures of furnaces and moving bodies.

Manometer (pressure). A U-tube holds a liquid of density $\rho$. One limb is connected to the pressure and the height difference $h$ of the liquid gives $P=\rho g h$ by hydrostatic balance.

Pressure relations. $P_{abs}=P_{atm}+P_{gauge}$ for pressures above atmospheric, and $P_{abs}=P_{atm}-P_{vacuum}$ for pressures below atmospheric. Absolute pressure is measured from perfect vacuum (zero) and gauge pressure from atmosphere.

Pitot tube (velocity). A tube with its open end facing the flow brings the fluid to rest at the stagnation point, converting kinetic energy into pressure energy. A second tube gives static pressure, and the liquid column difference is $h$. Bernoulli between free stream and stagnation point gives:

$$v = C_v\sqrt{2gh}$$

with $C_v$ the velocity coefficient (about 0.98 to 1). Example: $h=0.5$ m gives $v=\sqrt{2\times9.81\times0.5}=3.13$ m/s for $C_v=1$.

Other quantities. Flow is measured by orifice, venturi meter or rotameter, which use pressure difference. Strain is measured by an electrical strain gauge whose resistance changes with strain: gauge factor $GF=(\Delta R/R)/\varepsilon$. Force is measured by a load cell or spring balance, and torque by a strain-gauge shaft or a dynamometer.

Answer frame. Temperature: open with the list of methods, draw the thermocouple circuit, explain the Seebeck effect, construction, operation, uses. Manometer: define, draw U-tube plus pressure datum, derive $P=\rho g h$, give both relations. Pitot: state principle, draw the tube, apply Bernoulli, end with $v=C_v\sqrt{2gh}$.

Pitfall: Writing $P_{abs}=P_{atm}+P_{vacuum}$; vacuum pressure is subtracted.

Asked: [7 marks] (Jun 2022, Jun 2025) What are the various methods used for temperature measurement? Explain any one of them. Explain working of a thermocouple and radiation pyrometer with neat sketches. Asked: [7 marks] (Nov 2022) Explain the working of a manometer for pressure measurement. Also relate absolute pressure, gauge pressure and vacuum pressure. Asked: [8 marks] (Jun 2023) Explain the process of Pitot tube for velocity measurement of a fluid with neat sketch.

Vernier caliper, micrometer, dial gauge, slip gauge, sine bar and combination set

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>

Vernier caliper. <mark>A vernier caliper measures external, internal and depth dimensions using a main scale and a sliding vernier scale, to an accuracy equal to its least count.</mark>

  1. Parts: main scale, vernier scale, fixed and sliding jaws, internal jaws, depth rod, clamp screw.
  2. Least count $LC = 1\,\text{MSD} - 1\,\text{VSD}$; if 50 VSD equal 49 MSD of 1 mm, $LC=1-\tfrac{49}{50}=0.02$ mm.
  3. Reading = main scale reading + (coinciding vernier division $\times$ LC) $\pm$ zero error.
  4. Procedure: close the jaws to check zero error, place the job between the jaws, tighten the clamp, read the main scale just before the vernier zero, then add the coinciding division times LC.
  5. Zero error is positive if the zero of the vernier lies right of the main zero (subtract it), and negative if left (add it).

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Micrometer. It works on the screw and nut principle: one rotation of the spindle moves it by the pitch. Parts are frame, anvil, spindle, sleeve (barrel), thimble and ratchet stop. $LC=\text{pitch}/\text{thimble divisions}=0.5/50=0.01$ mm. The ratchet stop gives uniform measuring pressure.

Dial gauge. It is a comparator: plunger movement is magnified by a rack and pinion and gear train to a pointer on a dial, reading typically 0.01 mm. It checks flatness, parallelism, run-out and alignment against a reference.

Slip gauge. Slip gauges are hardened, lapped steel blocks of exact thickness used as length standards. Wringing joins them: clean, place two crosswise with light pressure, rotate until aligned so molecular adhesion holds them. Use the fewest blocks, wring gently, and handle at standard $20^\circ$C.

Sine bar. ==A sine bar is a hardened bar with two rollers of equal diameter a fixed distance $L$ apart, used with slip gauges to set or measure angles by $\sin\theta = h/L$.==

  1. Place the bar on the surface plate with the job on it; pack slip gauges of height $h$ under one roller.
  2. Adjust $h$ until a dial indicator reads zero along the job surface, then $\theta=\sin^{-1}(h/L)$.
  3. Example: $L=200$ mm, $h=50$ mm, so $\sin\theta=0.25$ and $\theta=14.48^\circ$.

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Combination set. It has a steel rule, square head, protractor head and centre head on one blade; it checks squareness, $45^\circ$ and $90^\circ$ angles, and finds centres.

Answer frame. Vernier: define, sketch, give LC, reading steps, zero error, closing with uses. Sine bar: principle, sketch with slip gauges, procedure, formula, example. Slip gauge: introduce, sketch three wringing stages, explain wringing, close with precautions.

Pitfall: Forgetting to apply the zero error; subtract positive, add negative.

Asked: [7 marks] (Nov 2022, Dec 2023) Explain the measurement process using vernier caliper with neat sketch; discuss applications and method. Asked: [6 marks] (Dec 2023, Jun 2025) Explain the method of using sine bar with neat sketch; construction and use, measuring taper. Asked: [14 marks] (Jun 2022) Short notes (any two): casting defects, micrometre, dial gauge, different types of fluids. Asked: [14 marks] (Nov 2022) Short notes (any two): welding defects, dial gauge, sine bar, thermodynamic properties. Asked: [6 marks] (Jun 2023) Explain the method of using slip gauge with the neat sketch. Asked: [7 marks] (Dec 2024) Short notes: vernier caliper, micrometer, dial gauge.

Elementary aspects of production processes: casting, carpentry, welding

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Medium weight</span>

Welding. <mark>Welding is a permanent joining process in which metals are joined by heat, or pressure, with or without filler material.</mark>

Key points.

  1. Fusion welding melts the parent metal (gas, arc, thermit), while solid-state welding joins below melting (resistance, friction, forge).
  2. In arc welding, an electric arc between electrode and work produces heat of about 3500 $^\circ$C that melts both and the filler.
  3. In shielded metal arc welding (SMAW), the flux coating melts to give shielding gas and slag that protects the weld pool from oxidation.
  4. The circuit is a power source (AC or DC), cables, electrode holder, and work connected to ground; it is used for structures, pipes and repair.
  5. Common weld defects are porosity, undercut, slag inclusion, lack of fusion and cracks.

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Sand casting flow.

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Pattern making is followed by sand and core preparation, moulding, pouring of molten metal, cooling, removal of the casting, cleaning and inspection. Carpentry uses wood, sawing, planing and joints. Casting defects are blowholes, shrinkage cavity, cold shut, misrun and hot tears.

Answer frame. Welding: open with the definition, give the classification, draw the SMAW circuit, explain the arc, close with applications. Casting: draw the flow chart and give one line per box.

Asked: [7 marks] (Jun 2022) Define welding. Classify welding processes. Explain arc welding process. Asked: [7 marks] (Dec 2024) Give in detail the flow chart followed in preparation of sand casting.

Lathe and drilling machines and their operations

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>

Lathe. <mark>A lathe is a machine tool that rotates the workpiece against a fixed single-point cutting tool to remove material.</mark>

Lathe parts.

  1. The bed is the rigid cast-iron base with guideways that carries all other parts.
  2. The headstock at the left holds the spindle, chuck and gear box for speed.
  3. The tailstock slides on the bed and supports long work with a centre or holds drills.
  4. The carriage has saddle, apron, cross-slide, compound rest and tool post, and carries the tool.
  5. The lead screw moves the carriage for thread cutting, and the feed rod for turning.

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Lathe operations. Turning reduces diameter along the axis. Facing feeds the tool perpendicular to the axis to machine a flat end face. Knurling presses hardened rollers on the revolving job to impress a diamond or straight pattern for grip. Thread cutting uses the lead screw with a threading tool.

Drilling machine. <mark>A drilling machine produces round holes by a rotating drill that is fed axially into a stationary workpiece.</mark>

Drilling parts.

  1. The base carries the machine and clamps the work.
  2. The column is the vertical member that supports the table and head.
  3. The table is adjustable in height and holds the work.
  4. The spindle head has the motor, spindle, and chuck that grips the drill.
  5. The feed lever moves the spindle down.

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Operations. Reaming finishes a drilled hole to exact size. Counterboring enlarges the top of a hole for a bolt head. Countersinking makes a conical recess. Tapping cuts internal threads.

Feed mechanism. Feed is the axial advance of the drill per revolution. In hand feed, a lever turns a pinion which moves a rack on the spindle sleeve. In power feed, a gear train drives a worm and worm gear connected to the pinion shaft, giving feed in mm/rev.

Answer frame. Lathe: draw with labelled parts, explain each part in one line, then describe two operations with sketches. Drilling: state principle (rotation plus axial feed), draw, list parts, then reaming and boring or tapping.

Asked: [7 marks] (Jun 2022, Jun 2025) With a neat diagram explain the main parts of a drilling machine. Explain any two operations. Asked: [7 marks] (Nov 2022) With a neat diagram explain the main parts of a lathe machine. Explain any two operations. Asked: [6 marks] (Dec 2023) With a neat sketch, explain lathe operations: facing, knurling. Asked: [6 marks] (Jun 2023) Explain feed mechanism used in a drilling machine. Asked: [7 marks] (Dec 2024) Explain the working principles of drilling machine and different parts with line diagram.

Last-minute revision

  1. Measurement compares an unknown quantity with a standard; error is measured minus true value.
  2. Vernier least count is 1 MSD minus 1 VSD, commonly 0.02 mm.
  3. Micrometer least count is pitch divided by thimble divisions, commonly 0.01 mm.
  4. Dial gauge is a comparator using rack and pinion; sine bar uses $\sin\theta=h/L$.
  5. Slip gauges are joined by wringing.
  6. Thermocouple works on the Seebeck effect; pyrometer measures radiation without contact.
  7. Manometer gives $P=\rho g h$; $P_{abs}=P_{atm}+P_{gauge}$ and $P_{abs}=P_{atm}-P_{vacuum}$.
  8. Pitot tube velocity is $v=C_v\sqrt{2gh}$.
  9. Welding is permanent joining; SMAW uses a flux-coated electrode.
  10. Lathe rotates the work; drilling rotates the tool.

Memory hooks

  1. Vernier: "Main minus Vernier" gives least count.
  2. Sine bar: sine equals height over length, "SHL".
  3. Lathe parts: "Bed Head Tail Carriage".
  4. Sand casting order: pattern, sand, core, mould, pour, cool, clean, inspect.

Coverage checklist

  • Concept of measurements: no past questions.
  • errors in measurement: no past questions.
  • Temperature, Pressure, Velocity, Flow strain, Force and torque measurement: temperature methods, manometer, Pitot tube.
  • Vernier caliper, Micrometer, Dial gauge, Slip gauge, Sine-bar and Combination set: vernier, sine bar, slip gauge, short notes.
  • Elementary theoretical aspects of production processes like casting, carpentry, welding etc: welding, sand casting flow chart.
  • Introduction to Lathe and Drilling machines and their various operations: drilling parts, lathe parts, facing and knurling, feed mechanism.
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