Assam ASSEB HS Skill Mechanical Engineering Technology cheat sheet

First-angle projection (common in India): the object sits between observer and plane, so the top view is placed below the front view and the left-side view to the right of the front view.

Third-angle projection: the plane sits between observer and object, so the top view is above the front view; the symbol shows a truncated cone with the small circle nearer the front view.

A line parallel to a plane projects true length on that plane; a line inclined to a plane projects foreshortened, and its true length is found by the auxiliary-view or rotation method.

An inclined line's true inclination to the HP is found from a view taken perpendicular to the line; true length appears only in a view parallel to the line.

Representative fraction (RF) = drawing length / actual length; for a scale of 1:50, RF = 0.02 and a 2 m object is drawn 40 mm long.

Plain scale shows two units (e.g., m and dm); diagonal scale uses similar triangles to read a third unit and is used for RF below about 1/50.

In isometric projection all three axes are inclined 120° to each other and are foreshortened by 0.816; in isometric drawing true lengths are used along the axes.

CAD: orthographic views are generated from a 3D model by projecting along axes; layers control line type/colour, and line-weight conventions (thick visible, thin hidden/centre) match manual drafting standards.

Limits and fits: clearance fit has allowance positive, interference fit negative; hole-basis system varies the shaft while the hole lower deviation is zero (H).

Carpentry joints: mortise-and-tenon resists tension/compression, dovetail resists pulling apart, and halving joints are used for right-angle frame corners.

Fitting: a file cuts on the forward stroke only; a bastard file is coarser than a second-cut, which is coarser than a smooth file; draw filing produces a fine finish.

Marking-out uses datum surfaces; a try square checks 90°, a scriber marks metal, and a centre punch locates drill points to prevent wander.

Drilling: cutting speed V = πDN/1000 (V in m/min, D in mm, N in rpm); for HSS in mild steel, V ≈ 25–30 m/min.

Lathe: cutting speed depends on work diameter, feed is mm/rev, depth of cut is half the diameter reduction; turning reduces diameter by 2 × depth of cut.

Welding: in arc welding the electrode may be consumable (SMAW) or non-consumable (TIG tungsten); gas welding uses a neutral, oxidising, or carburising flame set by O2:C2H2 ratio.

Casting: a pattern is made oversized by shrinkage allowance (about 10–20 mm/m for cast iron) plus draft and machining allowances; a riser feeds shrinkage, a chill speeds solidification.

Sheet metal: bend allowance accounts for the neutral axis; a clearance between punch and die of about 5–10% of sheet thickness is typical for shearing.

Milling: up-milling produces chip thickness from zero to maximum and tends to lift the work; down-milling starts with maximum chip and gives a better finish on rigid machines.

Hooke's law holds only in the elastic region; the modulus of elasticity E is the slope of the linear part of the stress–strain curve.

For a ductile metal, yield point (or 0.2% proof stress) marks the end of elasticity; ultimate tensile strength is the maximum stress, and fracture stress is lower than UTS for ductile steels.

Brinell test: hard steel ball, load/diameter² ratio constant (30 for steel, 10 for aluminium); Vickers uses a diamond pyramid and is load-independent; Rockwell uses depth of indentation.

Izod and Charpy tests measure impact toughness; a V-notch concentrates stress, and absorbed energy falls as temperature drops (ductile-to-brittle transition).

Carbon steel: up to 0.8% C is hypoeutectoid (ferrite + pearlite), 0.8% C is eutectoid pearlite, above 0.8% C is hypereutectoid (pearlite + cementite).

Cast iron types: grey (flake graphite, good damping), white (cementite, hard and brittle), malleable (heat-treated white iron), nodular/SG (spheroidal graphite, ductile).

Brass is Cu–Zn, bronze is Cu–Sn; adding Zn raises strength and lowers cost, adding Sn improves corrosion resistance and bearing properties.

Hardness and tensile strength are roughly proportional for steels (UTS ≈ 3.4 × HB in MPa); hardness testing is non-destructive and is used for acceptance checks.

Heat treatment: annealing softens and relieves stress, normalising refines grain, quenching hardens, and tempering restores toughness after quenching.

Resultant of two forces: R = √(P² + Q² + 2PQ cos θ); if θ = 90°, R = √(P² + Q²).

Lami's theorem applies to three coplanar concurrent forces in equilibrium: P/sin α = Q/sin β = R/sin γ, where each angle is opposite its force.

Varignon's theorem: the moment of a force about a point equals the algebraic sum of moments of its components about that point.

Equilibrium of a rigid body: ΣFx = 0, ΣFy = 0, ΣM = 0; a three-force body in equilibrium has concurrent lines of action.

Friction: limiting friction F = μN; the angle of friction φ satisfies tan φ = μ, and the angle of repose equals φ.

A ladder against a wall: reactions are found from ΣM about the base and ΣM about the wall contact; the wall reaction is horizontal and the ground reaction has normal and frictional components.

Centroid of a semicircle is 4r/3π from the diameter; of a triangle, h/3 from the base; of a quarter circle, 4r/3π from each axis.

Moment of inertia of a rectangle about its centroidal axis is bd³/12; parallel-axis theorem: I = I_G + A d².

Projectile: range R = u² sin 2θ / g, maximum height H = u² sin²θ / 2g, and time of flight T = 2u sin θ / g.

Stress σ = P/A; strain ε = δ/L; E = σ/ε; a bar's elongation δ = PL/AE, and for stepped or varying sections sum PL/AE for each part.

Thermal stress in a fully constrained bar: σ = α E ΔT (compressive on heating); if free to expand, δ = α L ΔT and stress is zero.

Shear stress τ = V/A; for a circular shaft, τ_max = 16T/πd³ and angle of twist θ = TL/GJ with J = πd⁴/32.

Bending: M/I = σ/y = E/R; for a rectangle σ_max = 6M/bd² and for a circle σ_max = 32M/πd³.

A simply supported beam with central point load W has maximum BM = WL/4 at mid-span and maximum deflection = WL³/48EI.

A cantilever with end load W has maximum BM = WL at the fixed end and maximum deflection = WL³/3EI.

A simply supported beam under UDL w per unit length has maximum BM = wL²/8 and maximum shear force = wL/2 at the supports.

Simple machines: MA = load/effort, VR = distance moved by effort / distance moved by load, efficiency η = MA/VR; for a screw jack VR = 2πL/p.

A pulley system with n rope segments supporting the load has VR = n; a Weston differential pulley has VR = 2D/(D − d).

First law for a closed system: Q − W = ΔU; for a steady-flow open system: Q − W = Δh + ΔKE + ΔPE.

Ideal gas: PV = mRT; cp − cv = R; γ = cp/cv; for air R ≈ 0.287 kJ/kg·K and γ = 1.4.

Process relations: isothermal PV = C and W = P₁V₁ ln(V₂/V₁); adiabatic PV^γ = C and W = (P₁V₁ − P₂V₂)/(γ − 1); isochoric W = 0 and Q = m cv ΔT.

Carnot efficiency η = 1 − T₂/T₁ (absolute temperatures); no engine operating between the same limits can exceed it.

Otto cycle efficiency η = 1 − 1/r^(γ−1), where r is compression ratio; Diesel cycle efficiency also depends on cut-off ratio and is lower than Otto at equal r.

Four-stroke SI engine: suction, compression, power, exhaust, two revolutions per cycle; valve overlap improves volumetric efficiency.

IC engine performance: indicated power IP = Pmep × L × A × N (per cycle factor), brake power BP = 2πNT/60, and mechanical efficiency = BP/IP.

Boiler: equivalent evaporation = actual evaporation × (h − h_f)/2257 kJ/kg; boiler efficiency = heat absorbed / heat supplied in the fuel.

A boiler mountings are safety/working fittings (safety valve, water level indicator, pressure gauge, fusible plug); accessories improve efficiency (economiser, superheater, air preheater).

Hydrostatic pressure p = ρgh; pressure in a liquid increases linearly with depth and acts equally in all directions at a point.

Continuity: A₁V₁ = A₂V₂ for incompressible flow; velocity rises where area falls.

Bernoulli's equation: P/ρg + V²/2g + z = constant along a streamline for steady, frictionless, incompressible flow.

Reynolds number Re = ρVD/μ = VD/ν; flow is laminar below about 2000 in pipes and turbulent above about 4000.

Darcy–Weisbach head loss h_f = fLV²/2gD; friction factor f = 64/Re for laminar pipe flow.

Orifice: actual discharge Q = Cd A√(2gh); Cd = Cc × Cv, with Cd around 0.6 for a sharp-edged orifice.

A venturimeter uses the same principle with Q = Cd A₁A₂√(2gh)/√(A₁² − A₂²), where h is the differential head.

Centrifugal pump: head H = (P₂ − P₁)/ρg + (V₂² − V₁²)/2g + z; specific speed Ns = N√Q/H^(3/4) classifies the impeller type.

Pelton wheel is an impulse turbine for high head and low discharge; Francis and Kaplan are reaction turbines for medium and low heads respectively.

Ohm's law V = IR; power P = VI = I²R = V²/R; for three-phase systems P = √3 V_L I_L cos φ.

Series resistances add (R = R₁ + R₂); parallel resistances satisfy 1/R = 1/R₁ + 1/R₂, so total resistance is less than the smallest branch.

A single-phase induction motor is not self-starting; a capacitor or shaded pole provides the starting phase shift, which is why capacitor-start motors are common on small pumps.

A three-phase induction motor's synchronous speed Ns = 120f/P; slip s = (Ns − N)/Ns and rotor frequency = s × supply frequency.

A star–delta starter reduces starting voltage per phase by 1/√3 and starting torque to one-third, limiting supply current.

A relay is an electrically operated switch; a contactor switches motor loads; a thermal overload relay protects a motor by sensing sustained overcurrent.

A thermocouple uses the Seebeck effect (two dissimilar metals generate an EMF proportional to temperature difference) and is used for temperature measurement in machines.

A diode conducts in forward bias and blocks in reverse; a bridge rectifier converts AC to DC for battery charging and DC motor drives.

A PLC reads digital/analogue inputs, executes a ladder-logic program, and drives outputs to actuators; interlocks and emergency-stop circuits must fail safe.

Fire triangle: fuel, oxygen, and heat; removing any one extinguishes the fire. Class A = solids, B = flammable liquids, C = gases, D = metals, and electrical fires need CO₂ or dry powder, never water.

A permit-to-work system is required for hot work, confined-space entry, and work on energised or isolated equipment.

Lockout/tagout: isolate the energy source, lock it, tag it, release stored energy, and verify isolation before work begins.

Machine guarding must prevent access to nip points and rotating parts; an interlock stops the machine when a guard is opened.

Personal protective equipment order of control: elimination, substitution, engineering controls, administrative controls, then PPE as the last line of defence.

Noise exposure limits are commonly 90 dB(A) for 8 hours with a 3 dB exchange rate, so 93 dB(A) halves the permitted exposure time.

Confined-space hazards include oxygen deficiency (below 19.5%), toxic gases, and flammable vapours; test the atmosphere before and during entry.

Ergonomic risk: keep loads close to the body, avoid twisting while lifting, and use mechanical aids above about 25 kg for a single person.

Environmental controls: effluent treatment before discharge, scrubbers/filters for stack emissions, and segregation of hazardous waste from general waste.

Break-even quantity = fixed cost / (selling price − variable cost per unit); contribution per unit must be positive for the venture to be viable.

Working capital = current assets − current liabilities; a positive value means short-term obligations can be met from short-term resources.

A small-scale enterprise in India commonly registers as a sole proprietorship, partnership, LLP, or private limited company; Udyam registration gives MSME benefits.

Depreciation reduces taxable profit: straight-line spreads cost equally, while written-down value applies a fixed percentage to the reducing balance.

A project report should contain market analysis, technical feasibility, cost of production, means of finance, and projected profit and cash flow.

A good technical drawing or report is traceable: revision number, date, author, and approval signature; unapproved drawings must not be released for manufacture.

Professional ethics: declare conflicts of interest, keep client data confidential, and do not certify work you have not personally checked.

Employability: a résumé should state measurable outcomes (e.g., reduced cycle time by 12%), and interview answers should use situation–action–result structure.

Costing: prime cost = direct material + direct labour + direct expenses; factory cost adds factory overheads, and total cost adds administration and selling overheads.