Maharashtra HSC Mechanical Maintenance (Bifocal) cheat sheet

First-angle projection (IS 15021 / ISO-E): the object sits between observer and plane, so the view from the front is placed below the top view and the left-side view is placed on the right.

Third-angle projection (ANSI): the plane sits between observer and object, so the top view is above the front view and the right-side view is on the right.

Scale formulas: Representative Fraction = drawing length / actual length; for a reducing scale RF < 1, enlarging scale RF > 1, full size RF = 1.

Plain scale: length of scale L = RF × maximum length to be measured; divide into major divisions, then subdivide the first division for the smallest readable unit.

Diagonal scale reads a unit plus two sub-divisions by dividing a small vertical into 10 equal parts; use it when the smallest distance is smaller than a normal subdivision.

Thread designation M20 × 2.5 means metric coarse-series external thread, nominal diameter 20 mm and pitch 2.5 mm; a fine series is written M20 × 1.5.

Tolerance stack: maximum clearance = hole upper limit − shaft lower limit; minimum clearance = hole lower limit − shaft upper limit; negative values mean interference.

Workshop calculations: cutting speed V = πDN/1000 (V in m/min, D in mm, N in rpm); machining time T = L/(f × N) with feed f in mm/rev and L the total tool travel.

Taper turning by tailstock offset = (L × (D − d)) / (2 × l), where L is total job length, l the tapered length, D and d the large and small diameters.

Plain-carbon steel classification by carbon: low carbon up to about 0.3% C (ductile, weldable), medium carbon 0.3–0.6% C (hardenable by quenching), high carbon 0.6–1.4% C (springs, tools).

Cast iron types: grey (flake graphite, good damping, brittle), white (cemented carbide Fe3C, hard, wear-resistant), malleable (heat-treated white iron, some ductility), spheroidal/ductile (nodular graphite from Mg or Ce addition).

Critical temperatures: lower critical (A1) near 723 °C for plain-carbon steels; upper critical A3 rises as carbon falls below the eutectoid 0.8% C.

Annealing: heat above A3/A1, soak, then furnace-cool slowly - softens, relieves internal stress, refines grain and improves machinability.

Normalising: heat above A3/A1 and cool in still air - finer pearlite than annealing, higher strength, used after forging or cold working.

Hardening: austenitise, then quench in water, oil or brine; quenching rate must exceed the critical cooling rate to form martensite.

Tempering follows hardening: reheat below A1 (typically 150–650 °C), hold, air-cool; low-temperature tempering keeps hardness, high-temperature tempering gives toughness.

Case hardening methods: carburising (add carbon to low-carbon steel surface), cyaniding and nitriding (add nitrogen), induction and flame hardening (surface only, no chemistry change).

Non-ferrous: aluminium alloys are light and corrosion-resistant but low melting; brass is Cu-Zn, bronze is Cu-Sn; bearing bronzes contain tin and lead for conformability.

Least count of a vernier caliper = 1 main-scale division − 1 vernier-scale division; for a 0.02 mm caliper, 49 main divisions equal 50 vernier divisions.

Least count of a micrometer = pitch of screw / number of divisions on thimble; a 0.5 mm pitch with a 50-division thimble gives 0.01 mm.

Vernier reading = main-scale reading at the zero of the vernier + (coinciding vernier division × least count).

Micrometer reading = barrel (sleeve) reading + (thimble division × least count); always check the zero error before use.

Zero error: positive when the zero of the vernier/thimble is ahead of the main-scale zero; subtract it. Negative error is added to the measured value.

Dial indicator (plunger type) measures small displacements, runout, ovality and flatness; its least count is commonly 0.01 mm or 0.001 mm.

Slip gauges are wrung together to build a dimension; choose the minimum number of blocks and start with the last decimal place.

Comparators (mechanical, pneumatic, electrical, optical) give magnification of small differences; they need a master setting piece, not absolute scales.

Limits and fits: clearance fit (hole larger than shaft), interference fit (shaft larger than hole), transition fit (either may result); IS 919 defines IT grades.

A cold chisel is made of high-carbon or alloy steel, hardened and tempered at the cutting end only; the head stays soft to resist splitting.

Chisel wedge angle: about 70° for hard metal, 60° for mild steel, 40° for brass and copper, 30° for wood - softer material needs a sharper angle.

Files are classified by cut (single, double, rasp), grade (rough, bastard, second cut, smooth) and shape (flat, half-round, round, square, triangular, knife).

File length is measured from heel to point, excluding the tang; never use a file without a handle fitted.

Hacksaw blade pitch: 14 TPI for thick sections, 18 TPI general work, 24–32 TPI for thin tube and sheet; at least three teeth must contact the work.

A screwdriver tip must match the slot width and length; a loose fit damages both the slot and the tip and can slip and cause injury.

Spanners: open-end for two-point access, ring for full grip and high torque, socket with ratchet for speed, torque wrench for specified tightening.

Punches: centre punch has a 90° point for starting a drill; prick punch has a sharper 30–60° point for marking location; dot punch marks layout lines.

Marking-out needs a surface plate, scribing block, V-block, angle plate, centre punch and steel rule; datum faces are chosen first and all dimensions referenced from them.

Filing sequence: select the file for finish, clamp work just above jaw height, use full strokes with even pressure, and cross-check flatness with a try square and straight edge.

Hacksawing: cut on the forward stroke, apply pressure only on the forward stroke, keep the blade square to the work, and let the blade cool.

Chipping: hold the chisel at about 35–40° to the work surface, cut from the edge inward, and use a guard or shield for eye protection.

Drilling: centre-punch the mark, pilot-drill for large holes, keep the drill perpendicular, and use coolant; drill speed V = πDN/1000 with D in mm.

Tapping: tap drill size = major diameter − pitch (metric); use taper, intermediate and plug taps in sequence with cutting fluid and back off a quarter turn to break chips.

Reaming removes a small amount of material to give an accurate, smooth hole; the reamed hole is slightly larger than the reamer and the drill size is left undersize.

Scraping produces bearing surfaces and aligns machine ways; blueing (marking compound) shows high spots, which are scraped until the contact spots are evenly distributed.

Lapping uses loose abrasive between a lap and the work to reach very fine surface finish and tight tolerance; it removes only a few micrometres.

Lathe operations: turning, facing, chamfering, parting off, knurling, thread cutting, taper turning, boring and drilling; work rotates, the single-point tool feeds along or across.

Lathe cutting speed V = πDN/1000 m/min; feed is mm/rev; depth of cut = (D − d)/2; machining time = length / (feed × rpm).

Taper turning methods: tailstock offset, compound rest swivel, taper turning attachment, and form tool; the taper attachment suits long tapers with small angles.

Thread cutting on a lathe requires the lead screw and half-nut engaged with the spindle through gearing; the thread pitch is set by change gears or a gearbox.

Milling: up (conventional) milling produces a chip from minimum to maximum thickness; down (climb) milling starts at maximum thickness and needs a backlash eliminator.

Shaping and planing are single-point reciprocating processes: shaping moves the tool, planing moves the work; feed is applied on the return or cross stroke.

Grinding uses an abrasive wheel; wheel grade (hard or soft) refers to bond strength, not abrasive hardness; a soft wheel is used on hard material.

Drilling machine types: sensitive (bench), pillar, radial, gang and multi-spindle; radial drills allow the head to move over large work.

Welding processes: arc welding uses a consumable electrode and a power source with drooping characteristic; gas welding uses an oxy-acetylene flame with a neutral, oxidising or carburising adjustment.

Preventive maintenance is scheduled on running hours or calendar intervals; predictive maintenance is triggered by measured condition such as vibration, temperature or oil analysis.

Breakdown maintenance is reactive and is used only where failure cost is low or redundancy exists; it is not a substitute for planned maintenance.

Vibration analysis: imbalance gives a large 1× running-speed peak; misalignment gives 2× and axial 1×; looseness gives many harmonics; bearing defects give high-frequency peaks and sidebands.

Wear limits and alignment: check coupling alignment with a dial indicator or laser; soft foot, angular and parallel misalignment are corrected by shimming.

Belt drive troubleshooting: slipping means low tension or oil contamination; squealing means glazing; whip means the belt is too long or the pulleys are misaligned.

Gearbox troubleshooting: whining noise indicates tooth wear or poor contact pattern; oil leaks indicate seal or breather faults; overheating indicates overfill, wrong oil or overload.

Pump troubleshooting: cavitation gives noise and reduced flow, caused by low NPSH available; air ingress gives erratic flow; worn impeller gives reduced head.

Maintenance records: keep history cards with failure mode, downtime, spare used and cost; use them to set the maintenance interval and spare-parts stock levels.

Lockout/tagout: isolate the energy source, lock it, tag it, release stored energy (springs, pressure, capacitors) and verify zero energy before work begins.

Lubrication regimes: boundary (thin film, asperity contact, needs EP additives), mixed, and hydrodynamic (full fluid film, viscosity dominates).

Viscosity is the most important lubricant property; it falls as temperature rises (viscosity index) and rises as pressure rises.

Grease = base oil + thickener (usually lithium or calcium soap) + additives; NLGI grades 0 (soft) to 6 (hard); grade 2 is general purpose.

Bearing types: ball bearings carry radial and some axial load; cylindrical roller bearings carry radial load only; tapered roller bearings carry combined radial and axial load.

Bearing designation 6205: 6 = deep-groove ball bearing, 2 = light series, 05 = 5 × 5 = 25 mm bore; 04 and above multiply the last two digits by 5.

Bearing mounting: press fit on the rotating shaft, free fit in the housing for the non-locating bearing; allow thermal expansion, never preload a deep-groove bearing excessively.

Belt length for an open drive L = 2C + 1.57(D + d) + (D − d)²/(4C), where C is the centre distance and D, d the pulley diameters.

Gear ratio = number of teeth on the driven gear / number of teeth on the driving gear = speed of driver / speed of driven; for a compound train multiply the stage ratios.

Chain drive: keep slack on the return side, align sprockets, and replace the chain and sprockets together when worn; a stretched chain damages the sprocket teeth.

Personal protective equipment: safety goggles for chipping and grinding, face shield for grinding, gloves for handling, safety shoes, ear protection above about 85 dB(A) exposure.

Machine guarding: fixed guards for belts and gears, interlocked guards for doors, and a push-stick or guard for the drill press; never remove a guard to save time.

Fire classes: A solid, B flammable liquid, C gas, D metal; use water (A), foam or CO2 (B), dry powder (C), special powder (D); never water on electrical fires.

5S = Seiri (sort), Seiton (set in order), Seiso (shine), Seiketsu (standardise), Shitsuke (sustain); it is the base of housekeeping and visual management.

Housekeeping rules: keep aisles clear, store material in racks, clean oil spills immediately, and keep the workbench free of chips and tools at the end of the shift.

Environmental practice: segregate metal scrap, oily waste, used coolant and e-waste; used oil goes to authorised recyclers, never to drains or soil.

Waste hierarchy: prevent, reduce, reuse, recycle, recover, dispose; source segregation at the machine is cheaper than sorting later.

Ergonomics: keep the work at about elbow height, lift with the legs and a straight back, and rotate repetitive tasks to reduce musculoskeletal strain.

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

Ammeter is connected in series, voltmeter in parallel; a clamp meter reads current without breaking the circuit.

Continuity and insulation testing: a multimeter checks continuity; a megger (insulation tester) applies 500 V or 1000 V DC and reads in megohms.

Motor protection: overload relay protects against sustained overcurrent, fuse or MCB against short circuit, earth-leakage device against earth fault.

Star-delta starting reduces the starting voltage per winding by 1/√3, so starting torque falls to one-third of the direct-on-line value.

Single-phase motor with a capacitor: a failed capacitor gives no starting torque and humming; a centrifugal switch fault gives similar symptoms.

Sensors in maintenance: thermocouple and RTD for temperature, proximity and limit switches for position, pressure switch for hydraulic and pneumatic interlocks.

Electrical safety: isolate, lock and tag, prove dead with a tested voltage detector, discharge capacitors, and earth the circuit before touching conductors.

Instrument loop: a 4–20 mA signal represents the measured range; 4 mA is the live zero, so a broken wire reads below 4 mA and is detected.

Industrial visit report structure: plant profile, layout and departments, machines observed, processes seen, safety practices, and lessons linked to the syllabus.

Project report components: title, aim and objective, literature or market survey, design and calculations, material and cost estimate, fabrication, testing, results and conclusion.

Project selection criteria: relevance to the trade, feasibility with available machines and time, safety, cost within budget, and a measurable outcome.

Cost estimate: total cost = material + labour + machining + bought-out parts + overheads + contingency (typically 5–10%).

Break-even point in units = fixed cost / (selling price per unit − variable cost per unit); above it the venture makes a profit.

Entrepreneurship basics: identify an opportunity, prepare a business plan, arrange finance (own funds, bank loan, government schemes), register the unit, then market the product.

Small-scale registration and support in Maharashtra: Udyam registration, DIC (District Industries Centre) guidance, and MSME schemes for finance and subsidy.

Depreciation and payback are used to judge a machine purchase; simple payback period = initial investment / annual net saving.

Isometric drawing uses equal 30° axes from the horizontal, with all three axes foreshortened equally; isometric length = true length × 0.816, but drawing practice uses true length on all axes.

Orthographic views: a maximum of six views is possible; the minimum needed to describe an object fully is usually two or three.

Sectional views show internal features; hatching lines are thin and evenly spaced at 45°, and adjacent parts are hatched in opposite directions.

Limits and fits notation: 25 H7/g6 means basic size 25 mm, hole tolerance H7 and shaft tolerance g6; H is the basic hole and h the basic shaft.

Surface roughness Ra is the arithmetic mean deviation in micrometres; a lower Ra means a smoother surface; Rz is the ten-point height.

Geometric tolerancing symbols: straightness, flatness, circularity, cylindricity, parallelism, perpendicularity, angularity, position, concentricity, symmetry and runout.

Development of a cylinder: circumference = πD; a cone's slant height l = √(r² + h²), and the developed sector angle = 360° × r / l.

Workshop calculation of weight: mass = volume × density; steel ≈ 7850 kg/m³, aluminium ≈ 2700 kg/m³, cast iron ≈ 7200 kg/m³.

Percentage and ratio problems in workshop costing: percentage scrap = (scrap mass / blank mass) × 100; material utilisation = (finished mass / blank mass) × 100.