Tamil Nadu HSE First Year Basic Mechanical Engineering cheat sheet

Ferrous metals contain iron as the base element; cast iron (2–4% C), mild steel (0.15–0.30% C), medium carbon steel (0.30–0.60% C) and high carbon steel (0.60–1.5% C) are classified by carbon content.

Increasing carbon content raises hardness and tensile strength but lowers ductility, weldability and machinability; mild steel is the general workshop default for bolts, keys and shafts.

Pig iron is the blast-furnace product (about 3–4.5% C with Si, Mn, S, P) and is remelted in a cupola to make cast iron; cast iron is brittle in tension but strong in compression and good for damping vibration.

Non-ferrous examples: aluminium (light, corrosion resistant, good conductor), copper (excellent electrical and thermal conductor), brass (copper + zinc), bronze (copper + tin), and zinc used for galvanising.

Alloy steel adds elements such as Ni, Cr, Mn, Mo, V or W to improve hardenability, wear resistance or heat resistance; stainless steel relies on chromium (typically above 11–12%) forming a passive oxide layer.

Mechanical properties vocabulary: elasticity (return to shape), plasticity (permanent deformation), ductility (drawing into wire), malleability (hammering into sheets), toughness (energy absorbed before fracture), brittleness, hardness and fatigue resistance.

Hardness test methods: Brinell (10 mm ball, softer/larger parts), Vickers (diamond pyramid, wide range), Rockwell (indenter plus load, direct dial reading).

Heat treatment: annealing softens and relieves stress by slow cooling; normalising refines grain with air cooling; hardening uses rapid quenching; tempering after hardening reduces brittleness; case hardening adds a hard skin over a tough core.

Bench vice parts: fixed jaw, movable jaw, spindle with square thread, handle, guide bars and swivel base; the workpiece is gripped low in the jaws so clamping force acts close to the slide.

Files are classified by length, cut (single, double, rasp), shape (flat, half-round, round, square, triangular, knife) and grade (rough, bastard, second cut, smooth); a smooth file removes least metal per stroke.

Filing rules: use a file card to clean the teeth, never strike a file or use it without a handle, apply pressure on the forward stroke only, and lift the file on the return stroke.

Hacksaw blade teeth point forward and cut on the push stroke; blade selection depends on section thickness - 14 or 18 TPI for thick sections, 24 or 32 TPI for thin tube and sheet to avoid tooth snagging.

Chisels: cold chisel for metals (flat, cross-cut, half-round, diamond point), firmer and mortise chisels for wood; a chisel is struck with a ball-peen hammer, and a chipped head must be reground, not used.

Punches: centre punch (dot mark for drilling), prick punch (fine locating mark), dot punch and pin punch; the punch is held vertically and struck once to mark layout lines.

Scribers, dividers, trammels, surface plate, V-block, try square, steel rule and vernier are layout tools; layout blue (marking ink) improves visibility of scribed lines on bright metal.

Screwdriver and spanner selection: blade width must match the screw slot, and the spanner jaw must fit the nut flats exactly - an undersized spanner rounds the corners.

Marking-out sequence: clean the surface, apply marking medium, establish a datum face, scribe lines, then punch them; all dimensions are taken from the datum, never from a rough edge.

Sawing, chipping, filing and drilling are the main bench operations; each removes metal to bring the job to the layout lines, with final finishing by smooth file or scraper.

Drilling: drill size for a tap equals the major diameter minus the pitch (for M10 × 1.5, use an 8.5 mm drill); centre drill first, use cutting fluid, and clamp the work in a machine vice.

Tapping: taper tap starts the thread, intermediate tap cuts the full form, bottoming tap finishes blind holes; the tap is reversed a quarter turn periodically to break chips.

Reaming produces a close-tolerance, smooth hole after drilling undersize; a reamer is turned clockwise only and never reversed, to avoid damaging the cutting edges.

Scraping and lapping are hand finishing processes for flat surfaces and precision fits; blueing (marker compound) shows high spots when two surfaces are rubbed together.

Limits and fits: clearance fit (hole always larger than shaft), interference fit (shaft larger than hole), transition fit; allowance is the intentional difference between the basic sizes.

Threads and fasteners: V-thread included angle 60°, BSW 55°, pitch is the axial distance between crests, and lead equals pitch times number of starts (single-start thread: lead = pitch).

Sheet metal gauge numbers run opposite to thickness: a higher gauge number means thinner sheet; standard wire gauge (SWG) values are used for selection.

Key operations: shearing, blanking, piercing, bending, folding, rolling, seaming and riveting; a burr is removed with a file or deburring tool before folding.

Bend allowance must be added to the flat pattern; a simple estimate for a 90° bend uses the neutral axis at roughly half the thickness from the inner surface.

Edges are stiffened by folding a hem (single or double), wiring, or beading; a wired edge prevents sharp edges and adds rigidity to buckets and trays.

Seams: lap seam (soldered or riveted), grooved seam (double locked), and Pittsburgh lock; the grooved seam is formed on a folding bar and locked with a seam setter.

Rivets: snap head, pan head, countersunk and pop rivets; hole diameter is slightly larger than the rivet shank, and the rivet length must cover the plates plus the head allowance.

Tools: snips (straight, curved, aviation), ball-peen hammer, mallet, stakes, shears, folding bar, groovers, hand rolls and soldering iron; a mallet is used on soft sheet to avoid stretching marks.

Sheet metal is joined by soft soldering with a flux (zinc chloride for galvanised, resin for electrical work); the joint must be clean and tinned before the solder flows.

Wood classification: hardwoods come from broad-leaved deciduous trees (teak, rosewood, sal), softwoods from conifers (pine, deodar, fir); hardness is not determined by this botanical split alone.

Seasoning reduces moisture content to prevent shrinkage, warping and splitting; air seasoning is slow and cheap, kiln seasoning is fast and gives controlled moisture content.

Wood defects: knots, shakes, splits, twist, cup, bow, waney edge and decay; a knot is the base of a branch embedded in the stem and weakens the board.

Carpentry joints: lap, mortise and tenon, dovetail, dowel, halving, bridle, housing and butt; the mortise-and-tenon joint is standard for frames and requires a mortise chisel.

Marking and cutting tools: try square, marking gauge, mortise gauge, bevel, rip saw (cut along the grain), cross-cut saw (across the grain), tenon saw (fine work), and a jack plane for surfacing.

Plane parts: body, sole, cutting iron (bevel down), cap iron, lever cap, adjusting screw; the cap iron is set close to the edge for rough work to break chips.

Abrasive finishing follows the grain; sandpaper grit numbers increase with fineness, and varnish or polish is applied only after the surface is dust free.

Wood adhesives: animal glue (hot), PVA (white glue, interior), epoxy and synthetic resin; joints must be close fitting because glue lines are weak in thick gaps.

Smithy operations: upsetting (increasing cross-section by end blows), drawing down (reducing section and lengthening), bending, twisting, punching, drifting, swaging and forge welding.

The forge fire is controlled by air blast; correct temperature is judged by colour - cherry red around 800 °C, orange to yellow around 1000–1200 °C for forging, and white heat risks burning the steel.

Overheating causes coarse grain and burning causes irrecoverable loss of strength; the metal must not be forged below the critical range (blue brittle region) or it may crack.

Tools: anvil (with horn, face and hardy hole), swage block, tongs (flat, pick-up, hollow bit), hammers (sledge and hand), hot set, hardie, punch, drift and fullers.

Forge welding joins two heated pieces by hammering; the surfaces must be clean, fluxed (borax or sand) and brought to welding heat so that slag is squeezed out.

Upsetting is used to form bolt heads; drawing down is used to taper a bar to a point; swaging shapes the work in a swage block or top and bottom fullers.

Safety in smithy: wear leather gloves, goggles and apron, keep the anvil face clean and dry, stand clear of the hammer arc, and quench tongs to keep handles cool.

Casting steps: pattern making, moulding (sand rammed around the pattern in a flask), core setting, gating, melting, pouring, cooling, shakeout and fettling.

Green sand moulding sand contains silica sand, clay binder, water and additives; it needs permeability, refractoriness, cohesiveness and collapsibility.

Pattern types: solid, split, match plate, loose piece, sweep and skeleton; a pattern is made slightly larger than the casting by the shrinkage allowance (cast iron about 10 mm/m, aluminium about 16 mm/m).

Machining allowance is added to surfaces to be machined; draft (taper) allows easy pattern withdrawal; a core print supports the core and forms internal cavities.

Gating system parts: pouring basin, sprue, runner, ingate and riser; a riser (feeder) supplies liquid metal to compensate shrinkage during solidification.

Cupola is used for cast iron, induction and electric arc furnaces for steel; pouring temperature must be high enough to fill the mould but not so high as to cause gas defects.

Casting defects: blowholes and porosity (gas), shrinkage cavity (insufficient feeding), cold shut (low pouring temperature), misrun, hot tear, sand inclusion, and drop (mould wall collapse).

Classification: fusion welding (arc, gas, resistance, thermit) melts the parent metal; solid-state welding (friction, forge, ultrasonic) joins without melting; brazing and soldering use a filler melting below the parent metal.

Arc welding uses a consumable coated electrode; the coating stabilises the arc, shields the molten pool and adds alloying elements; electrode diameter is chosen by plate thickness and current.

Polarity: DC straight polarity means the electrode is negative and the work positive (more heat at the work); DC reverse polarity puts more heat at the electrode, suited to thin sheet and some electrodes.

Oxy-acetylene flame types: neutral (ratio about 1:1, used for steel), oxidising (excess oxygen, used for brass) and carburising (excess acetylene, used for high carbon steel and hard surfacing).

Gas welding temperatures: acetylene–oxygen flame reaches about 3200 °C; the inner cone tip is held just above the pool, and the torch is moved at a steady speed to avoid undercut.

Resistance welding (spot and seam) uses pressure plus I²Rt heating; thermit welding uses the exothermic reaction of aluminium powder with iron oxide for rail and heavy sections.

Weld defects: slag inclusion, porosity, incomplete penetration, lack of fusion, undercut, spatter, distortion and cracks; distortion is reduced by back-step welding, clamping and balanced sequence.

Safety: shield the arc with a helmet of correct shade, keep cylinders upright and away from heat, never use oil on oxygen fittings, and open the acetylene cylinder valve only about one turn.

Lathe parts: bed, headstock, tailstock, carriage with cross slide and compound rest, tool post, lead screw and feed rod; the workpiece rotates while a single-point tool feeds along or across.

Lathe operations: facing, turning, taper turning (swivelling compound rest, offset tailstock, taper turning attachment), thread cutting, drilling, boring, knurling, chamfering and parting off.

Cutting speed V = πDN/1000 (m/min) with D in mm and N in rpm; feed is in mm/rev, and depth of cut is half the diameter reduction.

Drilling machine types: sensitive (bench), pillar, radial, gang and multi-spindle; a drill held in a sleeve and socket or chuck, and the spindle speed is set from the cutting-speed formula.

Milling: up (conventional) milling produces chip thickness from zero to maximum and tends to lift the work; down (climb) milling starts with maximum chip thickness and gives better finish on machines with backlash eliminators.

Grinding uses an abrasive wheel; wheel grade, grit and bond are selected by material, and the wheel is dressed to restore sharpness and true form; the wheel guard must be in place.

Shaper and planer: the shaper uses a reciprocating single-point tool with a quick-return mechanism and clapper box; the planer moves the work past a stationary tool for large flat surfaces.

Tool geometry and materials: rake angle, clearance angle, cutting edge angle and nose radius control cutting force and finish; high-speed steel, carbide, ceramic and diamond represent increasing hot hardness.

Velocity ratio of a belt drive = speed of driver / speed of driven = diameter of driven / diameter of driver (for equal belt thickness); crossed belts reverse direction, open belts keep the same direction.

Gear trains: for a simple train, velocity ratio = product of teeth on driven gears / product of teeth on driving gears; a compound train multiplies individual stage ratios.

Gear types: spur (parallel shafts), helical (smoother, thrust load), bevel (intersecting shafts), worm and worm wheel (large reduction, self-locking in some cases) and rack and pinion (rotary to linear).

Four-bar linkage, slider-crank, double slider, quick-return and toggle mechanisms are simple mechanisms; the slider-crank converts rotary motion to reciprocating motion in engines and compressors.

A cam converts rotary motion into specified follower motion (uniform velocity, simple harmonic, uniform acceleration); a roller follower reduces wear compared with a knife-edge follower.

Bearings: sliding (journal) bearings rely on an oil film; rolling bearings are ball, cylindrical roller, taper roller and needle; taper rollers accept combined radial and thrust loads.

Clutches and brakes transmit or absorb torque by friction; a single-plate clutch transmits torque T = μ·W·r with r the effective friction radius, and a centrifugal clutch engages with speed.

Chain drives need no slip and tolerate shorter centre distances than belts; belt slip protects against overload but reduces the exact velocity ratio.

Least count of a vernier calliper = 1 main scale division − 1 vernier scale division (for a 0.02 mm calliper, 49 main scale divisions equal 50 vernier divisions); reading = main scale + vernier coincidence × least count.

Micrometer least count = pitch of screw / number of divisions on thimble (commonly 0.5 mm / 50 = 0.01 mm); the ratchet gives consistent measuring pressure.

Other instruments: steel rule (0.5 mm), try square (squareness), feeler gauge (gaps), dial indicator (runout), sine bar (angles), surface plate (datum), and limit gauges (go/no-go).

Personal protective equipment: safety shoes, goggles for chipping and grinding, gloves for handling hot or rough stock (not near rotating machines), ear protection and a helmet where required.

Machine safety rules: never leave a chuck key in the chuck, stop the machine before measuring, keep guards in place, tie back loose clothing and hair, and never remove chips by hand.

Fire classes and extinguishers: Class A solids (water), Class B flammable liquids (foam, CO2, dry powder), Class C gases, and electrical fires need CO2 or dry powder, never water.

First aid basics: cool burns with clean running water, cover cuts with sterile dressing, and report every injury; a workshop must have a first-aid box and an emergency exit clear of obstruction.

Care of instruments: wipe callipers after use, store in a case, avoid magnetic chucks near instruments, and never use a micrometer as a clamp or spanner.

Fits and limits use the hole-basis system: H7/h6 is a common sliding fit, and the tolerance zone is the difference between upper and lower limits.

Ferrous metals contain iron as the base element; carbon content separates cast iron (2–4% C), mild steel (0.15–0.30% C), medium carbon steel (0.30–0.60% C) and high carbon steel (0.60–1.5% C).

Raising carbon content increases hardness and tensile strength but reduces ductility, weldability and machinability; mild steel is the workshop default for bolts, keys and shafts.

Pig iron is the blast-furnace product (about 3–4.5% C with Si, Mn, S, P) and is remelted in a cupola to make cast iron; cast iron is brittle in tension but strong in compression and damps vibration well.

Non-ferrous examples: aluminium (light, corrosion resistant), copper (excellent electrical and thermal conductor), brass (copper + zinc), bronze (copper + tin) and zinc used for galvanising.

Alloy steel adds Ni, Cr, Mn, Mo, V or W to improve hardenability, wear resistance or heat resistance; stainless steel relies on chromium above about 11–12% forming a passive oxide layer.

Mechanical properties vocabulary: elasticity, plasticity, ductility (wire drawing), malleability (sheet hammering), toughness, brittleness, hardness and fatigue resistance.

Hardness tests: Brinell (10 mm ball), Vickers (diamond pyramid, wide range) and Rockwell (indenter plus load, direct dial reading).

Heat treatment: annealing softens and relieves stress by slow cooling; normalising refines grain with air cooling; hardening uses rapid quenching; tempering reduces post-hardening brittleness; case hardening gives a hard skin over a tough core.

Bench vice parts: fixed jaw, movable jaw, square-thread spindle, handle, guide bars and swivel base; the workpiece is gripped low in the jaws so clamping force acts close to the slide.

Files are classified by length, cut (single, double, rasp), shape (flat, half-round, round, square, triangular, knife) and grade (rough, bastard, second cut, smooth); a smooth file removes least metal per stroke.

Filing rules: clean teeth with a file card, never use a file without a handle, apply pressure on the forward stroke only, and lift the file on the return stroke.

Hacksaw blade teeth point forward and cut on the push stroke; use 14 or 18 TPI for thick sections and 24 or 32 TPI for thin tube and sheet to avoid tooth snagging.

Chisels: cold chisel for metals (flat, cross-cut, half-round, diamond point), firmer and mortise chisels for wood; a cold chisel is struck with a ball-peen hammer and a chipped head must be reground.

Punches: centre punch (dot mark for drilling), prick punch (fine locating mark) and pin punch; the punch is held vertically and struck once to mark layout lines.

Scribers, dividers, trammels, surface plate, V-block, try square, steel rule and vernier are layout tools; layout blue improves visibility of scribed lines on bright metal.

Screwdriver and spanner selection: blade width must match the screw slot, and the spanner jaw must fit the nut flats exactly - an undersized spanner rounds the corners.

Marking-out sequence: clean the surface, apply marking medium, establish a datum face, scribe lines, then punch them; all dimensions are taken from the datum, never from a rough edge.

Sawing, chipping, filing and drilling are the main bench operations; each removes metal to bring the job to the layout lines, with final finishing by smooth file or scraper.

Drilling: drill size for a tap equals the major diameter minus the pitch (for M10 × 1.5, use an 8.5 mm drill); centre drill first, use cutting fluid and clamp the work in a machine vice.

Tapping: the taper tap starts the thread, the intermediate tap cuts the full form and the bottoming tap finishes blind holes; reverse the tap a quarter turn periodically to break chips.

Reaming produces a close-tolerance, smooth hole after drilling undersize; a reamer is turned clockwise only and never reversed, to avoid damaging the cutting edges.

Scraping and lapping are hand finishing processes for flat surfaces and precision fits; blueing compound shows high spots when two surfaces are rubbed together.

Limits and fits: clearance fit (hole always larger than shaft), interference fit (shaft larger than hole) and transition fit; allowance is the intentional difference between basic sizes.

Threads: V-thread included angle 60°, BSW 55°, pitch is the axial distance between crests, and lead equals pitch times number of starts (single-start: lead = pitch).

Sheet metal gauge numbers run opposite to thickness: a higher gauge number means thinner sheet; standard wire gauge (SWG) values are used for selection.

Key operations: shearing, blanking, piercing, bending, folding, rolling, seaming and riveting; burrs are removed with a file or deburring tool before folding.

Bend allowance must be added to the flat pattern; a simple estimate for a 90° bend places the neutral axis at roughly half the thickness from the inner surface.

Edges are stiffened by a hem (single or double), wiring or beading; a wired edge prevents sharp edges and adds rigidity to buckets and trays.

Seams: lap seam (soldered or riveted), grooved seam (double locked) and Pittsburgh lock; the grooved seam is formed on a folding bar and locked with a seam setter.

Rivets: snap head, pan head, countersunk and pop rivets; the hole is slightly larger than the shank, and rivet length must cover the plates plus head allowance.

Tools: snips (straight, curved, aviation), ball-peen hammer, mallet, stakes, shears, folding bar, groovers, hand rolls and soldering iron; a mallet is used on soft sheet to avoid stretch marks.

Sheet metal is joined by soft soldering with a flux (zinc chloride for galvanised, resin for electrical work); the joint must be clean and tinned before solder flows.

Hardwoods come from broad-leaved deciduous trees (teak, rosewood, sal) and softwoods from conifers (pine, deodar, fir); the botanical split alone does not fix hardness.

Seasoning reduces moisture content to prevent shrinkage, warping and splitting; air seasoning is slow and cheap, kiln seasoning is fast and gives controlled moisture content.

Wood defects: knots, shakes, splits, twist, cup, bow, waney edge and decay; a knot is a branch base embedded in the stem and weakens the board.

Carpentry joints: lap, mortise and tenon, dovetail, dowel, halving, bridle, housing and butt; the mortise-and-tenon joint is standard for frames and needs a mortise chisel.

Marking and cutting tools: try square, marking gauge, mortise gauge, bevel, rip saw (along the grain), cross-cut saw (across the grain), tenon saw (fine work) and jack plane for surfacing.

Plane parts: body, sole, cutting iron (bevel down), cap iron, lever cap and adjusting screw; the cap iron is set close to the edge for rough work to break chips.

Abrasive finishing follows the grain; sandpaper grit numbers increase with fineness, and varnish or polish is applied only after the surface is dust free.

Wood adhesives: animal glue (hot), PVA (interior), epoxy and synthetic resin; joints must be close fitting because glue lines are weak across thick gaps.

Smithy operations: upsetting (increasing cross-section by end blows), drawing down (reducing section and lengthening), bending, twisting, punching, drifting, swaging and forge welding.

Forge fire is controlled by air blast; temperature is judged by colour - cherry red near 800 °C, orange to yellow around 1000–1200 °C for forging, and white heat risks burning the steel.

Overheating causes coarse grain and burning causes irrecoverable loss of strength; forging below the critical range (blue brittle region) may crack the metal.

Tools: anvil (horn, face, hardy hole), swage block, tongs (flat, pick-up, hollow bit), hammers (sledge and hand), hot set, hardie, punch, drift and fullers.

Forge welding joins two heated pieces by hammering; surfaces must be clean, fluxed (borax or sand) and brought to welding heat so slag is squeezed out.

Upsetting forms bolt heads, drawing down tapers a bar to a point, and swaging shapes work in a swage block or between top and bottom fullers.

Safety: wear leather gloves, goggles and apron, keep the anvil face clean and dry, stand clear of the hammer arc, and quench tongs to keep handles cool.

Casting steps: pattern making, moulding (sand rammed around the pattern in a flask), core setting, gating, melting, pouring, cooling, shakeout and fettling.

Green sand contains silica sand, clay binder, water and additives; it must have permeability, refractoriness, cohesiveness and collapsibility.

Pattern types: solid, split, match plate, loose piece, sweep and skeleton; the pattern is enlarged by shrinkage allowance (cast iron about 10 mm/m, aluminium about 16 mm/m).

Machining allowance is added to surfaces to be machined; draft (taper) allows easy pattern withdrawal; core prints support the core that forms internal cavities.

Gating system parts: pouring basin, sprue, runner, ingate and riser; the riser (feeder) supplies liquid metal to compensate shrinkage during solidification.

Cupola is used for cast iron, while induction and electric arc furnaces melt steel; pouring temperature must fill the mould without causing gas defects.

Casting defects: blowholes and porosity (gas), shrinkage cavity (insufficient feeding), cold shut (low pouring temperature), misrun, hot tear, sand inclusion and drop (mould wall collapse).

Fusion welding (arc, gas, resistance, thermit) melts the parent metal; solid-state welding (friction, forge, ultrasonic) joins without melting; brazing and soldering use a filler melting below the parent metal.

Arc welding uses a coated consumable electrode; the coating stabilises the arc, shields the molten pool and adds alloying elements; electrode diameter is chosen by plate thickness and current.

Polarity: DC straight polarity has the electrode negative and the work positive (more heat at the work); DC reverse polarity puts more heat at the electrode, suited to thin sheet.

Oxy-acetylene flames: neutral (about 1:1, used for steel), oxidising (excess oxygen, used for brass) and carburising (excess acetylene, used for high carbon steel and hard surfacing).

The acetylene–oxygen flame reaches about 3200 °C; the inner cone tip is held just above the pool and the torch is moved steadily to avoid undercut.

Resistance welding (spot and seam) uses pressure plus I²Rt heating; thermit welding uses the exothermic reaction of aluminium powder with iron oxide for rails and heavy sections.

Weld defects: slag inclusion, porosity, incomplete penetration, lack of fusion, undercut, spatter, distortion and cracks; distortion is reduced by back-step welding, clamping and balanced sequence.

Safety: shield the arc with a helmet of correct shade, keep cylinders upright and away from heat, never use oil on oxygen fittings, and open the acetylene valve only about one turn.

Lathe parts: bed, headstock, tailstock, carriage with cross slide and compound rest, tool post, lead screw and feed rod; the work rotates while a single-point tool feeds along or across.

Lathe operations: facing, turning, taper turning (swivelling compound rest, offset tailstock, taper turning attachment), thread cutting, drilling, boring, knurling, chamfering and parting off.

Cutting speed V = πDN/1000 (m/min) with D in mm and N in rpm; feed is in mm/rev, and depth of cut is half the diameter reduction.

Drilling machine types: sensitive (bench), pillar, radial, gang and multi-spindle; the drill is held in a sleeve and socket or chuck, and spindle speed is set from the cutting-speed formula.

Milling: up (conventional) milling produces chip thickness from zero to maximum and tends to lift the work; down (climb) milling starts with maximum chip thickness and gives better finish with a backlash eliminator.

Grinding uses an abrasive wheel; grit, grade and bond are selected by material, the wheel is dressed to restore sharpness and form, and the wheel guard must be in place.

Shaper and planer: the shaper uses a reciprocating single-point tool with quick-return mechanism and clapper box; the planer moves the work past a stationary tool for large flat surfaces.

Tool geometry and materials: rake angle, clearance angle, cutting edge angle and nose radius control cutting force and finish; HSS, carbide, ceramic and diamond show increasing hot hardness.

Belt drive velocity ratio = speed of driver / speed of driven = diameter of driven / diameter of driver (for equal belt thickness); crossed belts reverse direction, open belts keep the same direction.

Simple gear train velocity ratio = product of teeth on driven gears / product of teeth on driving gears; a compound train multiplies individual stage ratios.

Gear types: spur (parallel shafts), helical (smoother, thrust load), bevel (intersecting shafts), worm and worm wheel (large reduction, sometimes self-locking) and rack and pinion (rotary to linear).

Four-bar linkage, slider-crank, double slider, quick-return and toggle mechanisms are simple mechanisms; the slider-crank converts rotary to reciprocating motion in engines and compressors.

A cam converts rotary motion into specified follower motion (uniform velocity, simple harmonic, uniform acceleration); a roller follower reduces wear compared with a knife-edge follower.

Bearings: sliding (journal) bearings rely on an oil film; rolling bearings are ball, cylindrical roller, taper roller and needle; taper rollers accept combined radial and thrust loads.

Clutches and brakes transmit or absorb torque by friction; a single-plate clutch transmits torque T = μ·W·r with r the effective friction radius, and a centrifugal clutch engages with speed.

Chain drives do not slip and tolerate shorter centre distances than belts; belt slip protects against overload but reduces the exact velocity ratio.

Vernier calliper least count = 1 main scale division − 1 vernier scale division (a 0.02 mm calliper has 49 main scale divisions equal to 50 vernier divisions); reading = main scale + vernier coincidence × least count.

Micrometer least count = pitch of screw / number of thimble divisions (commonly 0.5 mm / 50 = 0.01 mm); the ratchet gives consistent measuring pressure.

Other instruments: steel rule (0.5 mm), try square (squareness), feeler gauge (gaps), dial indicator (runout), sine bar (angles), surface plate (datum) and limit gauges (go/no-go).

PPE: safety shoes, goggles for chipping and grinding, gloves for hot or rough stock (not near rotating machines), ear protection and a helmet where required.

Machine safety rules: never leave a chuck key in the chuck, stop the machine before measuring, keep guards in place, tie back loose clothing and hair, and never remove chips by hand.

Fire classes and extinguishers: Class A solids (water), Class B flammable liquids (foam, CO2, dry powder), Class C gases, and electrical fires need CO2 or dry powder, never water.

First aid: cool burns with clean running water, cover cuts with sterile dressing and report every injury; a workshop must have a first-aid box and an exit clear of obstruction.

Care of instruments: wipe callipers after use, store in a case, keep them away from magnetic chucks, and never use a micrometer as a clamp or spanner.

Fits and limits use the hole-basis system: H7/h6 is a common sliding fit, and the tolerance zone is the difference between upper and lower limits.

Ferrous metals contain iron as the base element; carbon content separates cast iron (2–4% C), mild steel (0.15–0.30% C), medium carbon steel (0.30–0.60% C) and high carbon steel (0.60–1.5% C).

Raising carbon content increases hardness and tensile strength but reduces ductility, weldability and machinability; mild steel is the workshop default for bolts, keys and shafts.

Pig iron is the blast-furnace product (about 3–4.5% C with Si, Mn, S, P) and is remelted in a cupola to make cast iron; cast iron is brittle in tension but strong in compression and damps vibration well.

Non-ferrous examples: aluminium (light, corrosion resistant), copper (excellent electrical and thermal conductor), brass (copper + zinc), bronze (copper + tin) and zinc used for galvanising.

Alloy steel adds Ni, Cr, Mn, Mo, V or W to improve hardenability, wear resistance or heat resistance; stainless steel relies on chromium above about 11–12% forming a passive oxide layer.

Mechanical properties vocabulary: elasticity, plasticity, ductility (wire drawing), malleability (sheet hammering), toughness, brittleness, hardness and fatigue resistance.

Hardness tests: Brinell (10 mm ball), Vickers (diamond pyramid, wide range) and Rockwell (indenter plus load, direct dial reading).

Heat treatment: annealing softens and relieves stress by slow cooling; normalising refines grain with air cooling; hardening uses rapid quenching; tempering reduces post-hardening brittleness; case hardening gives a hard skin over a tough core.

Bench vice parts: fixed jaw, movable jaw, square-thread spindle, handle, guide bars and swivel base; the workpiece is gripped low in the jaws so clamping force acts close to the slide.

Files are classified by length, cut (single, double, rasp), shape (flat, half-round, round, square, triangular, knife) and grade (rough, bastard, second cut, smooth); a smooth file removes least metal per stroke.

Filing rules: clean teeth with a file card, never use a file without a handle, apply pressure on the forward stroke only, and lift the file on the return stroke.

Hacksaw blade teeth point forward and cut on the push stroke; use 14 or 18 TPI for thick sections and 24 or 32 TPI for thin tube and sheet to avoid tooth snagging.

Chisels: cold chisel for metals (flat, cross-cut, half-round, diamond point), firmer and mortise chisels for wood; a cold chisel is struck with a ball-peen hammer and a chipped head must be reground.

Punches: centre punch (dot mark for drilling), prick punch (fine locating mark) and pin punch; the punch is held vertically and struck once to mark layout lines.

Scribers, dividers, trammels, surface plate, V-block, try square, steel rule and vernier are layout tools; layout blue improves visibility of scribed lines on bright metal.

Screwdriver and spanner selection: blade width must match the screw slot, and the spanner jaw must fit the nut flats exactly - an undersized spanner rounds the corners.

Marking-out sequence: clean the surface, apply marking medium, establish a datum face, scribe lines, then punch them; all dimensions are taken from the datum, never from a rough edge.

Sawing, chipping, filing and drilling are the main bench operations; each removes metal to bring the job to the layout lines, with final finishing by smooth file or scraper.

Drilling: drill size for a tap equals the major diameter minus the pitch (for M10 × 1.5, use an 8.5 mm drill); centre drill first, use cutting fluid and clamp the work in a machine vice.

Tapping: the taper tap starts the thread, the intermediate tap cuts the full form and the bottoming tap finishes blind holes; reverse the tap a quarter turn periodically to break chips.

Reaming produces a close-tolerance, smooth hole after drilling undersize; a reamer is turned clockwise only and never reversed, to avoid damaging the cutting edges.

Scraping and lapping are hand finishing processes for flat surfaces and precision fits; blueing compound shows high spots when two surfaces are rubbed together.

Limits and fits: clearance fit (hole always larger than shaft), interference fit (shaft larger than hole) and transition fit; allowance is the intentional difference between basic sizes.

Threads: V-thread included angle 60°, BSW 55°, pitch is the axial distance between crests, and lead equals pitch times number of starts (single-start: lead = pitch).

Sheet metal gauge numbers run opposite to thickness: a higher gauge number means thinner sheet; standard wire gauge (SWG) values are used for selection.

Key operations: shearing, blanking, piercing, bending, folding, rolling, seaming and riveting; burrs are removed with a file or deburring tool before folding.

Bend allowance must be added to the flat pattern; a simple estimate for a 90° bend places the neutral axis at roughly half the thickness from the inner surface.

Edges are stiffened by a hem (single or double), wiring or beading; a wired edge prevents sharp edges and adds rigidity to buckets and trays.

Seams: lap seam (soldered or riveted), grooved seam (double locked) and Pittsburgh lock; the grooved seam is formed on a folding bar and locked with a seam setter.

Rivets: snap head, pan head, countersunk and pop rivets; the hole is slightly larger than the shank, and rivet length must cover the plates plus head allowance.

Tools: snips (straight, curved, aviation), ball-peen hammer, mallet, stakes, shears, folding bar, groovers, hand rolls and soldering iron; a mallet is used on soft sheet to avoid stretch marks.

Sheet metal is joined by soft soldering with a flux (zinc chloride for galvanised, resin for electrical work); the joint must be clean and tinned before solder flows.

Hardwoods come from broad-leaved deciduous trees (teak, rosewood, sal) and softwoods from conifers (pine, deodar, fir); the botanical split alone does not fix hardness.

Seasoning reduces moisture content to prevent shrinkage, warping and splitting; air seasoning is slow and cheap, kiln seasoning is fast and gives controlled moisture content.

Wood defects: knots, shakes, splits, twist, cup, bow, waney edge and decay; a knot is a branch base embedded in the stem and weakens the board.

Carpentry joints: lap, mortise and tenon, dovetail, dowel, halving, bridle, housing and butt; the mortise-and-tenon joint is standard for frames and needs a mortise chisel.

Marking and cutting tools: try square, marking gauge, mortise gauge, bevel, rip saw (along the grain), cross-cut saw (across the grain), tenon saw (fine work) and jack plane for surfacing.

Plane parts: body, sole, cutting iron (bevel down), cap iron, lever cap and adjusting screw; the cap iron is set close to the edge for rough work to break chips.

Abrasive finishing follows the grain; sandpaper grit numbers increase with fineness, and varnish or polish is applied only after the surface is dust free.

Wood adhesives: animal glue (hot), PVA (interior), epoxy and synthetic resin; joints must be close fitting because glue lines are weak across thick gaps.

Smithy operations: upsetting (increasing cross-section by end blows), drawing down (reducing section and lengthening), bending, twisting, punching, drifting, swaging and forge welding.

Forge fire is controlled by air blast; temperature is judged by colour - cherry red near 800 °C, orange to yellow around 1000–1200 °C for forging, and white heat risks burning the steel.

Overheating causes coarse grain and burning causes irrecoverable loss of strength; forging below the critical range (blue brittle region) may crack the metal.

Tools: anvil (horn, face, hardy hole), swage block, tongs (flat, pick-up, hollow bit), hammers (sledge and hand), hot set, hardie, punch, drift and fullers.

Forge welding joins two heated pieces by hammering; surfaces must be clean, fluxed (borax or sand) and brought to welding heat so slag is squeezed out.

Upsetting forms bolt heads, drawing down tapers a bar to a point, and swaging shapes work in a swage block or between top and bottom fullers.

Safety: wear leather gloves, goggles and apron, keep the anvil face clean and dry, stand clear of the hammer arc, and quench tongs to keep handles cool.

Casting steps: pattern making, moulding (sand rammed around the pattern in a flask), core setting, gating, melting, pouring, cooling, shakeout and fettling.

Green sand contains silica sand, clay binder, water and additives; it must have permeability, refractoriness, cohesiveness and collapsibility.

Pattern types: solid, split, match plate, loose piece, sweep and skeleton; the pattern is enlarged by shrinkage allowance (cast iron about 10 mm/m, aluminium about 16 mm/m).

Machining allowance is added to surfaces to be machined; draft (taper) allows easy pattern withdrawal; core prints support the core that forms internal cavities.

Gating system parts: pouring basin, sprue, runner, ingate and riser; the riser (feeder) supplies liquid metal to compensate shrinkage during solidification.

Cupola is used for cast iron, while induction and electric arc furnaces melt steel; pouring temperature must fill the mould without causing gas defects.

Casting defects: blowholes and porosity (gas), shrinkage cavity (insufficient feeding), cold shut (low pouring temperature), misrun, hot tear, sand inclusion and drop (mould wall collapse).

Fusion welding (arc, gas, resistance, thermit) melts the parent metal; solid-state welding (friction, forge, ultrasonic) joins without melting; brazing and soldering use a filler melting below the parent metal.

Arc welding uses a coated consumable electrode; the coating stabilises the arc, shields the molten pool and adds alloying elements; electrode diameter is chosen by plate thickness and current.

Polarity: DC straight polarity has the electrode negative and the work positive (more heat at the work); DC reverse polarity puts more heat at the electrode, suited to thin sheet.

Oxy-acetylene flames: neutral (about 1:1, used for steel), oxidising (excess oxygen, used for brass) and carburising (excess acetylene, used for high carbon steel and hard surfacing).

The acetylene–oxygen flame reaches about 3200 °C; the inner cone tip is held just above the pool and the torch is moved steadily to avoid undercut.

Resistance welding (spot and seam) uses pressure plus I²Rt heating; thermit welding uses the exothermic reaction of aluminium powder with iron oxide for rails and heavy sections.

Weld defects: slag inclusion, porosity, incomplete penetration, lack of fusion, undercut, spatter, distortion and cracks; distortion is reduced by back-step welding, clamping and balanced sequence.

Safety: shield the arc with a helmet of correct shade, keep cylinders upright and away from heat, never use oil on oxygen fittings, and open the acetylene valve only about one turn.

Lathe parts: bed, headstock, tailstock, carriage with cross slide and compound rest, tool post, lead screw and feed rod; the work rotates while a single-point tool feeds along or across.

Lathe operations: facing, turning, taper turning (swivelling compound rest, offset tailstock, taper turning attachment), thread cutting, drilling, boring, knurling, chamfering and parting off.

Cutting speed V = πDN/1000 (m/min) with D in mm and N in rpm; feed is in mm/rev, and depth of cut is half the diameter reduction.

Drilling machine types: sensitive (bench), pillar, radial, gang and multi-spindle; the drill is held in a sleeve and socket or chuck, and spindle speed is set from the cutting-speed formula.

Milling: up (conventional) milling produces chip thickness from zero to maximum and tends to lift the work; down (climb) milling starts with maximum chip thickness and gives better finish with a backlash eliminator.

Grinding uses an abrasive wheel; grit, grade and bond are selected by material, the wheel is dressed to restore sharpness and form, and the wheel guard must be in place.

Shaper and planer: the shaper uses a reciprocating single-point tool with quick-return mechanism and clapper box; the planer moves the work past a stationary tool for large flat surfaces.

Tool geometry and materials: rake angle, clearance angle, cutting edge angle and nose radius control cutting force and finish; HSS, carbide, ceramic and diamond show increasing hot hardness.

Belt drive velocity ratio = speed of driver / speed of driven = diameter of driven / diameter of driver (for equal belt thickness); crossed belts reverse direction, open belts keep the same direction.

Simple gear train velocity ratio = product of teeth on driven gears / product of teeth on driving gears; a compound train multiplies individual stage ratios.

Gear types: spur (parallel shafts), helical (smoother, thrust load), bevel (intersecting shafts), worm and worm wheel (large reduction, sometimes self-locking) and rack and pinion (rotary to linear).

Four-bar linkage, slider-crank, double slider, quick-return and toggle mechanisms are simple mechanisms; the slider-crank converts rotary to reciprocating motion in engines and compressors.

A cam converts rotary motion into specified follower motion (uniform velocity, simple harmonic, uniform acceleration); a roller follower reduces wear compared with a knife-edge follower.

Bearings: sliding (journal) bearings rely on an oil film; rolling bearings are ball, cylindrical roller, taper roller and needle; taper rollers accept combined radial and thrust loads.

Clutches and brakes transmit or absorb torque by friction; a single-plate clutch transmits torque T = μ·W·r with r the effective friction radius, and a centrifugal clutch engages with speed.

Chain drives do not slip and tolerate shorter centre distances than belts; belt slip protects against overload but reduces the exact velocity ratio.

Vernier calliper least count = 1 main scale division − 1 vernier scale division (a 0.02 mm calliper has 49 main scale divisions equal to 50 vernier divisions); reading = main scale + vernier coincidence × least count.

Micrometer least count = pitch of screw / number of thimble divisions (commonly 0.5 mm / 50 = 0.01 mm); the ratchet gives consistent measuring pressure.

Other instruments: steel rule (0.5 mm), try square (squareness), feeler gauge (gaps), dial indicator (runout), sine bar (angles), surface plate (datum) and limit gauges (go/no-go).

PPE: safety shoes, goggles for chipping and grinding, gloves for hot or rough stock (not near rotating machines), ear protection and a helmet where required.

Machine safety rules: never leave a chuck key in the chuck, stop the machine before measuring, keep guards in place, tie back loose clothing and hair, and never remove chips by hand.

Fire classes and extinguishers: Class A solids (water), Class B flammable liquids (foam, CO2, dry powder), Class C gases, and electrical fires need CO2 or dry powder, never water.

First aid: cool burns with clean running water, cover cuts with sterile dressing and report every injury; a workshop must have a first-aid box and an exit clear of obstruction.

Care of instruments: wipe callipers after use, store in a case, keep them away from magnetic chucks, and never use a micrometer as a clamp or spanner.

Fits and limits use the hole-basis system: H7/h6 is a common sliding fit, and the tolerance zone is the difference between upper and lower limits.

Ferrous metals contain iron as the base element; carbon content separates cast iron (2–4% C), mild steel (0.15–0.30% C), medium carbon steel (0.30–0.60% C) and high carbon steel (0.60–1.5% C).

Raising carbon content increases hardness and tensile strength but reduces ductility, weldability and machinability; mild steel is the workshop default for bolts, keys and shafts.

Pig iron is the blast-furnace product (about 3–4.5% C with Si, Mn, S, P) and is remelted in a cupola to make cast iron; cast iron is brittle in tension but strong in compression and damps vibration well.

Non-ferrous examples: aluminium (light, corrosion resistant), copper (excellent electrical and thermal conductor), brass (copper + zinc), bronze (copper + tin) and zinc used for galvanising.

Alloy steel adds Ni, Cr, Mn, Mo, V or W to improve hardenability, wear resistance or heat resistance; stainless steel relies on chromium above about 11–12% forming a passive oxide layer.

Mechanical properties vocabulary: elasticity, plasticity, ductility (wire drawing), malleability (sheet hammering), toughness, brittleness, hardness and fatigue resistance.

Hardness tests: Brinell (10 mm ball), Vickers (diamond pyramid, wide range) and Rockwell (indenter plus load, direct dial reading).

Heat treatment: annealing softens and relieves stress by slow cooling; normalising refines grain with air cooling; hardening uses rapid quenching; tempering reduces post-hardening brittleness; case hardening gives a hard skin over a tough core.

Bench vice parts: fixed jaw, movable jaw, square-thread spindle, handle, guide bars and swivel base; the workpiece is gripped low in the jaws so clamping force acts close to the slide.

Files are classified by length, cut (single, double, rasp), shape (flat, half-round, round, square, triangular, knife) and grade (rough, bastard, second cut, smooth); a smooth file removes least metal per stroke.

Filing rules: clean teeth with a file card, never use a file without a handle, apply pressure on the forward stroke only, and lift the file on the return stroke.

Hacksaw blade teeth point forward and cut on the push stroke; use 14 or 18 TPI for thick sections and 24 or 32 TPI for thin tube and sheet to avoid tooth snagging.

Chisels: cold chisel for metals (flat, cross-cut, half-round, diamond point), firmer and mortise chisels for wood; a cold chisel is struck with a ball-peen hammer and a chipped head must be reground.

Punches: centre punch (dot mark for drilling), prick punch (fine locating mark) and pin punch; the punch is held vertically and struck once to mark layout lines.

Scribers, dividers, trammels, surface plate, V-block, try square, steel rule and vernier are layout tools; layout blue improves visibility of scribed lines on bright metal.

Screwdriver and spanner selection: blade width must match the screw slot, and the spanner jaw must fit the nut flats exactly - an undersized spanner rounds the corners.

Marking-out sequence: clean the surface, apply marking medium, establish a datum face, scribe lines, then punch them; all dimensions are taken from the datum, never from a rough edge.

Sawing, chipping, filing and drilling are the main bench operations; each removes metal to bring the job to the layout lines, with final finishing by smooth file or scraper.

Drilling: drill size for a tap equals the major diameter minus the pitch (for M10 × 1.5, use an 8.5 mm drill); centre drill first, use cutting fluid and clamp the work in a machine vice.

Tapping: the taper tap starts the thread, the intermediate tap cuts the full form and the bottoming tap finishes blind holes; reverse the tap a quarter turn periodically to break chips.

Reaming produces a close-tolerance, smooth hole after drilling undersize; a reamer is turned clockwise only and never reversed, to avoid damaging the cutting edges.

Scraping and lapping are hand finishing processes for flat surfaces and precision fits; blueing compound shows high spots when two surfaces are rubbed together.

Limits and fits: clearance fit (hole always larger than shaft), interference fit (shaft larger than hole) and transition fit; allowance is the intentional difference between basic sizes.

Threads: V-thread included angle 60°, BSW 55°, pitch is the axial distance between crests, and lead equals pitch times number of starts (single-start: lead = pitch).

Sheet metal gauge numbers run opposite to thickness: a higher gauge number means thinner sheet; standard wire gauge (SWG) values are used for selection.

Key operations: shearing, blanking, piercing, bending, folding, rolling, seaming and riveting; burrs are removed with a file or deburring tool before folding.

Bend allowance must be added to the flat pattern; a simple estimate for a 90° bend places the neutral axis at roughly half the thickness from the inner surface.

Edges are stiffened by a hem (single or double), wiring or beading; a wired edge prevents sharp edges and adds rigidity to buckets and trays.

Seams: lap seam (soldered or riveted), grooved seam (double locked) and Pittsburgh lock; the grooved seam is formed on a folding bar and locked with a seam setter.

Rivets: snap head, pan head, countersunk and pop rivets; the hole is slightly larger than the shank, and rivet length must cover the plates plus head allowance.

Tools: snips (straight, curved, aviation), ball-peen hammer, mallet, stakes, shears, folding bar, groovers, hand rolls and soldering iron; a mallet is used on soft sheet to avoid stretch marks.

Sheet metal is joined by soft soldering with a flux (zinc chloride for galvanised, resin for electrical work); the joint must be clean and tinned before solder flows.

Hardwoods come from broad-leaved deciduous trees (teak, rosewood, sal) and softwoods from conifers (pine, deodar, fir); the botanical split alone does not fix hardness.

Seasoning reduces moisture content to prevent shrinkage, warping and splitting; air seasoning is slow and cheap, kiln seasoning is fast and gives controlled moisture content.

Wood defects: knots, shakes, splits, twist, cup, bow, waney edge and decay; a knot is a branch base embedded in the stem and weakens the board.

Carpentry joints: lap, mortise and tenon, dovetail, dowel, halving, bridle, housing and butt; the mortise-and-tenon joint is standard for frames and needs a mortise chisel.

Marking and cutting tools: try square, marking gauge, mortise gauge, bevel, rip saw (along the grain), cross-cut saw (across the grain), tenon saw (fine work) and jack plane for surfacing.

Plane parts: body, sole, cutting iron (bevel down), cap iron, lever cap and adjusting screw; the cap iron is set close to the edge for rough work to break chips.

Abrasive finishing follows the grain; sandpaper grit numbers increase with fineness, and varnish or polish is applied only after the surface is dust free.

Wood adhesives: animal glue (hot), PVA (interior), epoxy and synthetic resin; joints must be close fitting because glue lines are weak across thick gaps.

Smithy operations: upsetting (increasing cross-section by end blows), drawing down (reducing section and lengthening), bending, twisting, punching, drifting, swaging and forge welding.

Forge fire is controlled by air blast; temperature is judged by colour - cherry red near 800 °C, orange to yellow around 1000–1200 °C for forging, and white heat risks burning the steel.

Overheating causes coarse grain and burning causes irrecoverable loss of strength; forging below the critical range (blue brittle region) may crack the metal.

Tools: anvil (horn, face, hardy hole), swage block, tongs (flat, pick-up, hollow bit), hammers (sledge and hand), hot set, hardie, punch, drift and fullers.

Forge welding joins two heated pieces by hammering; surfaces must be clean, fluxed (borax or sand) and brought to welding heat so slag is squeezed out.

Upsetting forms bolt heads, drawing down tapers a bar to a point, and swaging shapes work in a swage block or between top and bottom fullers.

Safety: wear leather gloves, goggles and apron, keep the anvil face clean and dry, stand clear of the hammer arc, and quench tongs to keep handles cool.

Casting steps: pattern making, moulding (sand rammed around the pattern in a flask), core setting, gating, melting, pouring, cooling, shakeout and fettling.

Green sand contains silica sand, clay binder, water and additives; it must have permeability, refractoriness, cohesiveness and collapsibility.

Pattern types: solid, split, match plate, loose piece, sweep and skeleton; the pattern is enlarged by shrinkage allowance (cast iron about 10 mm/m, aluminium about 16 mm/m).

Machining allowance is added to surfaces to be machined; draft (taper) allows easy pattern withdrawal; core prints support the core that forms internal cavities.

Gating system parts: pouring basin, sprue, runner, ingate and riser; the riser (feeder) supplies liquid metal to compensate shrinkage during solidification.

Cupola is used for cast iron, while induction and electric arc furnaces melt steel; pouring temperature must fill the mould without causing gas defects.

Casting defects: blowholes and porosity (gas), shrinkage cavity (insufficient feeding), cold shut (low pouring temperature), misrun, hot tear, sand inclusion and drop (mould wall collapse).

Fusion welding (arc, gas, resistance, thermit) melts the parent metal; solid-state welding (friction, forge, ultrasonic) joins without melting; brazing and soldering use a filler melting below the parent metal.

Arc welding uses a coated consumable electrode; the coating stabilises the arc, shields the molten pool and adds alloying elements; electrode diameter is chosen by plate thickness and current.

Polarity: DC straight polarity has the electrode negative and the work positive (more heat at the work); DC reverse polarity puts more heat at the electrode, suited to thin sheet.

Oxy-acetylene flames: neutral (about 1:1, used for steel), oxidising (excess oxygen, used for brass) and carburising (excess acetylene, used for high carbon steel and hard surfacing).

The acetylene–oxygen flame reaches about 3200 °C; the inner cone tip is held just above the pool and the torch is moved steadily to avoid undercut.

Resistance welding (spot and seam) uses pressure plus I²Rt heating; thermit welding uses the exothermic reaction of aluminium powder with iron oxide for rails and heavy sections.

Weld defects: slag inclusion, porosity, incomplete penetration, lack of fusion, undercut, spatter, distortion and cracks; distortion is reduced by back-step welding, clamping and balanced sequence.

Safety: shield the arc with a helmet of correct shade, keep cylinders upright and away from heat, never use oil on oxygen fittings, and open the acetylene valve only about one turn.

Lathe parts: bed, headstock, tailstock, carriage with cross slide and compound rest, tool post, lead screw and feed rod; the work rotates while a single-point tool feeds along or across.

Lathe operations: facing, turning, taper turning (swivelling compound rest, offset tailstock, taper turning attachment), thread cutting, drilling, boring, knurling, chamfering and parting off.

Cutting speed V = πDN/1000 (m/min) with D in mm and N in rpm; feed is in mm/rev, and depth of cut is half the diameter reduction.

Drilling machine types: sensitive (bench), pillar, radial, gang and multi-spindle; the drill is held in a sleeve and socket or chuck, and spindle speed is set from the cutting-speed formula.

Milling: up (conventional) milling produces chip thickness from zero to maximum and tends to lift the work; down (climb) milling starts with maximum chip thickness and gives better finish with a backlash eliminator.

Grinding uses an abrasive wheel; grit, grade and bond are selected by material, the wheel is dressed to restore sharpness and form, and the wheel guard must be in place.

Shaper and planer: the shaper uses a reciprocating single-point tool with quick-return mechanism and clapper box; the planer moves the work past a stationary tool for large flat surfaces.

Tool geometry and materials: rake angle, clearance angle, cutting edge angle and nose radius control cutting force and finish; HSS, carbide, ceramic and diamond show increasing hot hardness.

Belt drive velocity ratio = speed of driver / speed of driven = diameter of driven / diameter of driver (for equal belt thickness); crossed belts reverse direction, open belts keep the same direction.

Simple gear train velocity ratio = product of teeth on driven gears / product of teeth on driving gears; a compound train multiplies individual stage ratios.

Gear types: spur (parallel shafts), helical (smoother, thrust load), bevel (intersecting shafts), worm and worm wheel (large reduction, sometimes self-locking) and rack and pinion (rotary to linear).

Four-bar linkage, slider-crank, double slider, quick-return and toggle mechanisms are simple mechanisms; the slider-crank converts rotary to reciprocating motion in engines and compressors.

A cam converts rotary motion into specified follower motion (uniform velocity, simple harmonic, uniform acceleration); a roller follower reduces wear compared with a knife-edge follower.

Bearings: sliding (journal) bearings rely on an oil film; rolling bearings are ball, cylindrical roller, taper roller and needle; taper rollers accept combined radial and thrust loads.

Clutches and brakes transmit or absorb torque by friction; a single-plate clutch transmits torque T = μ·W·r with r the effective friction radius, and a centrifugal clutch engages with speed.

Chain drives do not slip and tolerate shorter centre distances than belts; belt slip protects against overload but reduces the exact velocity ratio.

Vernier calliper least count = 1 main scale division − 1 vernier scale division (a 0.02 mm calliper has 49 main scale divisions equal to 50 vernier divisions); reading = main scale + vernier coincidence × least count.

Micrometer least count = pitch of screw / number of thimble divisions (commonly 0.5 mm / 50 = 0.01 mm); the ratchet gives consistent measuring pressure.

Other instruments: steel rule (0.5 mm), try square (squareness), feeler gauge (gaps), dial indicator (runout), sine bar (angles), surface plate (datum) and limit gauges (go/no-go).

PPE: safety shoes, goggles for chipping and grinding, gloves for hot or rough stock (not near rotating machines), ear protection and a helmet where required.

Machine safety rules: never leave a chuck key in the chuck, stop the machine before measuring, keep guards in place, tie back loose clothing and hair, and never remove chips by hand.

Fire classes and extinguishers: Class A solids (water), Class B flammable liquids (foam, CO2, dry powder), Class C gases, and electrical fires need CO2 or dry powder, never water.

First aid: cool burns with clean running water, cover cuts with sterile dressing and report every injury; a workshop must have a first-aid box and an exit clear of obstruction.

Care of instruments: wipe callipers after use, store in a case, keep them away from magnetic chucks, and never use a micrometer as a clamp or spanner.

Fits and limits use the hole-basis system: H7/h6 is a common sliding fit, and the tolerance zone is the difference between upper and lower limits.