Agricultural Engineering · Foundations · also Workshop Practice, Manufacturing Technology
How farm machines are made: joining, casting, machining, heat treatment and measurement, up to building a prototype.
In the exams: ICAR AICE-JRF/SRF subject 39 (FMPE) Unit 7 Manufacturing Technology; ASRB NET Farm Machinery & Power; machinery fabrication in PRC ABE
Workshop technology is the study of how metal and wood become working parts: how a material is specified, cast, forged, pressed, cut, joined and heat treated, and how the result is measured against a drawing. For an agricultural engineer it is concrete. A cultivator frame is cut, drilled and welded; a ploughshare is forged or pressed from hardenable steel, then hardened and tempered; a thresher shaft is turned; a hopper is sheared, bent and seamed. When these parts fail in the field, they are repaired with the same processes.
Most agricultural engineering degrees teach the subject first as a practical course (fitting, carpentry, smithy, welding, sheet metal, foundry, machine shop) and later as manufacturing technology, with calculations of cutting speed, machining time, press force, weld strength and fits, and the planning of a prototype. This manual follows that path and treats safety inside each process, where accidents happen.
Entrance and degree papers ask process selection: which flame for brass, which wheel for cast iron, why a sprue is tapered. Postgraduate and licensing papers, such as the manufacturing unit of the Indian PhD entrance in farm machinery and power, the Indian agricultural scientist eligibility test and the Philippine agricultural and biosystems engineering board, add short calculations. Standards differ by country (Indian, ISO and EN, ASTM and SAE, JIS), so the manual gives the principle and notes where designations differ.
Every farm machine part is a compromise between strength, toughness, wear resistance, ease of manufacture and cost. A ploughshare needs a hard edge that will not shatter on a stone, so it is made from a hardenable medium- or high-carbon or boron steel, then hardened and tempered. A frame needs weldability and adequate yield strength, so it is low-carbon structural steel. A gearbox housing has complex shape and must damp vibration, so it is grey cast iron.
Yield strength is the stress at which permanent deformation begins; ultimate tensile strength is the highest engineering stress in a tensile test; elongation and reduction of area measure ductility. Toughness is the energy absorbed in fracturing a notched bar in a Charpy or Izod impact test. Hardness is resistance to indentation. The Brinell test presses a hardened ball (commonly 10 mm under 3000 kgf for steel) and divides load by the curved area of the dent; the Vickers test uses a diamond pyramid with 136° between faces; the Rockwell test measures depth, the C scale with a 120° diamond cone and the B scale with a small ball. For plain steels, tensile strength in MPa is roughly 3.45 times the Brinell number, an estimate only.
Steels are iron–carbon alloys with up to about 2% carbon; cast irons usually carry 2–4%. Low-carbon (mild) steel, up to about 0.25% carbon, is ductile, welds easily and cannot be usefully hardened by quenching; it is used for frames, brackets, sheet and bar. Medium-carbon steel (about 0.3–0.6%) responds to hardening and tempering and is used for shafts, axles and gears. High-carbon steel (above about 0.6%) is used for springs, blades and shares. Alloying with chromium, molybdenum, nickel, manganese or boron mainly raises hardenability and toughness; boron, at a few thousandths of a percent, does this cheaply and is common in tillage wear parts.
In grey cast iron the carbon forms graphite flakes: it machines well, damps vibration and is strong in compression but brittle in tension. White cast iron holds carbon as cementite and is very hard. Malleable iron is annealed white iron with compact graphite nodules. Spheroidal graphite (ductile) iron, treated with magnesium or cerium, has graphite spheres and real ductility, and suits castings that see shock. Stainless steels contain at least about 10.5% chromium; austenitic grades with about 18% chromium and 8% nickel are the usual food-contact and sprayer choice.
Aluminium alloys are light and corrosion resistant, used as extrusions, sheet and castings. Brass (copper–zinc) is used for fittings and valves; bronze (copper–tin, sometimes with phosphorus or lead) for bushes and worm wheels because it runs well against steel. Zinc alloys are die cast for small parts. Among polymers, high-density polyethylene and polypropylene make tanks, hoppers and seed tubes; nylon and acetal make gears, rollers and bushes; ultra-high-molecular-weight polyethylene lines chutes. Polymers creep under sustained load, soften with heat and degrade in sunlight unless stabilised.
A drawing names a material by standard designation, never by trade name, and systems differ by country. In the SAE–AISI four-digit system the first two digits give the alloy family and the last two the carbon in hundredths of a percent: 1045 is plain carbon steel with about 0.45% carbon, 4140 a chromium–molybdenum steel with about 0.40%, and a B after the first two digits marks a boron steel. In the EN system C45 is a non-alloy steel with about 0.45% carbon and 42CrMo4 gives carbon (0.42%) then the main alloying elements. EN 10025 names structural steels by minimum yield strength (S235, S275, S355, in MPa); the Indian structural standard does the same (E250, E350). The Japanese system names structural steel by tensile strength (SS400) and machine carbon steel as, for example, S45C.
Cast irons are named by property: grey iron EN-GJL-200 (FG 200 in the Indian standard) has about 200 MPa minimum tensile strength; ductile iron EN-GJS-400-15 has 400 MPa and 15% elongation. Equivalent grades in different systems are similar, not identical, so a substitution is checked against both standards.
Heat treatment changes microstructure, and so properties, by controlled heating and cooling, using the iron–carbon diagram as the map. A slowly cooled plain carbon steel consists of ferrite (nearly pure iron, soft) and cementite (Fe3C, hard and brittle). Above a critical temperature it becomes austenite, a face-centred cubic phase that dissolves much more carbon. At 727 °C the eutectoid reaction turns austenite of about 0.76–0.8% carbon (texts differ slightly) into pearlite, a lamellar mixture of ferrite and cementite.
Hypoeutectoid steels (below eutectoid carbon) are ferrite plus pearlite; hypereutectoid steels are pearlite plus cementite. The lower critical temperature A1 is the eutectoid line; the upper is A3 for hypoeutectoid and Acm for hypereutectoid steels. The lever rule gives phase fractions: for 0.4% carbon steel just below 727 °C, with eutectoid carbon 0.76% and ferrite carbon 0.022%, pearlite is (0.4 − 0.022)/(0.76 − 0.022), about 51%. Beyond about 2% carbon lies the cast-iron region, where a eutectic forms at about 4.3% carbon and 1147 °C.
Full annealing heats a hypoeutectoid steel about 30–50 °C above A3, soaks it and cools it in the furnace, giving coarse pearlite and ferrite: soft, ductile and machinable. Process annealing below A1 removes cold-work hardening between forming stages; spheroidising near A1 gives the softest state for high-carbon steel; stress relieving at a few hundred degrees reduces residual stress after welding. Normalising heats to a similar temperature and cools in still air, giving finer pearlite and grain, so the part is a little stronger than annealed; it refines forgings and castings.
Hardening heats a hypoeutectoid steel above A3 (a hypereutectoid steel only above A1, so some hard carbide remains and grain growth is limited) and quenches it in water, brine, oil or polymer. Austenite then shears without diffusion into martensite, a body-centred tetragonal phase that is very hard and brittle. Water is severe and risks cracking; oil suits alloy steels of higher hardenability.
Tempering reheats below A1 to trade hardness for toughness: roughly 150–250 °C for tools and wear edges, roughly 400–650 °C for tough shafts and axles (quench-and-temper). Some steels lose toughness in an intermediate band, so ranges come from the data sheet. Austempering holds in a salt bath above martensite-start until bainite forms, giving toughness with little distortion.
Hardenability is the depth to which a steel forms martensite; maximum hardness depends mainly on carbon. It is measured by the Jominy end-quench test: a heated bar is water-sprayed on one end and hardness is read along its length. Alloying and boron raise it. A share of boron steel can harden through a section that, in plain carbon steel of the same carbon content, would harden only near the surface.
Many farm parts need a hard skin on a tough core. Carburising adds carbon to low-carbon steel at roughly 900–950 °C, followed by quench and low temper, for gears and pins. Nitriding diffuses nitrogen into alloy steels at about 500–570 °C, needs no quench and gives very little distortion. Flame and induction hardening heat only the surface of a medium-carbon steel and quench it, for shafts, sprocket teeth and tool edges. In the farm workshop, a forged share is re-hardened and tempered after reshaping, and a spring tine is never heated to straighten it because that destroys its temper.
A measurement is useful only if it is traceable through calibrations to the national standard of length. Line standards carry length between engraved lines (a rule); end standards between flat faces (slip gauges, a micrometer's anvils). The reference temperature for dimensional measurement is 20 °C: a 500 mm steel part at 30 °C is about 500 × 11.5 × 10⁻⁶ × 10 = 0.058 mm longer, more than many tolerances.
Accuracy is closeness to the true value; precision is closeness of repeated readings to each other. An instrument with a zero error can be precise but inaccurate. Errors come from parallax, measuring force, dirt, temperature and Abbe error, which arises when the scale is not in line with the dimension. A micrometer obeys Abbe's principle; a vernier caliper does not, so its jaws are kept square and close to the beam. Calibration certificates state each instrument's uncertainty, and an instrument whose uncertainty is a large fraction of the tolerance it checks is not fit for that job.
A vernier places n vernier divisions over (n − 1) main-scale divisions; the least count is one main division minus one vernier division. Fifty vernier divisions over 49 mm give 0.02 mm. With 24 mm on the main scale and the 17th line coinciding, the reading is 24 + 17 × 0.02 = 24.34 mm. Vernier height gauges are used on a surface plate; bevel protractors measure angles.
A micrometer's least count is screw pitch divided by thimble divisions: 0.5/50 = 0.01 mm. The reading is sleeve value plus thimble division times least count, minus any positive zero error; a ratchet gives constant measuring force. Dial indicators magnify small movements and are used to check shaft runout and set work in a four-jaw chuck. A comparator shows the difference between the work and a master set with slip gauges; it may be mechanical, optical, pneumatic or electrical (the linear variable differential transformer).
Slip gauges are hardened, lapped blocks with flat, parallel faces. Clean blocks wrung together adhere, and a stack builds any length; to use the fewest blocks, eliminate the last decimal place first. ISO 3650 grades K, 0, 1 and 2 separate calibration masters from workshop sets.
A sine bar sits on two rollers at centre distance L; a stack of height h under one roller tilts it so sin θ = h/L. A 200 mm sine bar at 15° needs 200 sin 15° = 51.76 mm. Sine bars lose accuracy above about 45°. Surface roughness is most often stated as Ra, the arithmetic mean deviation of the profile from its mean line in micrometres; grinding and lapping give lower Ra than turning, milling and shaping. Fine finish is specified only where function needs it, because it costs time. Slip gauges are handled by their sides, cleaned before wringing and not left wrung together for long, because the faces can corrode and become hard to separate.
A drawing gives a basic size and limits. Tolerance is the difference between upper and lower limits; deviation is the difference between a limit and basic size. In the ISO system (ISO 286) grades IT01 to IT18 set the size of the tolerance and letters set its position, capitals for holes and small letters for shafts. The standard tolerance unit is i = 0.45∛D + 0.001D µm, D being the geometric mean of the diameter step; IT7 = 16i and IT8 = 25i. For the 30–50 mm step, D = 38.7 mm, i ≈ 1.56 µm and IT7 ≈ 25 µm.
A clearance fit always leaves a gap (running shafts); an interference fit needs force or heat to assemble (hubs pressed on shafts); a transition fit may give either (accurate location). The hole-basis system keeps the hole at H (zero lower deviation) and varies the shaft; it is preferred because holes are made with fixed-size drills and reamers and checked with plug gauges. H7/g6 is a close running fit, H7/k6 transition, H7/p6 interference. For a hole of 50.000–50.025 mm and shaft of 49.950–49.975 mm, maximum clearance is 0.075 mm and minimum 0.025 mm.
Most workshop injuries are predictable: eyes hit by chips and sparks, hands caught in chucks and belts, burns, crushing by falling work, and lungs damaged by fume and dust. Hazards are controlled in order: eliminate or substitute, guard, organise the work, and only then rely on personal protective equipment. Rotating machines are guarded and stopped before adjusting or measuring; a chuck key is never left in a chuck; loose clothing, gloves near spindles, rings and long hair are kept away from rotating parts; everyone in the shop wears eye protection. Before maintenance a machine is isolated, locked off and tagged, and stored energy in springs, hydraulics and raised implements is released or blocked. Good housekeeping prevents falls, and hot work is kept clear of combustibles. Laws differ by country, but this hierarchy and the duty to guard moving parts are common to most. Noise from grinding, hammering and sheet work damages hearing over years, so hearing protection is worn where levels are high.
Work is held in a bench vice at about elbow height. Marking out starts from a datum edge, with surface plate, try square, scriber, dividers, odd-leg calipers and layout dye. A light prick or dot punch makes witness marks; a blunter centre punch locates a drill.
A hacksaw cuts on the forward stroke, so teeth point away from the handle. At least about three teeth should be in the cut: a coarse blade for thick soft stock, a fine one (about 32 teeth per 25 mm) for thin sheet and tube. Single-cut files have one set of teeth; double-cut files have two crossing sets and cut faster; rasps are for wood. Grades run from rough and bastard through second-cut to smooth. A hand file has one uncut safe edge for filing into corners. Draw-filing, pushing the file sideways along the work, gives a fine flat finish.
Internal threads are cut with taper, intermediate and plug (bottoming) taps, the taper tap first. The tap drill for a metric coarse thread is about major diameter minus pitch: M12 × 1.75 needs about 10.2 mm. Dies cut external threads. Reamers finish a drilled hole to size, removing only a small allowance.
Wood is still used on farms for handles, yokes, carts, sheds, crates and foundry patterns. Softwoods come from conifers and hardwoods from broad-leaved trees; the names are botanical, not a measure of hardness. Seasoning lowers moisture toward the level the wood reaches in service, which stabilises size and improves strength and decay resistance; air seasoning is slow and cheap, kiln seasoning fast and controlled. Defects include knots, shakes (heart, star, ring), checks, warping (cup, bow, twist) and decay.
A rip saw has chisel-like teeth for cutting along the grain; a cross-cut saw has knife-like teeth for cutting across it. Jack planes do general work and smoothing planes finish; chisels are struck with a wooden mallet. The mortise and tenon suits load-bearing frames and handles; the dovetail's flared shape resists being pulled apart; lap, halving and bridle joints serve simpler frames. Several traditional wood preservatives are restricted in many countries, so local rules apply.
Hand forging uses a hearth with blower, an anvil (face, horn for bending, hardie hole for cutting and swaging tools, pritchel hole for punching), tongs, hammers and set tools: fullers to groove and draw down, flatters to smooth, swages to round. Operations are drawing down (lengthening and thinning), upsetting (shortening and thickening, as when heading a bolt), setting down, bending, punching and drifting, and forge welding of scarfed, fluxed ends at welding heat. Volume is conserved: a 50 mm diameter bar 100 mm long upset to 50 mm height becomes about 70.7 mm in diameter.
Forging is hot working, above the recrystallisation temperature, so the metal does not work-harden. Its grain flow follows the shape, improving fatigue and impact strength over a part machined from bar. Steel is not worked once it cools to a dull red, which causes cracks, nor overheated until it burns. Farm smiths draw out worn shares and points and then re-harden the edge. Industry uses power hammers, presses and closed-die drop forging, where excess metal escapes as flash and is trimmed.
Casting pours molten metal into a shaped cavity, the cheapest way to make complex shapes with internal passages such as pump casings, gearbox housings, flywheels and crusher rollers. In sand casting the cavity is formed around a pattern in a flask whose upper half is the cope and lower half the drag.
Patterns of wood, metal, plastic or wax are larger than the casting. The shrinkage allowance compensates for solid contraction and differs by metal (a 400 mm casting with a stated allowance of 21 mm per metre needs a 408.4 mm pattern). The machining allowance adds metal on machined faces. The draft allowance tapers vertical faces so the pattern can be withdrawn. The distortion allowance pre-bends shapes that warp. The shake (rapping) allowance is negative, because rapping slightly enlarges the cavity. Pattern types include single-piece, split, loose-piece, match-plate (both halves on one plate for machine moulding) and sweep. Wooden patterns are cheap for a few castings; metal patterns last for production runs.
Green sand is silica sand with a clay binder such as bentonite and water. It needs permeability to vent gas, refractoriness, green and dry strength, collapsibility so the casting can contract, and flowability. Cores form internal cavities; they sit in core prints and, where unsupported, on chaplets that fuse into the casting.
Metal flows from a pouring basin down a sprue to a sprue well, along a runner and through ingates into the cavity. The sprue is tapered, narrower at the bottom, because the falling stream accelerates; by continuity the area must fall as velocity rises, or the stream leaves the walls and aspirates air and sand. Areas follow A_top/A_bottom = √(h_bottom/h_top), heads measured from the basin surface. Velocity at the sprue base, ignoring losses, is √(2gh): 200 mm of head gives about 1.98 m/s. Risers must solidify after the casting so that they feed it as it shrinks. The gating ratio of sprue, runner and ingate areas decides whether the system is pressurised (ingates smallest) or unpressurised (choke at the sprue, larger runners and ingates).
Chvorinov's rule gives solidification time t = B(V/A)², where V/A is the modulus and B depends on metal and mould. A riser must have a larger modulus than the section it feeds: to solidify 25% later than a section of modulus 10 mm, it needs 10 × √1.25 = 11.2 mm. A sphere has the largest modulus for its volume; a cylinder with height equal to diameter is the practical compromise.
Defects and usual causes: blowholes and gas porosity (moisture, low permeability), shrinkage cavities (poor feeding), misruns and cold shuts (metal too cold or poured too slowly), hot tears (contraction restrained by a hard mould or core), sand inclusions and scabs (weak mould), and shifts (misaligned cope and drag).
Grey iron is traditionally melted in a cupola, a shaft furnace charged with coke, pig iron, scrap and limestone flux, with air blown through tuyeres; induction furnaces are now common, and crucibles for small non-ferrous melts. Shell moulding uses resin-bonded sand cured on a heated pattern. Investment (lost-wax) casting coats a wax pattern in ceramic, melts out the wax and gives accurate, complex parts. Permanent-mould casting pours into metal moulds. Pressure die casting injects metal into steel dies: hot-chamber machines suit low-melting zinc, while aluminium needs cold-chamber machines because it attacks a submerged iron injector. True centrifugal casting spins the mould to make pipes and cylinder liners without a core.
Press working shapes cold sheet between punch and die. In shearing, the punch drives the sheet into the die until cracks from both edges meet. Clearance, usually a few percent of thickness per side and more for harder and thicker sheet, controls burr and edge quality.
Blanking keeps the piece cut out (the blank); piercing or punching keeps the sheet with its hole, and the slug is scrap. In blanking the die is made to blank size and the punch is smaller by twice the clearance; in piercing the punch is made to hole size and the die is larger. A 50 mm blank with 0.1 mm clearance per side needs a 50.0 mm die and a 49.8 mm punch.
Force is perimeter × thickness × shear strength: a 50 mm blank from 2 mm sheet at 300 MPa needs π × 50 × 2 × 300 = 94.2 kN. Grinding shear on the punch (for piercing) or die (for blanking) spreads the cut over part of the stroke and lowers peak force but not total work. A stripper pulls the sheet off the punch on the return stroke.
In bending, outer fibres stretch, inner fibres compress and the neutral axis keeps its length, lying a fraction k of the thickness from the inside (k from tables, often about 0.3–0.5). Bend allowance is θ(R + kt), θ in radians: a 90° bend in 2 mm sheet with 5 mm inside radius and k = 0.33 needs π/2 × (5 + 0.66) = 8.89 mm. The sheet springs back on release, so dies over-bend; minimum bend radius depends on ductility and on grain direction.
Deep drawing pulls a blank into a die with a punch while a blank holder prevents wrinkling. With constant surface area, a flat-bottomed cup of diameter d and height h needs a blank D = √(d² + 4dh); a 50 mm × 30 mm cup needs about 92 mm. One draw is limited to a blank-to-cup ratio of roughly 2 for good drawing steels; deeper parts are redrawn. Coining squeezes metal in a closed die to fill fine detail; embossing raises shallow patterns.
Mechanical presses deliver full force near the bottom of the stroke and are fast, suiting blanking; hydraulic presses give full force throughout the stroke and suit drawing. A progressive die performs successive operations at several stations as the strip advances each stroke. A compound die performs two or more cutting operations, such as blanking and piercing a washer, at one station in one stroke. A combination die joins a cutting and a non-cutting operation at one station.
Much farm sheet work uses snips, bench shears, bar folders, press brakes, slip rolls, stakes and mallets, with lap and grooved seams, hems and wired edges to join and stiffen hoppers, guards and bins. Thickness is specified in millimetres, because gauge numbers differ by country and by metal. Power presses need guards or two-hand controls. The blank layout on the strip sets the scrap bridge between parts and the material utilisation, which often dominates part cost.
Welding fuses metals, with or without filler, into a joint as strong as the parent metal. The five basic joints are butt, lap, tee, corner and edge. Fillet welds fill the corner of a lap or tee; groove welds join butted edges, which are square on thin plate and prepared as single-V, double-V, U, J or bevel on thicker plate. Double-sided preparations use less filler and balance distortion; the thicknesses for each come from the applicable code. Positions are flat, horizontal, vertical and overhead; American test designations are 1G to 4G for plate grooves (flat to overhead), 5G and 6G for fixed pipe, and F for fillets; ISO 6947 uses letters such as PA (flat).
Fillet-weld strength is based on the throat, 0.707 × leg for an equal-leg fillet. A 6 mm fillet 100 mm long at an allowable shear of 90 MPa carries 0.707 × 6 × 100 × 90 = 38.2 kN. A full-penetration butt weld in tension carries thickness × length × allowable tensile stress. Allowable stresses differ between codes.
In shielded metal arc (stick) welding the arc burns between a flux-coated consumable electrode and the work. The coating shields the arc with gas, forms protective slag, stabilises the arc, deoxidises and can add alloy. Rutile electrodes run smoothly for general work; basic low-hydrogen electrodes give tough, crack-resistant welds on thicker or higher-carbon steel but must be kept dry or baked. In the AWS system E6013 is 60 ksi tensile, all-position, rutile; E7018 is 70 ksi, all-position, low-hydrogen with iron powder.
Stick welding uses a constant-current source, AC or DC, so arc-length changes alter current little. Arc length is kept about equal to core-wire diameter. Duty cycle, the percentage of ten minutes a set can weld at a given current, varies inversely with current squared: a set rated 60% at 300 A runs about 60 × (300/250)² = 86% at 250 A. Heat input is ηVI/v: at 25 V, 200 A, 5 mm/s and η = 0.8 it is 800 J/mm. Low heat input on hardenable steel cools fast and risks a hard, crack-prone heat-affected zone.
An oxy-acetylene flame reaches about 3100–3200 °C at the inner cone. A neutral flame (roughly equal oxygen and acetylene) welds most steels. A carburising flame has excess acetylene and a feather round the inner cone. An oxidising flame has excess oxygen and a short hissing cone; it is used on brass and bronze because the oxide film limits zinc fuming. Leftward (forehand) welding suits thin sheet; rightward (backhand) suits thicker plate.
Oxy-fuel cutting preheats steel to ignition and then burns it with a jet of pure oxygen; the oxide melts below the steel and is blown out. It cuts carbon and low-alloy steel but not cast iron, stainless steel or aluminium, which are cut with plasma. Brazing uses a non-ferrous filler melting above 450 °C but below the parent metal, drawn into a close joint by capillarity; soldering is below 450 °C. Braze welding with bronze is a classic cast-iron repair. Resistance spot and seam welding join sheet by current and pressure.
Arcs emit ultraviolet and infrared radiation: the welder's filter shade is chosen from the standard's table by process and current, and screens protect bystanders. Fume is extracted at source. Cylinders stand upright and secured. Acetylene is not used above about 1 bar (15 psi) gauge because it becomes unstable; oxygen must never meet oil or grease; flashback arrestors and check valves are fitted; leaks are found with soapy water, never a flame. Hot work stays clear of fuel tanks and crop residue, and a container that held fuel is never welded until made safe.
Gas metal arc welding (MIG with inert gas, MAG with active gas) feeds a continuous wire that is both electrode and filler, under shielding gas. It uses a constant-voltage source: if the arc lengthens, current falls and burn-off slows, so arc length self-regulates. Steels are welded with carbon dioxide or argon–CO2; aluminium with argon or argon–helium.
Short-circuit transfer at low current dips the wire into the pool many times a second; its low heat suits thin sheet and positional work. Globular transfer gives large drops and spatter. Spray transfer at high current in argon-rich gas gives deep penetration but only in flat and horizontal positions. Pulsed transfer gets spray-like transfer at lower average heat. Flux-cored wire is useful outdoors, where wind can blow away a gas shield. GMAW is fast, leaves little slag and dominates implement manufacture. With a constant-voltage source, a longer wire stick-out preheats the wire and lowers the current at the same wire feed.
Gas tungsten arc welding (TIG) strikes an arc from a non-consumable tungsten electrode under inert gas, usually argon, with filler rod fed by hand. It uses a constant-current source, often with high-frequency or lift-arc starting. Steels and stainless steels are welded on DC electrode negative, which puts most heat into the work and keeps the tungsten cool. Aluminium and magnesium are welded on AC: the electrode-positive half-cycle breaks the surface oxide (cleaning action). Thoriated tungsten is mildly radioactive, so ceriated or lanthanated electrodes are increasingly preferred. TIG gives the cleanest welds on thin stainless and aluminium but is slow. Submerged arc welding, under a blanket of granular flux, suits long heavy welds. A tungsten dipped into the pool is contaminated and must be reground before welding continues. Argon flow is set high enough to shield the pool and trailing bead but not so high that turbulence draws in air; stainless and reactive metals may also need a backing purge on the root side.
Common defects are porosity (contamination, moisture, lost shielding), slag inclusions (poor inter-pass cleaning), lack of fusion and incomplete penetration (too little heat, wrong angle, poor fit-up), undercut (a groove melted at the toe by excess current) and cracks. Hydrogen (cold) cracks appear in a hard heat-affected zone, sometimes hours later, when hydrogen, a susceptible microstructure and tensile stress coincide; low-hydrogen consumables, preheat and adequate heat input control them.
Preheat need is judged from carbon equivalent; the IIW formula is CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. A steel with 0.45% C and 0.75% Mn has CE = 0.575, high enough that preheat and low-hydrogen practice are normally advised; codes set thresholds by thickness and hydrogen level. Distortion is controlled by tacking, fixtures, balanced and back-step sequences, presetting and not over-welding. Undercut and overlap are found visually; lack of fusion inside a joint needs ultrasonic testing or sectioning.
Visual inspection comes first and always. Dye penetrant testing shows surface-breaking defects in any non-porous material. Magnetic particle testing finds surface and near-surface defects, but only in ferromagnetic metal. Ultrasonic testing detects internal planar defects such as cracks and lack of fusion; radiography records volumetric defects such as porosity and slag.
Destructive tests on test plates include transverse tensile tests, guided bend tests (face, root, side) that reveal lack of fusion and poor ductility, nick-break and fillet-break tests, macro-etch sections, hardness traverses of the heat-affected zone and Charpy impact tests. A welding procedure specification records the variables of a qualified procedure, and welders are qualified by test to a code such as the ISO, AWS or ASME standards. Acceptance levels for each defect come from the governing code or product standard, not the inspector's taste; a flaw acceptable in a trailer frame may be rejectable in a pressure vessel. Records of tests are kept with the job.
The lathe cuts rotating work with a single-point tool. Its parts are the bed; the headstock with spindle and speed gears; the tailstock, carrying a centre or drill and able to be offset; the carriage (saddle, cross slide, compound rest, apron); and the feed rod and lead screw. The feed rod drives turning and facing feeds; the lead screw, engaged through the half nut, is kept for threading to preserve its accuracy. A lathe is specified by swing over the bed, distance between centres, spindle bore, power and speeds.
A three-jaw self-centring chuck holds round and hexagonal stock quickly; a four-jaw independent chuck holds irregular work and allows accurate setting with a dial indicator; collets hold small bar concentrically; a faceplate takes large irregular work; centres with a carrier let work be removed and replaced concentrically. Slender work is supported by a steady or follower rest. The bed ways are kept clean and oiled, because their wear limits the accuracy of everything turned.
Operations include facing, plain and step turning, chamfering, grooving, parting off, knurling, drilling, boring, reaming and threading. Tapers are turned four ways. Swivelling the compound rest to the half-angle tan⁻¹[(D − d)/2L] suits short steep tapers fed by hand; for D = 50, d = 40 and L = 100 mm it is 2.86°. Offsetting the tailstock suits long gentle tapers between centres; offset = (D − d)L_w/(2L), where L_w is the whole work length, so the same taper on 300 mm work needs 15 mm. A taper attachment guides the cross slide and allows power feed. A form tool cuts short tapers directly.
For threading, the lead screw is geared to the spindle so the carriage advances one pitch per work revolution. With a 6 mm lead screw, a 1.5 mm pitch needs a ratio of 1.5/6 = 1/4 (for example 20-tooth driver, 80-tooth driven). A thread-chasing dial shows when to re-engage the half nut.
Cutting speed is V = πDN/1000 m/min, D in mm and N in rev/min: 50 mm work at 300 rev/min runs at 47.1 m/min. For a recommended 30 m/min on 80 mm stock, N = 1000V/(πD) = 119 rev/min. Recommended speeds depend on work and tool material, feed, depth and coolant.
Feed is tool advance per revolution; depth of cut is the radial thickness removed per pass. Time per pass is t = L/(fN), with L including approach and overrun: 200 mm at 0.25 mm/rev and 400 rev/min takes 2 min. Reducing 60 mm to 52 mm removes 4 mm on the radius, four passes at 1 mm. Material removal rate is about V f d: 60 m/min, 0.2 mm/rev and 2 mm give 24 000 mm³/min. Roughing cuts use the largest depth and feed the machine and work will stand; finishing cuts use a small depth and fine feed for size and finish.
Rake angles control chip flow and force; clearance angles stop the flank rubbing; nose radius affects finish and strength. Positive rake suits ductile metals; small or negative rake strengthens carbide edges for hard work and interrupted cuts. Tool materials in rising hot hardness are carbon tool steel, high-speed steel (hard to about 600 °C), cemented carbide, ceramic, cubic boron nitride and diamond; diamond is not used on ferrous metals because carbon diffuses into iron at cutting heat.
Chips are continuous in ductile metal at high speed, discontinuous in grey cast iron, and continuous with a built-up edge when soft metal welds to the tool at low speed. In orthogonal cutting the chip ratio r = t/t_c gives shear angle φ from tan φ = r cos α/(1 − r sin α); with t = 0.5, t_c = 1.25 mm and α = 10°, φ ≈ 22.9°. Taylor's equation VT^n = C: for n = 0.25 and C = 300, at 150 m/min T = (300/150)^4 = 16 min. Lathe safety: remove the chuck key, stop before measuring, and clear chips with a hook, never by hand.
Drilling machines rotate a tool and feed it axially. Pillar drills take small work; the radial drill's head moves along a swinging arm to reach holes on large castings without moving them; gang and multi-spindle drills make several holes in sequence or at once. A twist drill has a straight or Morse-taper shank, two helical flutes that clear chips, a guiding margin, a web and a point, commonly 118° for steel. The point cone adds travel: for a 20 mm drill it is about 10/tan 59° = 6 mm, so drilling through 40 mm at 0.2 mm/rev and 300 rev/min takes (40 + 6)/(0.2 × 300) = 0.77 min. A 12 mm drill at 24 m/min turns at 1000 × 24/(π × 12) = 637 rev/min.
Allied operations are reaming (sizing and finishing), boring, counterboring (a flat-bottomed recess for a bolt head), countersinking (a conical recess for a countersunk screw), spot facing (a flat seat on a rough casting) and tapping. Work is clamped or held in a vice, never by hand, because a drill can grab and spin thin work.
Milling feeds work past a rotating multi-tooth cutter. Horizontal machines carry cutters on an arbor; vertical machines carry end and face mills; a universal machine has a swivelling table for helical work. In up (conventional) milling the cutter turns against the feed, the chip goes thin to thick and the cutter tends to lift the work. In down (climb) milling it turns with the feed, the chip goes thick to thin, finish is often better, and the cutter pulls the work in, so a backlash eliminator is needed. Straddle milling uses two cutters on two faces; gang milling several cutters on one arbor.
Table feed is f_z × z × N: 0.1 mm per tooth, 8 teeth and 200 rev/min give 160 mm/min, so 320 mm of travel takes 2 min. A dividing head with a 40:1 worm indexes work; in simple indexing the crank turns 40/Z. For 27 divisions, 40/27 = 1 13/27: one turn plus 13 holes on a 27-hole circle. Differential indexing, with change gears, handles numbers simple indexing cannot.
A shaper moves a single-point tool across stationary work, cutting on the forward stroke, with the table feeding sideways between strokes; it makes flats, grooves and keyways. A crank and slotted-lever quick-return mechanism makes the return faster than the cut. With m as return time divided by cutting time, average cutting speed is V = LN(1 + m)/1000 m/min. For a 200 mm stroke, 30 strokes per minute and m = 0.5, V = 9 m/min. Shaping a 150 mm wide surface at 0.5 mm feed per stroke and 30 strokes per minute takes 150/(0.5 × 30) = 10 min. A planer moves large work under a stationary tool; a slotter is a vertical shaper for internal keyways. Shapers are slow and have largely given way to milling in production, but remain common in training and repair shops because one simple tool cuts keyways and flats. The ram stroke is set slightly longer than the work.
Grinding removes small amounts with a bonded abrasive wheel, giving accuracy and finish, even on hardened steel. Aluminium oxide grinds steels; silicon carbide grinds grey cast iron, non-ferrous metals and non-metals; cubic boron nitride and diamond grind very hard materials. ISO 525 marks a wheel in order: abrasive, grit size, grade (bond hardness, A soft to Z hard), structure and bond (vitrified V, resinoid B, rubber R, metal M). A soft wheel is used on hard material, so blunt grains break out, and a hard wheel on soft material. Dressing restores sharpness to a glazed or loaded wheel; truing restores its round shape. Surface speed is πDN: a 200 mm wheel at 1800 rev/min runs at 18.8 m/s, which must not exceed the maximum speed marked on the wheel.
A new vitrified wheel is ring-tested (a sound wheel rings; a cracked one sounds dull), mounted with blotters and proper flanges, and started with the operator standing aside. Guards stay on, the work rest is kept close (United States regulations, for example, require within about 3 mm), and eye protection is always worn.
A jig locates and holds the work and guides the tool, usually through hardened drill bushes; a fixture locates and holds but does not guide, the tool being set with setting blocks. Jigs serve drilling, reaming and tapping; fixtures serve milling, turning, grinding, welding and assembly.
A free body can translate along and rotate about three axes, six degrees of freedom (many textbooks count each direction separately and say twelve). The 3-2-1 principle locates a prismatic part with three points on the largest face, two on the next and one on the third; clamps then hold it against the locators. Extra locators on one face over-constrain the part, which rocks or distorts. Round work is located by V-blocks, mandrels and pins; a diamond (relieved) pin paired with a round pin locates on two holes without binding. Clamp directly over supports, let cutting thrust push into locators, make loading foolproof, leave chip clearance and use quick-acting cam or toggle clamps.
Drill jigs include template, plate, channel, leaf (hinged plate carrying the bushes), box (enclosing the work for drilling from several sides) and indexing jigs. Bushes are press-fitted for light work, renewable when they wear, or slip-renewable so a drill bush and reamer bush can be exchanged in the same hole.
Welding fixtures matter most in farm machinery. A cultivator frame, hitch mast or drawbar built in a fixture has the same hitch geometry and tine spacing every time, is held against distortion during welding and can be turned to weld flat. For small implement makers, a good welding fixture is often the most valuable tooling they own. Milling fixtures are keyed to the table slots so that they align with the feed without indicating. Turning fixtures are balanced so that the spindle does not vibrate, and assembly fixtures hold parts in position while they are bolted or pinned, so that the final machine matches the drawing.
Limit gauges accept or reject without measuring. Plug gauges check holes; ring and snap gauges check shafts. The GO end is made to the maximum-material limit (smallest hole, largest shaft) and the NO-GO end to the least-material limit. For a hole of 25.000–25.021 mm the GO plug is nominally 25.000 mm and the NO-GO 25.021 mm.
Taylor's principle: the GO gauge checks the whole mating form over the full length of engagement, because it stands for the mating part; the NO-GO gauge checks one dimension at a time. A full-form GO plug rejects a bent or out-of-round hole even if each diameter is within limits. Gauges are made to a gauge-maker's tolerance, commonly about a tenth of the work tolerance, and the GO gauge, which rubs on every accepted part, gets a wear allowance. Other gauges include thread, feeler, radius, taper and profile gauges. Gauging is fast and needs little skill, but it gives no size data for process control.
A prototype turns a design into a machine that can be tested. It starts from drawings (general arrangement and detailed parts with dimensions, tolerances, materials and finishes) and a bill of materials listing each part's material specification, stock size, quantity and whether it is made or bought; bearings, fasteners, chains and belts are bought to standard sizes. Using locally available stock sections makes later production possible. Drawings also state weld symbols, surface finish and any heat treatment, so the fabricator does not have to guess.
A route sheet lists each part's operations, machines, tooling and time. A cutting list groups parts by stock to cut bars and plates with least waste. Cost comes from material weight, machine and labour hours and overhead. Steel is about 7850 kg/m³: a 1.2 m × 0.6 m × 6 mm plate weighs about 33.9 kg, and 2 m of 50 × 10 mm flat bar weighs 7.85 kg.
A typical sequence is: cut stock (saw, shear, oxy-fuel or plasma); form; machine shafts, hubs and keyways; prepare edges; fit up and tack in a fixture; weld in a sequence planned against distortion; stress relieve if needed; machine faces that must be accurate after welding; clean, prime and paint; assemble. Bolt holes that must align across a welded frame are often drilled after welding or through a jig.
Shrink fitting mounts a hub by heating it until its bore grows by at least the interference: temperature rise = interference/(αD). For 0.05 mm on a 50 mm steel bore with α = 12 × 10⁻⁶ /°C it is about 83 °C, a little more in practice for assembly clearance. Safety is designed in: shafts, belts, chains and power take-off drives are guarded, and ISO 4254 sets general safety requirements for agricultural machinery.
A prototype is checked in the workshop (dimensions, free running, guards) and then in the field: draft or power, field capacity and efficiency, work quality and durability. National and international test codes set the procedures used by machinery testing institutes, so a designer plans prototype tests around them to get comparable results. Failures such as a cracked weld at a sharp corner, worn bushes or a deflecting shaft are fed back into revised drawings, and the build–test cycle repeats until the machine meets its specification.
The repair method follows from the material. Worn shares, sweeps and points are rebuilt by forging out and re-hardening, or by hardfacing: depositing a wear-resistant alloy, often chromium-carbide-rich, where wear occurs. Hardfacing one face of an edge, so the softer face wears faster, keeps the edge sharp (self-sharpening); which face depends on how the tool wears in that soil. Hardfacing is brittle and is laid in thin layers as its data sheet directs.
A cracked mild-steel frame is ground out to sound metal, drilled at each crack end to stop it running, welded and, if overload or fatigue caused it, reinforced with a fish plate; the root cause (a sharp corner, a weld toe at high stress) is corrected or the crack returns. Cast iron is welded hot, with the whole casting preheated and slowly cooled, or cold with nickel electrodes in short beads peened while hot; braze welding is a common alternative. Hardened spring tines and high-tensile bolts are replaced, not heated and straightened. Worn shafts are built up and re-machined or sleeved; worn bores are bushed. Repairs to hitches, drawbars and guards must restore at least the original strength.
In the first-year workshop you learn to mark out, file, saw, tap, forge, weld, cut sheet and turn a simple shaft, and you keep a record of each job with its drawing and the time taken. Later courses expect you to calculate machining time and spindle speed, read a material designation such as 1045 or C45, choose a heat treatment for a ploughshare, and pick between MIG, TIG and stick welding for a given joint. For exams, practise the short calculations until they are automatic: cutting speed and time, indexing, vernier and micrometer readings, fits, blanking force, bend allowance, fillet weld strength and Chvorinov's rule.
Five years into an implement firm, service centre or testing station, you write the route sheet for a new cultivator frame, specify the steel and the weld procedure, design the welding fixture, and decide which holes are drilled after welding. You approve hardfacing procedures for tillage wear parts, investigate cracked frames by finding the notch or weld toe where the crack began, and check incoming material certificates against the drawing. In a testing institute you inspect prototypes for weld quality, guards and conformity before field trials.
With twenty-five years' experience you set the manufacturing strategy: which parts are cast, forged, pressed or fabricated, which are bought, and where tooling such as progressive dies or robotic welding cells pays back at the volumes your market supports. You train fabricators across a region, write the company's welding and heat-treatment specifications, handle field-failure warranty claims by root-cause analysis, and balance cost against durability for farmers who cannot afford breakdowns at sowing time. You also judge when a design should change to suit local workshops that will repair it.
At research level you study wear of soil-engaging tools and the steels, boron additions, hardfacing alloys and surface treatments that extend their life; fatigue of welded implement frames under random field loads; distortion and residual stress in welded fabrications; and process innovation such as additive manufacturing of replacement parts or laser cladding of wear surfaces. You design experiments with controlled soil-bin and field tests, publish wear rates and fatigue data that designers can use, and advise standards committees on test methods for implement durability.
This manual does not reproduce tables of recommended cutting speeds, feeds, press clearances, pattern contraction allowances, filter shades, preheat temperatures or code allowable weld stresses; these vary by material, standard and country, and questions give them in the stem. It treats CNC machining, CAD/CAM, non-traditional machining (EDM, ECM, laser and water-jet), powder metallurgy, additive manufacturing and plastics moulding only in passing or not at all. It does not cover welding code requirements clause by clause, gear-cutting methods beyond simple indexing, or the details of any one country's welder-certification scheme.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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