Manufacture of engine blocks, crankshafts and gears

Why it matters

The engine block, crankshaft and transmission gears carry the highest loads in a vehicle and must be made to micrometre-level accuracy at volumes of hundreds of thousands per year. The route chosen — casting or forging, which heat treatment, which finishing process — fixes their strength, wear life, noise and cost. These parts are classic examples for casting, forging, machining-time and gear-cutting questions in university exams, GATE and powertrain-plant interviews.

Key ideas

Engine block — casting.

• Grey cast iron blocks (still common for diesels and commercial engines) are made by green-sand moulding with resin-bonded sand cores for the water jacket, oil galleries and cylinder bores. Grey iron has flake graphite, which gives good damping, machinability and wear resistance in the bore, and casts with little shrinkage. Compacted-graphite iron (CGI) is used for high-pressure diesels because it is stronger and stiffer.
• Aluminium alloy blocks (Al–Si alloys such as A319/A356 class) save weight. They are made by high-pressure die casting (fast, thin walls, but porosity limits heat treatment), low-pressure or gravity die casting, or lost-foam/precision sand casting. Bores get cast-in iron liners, thermally sprayed coatings, or hypereutectic Al–Si surfaces.
• Directional solidification, risers and chills are designed so the thick sections (main bearing bulkheads) feed properly; Chvorinov's rule shows that solidification time rises with the square of the volume-to-surface ratio.

Engine block — machining. Done on transfer lines or flexible lines of CNC machining centres, starting from cast locating pads:

1. Mill datum faces and drill/ream locating holes.
2. Rough and finish mill the deck (head) face, sump face and ends.
3. Rough and semi-finish bore cylinders; line-bore the crankshaft main bearing bores with the caps fitted (so all bores are coaxial).
4. Drill and tap hundreds of holes; drill long oil galleries (gun drilling).
5. Hone cylinders: abrasive stones expand while rotating and reciprocating, giving accurate roundness and straightness and a cross-hatch pattern; plateau honing removes peaks while keeping valleys to hold oil, cutting oil consumption and run-in time.
6. Wash, leak test (air-decay or helium) and measure on CMM.

Crankshaft.

• Forged steel (medium-carbon or micro-alloyed steel, e.g. 42CrMo4 or 38MnVS6 class) by closed-die (impression-die) forging for high-load and diesel engines. Grain flow follows the shape of webs and pins, giving better fatigue strength than casting. A flash gutter around the die cavity takes excess metal, and the flash is trimmed afterwards.
• Cast ductile (SG/spheroidal graphite) iron for many petrol engines: cheaper, lighter to machine and has good damping, but lower fatigue strength.
• Machining sequence: face and centre (end-to-end mass centring), turn or turn-broach main journals, mill or turn the eccentric crankpins (orbital/pin-chasing grinding on CNC), drill angled oil holes, harden the journals (induction hardening or gas/salt-bath nitriding), fillet rolling of journal fillets to induce compressive residual stress, grind journals and pins, dynamic balancing by drilling counterweights, and superfinish/polish to a low Ra.

Gears (transmission and final drive).

• Blanks are forged (often hot then ring-rolled for ring gears), normalised or isothermally annealed for machinability, then turned.
• Soft machining: teeth are cut by hobbing (continuous generating process — the hob is a worm with gashes; fast and versatile for spur and helical gears), shaping (gear shaper cutter; needed for internal gears and gears close to shoulders), or broaching for internal splines. Spiral-bevel and hypoid final-drive gears are cut on specialised bevel-gear generators.
• Shaving before heat treatment improves profile and lead and corrects expected distortion.
• Case carburising (e.g. 20MnCr5 or 16MnCr5 class steels) gives a hard, wear- and pitting-resistant case (around 58–62 HRC) over a tough core; quench distortion is controlled by press quenching for ring gears.
• Hard finishing: gear grinding or power honing after heat treatment for quiet, accurate gears; shot peening raises bending fatigue strength at the root.

Choosing routes. Casting suits complex hollow shapes (blocks, heads). Forging suits parts in cyclic bending and torsion (crankshafts, connecting rods, gears). Final accuracy and surface finish always come from finishing processes after heat treatment, because heat treatment distorts parts.

Formulas

t_s = B · (V / A)²

• Chvorinov's rule: t_s = solidification time (s); V = casting volume (mm³); A = surface area cooling the casting (mm²); B = mould constant (s/mm²), depends on metal and mould; find it from a known casting or a data book.

N = 1000 · V_c / (π · D)

• N = spindle speed (rpm); V_c = cutting speed (m/min); D = tool (boring) or workpiece diameter (mm).

T_m = (L + L_a) / (f · N)

• T_m = machining time per pass (min); L = length of cut (mm); L_a = approach plus overtravel (mm); f = feed (mm/rev).

d = m · Z, d_a = m · (Z + 2), h = 2.25 · m

• Gear pitch diameter, tip diameter and whole depth (mm) for a standard full-depth spur gear; m = module (mm); Z = number of teeth.

N_w = N_h · k / Z

• Work rpm in hobbing; N_h = hob rpm; k = number of hob starts.

T_hob = (b + L_a) / (f_a · N_w) = (b + L_a) · Z / (f_a · N_h · k)

• Hobbing time (min); b = face width (mm); f_a = axial feed per work revolution (mm/rev).

Worked examples

Example 1 (standard) — boring a cylinder. Given: bore 86 mm, bore length 140 mm, approach + overtravel 5 mm, cutting speed 150 m/min, feed 0.2 mm/rev, one pass.

1. N = 1000 × 150 / (π × 86) = 555.2 rpm
2. T_m = (140 + 5) / (0.2 × 555.2) = 145 / 111.0 = 1.31 min

Answer: about 1.31 min per bore (a four-cylinder block takes about 5.2 min on one spindle, which is why lines use multi-spindle heads or parallel machines).

Example 2 (GATE level) — hobbing a spur gear. Given: Z = 40, module 2.5 mm, face width 25 mm, approach + overrun 15 mm, single-start hob of 70 mm diameter at 60 m/min, axial feed 1.5 mm per work revolution.

1. d = m·Z = 2.5 × 40 = 100 mm; d_a = 2.5 × 42 = 105 mm; h = 2.25 × 2.5 = 5.625 mm
2. N_h = 1000 × 60 / (π × 70) = 272.8 rpm
3. N_w = N_h·k / Z = 272.8 × 1 / 40 = 6.82 rpm
4. T_hob = (25 + 15) / (1.5 × 6.82) = 40 / 10.23 = 3.91 min

Answer: pitch diameter 100 mm, tip 105 mm, hobbing time ≈ 3.91 min.

Example 3 (GATE level) — Chvorinov's rule for a block section. Given: a 50 mm cast-iron cube solidifies in 120 s in a sand mould. Find the solidification time of a 200 × 100 × 50 mm plate section in the same mould.

1. Cube: V/A = 50³ / (6 × 50²) = 50/6 = 8.333 mm → B = 120 / 8.333² = 1.728 s/mm²
2. Plate: V = 200 × 100 × 50 = 10⁶ mm³; A = 2(200×100 + 200×50 + 100×50) = 70,000 mm²; V/A = 14.29 mm
3. t_s = 1.728 × 14.29² = 352.7 s

Answer: about 353 s (5.9 min).

Common mistakes

• Taking hob rpm as work rpm in hobbing; the work turns Z/k times slower.
• Forgetting approach and overtravel in machining time.
• In Chvorinov's rule, including faces that are not in contact with the mould, or using V/A without squaring it.
• Believing cast crankshafts are always inferior: ductile iron is adequate for many petrol engines; forging is chosen where fatigue loads are high.
• Grinding gears before carburising — hard finishing must come after heat treatment because quenching distorts the teeth.
• Confusing honing (bore finishing with abrasive stones, cross-hatch) with lapping or grinding.

For GATE ME

Expect numericals on solidification time and riser design (Chvorinov), machining time for turning, boring and drilling, gear geometry from module and teeth, and occasionally hobbing time. Conceptual questions test which process suits which part, the purpose of honing, induction hardening versus carburising, and why forging gives better fatigue strength. Practise the units in machining-time problems (m/min to rpm).

Quick check

1. Why are crankshaft main bearing bores line-bored with the caps fitted?
2. Number of teeth and module for a gear of 120 mm pitch diameter with module 3 mm?
3. If the V/A ratio of a casting doubles, how does its solidification time change?
4. Which gear-cutting process is used for an internal ring gear?
5. What is plateau honing meant to achieve?

Answers: 1. So that all bores are coaxial with the caps in their final position. 2. Z = 120 / 3 = 40 teeth. 3. It becomes four times longer. 4. Gear shaping (or broaching). 5. A bore surface with peaks removed but oil-holding valleys kept, reducing oil consumption and run-in.