Cost estimation of castings, forgings and machined parts

Why it matters

Before a component is quoted, a production engineer must say what it will cost to cast, forge or machine it. The estimate drives the quotation, the choice between processes, and make-or-buy decisions. Estimating is systematic: work out how much material must be bought (including all losses), how long each operation takes, and then apply labour and overhead rates.

Key ideas

Estimating vs costing. Estimating predicts the cost before production from drawings, standard data and experience. Costing records what was actually spent. A good estimate breaks the job into material, operation times and overheads so that each part can be checked.

Procedure for any component.

1. Study the drawing; split the part into simple solids (cylinders, prisms, cones, rings) to find its volume.
2. Add process allowances to get the gross (as-made) volume and mass.
3. Add losses to find the material that must be bought or melted.
4. Estimate time for each operation: set-up, handling and machining.
5. Cost = material + labour + direct expenses (patterns, dies, special tools) + overheads.

Castings. The mass of the casting includes the finished part plus machining allowances on every surface to be machined. The pattern is made larger again by shrinkage allowance, but shrinkage does not add metal to the casting — the casting shrinks back to its drawing size plus machining allowance. Metal to be melted is greater than casting mass because of the gating system, risers and spillage; this is expressed by the casting yield = casting mass / metal poured. Returns (runners, risers) are remelted and are often credited at scrap value. Other cost items: pattern and core boxes (direct expense spread over the batch), moulding and core-making labour, melting (fuel or power), fettling and cleaning, inspection, and foundry overheads.

Forgings. Stock mass = net forging mass + losses:

• scale (oxidation) loss when the stock is heated, often taken as a percentage of net mass;
• flash loss around the die parting line (closed-die forging), proportional to the flash land periphery and thickness;
• tonghold loss — the extra length held by tongs and cut off;
• sprue loss — the connector between the forging and the tonghold;
• shear (cutting) loss when the bar is cut to length. Values of these percentages come from the shop's data book; use the figures given in a problem. Forging cost also includes heating, die cost per piece, forging labour, trimming and overheads.

Machined parts. Machining time for each operation is computed from the cutting conditions, then multiplied by a combined machine-and-labour rate. Total time per piece = set-up time / batch size + handling time + machining time. Non-cutting distances (approach, overrun) must be included in the tool travel.

Formulas

• Mass: m = ρ·V
• Casting yield: η_y = m_casting / m_poured, so m_poured = m_casting / η_y
• Forging stock mass: m_stock = m_net + m_scale + m_flash + m_tonghold + m_sprue + m_shear
• Stock length from bar of area A: L = m_stock / (ρ·A)
• Spindle speed: N = 1000·V / (π·D)
• Turning time per pass: t = (L + approach + overrun) / (f·N)
• Number of passes: passes = (D₀ − D₁) / (2·d) (round up)
• Drilling time: t = (hole depth + 0.3·d_drill + approach) / (f·N)
• Time per piece: T = t_set-up / batch + t_handling + Σ t_machining
• Operation cost: C_op = T × (machine rate + labour rate)

Symbols: ρ = density (kg/m³ or g/cm³); V = volume; V_c = cutting speed (m/min); D = workpiece diameter (mm); f = feed (mm/rev); N = spindle speed (rev/min); L = length of cut (mm); d = depth of cut (mm); times in minutes; rates in ₹/h.

Worked examples

Example 1 (standard) — casting material cost. A grey cast-iron bush is finished to OD 200 mm, ID 120 mm, length 250 mm. A machining allowance of 3 mm is given on every surface. Density 7.2 g/cm³, casting yield 80%, cost of molten metal ₹60/kg. Find the casting mass and the cost of metal melted.

1. Cast dimensions: OD = 200 + 2 × 3 = 206 mm; ID = 120 − 2 × 3 = 114 mm; length = 250 + 2 × 3 = 256 mm.
2. V = (π/4)(206² − 114²) × 256 mm³ = 59,19,263 mm³ = 5,919.3 cm³.
3. Casting mass = 5,919.3 × 7.2 = 42,619 g = 42.62 kg (the finished bush is 36.19 kg).
4. Metal poured = 42.62 / 0.80 = 53.27 kg.
5. Cost of metal = 53.27 × 60 = ₹3,196.

Example 2 (GATE level) — machining time and cost. A steel bar is turned from Ø50 mm to Ø44 mm over a length of 200 mm. Depth of cut 1.5 mm per pass, feed 0.25 mm/rev, cutting speed 100 m/min (spindle speed set for Ø50 and kept for all passes), approach plus overrun 5 mm. Loading and unloading take 3 min. The combined machine and operator rate is ₹900/h. Find the cost of the operation.

1. Passes = (50 − 44) / (2 × 1.5) = 2.
2. N = 1000 × 100 / (π × 50) = 636.6 rev/min.
3. Time per pass = (200 + 5) / (0.25 × 636.6) = 1.288 min; two passes = 2.576 min.
4. Total time = 2.576 + 3 = 5.576 min.
5. Cost = 5.576 × 900 / 60 = ₹83.6 per piece.

Example 3 — forging stock. A closed-die steel forging has a net mass of 3.0 kg. Scale loss is 5% and flash loss 6% of net mass; tonghold loss is 0.15 kg. Stock is Ø40 mm bar, density 7.85 g/cm³. Stock mass = 3.0 × 1.11 + 0.15 = 3.48 kg. Bar area = (π/4) × 4² = 12.566 cm². Length = 3,480 g / (7.85 × 12.566) g/cm = 35.3 cm.

Common mistakes

• Adding shrinkage allowance to the casting mass. Shrinkage affects pattern size only; machining allowance adds metal.
• Forgetting that a bore loses diameter when an allowance is added (ID gets smaller on the casting).
• Using finished mass instead of poured mass for melting cost, or ignoring yield.
• Leaving out approach and overrun, or the second pass, in machining time.
• Mixing units: mm³ to cm³ is ÷ 1,000; mm³ to m³ is ÷ 10⁹.
• Charging the full pattern or die cost to one piece instead of spreading it over the batch.

For GATE PI

Questions usually give dimensions and ask for the mass or material cost of a casting or forging, the stock length for a forging, or machining time and cost for turning, facing or drilling. Practise volume of composite solids, careful allowance bookkeeping, and the N–V–D relation with consistent units.

Quick check

1. Does shrinkage allowance increase the mass of metal in the finished casting?
2. A casting weighs 24 kg and the yield is 75%. How much metal is poured?
3. Turning 300 mm at f = 0.2 mm/rev and N = 500 rev/min (no approach) takes how long per pass?
4. Name three losses added to net forging mass.

Answers: 1. No, it only enlarges the pattern; 2. 32 kg; 3. 300 / (0.2 × 500) = 3 min; 4. scale, flash and tonghold (also sprue and shear) losses.