Economics and Production Planning
This domain tests whether you can compare processes and joint designs on cost per metre rather than on wire price, and plan capacity realistically.
Cost model
Cost per metre of completed weld = labour + filler metal + shielding gas + power + consumables (flux, tips, liners) + equipment depreciation + overhead + inspection + rework. Labour usually dominates for manual processes because operating factor (arc-on time / paid time) is low, often 20–40% for SMAW and higher for mechanised SAW. Deposition efficiency (deposited mass / consumed filler mass) is high for solid wire, lower for SMAW because of stub loss and spatter.
Planning
Capacity planning uses deposition rate, operating factor and available hours. Joint design affects cost strongly: a 60° V-groove needs far more filler than a narrow-gap or double-sided preparation. Submerged arc and narrow gap reduce cost per metre in thick plate despite higher equipment cost.
Worked example
A butt weld has cross-sectional area 30 mm² and length 1 m. Volume = 30 × 1000 = 30,000 mm³ = 30 cm³. Mass = 30 × 7.85 = 235.5 g. At 90% deposition efficiency, filler consumed = 235.5/0.9 = 262 g. At 4 kg/h deposition rate, arc time = 0.262/4 = 0.0654 h = 3.9 min. With an operating factor of 0.5, paid time is about 7.8 min/m. Reasoning: always convert area to mass, apply efficiency before dividing by deposition rate, then apply operating factor to get labour time.
Traps
Comparing filler price per kilogram without deposition efficiency. Confusing deposition rate with melt-off rate. Ignoring rework and NDT cost. Using arc time as paid time.