SOLAS Metal Fabrication cheat sheet

Marking-out datum: all dimensions are taken from a single reference edge or face; cumulative error grows with each chained measurement, so transfer dimensions from the datum, not from the previous cut.

Cutting allowance rule: rough stock length = finished length + saw kerf + facing allowance (typically 1–2 mm per machined end); kerf for a hacksaw blade is about 1.5 mm, for a cold saw 2–3 mm.

Bend allowance: BA = θ(π/180)(R + K·T), where θ is bend angle in degrees, R inside bend radius, T material thickness, K the neutral-axis factor (≈0.33–0.5 depending on R/T).

Flat length for a single bend = leg A + leg B − bend deduction, where bend deduction = 2 × outside setback − BA; outside setback = (R + T)·tan(θ/2).

Minimum bend radius guide: mild steel ≈ 1T, aluminium alloys ≈ 1–2T, austenitic stainless ≈ 1–2T; radii below this risk outside-face cracking and require bend testing.

Hole-to-bend rule: place hole edge at least 2T + R from the bend line, otherwise the hole distorts into an oval as the material stretches over the tooling.

Tolerance stack: for a chain of n dimensions each ±t, worst-case total = ±n·t; statistical (root-sum-square) stack = ±t·√n - use worst-case for fits and safety-critical joints.

Heat-tint colours on steel indicate temperature: pale straw ≈ 220 °C, brown ≈ 250 °C, purple ≈ 290 °C, blue ≈ 300 °C, grey ≈ 400 °C - used to judge tempering and welding heat input.

Apprentice sign-off: each competence is recorded only after a witnessed task to drawing tolerance; rework beyond the stated allowance is documented as a nonconformance, not silently corrected.

Workshop safety ratios: abrasive cut-off wheels must run at or below their marked maximum rpm; a wheel rated 4,400 rpm on a machine turning faster is a burst hazard.

Ventilation threshold: welding fume extraction is required for all arc processes indoors; manganese and hexavalent chromium fume from stainless and hardfacing consumables are the primary chronic-exposure concerns.

Hot-work permit control: a fire watch must remain in place during work and for at least 60 minutes after hot work ends, with the area checked again before leaving.

Lifting rule: for two-leg slings at included angle 2α, leg tension = load / (2·cos α); a 90° included angle raises each leg load to 0.71 × load, a 120° angle to 1.0 × load.

Cylinder handling: oxygen and fuel-gas cylinders are stored upright, separated by at least 3 m or a fire-rated barrier, with flashback arresters on both torch lines.

Distortion control: balanced welding (back-step, skip, or alternating sequence) and pre-setting the joint against expected shrinkage reduce angular distortion more effectively than post-straightening.

Measurement instrument choice: vernier callipers for external/internal sizes to 0.02 mm; micrometres to 0.01 mm for precision shafts; feeler gauges for gap measurement; a spirit level or dial indicator for flatness and runout.

Documentation: fabrication drawings take precedence in the order drawing → specification → standard practice; any conflict is raised as a technical query before cutting material.