SOLAS Sheet Metalworking cheat sheet

Sheet metal gauge numbers are inverse to thickness: a lower gauge number means thicker material, and each gauge system (mild steel, galvanised, aluminium) has its own thickness table, so always identify the standard before converting gauge to millimetres.

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

Bend deduction: BD = 2 × OSSB − BA, where OSSB (outside setback) = (R + T) × tan(θ/2); flat length = sum of outside leg dimensions − total bend deduction for all bends.

Minimum bend radius rule of thumb for mild steel is approximately 1 × thickness; for harder alloys such as 2024 aluminium or half-hard stainless it rises to 1.5–3 × thickness to avoid outside-face cracking.

Bend relief (a notch at the end of a bend line) must be at least the material thickness deep and equal to or wider than the bend radius, otherwise the flange tears at the corner.

Stretch flanging: the neutral axis shifts toward the inside of the bend, so developed lengths of sharply formed parts must be verified by trial bend rather than calculated alone.

Hemming (folding the edge back on itself) requires a set-back allowance of roughly 1.5 × thickness plus the inside radius of the fold to prevent the hem splitting.

Shearing clearance is typically 5–10% of material thickness per side; too little clearance causes secondary shear and burring, too much causes edge roll-over and poor cut quality.

Annealing (e.g. aluminium 2024 at about 400 °C, or softening copper by heating and quenching) restores ductility after work hardening so that further forming does not crack the metal.

Riveted lap joints: rivet diameter should be roughly 3 × sheet thickness for thin sheet, with edge distance not less than 2 × rivet diameter to avoid tear-out.

Pop-rivet grip range must cover the total clamped thickness; using a rivet too short for the stack leaves the mandrel head proud and the joint loose.

Spot welding current, time and electrode force are interdependent: doubling thickness roughly doubles the required current, and electrode tip diameter should be about 5 × √thickness in mm.

TIG welding thin sheet (0.8–1.5 mm) requires a backing purge or heat sink; DCEN with a thoriated tungsten electrode is used for steel and stainless, while aluminium needs AC.

Ductwork: rectangular duct pressure classes (low, medium, high) dictate seam type and stiffener spacing; transverse joints are generally spaced no more than about 1.5 m apart for rigidity.

HVAC duct leakage testing (EN 12237 / DW/144 classes A–D) is expressed as leakage factor f in l/(s·m²) at a stated test pressure, and the class required rises with duct pressure class.

Flashing and weathering laps must be installed shingle-fashion so that the upper sheet overlaps the lower, with a minimum lap of 100 mm on pitched roofs and 150 mm on low-pitch work.

Fixing centres for sheet cladding are typically 300–600 mm along supports, with fasteners set at least 25 mm from sheet edges to prevent pull-out and water ingress.

Dissimilar metals must be isolated (by neoprene tape, gaskets or bituminous coating) to prevent galvanic corrosion; the further apart in the galvanic series, the faster the attack.

Third-angle projection places the plan below the front elevation and the side view on the side it is viewed from; first-angle places the plan above the elevation, so always check the projection symbol on the title block.

A datum is a theoretically exact reference plane or axis; dimensions given from a datum (baseline dimensioning) stack tolerances additively, whereas chain dimensioning accumulates them - choose datum dimensioning where accuracy matters.

Tolerance stack-up: for independent dimensions the worst-case total tolerance is the sum of individual tolerances; for statistical (RSS) stack-up use √(t1² + t2² + … + tn²).

Fit designation uses upper-case letters for holes and lower-case for shafts (e.g. H7/h6 is a clearance fit; H7/p6 is interference), per ISO 286 limits and fits.

Geometric tolerancing symbols: flatness, straightness and circularity are form (no datum); perpendicularity, parallelism and angularity are orientation (datum required); position, concentricity and symmetry are location.

Vernier calliper resolution is main scale division ÷ number of vernier divisions; a 1 mm main scale with 50 vernier divisions reads to 0.02 mm.

Micrometer reading = barrel reading + thimble division × least count; a 0.5 mm pitch screw with a 50-division thimble has a least count of 0.01 mm.

Surface roughness Ra is the arithmetic mean deviation of the profile; Rz is the maximum height of profile, and Ra is roughly Rz/4 to Rz/5 for common machining.

Scale drawing: a 1:5 scale means 1 unit on paper represents 5 units in reality; a 20 mm dimension on a 1:5 drawing equals 100 mm actual.

COSHH requires an assessment before using any hazardous substance; safety data sheets (SDS) supply the hazard, exposure limits and control measures that the assessment must record.

Local exhaust ventilation (LEV) for welding fume must be positioned within one duct diameter of the arc and be subject to a statutory thorough examination at least every 14 months.

Welding eye protection shade number rises with current: about shade 10 for 60–160 A MMA, shade 11 for 160–250 A, and shade 12–13 above 250 A; gas cutting needs shade 3–4 for the visor.

Guillotine and press-brake safety: interlocked guards must prevent access to the blade or punch while the machine can cycle; press brakes require light curtains or two-hand controls.

Angle grinder guards must remain fitted, the rated speed of the disc must equal or exceed the spindle speed, and discs must be ring-tested for cracks before mounting.

Noise action values: lower exposure action value 80 dB(A) and upper 85 dB(A) for daily exposure, with hearing protection mandatory at or above the upper value.

Manual handling risk assessment uses the TILE principle - Task, Individual, Load, Environment - and the guideline lifting limit for a healthy adult male in ideal conditions is around 25 kg at waist height close to the body.

Abrasive-wheel and sheet-metal cutting generate sparks that ignite solvent vapours; flammable liquids must be stored in fire-rated cabinets and decanted away from hot work.

Hot work permits and a fire watch for at least 60 minutes after welding are standard controls where sparks can reach combustible materials.