Sheet Metal Work: Gauge, Operations and Development
Sheet metal work converts thin sheet into trays, funnels, ducts and guards by cutting, bending, folding and joining. The exam asks about sheet gauges, snips and stakes, operations such as blanking and piercing, and simple development length.
Materials and gauge
Black iron (uncoated), galvanised iron (zinc coated, resists rust), tinned plate (food containers), copper, aluminium and stainless steel. Gauge number and thickness are inversely related: 20 SWG is thicker than 26 SWG. Thickness is checked with a sheet gauge.
Operations
- Separating: shearing, blanking, piercing, notching, slitting. Blanking cuts out the required piece; piercing cuts a hole in it.
- Forming: bending, folding, seaming, crimping, wiring.
- Joining: riveting (snap or pop rivets), soldering, spot welding.
Tools
Straight and curved snips (aviation snips for curves), bench and beakhorn stakes for bending, boxwood mallet to shape soft sheet without stretching, ball peen hammer for riveting, groover and seamer for folded joints, rivet set and snap.
Worked example
Find the blank length for a channel with a 150 mm web, two 40 mm flanges, and two 90° bends of inside radius 2 mm in 1 mm sheet. Straight portions total 150 + 40 + 40 = 230 mm. Each bend adds a bend allowance of roughly (3.14/2)(R + 0.5t) = 1.571 × 2.5 ≈ 3.9 mm, so two bends add about 7.8 mm and the blank is about 238 mm. Mark bend lines from the centre outward so errors do not accumulate.
Traps
Assuming a bigger gauge number means thicker sheet. Using a steel hammer on soft aluminium, which stretches and marks it. Making the rivet hole smaller than the rivet shank. Forgetting the allowance taken up by a folded seam.