Materials and Hand Tools for Sheet Metal Work
This domain covers the common sheet metals, their properties and the hand tools used to cut, shape and finish them. Questions often ask you to select a material or tool for a stated job and to interpret standard wire gauge numbers.
Materials
- Mild steel is cheap, ductile and weldable but rusts. Galvanised iron has a zinc coating that protects it, but welding or grinding the coating produces harmful fumes, so ventilation is essential.
- Stainless steel resists corrosion but work-hardens quickly; use sharp tools and do not overwork a bend. Aluminium is light and soft, and it is best kept separate from steel tools to avoid scratching and contamination. Copper and brass are ductile and conduct heat and electricity well. Tinplate is thin steel coated with tin.
- In the Standard Wire Gauge system, a higher gauge number means thinner sheet. For example, 20 SWG is about 0.91 mm, 24 SWG about 0.61 mm and 26 SWG about 0.46 mm.
Hand tools
Straight snips cut straight lines; curved snips cut circles; aviation snips use compound leverage for thicker sheet. Use a ball-peen hammer for general work, a cross-peen hammer for stretching, and a mallet for soft sheet. Stakes such as beak, funnel and hatchet stakes support sheet during forming. Dividers, trammels, scribers, centre punches and rivet sets complete the basic kit.
Worked example
A job requires 0.8 mm galvanised sheet cut in a straight line. The nearest SWG is 21 SWG, about 0.81 mm. For a short straight cut, aviation snips are suitable. For a long production cut, a lever shear or guillotine is faster and keeps the edge straight. Reasoning: select the tool by thickness, cut length and edge quality, not by convenience.
Traps that cost marks
Thinking a higher SWG number is thicker; using a steel hammer directly on aluminium; cutting stainless with snips previously used on mild steel; and ignoring zinc fumes when heating galvanised sheet.