Cutting, Shearing and Hand Forming Operations
This domain covers separating sheet by shearing, snips, chisels, sawing and nibbling, and then forming it by hand. Expect questions on blade clearance, shear force, edge quality and the correct sequence for hand forming.
Cutting principles
- Shearing uses two blades. Correct clearance is roughly 5–10% of sheet thickness. Too little clearance causes secondary shearing and a rough edge; too much causes a rolled edge and burr.
- A guillotine gives straight cuts; a nibbler cuts curves and internal profiles; snips suit thin sheet and short cuts. Hold the sheet flat and keep fingers clear of the blade line.
- Hand forming uses a mallet over a stake, vice or sandbag. Stretching thins the metal, shrinking thickens it, crimping gathers an edge, and swaging offsets a joint. Form gradually and anneal work-hardened metal if permitted.
Worked example
A guillotine cuts mild steel 500 mm long and 1.5 mm thick. The ultimate shear strength is 300 N/mm². Shear force F = L × t × τ = 500 × 1.5 × 300 = 225,000 N, or 225 kN. Adding 20% for friction and blunt blades gives about 270 kN. Reasoning: force rises with cut length, thickness and shear strength, so a long cut in thick sheet needs a much larger machine.
Traps that cost marks
Using dull blades, which increase force and burr; forcing snips instead of using a larger shear; cutting without supporting the sheet; and forgetting that shearing leaves a burr that must be removed before forming.