Cutting, Forming, and Shaping Sheet Metal
What this domain tests
Here the exam checks process knowledge: which cut gives a clean edge, what die and tonnage a bend needs, and how metal behaves when rolled, stretched, or shrunk. Items often ask why a part cracked, distorted, or sprang back.
Cutting
- Hand snips cut on the line, and the blades must not be sprung by heavy gauge or wire. Notch corners before bending to prevent tearing.
- Shear blade clearance should be roughly 5 to 10 percent of thickness. Too much clearance on stainless or aluminum gives a ragged edge and edge cracking; too little overloads the machine.
- Thermal cutting leaves a heat-affected zone and a dross edge. Galvanized and coated material needs fume control before plasma or laser cutting.
- Keep hold-downs working and hands behind the guards; shear and brake injuries are the trade's most serious.
Forming
- Air-bend tonnage for mild steel: tons per foot = 575 x t squared / V, where t is thickness and V is die opening in inches. The die opening should be at least 8t.
- Springback rises with yield strength and bend radius. Overbend slightly or use bottoming when the angle matters.
- Slip rolling: pre-bend the ends to the radius before rolling the body, or the part stays flat at the ends.
- Rotary machine work (crimp, bead, turn, flange, wire edge) is rated in gauge. A wired edge stiffens the part and removes the sharp edge.
Shaping
- Hammer and dolly, shrinker/stretcher, and English wheel move metal by stretching or shrinking; planishing restores flatness.
- Heat softens and stress-relieves. Annealing copper and aluminum makes forming possible, but over-annealing weakens the part.
Worked example: press brake tonnage
Bend 0.060 in. (16 ga) mild steel, 10 ft long, in a 1/2 in. V die. Tons per foot = 575 x 0.060 squared / 0.50 = 575 x 0.0036 / 0.50 = 4.14 tons/ft. For 10 ft: 41.4 tons, so a 50-ton brake handles it. If the material were 304 stainless, multiply by about 1.5 to get 62 tons, which overloads that machine. Check the die: 8t = 0.48 in., so the 1/2 in. opening is the minimum acceptable.
Traps that cost marks
- Choosing a die opening smaller than 8t and blaming the material when it cracks.
- Ignoring springback and finishing with an 88 degree bend.
- Shearing stainless at mild-steel blade clearance.
- Cutting short or curved pieces on a shear without a backstop or gauge.
- Skipping deburring before forming; burrs become crack starters.