Forming sheet metal is the deliberate application of enough force to exceed the material's yield strength so it takes a new shape permanently, while staying below the point at which it tears. Every forming process lives in that window, and everything that goes wrong in fabrication is a consequence of being at one edge of it or the other.
Bending on a press brake
A punch presses the sheet into a die and the material folds about a line. Air bending, where the sheet does not touch the bottom of the die, is the flexible standard because one tool set produces a range of angles. Bottoming and coining press the material fully into the die, giving better angle control and needing far more force. Most shops air bend unless told otherwise.
Drawing, stretching and roll forming
Deep drawing pulls a blank into a cavity so it becomes a cup or box, thinning the walls and requiring a blank holder to prevent wrinkling. Stretch forming pulls the sheet over a form block. Roll forming passes strip through successive rollers to make a constant section continuously, and it is the cheapest way to make long formed sections in volume.
Springback and how it is handled
Metal is elastic before it is plastic, so it springs back partway when the force is released. The higher the strength of the material and the tighter the radius, the more it springs. Shops overbend to compensate using values tested against their own tooling and material, which is why supplying a folded model and letting them work out the flat pattern is better than supplying your own.
What forming does to the material
It work hardens the bend region, making it stronger and less ductile, which matters if a bend is later re formed or if a hole is punched through it. It thins the outside of the bend. And it introduces residual stress, which can show up as distortion after a later operation such as welding or machining removes material elsewhere.
Questions people ask about custom sheet metal forming
What is air bending?
Pressing the sheet into a vee die without forcing it to the bottom, so the angle is set by how far the punch descends. One tool set covers a range of angles, which is why it is the shop default.
What is springback?
The partial elastic recovery when forming force is released. It grows with material strength and with tighter radii, and shops overbend to compensate.
Can a bend be reformed?
It is risky. The bend region is work hardened and thinner, so reworking it can crack the outer fibre. Get the bend right the first time or make a new part.
Should I supply the flat pattern?
No. Supply the folded model and let the fabricator generate the flat using bend allowances tested against their own tooling and material.