Manufacturing Process
Bending
折弯
Forming sheet, tube and wire along a straight or curved axis - press-brake bending, roll bending and CNC tube bending - without cutting a die.
Process Steps
- Method matched to the part
- Tooling and bend sequence set
- Angle and radius verified
- Springback compensated
What "bending" covers
Direct answer: bending forms metal along a line without removing material. In a fabrication shop that means three different machines and three different sets of rules: a press brake or folder for sheet, roll benders for large-radius curves, and rotary-draw or mandrel benders for tube and pipe. The part decides the method, not the other way round.
Which bending method suits which part
| Method | What it does | Best for | Watch out for |
|---|---|---|---|
| Press-brake air bending | Straight bend lines anywhere on a flat blank | Prototypes to mid volume; boxes, brackets, channels | Minimum flange is set by the V-die opening |
| Bottoming / coining | Pushes the material fully into the die | Repeating angles on production parts | Needs far more tonnage and dedicated tooling |
| Folding | Clamps the sheet and wraps a beam over it | Long, thin panels and boxes at shallow angles | Not for short flanges or tight radii |
| Roll bending | Curves sheet through three rolls | Cylinders, arcs and segments far beyond a press brake | Sharp bends are impossible |
| Tube and pipe bending | Pulls tube around a former, usually with a mandrel | Lines, frames, handles, refrigeration pipe | Wall thins on the outside of a tight centreline radius |
Most parts on this page are quoted against the drawing by the method that holds the tolerance with the least tooling, which is usually press-brake air bending. Where a bend repeats in volume, or where bending has to combine with piercing and forming in one pass, the answer moves into a stamping die instead.
The three numbers behind every bent part
Bending stretches the outside of the material and compresses the inside while the neutral axis keeps its length. Three numbers follow from that, and getting any one wrong means the part is right on the drawing and wrong on the bench:
- Bend allowance — the neutral-axis length through the bend, added to the flat blank;
- K-factor — where that neutral axis sits inside the thickness;
- Springback — how far the part relaxes once the punch releases.
The working depth on those is on the metal bending page, and the arithmetic is in the bend allowance calculator. Tube has its own version of the same problem, because the centreline radius also sets wall thinning and ovality: see tube bending.
Tolerances a bent part holds
| Feature | Typical tolerance |
|---|---|
| Bend angle | ±0.5° – ±1.5° |
| Flange length | ±0.1 – ±0.3 mm |
| Hole-to-bend distance | ±0.15 – ±0.3 mm |
| Tube centreline radius | ±0.5° on angle; ovality and wall thinning agreed per part |
What the drawing should carry
- Bend lines or a folded view, and the inside radius you actually need rather than a nominal one;
- Which dimensions are functional — the flange that has to meet a mating part, not every dimension on the view;
- Grain or brush direction where the finish is visible;
- Whether the part is welded after bending, because weld distortion moves the last bend, not the first.
Frequently asked
What is the difference between bending and forming?
Bending deforms the material along a single line. Forming is the wider term: it also covers drawing a flange, curling, flanging a hole and the shaped bends a dedicated tool makes. A press brake is a bending machine; a stamping tool forms.
Can sheet be bent without a die?
A press brake always needs tooling, but it needs far less of it than a stamping die: a punch and a V-die cover a wide range of parts. That is why bending is the usual choice under about 10,000 pieces a year.
Does bending ever crack stainless?
Not if the inside radius is larger than the material's minimum. Annealed 304 bends at 1–2× thickness; harder tempers and thicker sheet need more. Where the drawing asks for a tighter radius than the material allows, the answer is a bend relief, an annealing step, or a different material.
How close to a bend can a hole be?
Keep it at least 2× thickness plus the bend radius away from the bend line. Closer than that, material flows into the bend and the hole distorts — which is a drawing issue, not a machine issue.
Related: sheet metal bending services · tube bending services · CNC tube bending · sheet cutting · sheet metal fabrication