Manufacturing Process
Metal Bending
金属折弯
Forming sheet metal along a straight axis with a press brake to create flanges, channels and enclosures.
Process Steps
- Tooling selected
- Air bending or bottoming
- Angle checked
- Springback compensated
What is metal bending?
Direct answer: bending forms sheet metal along a straight line to make a flange, a channel or an enclosure, usually on a press brake. The metal is plastically deformed past its yield point while the neutral axis stays the same length — and that last detail is what bend allowance is about.
The three numbers that decide a bent part
| Term | What it means | Rule of thumb |
|---|---|---|
| Bend allowance | The length of the neutral axis through the bend — what you must add to the flat blank | BA = angle(rad) × (R + K × T) |
| K-factor | Where the neutral axis sits inside the thickness | 0.33 when R < T, rising to 0.5 when R > 3T |
| Bend deduction | The difference between the sum of the outside dimensions and the flat length | Worked backwards from the blank size |
Get one of them wrong and the part is right on the drawing but wrong on the bench — which is why Balford publishes a bend allowance calculator rather than a lookup table.
Minimum radius and minimum flange
| Material | Minimum inside bend radius | Note |
|---|---|---|
| Mild / low-carbon steel | 0.5 – 1 × thickness | Below this the outside fibre cracks |
| Stainless steel (annealed) | 1 – 2 × thickness | Work-hardens; harder tempers need more |
| Aluminium 5052-H32 | 1 × thickness | Good forming temper |
| Aluminium 6061-T6 | 3 – 4 × thickness | Often needs a bend relief or annealing |
| Brass / copper | 0.5 – 1 × thickness | Soft tempers bend easily |
Minimum flange length is set by the die opening, not by the material: with a V-die opening of roughly 8× thickness, the shortest sensible flange is about 4× thickness. A shorter flange either needs a smaller V (more tonnage, more marking) or a different process.
What tolerance 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 |
| Overall flat pattern length | ±0.1 – ±0.25 mm (blanking) plus the bend stack-up |
Springback: the number that decides the angle
Every bent part springs back when the punch releases. Mild steel springs back 1–3°; stainless and high-strength aluminium can spring back considerably more. Three ways to deal with it:
- Over-bend. Program the angle past the target so it relaxes onto it. Cheap, effective, and how most air-bent parts are made.
- Bottoming or coining. Press the material fully into the die so the bend is defined by the tool. Tighter and more repeatable, but needs far more tonnage.
- In-die compensation. Where a stamping tool forms the bend, the tool is simply cut with the springback allowance built in — no process adjustment needed per batch.
Press brake or stamping die?
They are separate answers to the same question. A press brake is flexible: any angle, any flange, quick changes, low tooling cost — the right choice for prototypes and low volume. A stamping die forms the bend inside the tool, with no per-part handling: the right choice above roughly 10,000 pieces a year, and the only way to combine bending with deep drawing, piercing and tapping in one strip.
Frequently asked
What is bend allowance?
The length of the neutral axis through the bend, which is what you add to a flat blank so the finished part comes out the right size. It depends on the bend angle, the inside radius and the K-factor.
What is the minimum bend radius for sheet metal?
Roughly 0.5–1× the material thickness for mild steel, 1–2× for annealed stainless steel, and 3–4× for 6061-T6 aluminium. Tight radii crack the outside of the bend.
How accurate is a bent part?
Typically ±0.5°–1.5° on the angle and ±0.1–0.3 mm on flange length. Angle is dominated by springback and material batch; length is dominated by the blank and the die.
Why does bending sometimes remove a hole from tolerance?
Material flows into the bend, so a hole placed too close to the bend line distorts. Keep holes at least 2× thickness plus the bend radius away from the bend line, or relief them.
Related: bend allowance calculator · tube bending · laser cutting · deep drawing · precision metal stamping