Sheet metal bending forms flat cut blanks into angles, channels, boxes and brackets on a press brake. A punch presses the sheet into a V-shaped die; on a CNC press brake the back gauge positions each bend so repeat parts come out the same.
Quick facts
- Machine
- Press brake (CNC or NC), punch and V-die
- Typical thickness
- About 0.5–12 mm (0.0197–0.472 inch) (thicker on big machines)
- Angle tolerance
- About ±0.5–1°
- Size tolerance
- About ±0.2–0.5 mm (±0.0079–0.0197 inch) per bend
- Inside radius
- Roughly equal to the sheet thickness for mild steel
- Usually priced
- Per bend, per piece or per kg
Design rules
- Flange length: at least about 3–4 times the thickness, so the sheet sits on the die
- Keep holes at least about 2–3 times the thickness away from a bend, or they stretch
- Use the same inside radius on all bends where possible
- Bend across the rolling direction for thick or hard sheet to avoid cracks
- Add relief cuts where a bend meets an edge
Bend allowance
When metal bends, the outside stretches and the inside compresses, so the flat blank is shorter than the sum of the outside sizes. Bending shops calculate the flat length with a bend allowance (K-factor) for their tools. Send the bent drawing; let the shop make or confirm the flat pattern.
What to send
- A drawing of the bent part with inside or outside sizes clearly marked
- Material, grade and thickness
- Inside bend radius, if it matters
- Flat pattern (DXF) if you have it
- Quantity and finish (powder coating, plating)
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Questions
- What is the minimum bend radius for sheet metal?
- For mild steel, about one times the thickness on the inside is normal. Stainless steel and aluminium alloys such as 6061-T6 need a larger radius to avoid cracking; 5052 bends more easily.
- Why are my bent parts a little off size?
- Usually the flat length was calculated with a different bend allowance from the shop's tools, or the sheet springs back. Let the bending shop make the flat pattern for its own tools.
- Can holes be cut before bending?
- Yes, laser cutting holes before bending is normal, but keep them away from the bend line (about 2–3 times the thickness) so they do not distort.
The figures on this page are typical values for guidance, written by Diemeco from standard engineering practice. They are not a specification: confirm grades and properties with the relevant standard or the supplier's mill test certificate, and tolerances with your workshop. Last checked 15 October 2026.