In sheet metal bending operations, the K-factor is the most critical parameter. It defines the position of the neutral axis within the sheet thickness during bending and forms the basis of bend allowance calculation.
Using an incorrect K-factor means an error in the flat (blank) length calculation, resulting in the part dimension after bending deviating from the drawing. In tight-tolerance parts this error can accumulate and exceed acceptance limits.
K-Factor and Bend Allowance Calculation
The K-factor (ranging between 0 and 0.5) is the ratio of the neutral axis offset from the inner bend radius to the sheet thickness: K = t_neutral / t
Bend Allowance (BA) formula:
BA = (π/180) × A × (r + K×t)
Where A = bend angle (degrees), r = inside bend radius (mm), t = sheet thickness (mm).
| Material | Typical K-Factor | Note |
|---|---|---|
| Mild steel (S235/S275) | 0.33–0.38 | Reliable value for standard production |
| High-strength steel (S355/S420) | 0.38–0.42 | Springback must be considered |
| 304 Stainless steel | 0.38–0.45 | Springback is pronounced |
| Aluminium (5xxx series) | 0.38–0.45 | Varies with temper condition |
| Aluminium (6xxx series, T6) | 0.40–0.50 | r/t ≥ 2 recommended to avoid cracking |
Springback Management
Elastic springback after bending causes significant angular deviation, particularly in high-strength steels and stainless. Springback increases as yield strength rises (pronounced in S960QL), as inner bend radius increases, and as sheet thickness decreases. In press brake applications, springback compensation is applied as an angular correction by the CAM software. At ArmaWeld a test bend is performed for each new material and thickness combination, and the compensation factor is added to the calibration record.
EN ISO 2768 Tolerance Classes
Bending dimensional tolerances are evaluated according to EN ISO 2768. For angular tolerances:
| Class | ≤ 10 mm | 10–50 mm | 50–120 mm | Typical Application |
|---|---|---|---|---|
| f (fine) | ±0.05 mm | ±0.05 mm | ±0.08 mm | Precision machine parts |
| m (medium) | ±0.1 mm | ±0.1 mm | ±0.15 mm | General fabrication |
| c (coarse) | ±0.2 mm | ±0.3 mm | ±0.5 mm | Structural steel |
| v (very coarse) | ±0.5 mm | ±0.8 mm | ±1.2 mm | Secondary components |
EN 1090-2 tolerance requirements refer to EN ISO 13920 for dimensional accuracy. In EXC3, tighter tolerance classes (B or C) are generally required.
Practical Bending Recommendations
Key points to improve bending quality in manufacturing engineering: A minimum inside bend radius of r ≥ 1×t (mild steel) and r ≥ 1.5×t (stainless/high-strength) should be applied. Cracking risk is reduced when the bend line is perpendicular to the rolling direction — this is especially critical for high-strength and tempered steels. Bending sequence: for complex geometries such as closed boxes, inner bends must be made first and each step verified with a collision check.
Technical Evaluation for Your Bending Project
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