DIN 6935 Bend Allowance Calculator — Cold Bending of Flat Steel
DIN 6935 — Cold bending of flat products made of steel
DIN 6935 is the German standard for the cold bending of flat-rolled steel products. It defines the neutral-axis (K-factor) method for computing bend allowance and flat blank length — locating the neutral fibre at a fraction K of the sheet thickness from the inside surface, then integrating the arc length at that radius over the bend angle. The standard also tabulates minimum permissible inside bend radii as a function of sheet thickness, material tensile class, and bending direction relative to the rolling direction, giving designers the dimensional basis on which press-brake and folding-machine work is specified.
DIN 6935 is the method cited in German-speaking and European sheet-metal shops for flat-pattern layout: when a drawing callout references a K-factor or a minimum bend radius for S235, S355, or similar structural steel grades, it is this standard that governs. MechanixCalc runs the full DIN 6935 bend allowance calculation — plus Gardiner springback, punching force, deep drawing (Sachs), and forming-limit-diagram assessment — directly in the browser, with the substituted formulas and results exported in a shareable PDF engineering report.
Calculators that implement DIN 6935
What DIN 6935 covers
- K-factor neutral-axis method for bend allowance and flat blank length for cold-bent flat steel products
- Minimum permissible inside bend radii as a function of sheet thickness, material tensile strength class, and rolling direction
- Outside setback (OSSB) and bend deduction (BD) for flat-pattern layout in press-brake, box-and-pan, and folding-machine work
- Applicability to structural and engineering steels (S235, S355 and similar) in thicknesses used for cold forming
- Bend angle range — the geometric constraints on OSSB validity (bends up to approximately 170°) and the requirement to form in the valid yielding region
- Consistent with ISO 2768 general-tolerance practice for sheet-metal blanks and formed parts
Governing formulas
R_n = R + K · twhere R_n = neutral-axis bend radius (mm); R = inside bend radius (mm); K = K-factor, the fractional position of the neutral axis from the inside surface (typically 0.33–0.50; 0.38–0.40 for standard press-brake air bending of steel); t = sheet thickness (mm)
BA = (α · π / 180) · R_nwhere BA = bend allowance, the arc length of material consumed in the bend (mm); α = bend angle (°); R_n = neutral-axis radius (mm). For a 90° bend: BA = (π/2) · (R + K · t).
L_flat = L₁ + L₂ − BD, BD = 2 · OSSB − BA, OSSB = tan(α / 2) · (R + t)where L_flat = total flat blank length (mm); L₁, L₂ = flange lengths measured to the outside mould line (mm); BD = bend deduction (mm); OSSB = outside setback — the distance from the outside-mould-line intersection to the edge of the bend zone (mm). Valid for α < 180°; OSSB → ∞ as α → 180°.
Frequently asked questions
What is DIN 6935 used for?
DIN 6935 is used to calculate the flat blank length and flat-pattern layout for cold-bent sheet steel parts. It defines the K-factor neutral-axis method — locating the neutral fibre at a fraction K of the sheet thickness from the inside surface and integrating its arc length over the bend angle — so that the flat blank, when bent, yields the correct final dimensions. The standard also tabulates minimum inside bend radii for common structural steel grades to prevent cracking, based on thickness, tensile class and rolling direction.
What is the K-factor in DIN 6935 and how do I choose the right value?
The K-factor positions the neutral axis as a fraction of the sheet thickness from the inside of the bend: K = 0.33 for tight bends (small R/t) in soft material with air bending; K = 0.38–0.40 for typical press-brake work on mild and structural steel; K = 0.50 for large-radius bends and coining. For standard press-brake air bending of S235 or S355 with R/t ≈ 1–3, K = 0.40 is the most commonly cited value. An incorrect K is the largest single source of flat-blank length error in sheet-metal fabrication.
What is the difference between bend allowance and bend deduction?
Bend allowance (BA) is the length of material consumed in the bend zone — the arc length at the neutral-axis radius. Bend deduction (BD) is the amount by which the sum of the two flange lengths (measured to their outside-mould-line intersection) exceeds the flat blank length: BD = 2 · OSSB − BA. Flat blank length = L₁ + L₂ − BD. If you measure flanges to inside dimensions rather than outside-mould-line dimensions, you add BA instead of subtracting BD; both methods are equivalent when applied consistently.
Does DIN 6935 also cover springback?
DIN 6935 itself deals with the flat-pattern geometry (K-factor method, minimum radii) and does not define a springback prediction formula. Springback for air bending of flat steel is commonly calculated using the Gardiner (1957) elastic–perfectly-plastic plane-strain relation αf/αi = 1 − 3x + 4x³, where x = Sy·R/(E·t). The MechanixCalc sheet metal calculator applies this model on its springback tab — alongside the DIN 6935 flat-pattern calculation — and reports the required overbend angle and a validity warning when the operating point leaves the model's domain.
Is the DIN 6935 sheet metal calculator free?
You can use every tab of the sheet metal calculator for a free 30-minute preview with no sign-up required. A free 14-day account trial (no credit card needed) gives unlimited access to all calculators. The branded PDF engineering report showing the full DIN 6935 method with substituted values, and the ability to save and reload calculations, are part of a paid plan.
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