Sheet Metal K-Factor, Bend Allowance, Bend Deduction & Press-Brake Tonnage Calculator (DIN 6935)
K-Factor, Bend Allowance (BA), Bend Deduction (BD) & Air-Bend Tonnage Calculator
How This Sheet Metal K-Factor & Tonnage Calculator Works
When bending sheet metal on a CNC press brake, the inner face compresses while the outer face stretches, causing the neutral axis to shift inward to a distance t = K × T from the inside surface. To unfold a 3D bracket or enclosure into an accurate 1:1 flat pattern, engineers subtract the Bend Deduction (BD) from the sum of the outside flange lengths. Our interactive DIN 6935 calculator computes exact Bend Allowance (BA), Bend Deduction (BD), Outside Setback (OSSB), total flat blank length, optimal V-Die opening (6T–8T), and required air-bending tonnage.
Governing Mathematical Equations (DIN 6935 Cold Bending of Flat Steel Products & Press-Brake Air Bending Standard)
Computes the true arc length along the unstretched neutral fiber inside the bend zone.
Determines the exact amount of material elongation to subtract per bend when summing outside flange dimensions.
Calculates the required press brake force based on material ultimate tensile strength (UTS), thickness squared, bend length, and V-die width.
Standard Sheet Metal Air-Bending K-Factors, V-Die Openings & Tensile Strengths
| Material Grade | UTS (MPa) | Air-Bend K-Factor | Recommended V-Die | Natural Inside Radius (R) |
|---|---|---|---|---|
| Cold Rolled Mild Steel (CRS / A36) | 420 MPa | K = 0.42 | 6 × T to 8 × T | R ≈ 0.16 × V_die |
| 304 / 316 Austenitic Stainless Steel | 600 MPa | K = 0.44 | 8 × T to 10 × T | R ≈ 0.20 × V_die |
| 5052-H32 Marine Aluminum Alloy | 240 MPa | K = 0.40 | 6 × T | R ≈ 0.14 × V_die |
| 6061-T6 Structural Aluminum (Caution) | 310 MPa | K = 0.42 | 8 × T to 10 × T | R ≥ 2.0 × T (prevent cracking) |
Apply These Calculations in 1:1 Parametric CAD Studios
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Frequently Asked Questions — Sheet Metal K-Factor & Tonnage
What is the difference between Bend Allowance (BA) and Bend Deduction (BD)?
Bend Allowance (BA) is added to the flat lengths between tangent points, whereas Bend Deduction (BD) is subtracted directly from the sum of the outside flange lengths measured to the virtual sharp apex.
Why does doubling sheet thickness quadruple press-brake tonnage?
In the air-bending force equation F = (1.42 × UTS × T² × L) / V, bending resistance scales with the section modulus (thickness squared T²), unless you proportionally widen the bottom V-die opening (V).
What K-Factor should I use for standard 90-degree press-brake air bending?
For inside bend radii roughly equal to sheet thickness (R ≈ T), use K = 0.40 for soft aluminum, K = 0.42 for mild steel, and K = 0.44 for 304 stainless steel.