In precision sheet metal fabrication, few parameters cause as much scrap and assembly fitment issues as miscalculating the K-Factor and Bend Deduction (BD).
When CAD designers model a sheet metal bracket in SolidWorks, Autodesk Inventor, or PTC Creo, the software default is almost universally set to K = 0.5 or K = 0.44. On the press brake floor, however, trusting that generic default without accounting for punch radius, die opening, and material grain direction will routinely introduce a 0.5mm to 1.5mm cumulative error across three consecutive bends.
The Physics of the Neutral Axis
When a metal sheet is bent over a press brake tooling die:
- The outer fibers experience tensile elongation (stretching).
- The inner fibers undergo compressive deformation.
- Somewhere between the outer and inner surfaces lies a mathematical plane that experiences zero tension and zero compression: the neutral axis.
The K-Factor is simply the ratio that defines the location of this neutral axis relative to the sheet thickness ($T$):
$$K = \frac{t}{T}$$
Where:
- $T$ = Total material thickness
- $t$ = Distance from the inside bend radius to the neutral axis
Because metals exhibit greater yield strength in compression than in tension, the neutral axis always shifts inwards during plastic bending. Consequently, the actual K-factor typically ranges between 0.30 and 0.48, virtually never reaching the idealistic theoretical midpoint of 0.50.
Key Mathematical Formulations (DIN 6935)
To calculate the exact flat blank length ($L_{\text{flat}}$), precision shops calculate either the Bend Allowance (BA) or Bend Deduction (BD):
$$\text{BA} = \frac{\pi}{180} \times \alpha \times (R + K \times T)$$
$$\text{Outside Setback (OSSB)} = \tan\left(\frac{\alpha}{2}\right) \times (R + T)$$
$$\text{Bend Deduction (BD)} = 2 \times \text{OSSB} - \text{BA}$$
Where:
- $\alpha$ = Bend angle in degrees
- $R$ = Inside bend radius
- $T$ = Sheet thickness
Air Bending vs Bottoming: Why R/T Ratio Dictates K
In standard air bending, the inside radius $R$ is naturally formed by the tooling opening ($V$-die width), typically $V = 6T$ to $8T$. When the ratio of the inside radius to thickness ($R/T$) is small ($R/T < 1$), localized compression drives the neutral axis significantly inward ($K \approx 0.33 - 0.38$). When $R/T > 3$, the neutral axis approaches the sheet centerline ($K \approx 0.44 - 0.48$).
Online Engineering Calculators & Toolsets
To compute exact flat blank lengths, bend deductions, and reverse-engineer your shop's empirical K-factor from measured test bends, bookmark the CADGuide Sheet Metal K-Factor Calculator.
You can also read our deep-dive technical publications and shop-floor tutorials at CADGuide Technical Guides, or browse the main engineering portal at CADGuide.tools.
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