Understanding the Sheet Metal K-Factor
The **K-factor** is the ratio of the **neutral axis location** ($k$) to the **material thickness** ($T$). The neutral axis is the layer within the metal that neither stretches nor compresses during bending. Its location is crucial for accurate flat pattern development.
K-Factor Formula
The K-factor ($K$) is mathematically defined by the relationship derived from the Bend Allowance ($BA$) formula. By measuring the Bend Allowance for a known angle ($A$), radius ($R$), and thickness ($T$), you can isolate and calculate $K$.
| Symbol | Description | Units |
|---|---|---|
| K | K-factor (ratio of neutral axis location to thickness) | Dimensionless |
| BA | Bend Allowance (length of the neutral axis arc) | Length (e.g., inches) |
| T | Material Thickness | Length (e.g., inches) |
| R | Inner Bend Radius | Length (e.g., inches) |
| A | Bend Angle (in radians) | Radians |
The formula for K-factor, derived from the Bend Allowance formula $BA = A \cdot (R + K \cdot T)$, is: $$\mathbf{K} = \frac{\left( \frac{BA}{A_{\text{rad}}} \right) - R}{T}$$
Typical K-Factor Values and Use
- **Range:** The K-factor typically falls between **0.3 and 0.5**.
- **Default:** A value of **0.44** is often used as a standard default for a $90^\circ$ bend where the inner radius ($R$) equals the material thickness ($T$).
- **Factors Affecting K:** K-factor is influenced by the material type, the bending method (die type), and the ratio of the inner radius ($R$) to the thickness ($T$).
- **Application:** Once the K-factor is determined for a material/die combination, it can be used to accurately calculate the **Bend Allowance (BA)** and **Bend Deduction (BD)** for all other angles and radii with the same setup.
Note on Measurement:
Accurate calculation of the K-factor depends entirely on the **precision** of the measured **Bend Allowance (BA)** taken from a physical test piece. Minor errors in measuring BA will lead to significant changes in the calculated K-factor.