Table of Contents
What Is the K-Factor?
The K-factor is a dimensionless ratio that represents the location of the neutral axis (the plane in the sheet metal that neither compresses nor stretches during bending) relative to the material thickness. It is defined as the ratio of the distance from the inside face to the neutral axis divided by the material thickness.
Understanding the K-factor is essential for sheet metal fabrication because it directly affects the bend allowance calculation, which determines how much material is consumed in a bend. An accurate K-factor ensures that flat pattern layouts produce parts with correct finished dimensions.
The K-factor typically ranges from 0.25 to 0.50, with most materials falling between 0.33 and 0.50. A K-factor of 0.50 means the neutral axis is at the exact center of the material thickness, while lower values indicate the neutral axis is closer to the inside of the bend.
K-Factor Formulas
Where t is the distance from the inside bend surface to the neutral axis, and T is the material thickness.
Typical K-Factor Values
| Material | Soft / Annealed | Semi-Hard | Hard / Full Hard |
|---|---|---|---|
| Aluminum | 0.33 | 0.35 | 0.38 |
| Copper / Brass | 0.33 | 0.35 | 0.38 |
| Mild Steel | 0.38 | 0.42 | 0.45 |
| Stainless Steel | 0.40 | 0.44 | 0.46 |
| Titanium | 0.40 | 0.42 | 0.45 |
Understanding Bend Allowance
- Bend Allowance (BA): The arc length of the neutral axis through the bend. This is the amount of material consumed by the bend.
- Bend Deduction (BD): The difference between the sum of the two outside mold-line distances and the total flat length. Used to calculate flat pattern dimensions from outside dimensions.
- Outside Setback (OSSB): The distance from the tangent point of the bend to the apex (intersection of the outside mold lines). Important for dimensioning and tooling.
- Neutral Axis: The theoretical plane within the material that neither stretches nor compresses during bending. Its position shifts inward for tight bends.
Frequently Asked Questions
Why does the K-factor matter for sheet metal design?
The K-factor directly determines the bend allowance, which is the length of material used in a bend. If the K-factor is wrong, your flat pattern will be the wrong size, leading to parts that are either too large or too small after bending. Even small errors of 0.01 in the K-factor can cause measurable dimensional errors on parts with multiple bends.
How do I determine the K-factor for my material?
The most accurate method is empirical testing: bend a sample piece, measure the flat length and finished dimensions, then back-calculate the K-factor. Published K-factor tables (like the one above) provide good starting values based on material type and hardness. CAD software typically uses a default K-factor of 0.44 for steel.
Does the bend radius affect the K-factor?
Yes. For tight bends (radius less than the material thickness), the neutral axis shifts closer to the inside of the bend, resulting in a lower K-factor. For larger radii (radius greater than 2x thickness), the neutral axis approaches the center, and the K-factor approaches 0.50.
What K-factor should I use for air bending vs. bottom bending?
Air bending typically produces a K-factor closer to the published material values (0.33-0.45). Bottom bending (coining) tends to result in slightly lower K-factors because the material is more fully compressed. Always verify with test bends when precision is critical.