You bend a sheet to 90 degrees. You release it. It springs back to 87. Welcome to the world of springback — the effect that costs sheet metal shops hours every week.
What is springback?
When you bend a sheet, two things happen at the same time:
- A permanent plastic deformation (the bend you want)
- An elastic deformation that releases the moment you let go of the pressure
That elastic deformation makes the sheet "spring back" by a certain angle.
If you order 90 degrees, you get 87 or 88. If you actually want 90, you need to overbend to 92 or 93.
The 4 factors that drive springback
| Factor | Impact | Practical rule |
|---|---|---|
| Material | High | Stainless springs back more than mild steel |
| Thickness | Medium | Thin sheet springs back more, proportionally |
| Inside radius | High | A large radius increases springback |
| Yield strength | High | The harder the material, the more it springs back |
The K-factor: the heart of the calculation
The K-factor describes the position of the neutral axis inside the thickness — the fiber that is neither stretched nor compressed during bending. It is what lets you calculate the correct flat length of the part.
The K-factor typically ranges from 0.3 to 0.5. It depends on the material, the thickness and the inside radius. Mild steel sits around 0.38 to 0.42; stainless and hard aluminum tend to be lower.
Calculating the flat pattern
A bent part is longer when flat than the sum of its outside dimensions, because of the bend allowance. The bend allowance depends directly on the K-factor, the angle, the radius and the thickness. Designing without accounting for it produces wrong parts.
The flat pattern calculation is not optional. It is the difference between a part that fits and a part that gets scrapped.
Practical value table
| Material | Typical K-factor | Springback at 90° |
|---|---|---|
| Mild steel S235 | 0.40 | 1 to 2° |
| Stainless 304 | 0.35 | 2 to 4° |
| Aluminum 5754 | 0.40 | 1 to 3° |
| Aluminum 6082 (hard) | 0.33 | 3 to 5° |
These values are starting points. Every shop refines them with its own machines and tooling.
How DRAWLESS handles it
DRAWLESS integrates K-factor values by material and thickness directly into its geometry engine. When you describe a bent part, the flat pattern is calculated automatically, springback included. The DXF you get is the real flat length — not the theoretical sum of dimensions.