Sheet metal bend allowance calculation is important when converting a formed part into a flat pattern. Correct calculations help ensure that the finished part reaches the required dimensions after bending.
What is bend allowance?
Bend allowance represents the length of material along the neutral axis that participates in the bend. Its value depends on material thickness, inside bend radius, bend angle and the selected K-factor.
Basic bend allowance formula
A commonly used relationship is BA = angle in radians × (inside radius + K × thickness). The correct K-factor depends on material, tooling, thickness and forming process.
What is the K-factor?
The K-factor represents the position of the neutral axis through the sheet thickness. It is not a universal constant. In production, it should be validated against actual bends when accuracy is important.
Bend deduction
Bend deduction is another method used to calculate the flat length from outside dimensions. The selected method should match the CAD software setup and manufacturing convention.
Example
For a sheet with thickness 2 mm, inside radius 2 mm, a 90-degree bend and K-factor 0.33, the bend allowance is approximately 3.14 × (2 + 0.33 × 2) = 7.51 mm.
Why actual bending trials matter
Real tooling, springback, material grade, grain direction and machine settings can change the final result. For production parts, compare the CAD flat pattern with a sample and adjust the bend table or K-factor where necessary.
Sheet metal design checklist
- Choose a practical inside radius.
- Check minimum flange length.
- Add bend relief where needed.
- Use the correct material thickness.
- Validate the flat pattern with production.
Mastering sheet metal bend allowance calculation makes CAD flat patterns more predictable and reduces trial-and-error during fabrication.