During sheet metal bending, the inside bend surface is compressed and the outer surface is stretched. That results in increase of overall sheet length. This change in sheet length depends on position of neutral axis or k-factor. Best way to calculate k factor is by reverse engineering. You can run a simple experiment and use this calculator for K-factor calculation.
We suggest you to read a detailed article on k-factor and flat pattern calculations.
Cad tools use k-factor to calculate sheet-metal blank size. You can also use our flat pattern calculator for blank size calculation.
Reverse Engineering Approach
Step-1 : Cut a sample sheet and measure total flat length and sheet thickness in mm. Width of strip is not very critical.
Step-2 : Bend the sheet to 90 degrees and measure length A and B.
Please note bending of sample part is done in same manner as we are planning for production. Because different machines and bending tool gives different k factor values.
Step-3 : Value of inside bend radius depends on bending tool and die.
Value of k factor is always less than than 0.5. K factor value depends on material, sheet thickness and machine.