Allowance, setback and measurement datums
The neutral axis lies within the sheet thickness. This model places it at inside radius plus K times thickness. Bend allowance is that radius multiplied by bend deflection in radians.
Outside setback measures from a tangent point to the virtual sharp intersection. Subtracting bend deduction from two outside flange dimensions gives the flat blank length for one bend. A coupon measured with the same datums can estimate K for the forming process; it does not predict springback or select tooling.
