Bend Allowance Calculator

When sheet metal is bent, the inner surface is compressed while the outer surface is stretched. Somewhere between these two extremes lies the neutral axis—a theoretical layer that neither stretches nor compresses. The distance along this neutral axis during a bend is known as the bend allowance (BA). The primary formula used is: BA = B_angle * (pi / 180) * (R + K * T), where B_angle is the bend angle, R is the inside radius, T is the material thickness, and K is the K-factor. Additionally, the bend deduction (BD)—which tells you how much material to subtract from the sum of the flange lengths to find the flat pattern size—is calculated as: BD = 2 * (R + T) * tan(B_angle * pi / 360) - BA.

Imagine you are fabricating a bracket from a sheet of steel with a thickness (T) of 0.125 inches. You need a 90-degree bend (B_angle) with an inside radius (R) of 0.250 inches, using a material with a typical K-factor (K) of 0.33. First, calculate the bend allowance: BA = 90 * (3.14159 / 180) * (0.250 + 0.33 * 0.125) = 1.5708 * (0.250 + 0.04125) = 1.5708 * 0.29125 = 0.457 inches. Next, compute the bend deduction: BD = 2 * (0.250 + 0.125) * tan(90 * 3.14159 / 360) - 0.457 = 2 * (0.375) * tan(0.7854) - 0.457 = 0.750 * 1.0 - 0.457 = 0.293 inches. You can now use this bend deduction to precisely cut your flat blank.

Always perform a test bend on scrap material of the exact same alloy, thickness, and grain direction before cutting production runs, as minor material variations heavily impact the actual K-factor. Double-check your press brake tooling specifications, since air bending, bottoming, and coining all yield slightly different inside radii. Finally, remember that consistency in material thickness from your supplier is critical for maintaining accurate flat pattern dimensions across multiple parts.