Bend allowance is a measure of how much extra material is required to accommodate a bend in sheet metal. This is a function of the bend angle, bend radius, thickness of the material, and a variable called the K-factor. Bend allowances vary based on the angle of the bend, the type and thickness of the material, and the method used to create the bend. Calculating the bend allowance is necessary to create a finished product of the correct size.
When a piece of sheet metal is bent, the overall length of the piece changes because of compression on the inside of the bend and tension on the outside. A line through the thickness of the bend, called the neutral axis, does not change in length when the part is bent. Material to the inside of this line is in compression, while material on the outside of this line is in tension. The location of the neutral axis varies based on the angle of the bend and the radius of the bend.
The K-factor is a ratio of the distance of the neutral line from the inside edge of the material to the thickness of the material. Generally, the K-factor is not less than 0.25, and it cannot be greater than 0.5 because it is not physically possible for the compression on the inside of the bend to be greater than the tension on the outside. Essentially, the K-factor is an indirect measure of the ratio of compression to tension in the bend — the higher the K-factor, the greater the compression. The K-factor is dependent on the material used and the type of bend being created.
To calculate bend allowance, the K-factor is multiplied by the thickness of the material; that number is then added to the bend radius. This number is multiplied by the angle of the bend times pi over 180. The formula is bend allowance = (K-factor x thickness + radius) x angle x (pi/180). Once bend allowance is calculated, it is added to the required finished length to obtain the material length required to create the bent piece.
Machines commonly used to bend sheet metal include brake presses — also known as press brakes — and box-and-pan brakes. There are multiple methods for creating bends on each kind of brake. Materials commonly bent using brakes include aluminum and mild steels; some types of ductile plastics may also be bent using these machining methods.