Bend margin is the extra material needed for sheet metal bending, determined by bend angle, radius, material thickness, and K-factor. The K-factor is a ratio of the neutral line to material thickness. Bend allowance is calculated by adding the K-factor multiplied by thickness to the radius, then multiplying by the angle. Press brakes and box-and-pan brakes are used to bend materials like aluminum and mild steel.
Bend margin is a measure of how much extra material is needed to accommodate a sheet metal bend. This is a function of the bend angle, bend radius, material thickness, and a variable called the K-factor. Bend margins vary with bend angle, material type and thickness, and method used to create the fold. Bending allowance calculation 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 due to compression inside the bend and tension outside it. A line through the thickness of the bend, called the neutral axis, does not change in length when the part is bent. The material inside this line is in compression, while the material outside this line is in tension. The location of the neutral axis varies with bend angle and bend radius.
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 cannot be greater than 0.5 because it is not physically possible for the compression inside the curve to be greater than the tension outside. In essence, the K-factor is an indirect measure of the relationship between compression and tension in the crease: the higher the K-factor, the greater the compression. The K factor depends on the material used and the type of crease being created.
To calculate the bend allowance, the K factor is multiplied by the material thickness; that number is then added to the bend radius. This number is multiplied by the bend angle times pi over 180. The formula is bend allowance = (K-factor x thickness + radius) x angle x (pi/180). Once the bend allowance is calculated, it is added to the required finished length to obtain the length of material needed to create the bent part.
Machines commonly used to bend sheet metal include press brakes, also known as press brakes, and box-and-pan brakes. There are several methods for creating curves on each type of brake. Materials commonly bent using brakes include aluminum and mild steels; some types of ductile plastics can also be bent using these machining methods.
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