When the sheet metal is put through the process of bending the metal around the bend is deformed and stretched. As this happens you gain a small amount of total length in your part. Likewise when you are developing a flat pattern you will make a deduction from your desired part size to get the correct flat size. The deduction when determining your flat pattern length is known as the bend deduction.
The Bend Deduction (BD) formula takes into account the geometry of the bend and the properties of your metal to predict the material’s change. You will need to know your Material Thickness (MT), the Bend Angle (B<), the Inside Radius (IR), and the K-Factor (K). The material thickness will be measured in decimal form, not by the gauge number. It is important to convert from the included angle to the complimentary angle before performing any calculations. A more basic way to approach bend deduction is to show how it relates to the Flat Pattern Length (FPL) and the sum of the Flange Lengths (FL) as shown in the second equation.
Shown below a part with flange lengths of 2” and 3” with an inside radius of .250” at 90° will have leg lengths of 1.625” and 2.625” respectively. The material thickness is .125” and we are assuming a k-factor of .33. Using the above formula we can calculate the bend allowance to be .457”. In order to develop the flat pattern we add .457” to 1.625” and 2.625” to arrive at 4.707”. The bend deduction can be determined easily by subtracting the finished flange lengths from this flat pattern length.