When designing a sheet metal part which is folded you need to understand what happens when a flat sheet of metal is fabricated or folded. To calculate the actual pattern dimensions you can use the bend allowance or bend deduction method.
When the piece of sheet metal is put through the cold forming process of bending, the metal on the outside of the bend is often stretched while the inside is crushed. When this happens you can get a small extra amount of length of the part.
When developing the flat pattern you need to take into account the difference in length when this is formed to get the correct dimensions on the finished component.
By definition, the bend allowance is the arc length of the bend as measured along the neutral axis of the material you are using.
By definition, the bend deduction is the difference between the bend allowance and twice the outside setback.
Let us illustrate this below:
The Bend Allowance
The bend allowance defines the material you will need to add to the actual leg lengths of the part in order to get the flat pattern cut to a correct size.
The legs of the part are the sides outside of the bend which you can see on the image below.
To calculate this bend allowance, following formula can be used:
This formula takes into account the different geometries of the part you are forming and its properties.
The most important aspects of this calculation are the material thickness (MT), the bend angle (B), the inside bend radius (IR) and the K- factor of the material you are bending.
Note: the K-factor will typically range between 0 and .5 for most standard materials and thicknesses although some smaller and larger K-Factors are possible in certain applications.