What is sheet metal pattern?

When metal is bent, it stretches. Therefore, the flat pattern must be slightly smaller than the finished part. The pattern will also locate the center of each bend, because a typical bending machine (called a BRAKE) will press into the center of the bend area on the "bend line". The reason metal stretches during bending is this. Although the inside of the bend gets compressed while the outside gets stretched, metals are stronger in compression. Therefore, the stretching effect overcomes the compressing effect by a bit. If the stretching and compressing were equal, the flat pattern coefficient "k" would be 0.5, meaning that a line at half the thickness of the material would represent an accurate flat pattern. A typical "k" number for steel would be something like 0.45, meaning that the flat pattern lengths can be found with a line that is 45% from the inside edge of the bend.