For sheet metal bending and expansion, one side of the sheet will be elongated, the other side will be compressed. The affecting factors including material type, material thickness, heat treatment and bending angle.

In order to ensure the expected size of the parts after final bending by the press brake, the design of sheet metal parts will use a variety of algorithms to calculate the actual length of sheet metal under folding status.

One of the most commonly used methods is the simple “pinch rules”, which is based on their own experience in the algorithm.

On the other hand, with the emergence and popularization of computer technology, people are increasingly using computer-aided design means.

In general, there are two popular algorithms for the popular sheet metal bending algorithms that are widely adopted today, one based on the bending compensation algorithm and the other based on the algorithm of bending deduction.

**Bending compensation method**

The bending compensation algorithm describes the expansion length (LT) of a part as the sum of each length of the part after flattening and the length of the flattened bend area.

The length of the flattened bend area is expressed as the “bend compensation” value (BA). So the length of the entire part is expressed as follows: LT = D1 + D2 + BA

The bending area is theoretically deformed in the process of bending. In short, to determine the geometry of the unfolded parts, let’s consider the following steps:

- Cut the bend area out of the bending part
- Tile the remaining two flat part to a table
- Calculate the length of the bend area after its flattening
- Bonding the bending area after the flattened into the area between two flat sections, the result is the expanded parts that we needed.

**Bend deduction method**

Bend deduction usually refers to the amount of back, but also a different simple algorithm to describe the process of sheet metal bending.

The bend deduction method means that the flattening length (LT) of the part is equal to the sum of the length of the theoretical two-stage flat portion extending to the “sharp point” (the virtual intersection of the two flat portions) minus the bend deduction (BD).

**Expand Calculation Principle:**

- During the bending process, the outer layer is subjected to tensile stress, and the inner layers are subjected to compressive stress. There is a transition layer which is neither tensile nor pressure from pull to pressure, which is called the neutral layer. The length of the neutral layer remains unchanged in the bending process, the same as the length before the bending, so the neutral layer is the basis for calculating the expansion length of the bending piece.
- The position of the neutral layer is related to the degree of deformation.

When the bending radius is larger and the bending angle is small, the deformation degree is small, and the neutral layer is near the center of the sheet thickness; When the bending radius becomes smaller and the bending angle increases, the degree of deformation increases, and the position of the neutral layer moves toward the inner part of the bending center gradually. The distance from the neutral layer to the inside of the sheet is expressed in λ.

**Basic Formula for Expansion Calculation:**

Expansion length = inner layer + inner layer + compensation amount