When press brake sheet metal is bending and unfolding, one side of the material will be elongated and one side will be compressed. The factors affected are: material type, material thickness, material heat treatment and processing bending angle.

### Expansion calculation principle:

1. The outer layer of the sheet material is subjected to tensile stress during the bending process, and the inner layer is subjected to compressive stress. From tension to compression, there is a transition layer that is neither tension nor compression is called the neutral layer; the neutral layer is in the bending process The length is the same as before bending, so the neutral layer is the basis for calculating the unfolded length of the bending part.

2. The position of the neutral layer is related to the degree of deformation. When the bending radius is larger and the bending angle is smaller, the degree of deformation is smaller, and the position of the neutral layer is close to the center of the sheet thickness; when the bending radius becomes smaller, the bending angle increases When it is large, the degree of deformation increases, and the position of the neutral layer gradually moves to the inner side of the bending center. The distance from the neutral layer to the inner side of the sheet is represented by λ.

### The basic formula for expansion calculation:

Expanded length = inside material + inside material + compensation amount

#### 1) General bending: (R=0, θ=90°)

**L=A+B+K**

1. When 0≤T≤0.3, K=0

2. For iron materials:

a. When 0.3≤T≤1.5, K=0.4T

b. When 1.5≤T≤2.5, K=0.35T

c. When T＞2.5, K=0.3T

3. For other non-ferrous metal materials such as AL, CU:

When T＜0.3, K=0.4T

Note: When R≤2.0, treat as R=0.

#### 2) General bending (R≠0 θ=90°)

**L=A+B+K**

K value takes the arc length of the neutral layer

1. When T≤1.5, λ=0.5T

2. When T>1.5, λ=0.4T

#### 3) General bending (R=0 θ≠90°)

**L=A+B+K’**

1. When T≤0.3, K’=0

2. When T＜0.3, K’=(u/90)*K

Note: The compensation amount when K is 90∘

#### 4) General bending (R≠0 θ≠90°)

**L=A+B+K**

1. When T≤1.5, λ=0.5T

2. When T>1.5, λ=0.4T

The K value is the arc length of the neutral layer

Note: When R≤2.0, and the folding knife is used for processing, it is calculated as R=0, and A and B are taken according to the length of the straight edge after the zero angle is chamfered

#### 5) Z fold 1 (straight side difference)

1. When H＞5T, when forming in two times, calculate according to two 90° bends

2. When H≤5T, one-time molding, L=A+B+K

K value depends on the parameter in the attachment

#### 6) Z-fold 2 (hypotenuse difference)

1. When H≤2T, it is calculated according to the straight side segment difference, that is: unfolded length=total length before unfolding+K

K=0.2

2. When H＞2T, expand in two stages (R=0 θ≠90°).

#### 7) Hole extraction

The calculation principle of the hole size is the principle of constant volume, that is, the volume of the material before and after the hole is unchanged; generally the hole is calculated according to the following formula, and the parameters in the formula are shown on the right (set the pre-punched hole as X, and add the correction coefficient -0.1 ):

1. If the extraction hole is a extraction hole (tapping after extraction), then S shall be taken according to the following principles:

S=100%T when T≦0.5

S=70%T when 0.5<T<0.8

S=65%T when T≧0.8

Commonly used pre-punched teeth according to appendix 1

2. If the punching hole is used for riveting, then take S=50%T, H=T+T’+0.4 (Note: T’is the thickness of the plate that is riveted with it, and the gap between the punching hole and the salad hole is single 0.10~0.15)

3. If there is no mark or mark for the hole in the original drawing, the inner and outer diameter dimensions after the hole are guaranteed

4. When the calculated value of the pre-punched aperture is less than 1.0, always take 1.0

#### 8) Reverse fold and flatten

#### 9) N fold

1. When the N-fold processing method is the gasket reverse folding and flattening, it is calculated according to L=A+B+K, and the K value is taken according to the parameter in the appendix.

2. When N-fold is processed in other ways, please refer to “General Bending (R≠0 θ≠90°)” for the unfolding algorithm

Remarks:

a. The dimension design value of the marked tolerance: the middle value of the upper and lower limit dimensions is taken as the design standard value.

b. For the expansion of the square hole and the outer wrap angle, the corner treatment method refers to the <Product Development Process Treatment Standard>, and the straight wall part is bent and expanded at 90°.

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