When sheet metal is bent and flattened, one side of the material is elongated while the other side is compressed. The factors that affect this phenomenon include the type of material, material thickness, material heat treatment, and the angle of the bending process.
Principle of Unfolding Calculation:
1. During the bending process, the outer layer of the sheet metal experiences tensile stress, while the inner layer experiences compressive stress. Between the tension and compression zones, there is a neutral zone that is neither under tension nor compression. This neutral zone remains unchanged in length during the bending process and serves as the reference for calculating the unfolded length of the bent part.
2. The position of the neutral zone depends on the degree of deformation. When the bending radius is large and the bending angle is small, the deformation is minimal, and the neutral zone is located closer to the center of the material thickness. As the bending radius decreases and the bending angle increases, the degree of deformation increases, causing the neutral zone to gradually move toward the inner side of the bending center. The distance from the neutral zone to the inner side of the material is represented by λ.
Basic Formula for Unfolding Calculation:
Unfolded Length = Inside Length + Inside Length + Compensation
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 aluminum and copper:
When T < 0.3, K = 0.4T
Note: When R ≤ 2.0, it is treated as R=0.
2) General Bending (R≠0, θ=90°)
L = A + B + KK, where K is the length of the neutral zone arc.
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: K is the compensation value for θ=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
K, the length of the neutral zone arc, is used.
Note: When R ≤ 2.0 and the bending is done with a folding blade, it is calculated as R=0, and the values of A and B are taken as the lengths of the straight edges after folding.
5) Z Bend 1 (Straight Edge Difference)
1. When H > 5T and formed in two steps, it is calculated as two 90° bends.
2. When H ≤ 5T, it is calculated as a single-step forming: L = A + B + K
The value of K depends on the parameters given in the attachment.
6) Z Bend 2 (Diagonal Edge Difference)
1. When H ≤ 2T, it is calculated as a straight edge difference: Unfolded Length = Total Length before unfolding + K
K = 0.2
2. When H > 2T, it is calculated as a two-step bending unfold (R=0, θ≠90°).
7) Punching
The calculation principle for punching is based on the principle of constant volume, which means the volume of the material remains unchanged before and after punching. For general punching, the following formula is used, with parameters shown in the diagram on the right (assuming pre-punching is X, and adding a correction factor of -0.1):
1. If the punching is for tapping (creating threaded holes), S is determined as follows:
T ≤ 0.5: S = 100%T
0.5 < T < 0.8: S = 70%T
T ≥ 0.8: S = 65%T
For commonly used tapping and pre-punching, refer to Attachment 1 for values.
2. If the punching is for riveting, S = 50%T, and H = T + T' + 0.4 (Note: T' is the thickness of the plate to be riveted, and the gap between the punching and the riveted hole is 0.10-0.15 on one side).
3. If no specific markings or annotations are provided for the punching in the original drawing, the inner and outer diameters after punching should be maintained.
4. When the calculated value of the pre-punching diameter is less than 1.0, it should be taken as 1.0.
8) Reverse Folding and Flattening
L = A + B - 0.4T
1. When performing flattening, it should be considered whether to pre-press the line before bending based on the actual situation. The pre-pressing line is located in the middle of the deformation zone.
2. Reverse folding and flattening are generally performed in two steps: V-bending at 30° and reverse folding and flattening. Therefore, when drawing the unfolding diagram, a 30° bending line should be used.
9) N Bending
1. When N bending is done using the method of shimming and reverse folding and flattening, it is calculated as L = A + B + K, where K is determined based on the parameters given in the attachment.
2. When N bending is done using other methods, refer to "General Bending (R≠0, θ≠90°)" for the unfolding algorithm.
Note:
a. Design values for dimensions with specified tolerances: Take the average of the upper and lower limit dimensions as the design standard value.
b. For unfolding of square punching and external corner, refer to the "Product Unfolding Process Handling Standard." The straight wall part should be unfolded as a 90° bend.
