1. Principle of Calculation Expansion
During the bending process, the outer layer of the sheet is subjected to tensile stress, while the inner layer is subjected to compressive stress. Theoretically, there is a transition layer called the neutral layer between the inner and outer layers, which is neither under tension nor compression. The neutral layer is a hypothetical layer that remains unchanged in length during the bending process and serves as the reference for calculating the length of the bent part. The position of the neutral layer is related to the degree of deformation. When the bending radius is large and the bending angle is small, the degree of deformation is small, and the neutral layer is closer to the center of the sheet thickness. As the bending radius decreases and the bending angle increases, the degree of deformation increases, and the position of the neutral layer gradually moves towards the inner side of the bending center. The distance from the neutral layer to the inner side of the sheet is denoted as A.
2. Determination of Bending Method
There are two methods for bending: single-stroke punch bending and press brake bending. In single-stroke punch bending, the method and accuracy are determined by the mold. Therefore, as long as a qualified mold is made, qualified bent products can be produced. On the other hand, in press brake bending, not only the appropriate bending die needs to be selected, but also the bending parameters must be adjusted. Therefore, when using press brake bending, the bending method of the press brake must be considered when calculating the flat pattern size.
1. Single-bend method: This method is performed using general-purpose bending dies and includes bending at right angles, obtuse angles, and acute angles.
2. Two-bend method with forging allowance: This method is performed using special-purpose dies and has a higher difficulty level compared to ordinary bending.
3. Edge crushing: This bending method also requires the use of special dies.
4. Large R-radius bending: For this type of bending, if the R value is within a certain range, it can be formed using a dedicated R die. If the R value is too large, it may require multiple presses using smaller R dies.
The calculation of the flat pattern for these four bending methods is different. Therefore, when examining the drawings, the bending method to be used should be determined based on the dimensions of the bent part. The V-groove width of the general-purpose bending die matched with the press brake is usually 5-6 times the applicable plate thickness. If the single-bend method is used, the width of the V-groove (W1) and the distance from one side of the V-groove to the outer side of the die (L1) must be considered.
1. Empirical Values for Bending Height H based on Product Shape (Using 90 degrees as an example, obtuse angles and acute angles are similar to right angles):
1. Simple 90-degree single-side bending.
For this type of bending, the distance from the center of the lower die's V-groove to the positioning block of the bending machine needs to be considered to determine the bending height.
Usually, the value of H is H≥3.5T+R (R below 1mm).
2. U-shaped bending.
If the dimensions of this type of bending are too small, interference may occur due to the lack of suitable bending dies. Therefore, the width of the two vertical sides, L, should not be too small. The height of one vertical side, H, should also not be too large. In practice, the simulation can be done based on the shape of the bending die to determine the values of L and H, as follows:
3. Z-shaped bending.
After the first bending, when bending for the second time, the distance from the bending line to the positioning block of the bending machine must be greater than or equal to the sum of the distance from the center of the V-groove to the outer side of the die (L1) and the plate thickness (t). Therefore, the value of H is:
H≥5t+R (R below 1mm).
Three, Calculation Method for Flat Pattern:
1. 90° Bending (General Bending):
The length of the flat pattern is: L=LL+Ls-2t+coefficient a.
The empirical values for coefficient a are shown in the table below:
2. Edge Crushing:
Edge crushing is a bending shape where two layers overlap, usually used for reinforcement purposes. Therefore, edge crushing is rarely seen in plates above 2.0mm. It also requires special bending dies and multiple processes to form. The formula for calculating the flat pattern length of edge crushing bending is:
L=L1+L2-K.
The empirical values for K are shown in the table below:
3. Rib Bending:
1) Inclined Rib Bending:
This rib bending has a sloping surface, usually with a small H value. The formula for the flat pattern length is:
L=A+B+C+0.2 (Note: A, B, C are internal dimensions, 0.2 is a compensation value).
2) Right-angle Rib Bending:
The rib edge is an upright edge, and usually C has a larger value. The formula for the flat pattern length is:
L=A+B+C-4T+2a+0.5 (Note: A, B are external dimensions, C is the height including two layers of plate thickness, a is the coefficient for 90-degree bending, 0.5 is a compensation value).
3) Parallel Rib Bending:
The maximum value for rib height H is H=2t. The formula for the flat pattern length is:
L=A+B+H+0.2 (Note: A, B are internal dimensions, H is the rib height, 0.2 is a compensation value).
*Since the rib height mainly depends on the adjustment of the rib bending die's adjustment plate and the operators' individual experience may vary, sometimes there may be cases where the height meets the requirements after bending, but the overall flat pattern size is either too large or too small. In such cases, adjustments should be made based on the actual deviation.
4. Sharp Angle Bending:
The empirical formula is an inner diameter algorithm, where the inner diameter refers to the distance from the imaginary intersection point of the two inner sides of the bending edge to the other end. The formula for the flat pattern coefficient is as follows:
K=0.4txδ/90° (t<2.5).
However, when t≥2.5, the following formula should be used:
K=0.5txδ/90° (t≥2.5).
Therefore, the flat pattern calculation formula is:
L=L1+L2+K (Note: L is the flat pattern length, L1, L2 are inner diameter dimensions, K is the flat pattern coefficient).
During the bending process, the outer layer of the sheet is subjected to tensile stress, while the inner layer is subjected to compressive stress. Theoretically, there is a transition layer called the neutral layer between the inner and outer layers, which is neither under tension nor compression. The neutral layer is a hypothetical layer that remains unchanged in length during the bending process and serves as the reference for calculating the length of the bent part. The position of the neutral layer is related to the degree of deformation. When the bending radius is large and the bending angle is small, the degree of deformation is small, and the neutral layer is closer to the center of the sheet thickness. As the bending radius decreases and the bending angle increases, the degree of deformation increases, and the position of the neutral layer gradually moves towards the inner side of the bending center. The distance from the neutral layer to the inner side of the sheet is denoted as A.
2. Determination of Bending Method
There are two methods for bending: single-stroke punch bending and press brake bending. In single-stroke punch bending, the method and accuracy are determined by the mold. Therefore, as long as a qualified mold is made, qualified bent products can be produced. On the other hand, in press brake bending, not only the appropriate bending die needs to be selected, but also the bending parameters must be adjusted. Therefore, when using press brake bending, the bending method of the press brake must be considered when calculating the flat pattern size.
1. Single-bend method: This method is performed using general-purpose bending dies and includes bending at right angles, obtuse angles, and acute angles.
2. Two-bend method with forging allowance: This method is performed using special-purpose dies and has a higher difficulty level compared to ordinary bending.
3. Edge crushing: This bending method also requires the use of special dies.
4. Large R-radius bending: For this type of bending, if the R value is within a certain range, it can be formed using a dedicated R die. If the R value is too large, it may require multiple presses using smaller R dies.
The calculation of the flat pattern for these four bending methods is different. Therefore, when examining the drawings, the bending method to be used should be determined based on the dimensions of the bent part. The V-groove width of the general-purpose bending die matched with the press brake is usually 5-6 times the applicable plate thickness. If the single-bend method is used, the width of the V-groove (W1) and the distance from one side of the V-groove to the outer side of the die (L1) must be considered.
1. Empirical Values for Bending Height H based on Product Shape (Using 90 degrees as an example, obtuse angles and acute angles are similar to right angles):
1. Simple 90-degree single-side bending.
For this type of bending, the distance from the center of the lower die's V-groove to the positioning block of the bending machine needs to be considered to determine the bending height.
Usually, the value of H is H≥3.5T+R (R below 1mm).
2. U-shaped bending.
If the dimensions of this type of bending are too small, interference may occur due to the lack of suitable bending dies. Therefore, the width of the two vertical sides, L, should not be too small. The height of one vertical side, H, should also not be too large. In practice, the simulation can be done based on the shape of the bending die to determine the values of L and H, as follows:
3. Z-shaped bending.
After the first bending, when bending for the second time, the distance from the bending line to the positioning block of the bending machine must be greater than or equal to the sum of the distance from the center of the V-groove to the outer side of the die (L1) and the plate thickness (t). Therefore, the value of H is:
H≥5t+R (R below 1mm).
Three, Calculation Method for Flat Pattern:
1. 90° Bending (General Bending):
The length of the flat pattern is: L=LL+Ls-2t+coefficient a.
The empirical values for coefficient a are shown in the table below:
2. Edge Crushing:
Edge crushing is a bending shape where two layers overlap, usually used for reinforcement purposes. Therefore, edge crushing is rarely seen in plates above 2.0mm. It also requires special bending dies and multiple processes to form. The formula for calculating the flat pattern length of edge crushing bending is:
L=L1+L2-K.
The empirical values for K are shown in the table below:
3. Rib Bending:
1) Inclined Rib Bending:
This rib bending has a sloping surface, usually with a small H value. The formula for the flat pattern length is:
L=A+B+C+0.2 (Note: A, B, C are internal dimensions, 0.2 is a compensation value).
2) Right-angle Rib Bending:
The rib edge is an upright edge, and usually C has a larger value. The formula for the flat pattern length is:
L=A+B+C-4T+2a+0.5 (Note: A, B are external dimensions, C is the height including two layers of plate thickness, a is the coefficient for 90-degree bending, 0.5 is a compensation value).
3) Parallel Rib Bending:
The maximum value for rib height H is H=2t. The formula for the flat pattern length is:
L=A+B+H+0.2 (Note: A, B are internal dimensions, H is the rib height, 0.2 is a compensation value).
*Since the rib height mainly depends on the adjustment of the rib bending die's adjustment plate and the operators' individual experience may vary, sometimes there may be cases where the height meets the requirements after bending, but the overall flat pattern size is either too large or too small. In such cases, adjustments should be made based on the actual deviation.
4. Sharp Angle Bending:
The empirical formula is an inner diameter algorithm, where the inner diameter refers to the distance from the imaginary intersection point of the two inner sides of the bending edge to the other end. The formula for the flat pattern coefficient is as follows:
K=0.4txδ/90° (t<2.5).
However, when t≥2.5, the following formula should be used:
K=0.5txδ/90° (t≥2.5).
Therefore, the flat pattern calculation formula is:
L=L1+L2+K (Note: L is the flat pattern length, L1, L2 are inner diameter dimensions, K is the flat pattern coefficient).
