Bending Process Discussion
The bending process for sheet metal is an important component of flexible sheet metal processing. The quality of the R corner after bending directly affects the product's lifespan, making it a critical focus for quality control. Due to the need to produce products with different material thicknesses on the same bending machine, different bending tools with various R angles need to be purchased to achieve full coverage. This often results in the actual bending R angles deviating from the product drawings, directly impacting the quality of the R corner after bending.
The R tool for bending only comes in two specifications: R1 and R3. When producing thick sheet metal parts, quality defects such as small cracks and deformation often occur at the R corner, directly affecting the product's lifespan and delivery. Additionally, the quality of the blank material section has a significant impact on the quality of the R corner after bending.
Discussion on Section Quality
There are two methods for producing blanks for bending: shearing and laser cutting. Each corresponds to different types of blanks, and we will provide examples based on the lowest-cost blank preparation for each method.
1. Shearing: This method is mainly used for regular blank materials, such as rectangular blanks, square blanks, and right-angled trapezoidal blanks.
(a) Rectangular Blank (b) Trapezoidal Blank
2. Laser Cutting: This method is primarily used for cutting irregular-shaped blank materials that cannot be sheared.
Different cutting methods result in different section qualities for the blanks. Laser cutting provides a relatively better section quality, with no quality defects like burrs that would affect the bending process under normal circumstances. However, shearing is limited by the process itself, and the section undergoes different force modes at different times, resulting in lower section quality compared to laser cutting.
When the upper die plate contacts the sheet metal, the material is subjected to the pressure of the upper and lower die plate end faces (Fp and Fd), causing shear deformation between the points of force application. Due to the presence of a gap between the upper and lower die plates, Fp and Fd are not on the same line, resulting in a bending moment M on the material, causing it to warp (arch). As the material approaches the side of the upper die plate, the material beneath the upper die plate end face is forced into the gap between the die plates. The material is subjected to the lateral pressure (F1 and F2) from the die plates, as well as the frictional forces (μF1 and μF2) from the end face and side of the die plates, resulting in lateral extrusion deformation. This series of force interactions ultimately leads to a decrease in the section quality of the material after shearing.
To compare the effects of bending different blanks prepared by the two methods, one piece each of laser-cut and sheared blanks were selected for bending.
After bending, the laser-cut blank exhibits smooth deformation at the R corner with no significant quality defects. In contrast, the sheared blank shows good quality on one side of the R corner but small cracks have already appeared on the other side. There is a significant difference in quality between the two methods after bending, thus requiring an analysis of the causes of cracking in sheared blanks after bending.
Due to the limitations of the shearing process itself, after shearing, not only is the section quality relatively poor, but the burr directions at both ends of each blank are inconsistent. After shearing, the burrs on the material left on the lower die plate have a downward direction, while the burrs on the corresponding blank at the upper die plate have an upward direction. The different burr directions at both ends of a blank lead to a relatively better quality of the burr on the inner R during the bending process, while there is a higher probability of small cracks occurring on the side with the burr on the outer R.
From the above analysis, it can be seen that the section quality of the blank has a significant impact on the quality of the R corner after bending. If shearing is used for blank preparation, efforts should be made to minimize the height of the burrs on the sheared blank. If necessary, one side of the burr should be polished before bending to reduce the occurrence of cracks after bending.
Minimum Bending Radius
It can be observed that the deformation zone during bending mainly occurs at the curved corners, while the straight sections can be considered as not undergoing plastic deformation. In the deformation zone, the inner metal fibers are compressed and shortened, while the outer metal fibers are stretched and elongated.
From this, it can be seen that the smaller the bending radius, the greater the extent of tensile deformation on the outer surface layer of the deformation zone. If it exceeds the maximum allowable deformation of the material, it is prone to cracking, resulting in scrap. Therefore, it is necessary to control the tensile deformation of the deformation zone, and the magnitude of the tensile deformation mainly depends on the relative bending radius r/t. By calculating the relative bending radius, the minimum bending radius of the material can be determined, which ensures that the outer layer does not crack after bending, i.e., the minimum inner R.
The factors that affect the minimum bending radius are mainly two: material mechanical properties and fiber direction.
(1) Material mechanical properties:
Materials with good plasticity, high yield strength ratio, and elongation have a greater allowance for deformation of the outer fibers, resulting in a smaller minimum bending radius. Conversely, materials with poor plasticity have a relatively larger minimum bending radius.
(2) Fiber direction:
Based on on-site production experience, for the same batch of sheet metal, the plasticity index along the fiber direction is higher than that perpendicular to the fiber direction. Therefore, if the bending line is perpendicular to the fiber direction, the allowance for deformation of the outer fibers is greater, resulting in a smaller minimum bending radius. Conversely, if the bending line is parallel to the fiber direction, the allowance for deformation of the outer fibers will relatively decrease, resulting in an increased minimum bending radius.
Therefore, when formulating bending processes, it is advisable to make the bending line as perpendicular to the fiber direction of the blank as possible. For workpieces with multi-directional bending, the bending line can be set at a certain angle to the fiber direction of the blank to increase the degree of deformation and avoid cracking of the outer fibers. The minimum bending radius for commonly used materials is shown in Table 1.
Conclusion
This article analyzes and explains the issues of section quality and minimum bending radius regarding the cracking of thick sheet metal during bending. Through the analysis, it can be concluded that for bending thick sheet metal, it is necessary not only to ensure section quality and reduce burrs on the outer R during bending but also to ensure that the inner R of the bend is larger than the minimum bending radius of the blank.
The bending process for sheet metal is an important component of flexible sheet metal processing. The quality of the R corner after bending directly affects the product's lifespan, making it a critical focus for quality control. Due to the need to produce products with different material thicknesses on the same bending machine, different bending tools with various R angles need to be purchased to achieve full coverage. This often results in the actual bending R angles deviating from the product drawings, directly impacting the quality of the R corner after bending.
The R tool for bending only comes in two specifications: R1 and R3. When producing thick sheet metal parts, quality defects such as small cracks and deformation often occur at the R corner, directly affecting the product's lifespan and delivery. Additionally, the quality of the blank material section has a significant impact on the quality of the R corner after bending.
Discussion on Section Quality
There are two methods for producing blanks for bending: shearing and laser cutting. Each corresponds to different types of blanks, and we will provide examples based on the lowest-cost blank preparation for each method.
1. Shearing: This method is mainly used for regular blank materials, such as rectangular blanks, square blanks, and right-angled trapezoidal blanks.
(a) Rectangular Blank (b) Trapezoidal Blank
2. Laser Cutting: This method is primarily used for cutting irregular-shaped blank materials that cannot be sheared.
Different cutting methods result in different section qualities for the blanks. Laser cutting provides a relatively better section quality, with no quality defects like burrs that would affect the bending process under normal circumstances. However, shearing is limited by the process itself, and the section undergoes different force modes at different times, resulting in lower section quality compared to laser cutting.
When the upper die plate contacts the sheet metal, the material is subjected to the pressure of the upper and lower die plate end faces (Fp and Fd), causing shear deformation between the points of force application. Due to the presence of a gap between the upper and lower die plates, Fp and Fd are not on the same line, resulting in a bending moment M on the material, causing it to warp (arch). As the material approaches the side of the upper die plate, the material beneath the upper die plate end face is forced into the gap between the die plates. The material is subjected to the lateral pressure (F1 and F2) from the die plates, as well as the frictional forces (μF1 and μF2) from the end face and side of the die plates, resulting in lateral extrusion deformation. This series of force interactions ultimately leads to a decrease in the section quality of the material after shearing.
To compare the effects of bending different blanks prepared by the two methods, one piece each of laser-cut and sheared blanks were selected for bending.
After bending, the laser-cut blank exhibits smooth deformation at the R corner with no significant quality defects. In contrast, the sheared blank shows good quality on one side of the R corner but small cracks have already appeared on the other side. There is a significant difference in quality between the two methods after bending, thus requiring an analysis of the causes of cracking in sheared blanks after bending.
Due to the limitations of the shearing process itself, after shearing, not only is the section quality relatively poor, but the burr directions at both ends of each blank are inconsistent. After shearing, the burrs on the material left on the lower die plate have a downward direction, while the burrs on the corresponding blank at the upper die plate have an upward direction. The different burr directions at both ends of a blank lead to a relatively better quality of the burr on the inner R during the bending process, while there is a higher probability of small cracks occurring on the side with the burr on the outer R.
From the above analysis, it can be seen that the section quality of the blank has a significant impact on the quality of the R corner after bending. If shearing is used for blank preparation, efforts should be made to minimize the height of the burrs on the sheared blank. If necessary, one side of the burr should be polished before bending to reduce the occurrence of cracks after bending.
Minimum Bending Radius
It can be observed that the deformation zone during bending mainly occurs at the curved corners, while the straight sections can be considered as not undergoing plastic deformation. In the deformation zone, the inner metal fibers are compressed and shortened, while the outer metal fibers are stretched and elongated.
From this, it can be seen that the smaller the bending radius, the greater the extent of tensile deformation on the outer surface layer of the deformation zone. If it exceeds the maximum allowable deformation of the material, it is prone to cracking, resulting in scrap. Therefore, it is necessary to control the tensile deformation of the deformation zone, and the magnitude of the tensile deformation mainly depends on the relative bending radius r/t. By calculating the relative bending radius, the minimum bending radius of the material can be determined, which ensures that the outer layer does not crack after bending, i.e., the minimum inner R.
The factors that affect the minimum bending radius are mainly two: material mechanical properties and fiber direction.
(1) Material mechanical properties:
Materials with good plasticity, high yield strength ratio, and elongation have a greater allowance for deformation of the outer fibers, resulting in a smaller minimum bending radius. Conversely, materials with poor plasticity have a relatively larger minimum bending radius.
(2) Fiber direction:
Based on on-site production experience, for the same batch of sheet metal, the plasticity index along the fiber direction is higher than that perpendicular to the fiber direction. Therefore, if the bending line is perpendicular to the fiber direction, the allowance for deformation of the outer fibers is greater, resulting in a smaller minimum bending radius. Conversely, if the bending line is parallel to the fiber direction, the allowance for deformation of the outer fibers will relatively decrease, resulting in an increased minimum bending radius.
Therefore, when formulating bending processes, it is advisable to make the bending line as perpendicular to the fiber direction of the blank as possible. For workpieces with multi-directional bending, the bending line can be set at a certain angle to the fiber direction of the blank to increase the degree of deformation and avoid cracking of the outer fibers. The minimum bending radius for commonly used materials is shown in Table 1.
Conclusion
This article analyzes and explains the issues of section quality and minimum bending radius regarding the cracking of thick sheet metal during bending. Through the analysis, it can be concluded that for bending thick sheet metal, it is necessary not only to ensure section quality and reduce burrs on the outer R during bending but also to ensure that the inner R of the bend is larger than the minimum bending radius of the blank.
