Sheet metal processing is an indispensable part of mechanical manufacturing, particularly in industries such as aviation, home appliances, automotive, and elevators, where sheet metal parts are widely used. The bending process, as the only single-piece forming operation in sheet metal processing, is one of the most important procedures. The quality of the bending process directly affects the forming dimensions and appearance of the product. Therefore, how to ensure the forming dimensions and angles of the workpiece during the bending process is a key research topic in bending technology.
With economic development and the improvement of people's living standards, people's sensory pursuits are also climbing. In the metal decorations of some high-end venues, the more complex the shape of the bent sheet metal ornaments, the more they reflect the designer's level of design and trendy style, thereby attracting the attention of a broad customer base. At the same time, the workpiece must meet technological requirements such as having the smallest possible bending edge radius, no creases on the workpiece surface, and no indentations on the decorative surface. Traditional bending machines can no longer meet these special technological requirements, hence the process of planing and bending on metal sheets has emerged. This article mainly discusses the characteristics of the planing process and the methods of slotting, as well as how to ensure the dimensions and angles during the bending process.
Traditional Bending Methods and Their Limitations
The traditional bending process involves bending the metal sheet by the pressure of the upper and lower dies on the bending machine, using the two edges of the lower die opening and the edge of the upper die to bend the metal sheet through elastic deformation into plastic deformation. The bending angle is determined by the depth of the upper die entering the lower die, with the bending radius R being greater than or equal to the plate thickness t. In today's society, as demands for workpiece shapes increase, some complex-shaped workpieces can no longer be bent using traditional free bending, bending machine bending, or even three-point bending processes. Moreover, traditional bending methods cannot control the radius of the bending corners, making it difficult to achieve the requirements of traceless bending. Therefore, a new bending process—planing and bending—has emerged.
Features of the Planing and Bending Process
The planing and bending process is a procedure that first uses a planing machine to cut a V-shaped groove at the position on the metal sheet where bending is required, and then performs bending on a conventional bending machine according to needs. The features of the planing and bending process mainly include the following three aspects.
Small radius of the workpiece edges and no creases on the workpiece
As known from the bending process, the size of the radius of the arc at the edges of the workpiece after bending is directly proportional to the thickness of the plate; the thicker the plate, the larger the radius of the arc formed by bending. After V-grooving the metal sheet, the remaining thickness of the sheet becomes half or even less of the original thickness, which can significantly reduce the radius of the arc at the edges of the workpiece after bending. Moreover, since the remaining thickness of the sheet at the bend is thinner after grooving, the deformation force required during bending is also correspondingly reduced, which will not spread and affect the unbent area. Therefore, there are no creases on the surface of the workpiece after bending, and due to the thinner thickness at the bend, the reduced pressure needed for bending effectively avoids the risk of indentations on the decorative surface. This can meet the technological requirements of small radii on the edges of workpieces, no surface creases, and no indentations on the decorative surface in metal decorations in high-end places such as hotels, banks, commercial centers, and airports.
Reduced the tonnage required for bending the material
In the bending process, the bending force required for the metal sheet is proportional to its thickness. The greater the thickness of the metal sheet, the greater the required bending force, and consequently, the higher the tonnage of equipment needed. By V-grooving the metal sheet at the bending area before bending, the remaining thickness of the sheet at that area is significantly reduced. This reduction in thickness means that the force required to bend the sheet is also reduced, allowing thick plates to be bent on bending machines with lower tonnage. This approach not only reduces investment in equipment but also saves energy and space.
Bending of complex-shaped workpieces and control of springback
Parts that cannot be bent on a general bending machine can be bent by hand after a V-groove has been made at the bending location. Additionally, controlling the remaining thickness of the sheet can control the springback force and angle. If the remaining thickness after grooving is controlled at about 0.3mm, the springback angle can be made very small, to the point where it can be practically ignored.
V-groove slotting method
In sheet metal production, gantry planers and metal sheet grooving machines are commonly used to make V-grooves in metal sheets. The sheet to be grooved and bent is placed on the grooving machine for positioning, and then the thickness of the sheet is entered to automatically set the cutting tool for grooving. During grooving, the following two aspects need to be considered.
Setting of grooving depth and remaining thickness
With a given plate thickness, the grooving depth and the remaining thickness have a corresponding value. Based on the bending process requirements, a remaining thickness value is first set, usually by default at 0.8mm, with a minimum not less than 0.3mm. Then, according to the thickness of the sheet, the number of cutting passes and the depth of each cut are set. To control burrs and protect the cutting tool, the amount of each cut should not be too large, typically the depth of the first cut cannot exceed 0.8mm, and it should be divided into at least two cuts, not done in one go. For example, for a stainless steel sheet with a thickness of 1.2mm and a remaining thickness of 0.5mm after grooving, the first cut is set to a cutting depth of 0.5mm, and the second cut to a depth of 0.2mm. After two cuts, the remaining thickness reaches exactly 0.5mm, and the burrs are relatively small.
Setting of the grooving angle
As known from the bending process, metal sheets undergo various degrees of springback deformation when bent, resulting in a deviation in the bending angle. When making a V-groove, the grooving angle can be optimized based on the required bending angle of the workpiece. Generally, the angle of the V-groove should be 1° to 2° greater than the angle of the bend. For example, for a part that needs to be bent at 90°, the V-groove angle can be made 92° to effectively avoid the angle deviation caused by bending springback.
**Selection and Number of Slotting Cutters**
**Types and Selection of Slotting Cutters**
The types of slotting cutters mainly include diamond-point slotting cutters, square slotting cutters, triangular slotting cutters, and circular slotting cutters. When slotting, the appropriate tool can be selected according to the different shapes and angles of the V-groove. For general V-grooving, the tool angle should be less than the angle of the V-groove. For example, when the V-groove angle is between 45° and 60°, a diamond-point slotting cutter with a 35° angle should be chosen. For angles between 60° and 80°, a triangular slotting cutter should be selected. For angles between 80° and 90°, a diamond-point slotting cutter with an 80° angle should be used. For angles greater than 90°, a square slotting cutter is appropriate. For circular grooves, a circular slotting cutter should be used.
**Setting the Number of Tools**
When slotting deeper grooves on longer sheets, using a single tool for a continuous slotting path can easily cause tool overheating, wear, poorer slotting effects, and larger burrs. For instance, when slotting a 2m long stainless steel sheet to a depth of 2mm, if the first cut is set to a depth of 0.5mm and continues for 2m, the tool may overheat and soften, likely leading to a decline in slotting quality and increased burrs after 1.5m. If the cut depth is set to 0.2mm, completing a 2mm V-groove would require 10 cuts, significantly affecting processing efficiency. Therefore, when slotting longer sheets, in addition to setting the cut depth, it is also necessary to set the number of tools working simultaneously. In such cases, 3-4 tools are generally used at the same time, with each tool's cut depth differing by about 2mm. For example, if the first tool's cut depth is set to 5mm, then the second, third, and fourth tools' cut depths are set to 7mm, 9mm, and 11mm, respectively. This ensures both slotting quality and work efficiency.
**Quantity and Installation Method of Slotting Cutters**
How to Avoid Deviations in Bending Angles and Dimensions
For the bending process, the quality of the bend primarily depends on two critical parameters: the bending angle and dimensions. To ensure the accuracy of bending dimensions and angles, the following issues should be considered:
1. If the upper and lower dies are not centered, it can cause deviations in bending dimensions. Before bending, it is necessary to adjust the centering of the upper and lower die tools.
2. After moving the backstop block left or right, the relative position of the sheet metal to the lower die may change, affecting bending dimensions. Before bending, the position of the backstop block should be remeasured.
3. Insufficient parallelism between the workpiece and the lower die can cause springback, affecting the bending angle. Before bending, parallelism should be measured and adjusted.
4. Insufficient bending angle in the first bend will affect subsequent bends. Accumulated bending errors can lead to increased deviations in workpiece forming dimensions and angles. Therefore, ensuring the accuracy of single-sided bending is particularly important.
5. During bending, the size of the lower die's V-opening is inversely proportional to the bending pressure. When processing metal sheets of different thicknesses, it is necessary to select the appropriate lower die V-groove, generally choosing 6-8 times the thickness of the sheet is most appropriate.
6. When bending a workpiece on a bending machine after slotting, ensure that the upper die edge, the bottom edge of the workpiece's V-groove, and the bottom edge of the lower die's V-groove are in the same vertical plane.
7. To prevent the die from clamping when bending slotted workpieces, the upper die angle is best controlled at around 84°.
The slotting process, as a new processing technique in bending technology, is a result of market choice. With the continuous development of process technology, enterprises have increasingly higher demands on process personnel. As technicians, only by mastering various processing techniques can we manufacture superior products; only by continuously exploring and seeking new processes can we produce even better products. Market competition does not favor the weak; without innovation and breakthroughs, one can only be eliminated.
