Bending Methods
There are various ways in which CNC press brakes can bend sheet metal, and they can be classified based on different principles. This article will specifically introduce several common bending methods, focusing on the different relative positions of the upper and lower dies during the bending process and the resulting shapes. It will also provide detailed information about the bending process and considerations for each method.
Gap Bending and Bottoming Bending
Gap Bending:
In gap bending, the upper and lower dies of the CNC press brake are not tightly pressed together. The desired bending angle is achieved by adjusting the depth of the upper die entering the opening of the lower die. The deeper the upper die enters, the smaller the bending angle, and vice versa. Due to the elasticity of the material, excess bending is required to control the springback.
Gap Bending offers the advantages of using fewer molds and achieving various angles in the forming process. It also requires less processing pressure. To obtain the best bending results, the ratio of the material thickness (B) to the opening width (V) of the lower die can be selected as follows:
1. For material thickness below 12.7mm, B:V is 1:8.
2. For material thickness between 12.7mm and 22.2mm, B:V is 1:10.
3. For material thickness above 22.2mm, B:V is 1:12.
These ratios are standard mold ratios for low carbon steel with a material strength of 43.4kg/mm2. When creating bending programs, these parameters can be set in the CNC system, and the system will automatically generate the processing program.
Bottoming Bending:
In bottoming bending, the sheet metal is tightly pressed between the upper and lower dies of the CNC press brake to achieve the desired bending angle and bend radius. Bottoming bending is suitable for batch and large-scale production in enterprises, particularly for metal sheets with a thickness below 2mm. It offers small bending radii and high precision. It should be noted that the working pressure for bottoming bending is greater than that for gap bending, generally more than three times higher.
Bottoming Bending
The angle of the bottoming bending die should be suitable for the angle and material of the sheet metal. Typically, when bottoming bending low carbon steel, the angles of the upper and lower dies should match the required angle of the sheet metal. When using the bottoming bending method, the die ratio, which is the ratio of the material thickness (B) to the opening distance of the lower die (V), is B:V = 1:6.
Determining the working tonnage during the bending process involves applying force between the upper and lower dies onto the material, causing plastic deformation. The working tonnage refers to the bending pressure during the bending process. Factors influencing the determination of the working tonnage include the bending radius, bending method, die ratio, bend length, material thickness, and strength. The working tonnage can be selected based on the following table and set in the processing parameters.
1. The values in the table represent the bending pressure when the sheet metal length is one meter. For example, S=4mm, L=1000mm, V=32mm. According to the table, P=330kN.
2. The table is based on materials with a strength of σb=450N/mm2. When bending different materials, the bending pressure is obtained by multiplying the data in the table by the following coefficients: bronze (soft): 0.5; stainless steel: 1.5; aluminum (soft): 0.5; chrome-molybdenum steel: 2.0.
3. An approximate calculation formula for the bending pressure of the CNC press brake is P=650s2L/1000v, where the parameters have the following units: P—kN, S—mm, L—mm, V—mm.
Based on the different shapes formed after bending, the bending process can be divided into various forms such as L-bending, N-bending, Z-bending, reverse flattening, and hardware pressing. The characteristics and differences are as follows:
L-Bending:
It can be divided into 90° bending and non-90° bending. It is further classified into general bending (L>V/2) and special bending (L<V/2) based on the bending process.
L-Bending 90°
L-Bending less than 90°
L-Bending greater than 90°
1. The selection of the die depends on the material, plate thickness, and forming angle.
2. Positioning principles:
- The principle is to use two back gauges as the reference and position them according to the shape of the workpiece.
- When using a single back gauge, pay attention to any deviation and ensure that it aligns with the bending dimensions of the workpiece on the same centerline.
- For small bends, it is preferable to use a reverse back gauge.
- It is best to position the back gauge slightly below the centerline.
- The edge of the positioning should be closer to the back gauge.
- It is preferable to position along the longer edge.
- Auxiliary positioning with fixtures can be used for irregular oblique positioning.
3. Considerations:
Pay attention to the positioning method during processing and the movement of the back gauge in various positioning methods. When the die is installed correctly for bending, the back gauge should be pulled back to prevent deformation of the workpiece during bending. When bending large workpieces internally, the size of the workpiece is larger compared to the bending area, making it difficult to align the tool and the bending area, resulting in difficulties in workpiece positioning or damage during bending. To avoid these situations, a positioning point can be added in the longitudinal direction of the processing. This allows for dual-directional positioning during processing, making the positioning more convenient and improving processing safety, thus avoiding workpiece damage and enhancing production efficiency.
N-Bending:
Different processing methods should be used for N-bending based on different shapes. During bending, the inner dimension of the material should be larger than 4mm, and the size of the X dimension is limited by the shape of the die. If the inner dimension of the material is smaller than 4mm, special methods should be used for processing.
N-Bending
N-Bending
1. Select the die based on the material thickness, dimensions, material, and bending angle.
2. Positioning principles: Ensure that the workpiece does not interfere with the tool.
- Ensure that the positioning angle is slightly smaller than 90 degrees.
- It is preferable to use two back gauges for positioning, except in special cases.
3. Considerations:
- After bending an L-shape, ensure that the angle is 90 degrees or slightly smaller to facilitate positioning during processing.
- For the second bending operation, the positioning position should be centered on the processing surface.
Z-Bending:
Z-bending, also known as step bending, involves bending in opposite directions. It can be classified into oblique step bending and straight step bending based on the angle. The minimum size for bending is limited by the processing die, while the maximum size is determined by the machine's external shape. Generally, when the inner dimension of the Z-bending material is less than 3.5T, step bending die is used for processing. When it is greater than 3.5T, the normal processing method is adopted.
Straight Step Z-Bending
Oblique Step Z-Bending
1. Positioning principles:
- Convenient and stable positioning.
- Generally, the positioning is the same as L-bending.
- The secondary positioning requires the processed workpiece to be flush with the lower die.
2. Considerations:
- The bending angle for L-bending must be accurate, usually ranging from 89.5 to 90 degrees.
- When pulling back the back gauge, pay attention to the deformation of the workpiece.
- The processing sequence must be correct.
- For special processing, the following methods can be used: centerline separation method (eccentric processing), small V-bending (requires increasing the bending coefficient), easy die forming, and grinding of the lower die.
Reverse Flattening
Reverse flattening, also known as edge crushing, involves the following processing steps: first, bending to an angle of approximately 35 degrees, and then using a flattening die to flatten and make it flush.
Reverse Flattening
Reverse Flattening
1. Die selection: Select the depth of the lower die's V-groove to be 5-6 times the material thickness at an angle of 30 degrees. Select the upper die based on the specific situation of the edge crushing.
2. Considerations: Pay attention to the parallelism of both sides of the edge. When the length of the edge crushing dimension is long, the flattening edge can be bent slightly before flattening. For shorter edge crushings, padding can be used during processing.
Hardware Pressing
Using a press brake to join hardware components, usually with the assistance of concave dies and fixtures. Common applications include pressing nuts, pressing bolts, pressing screws, and other hardware components.
Pressing Nuts
Pressing Bolts
1. Considerations:
- When avoiding interference during processing due to the workpiece's shape, appropriate avoidance measures should be taken.
- After processing, check whether the torque and thrust meet the standards and ensure that the hardware components are flush and tight with the workpiece.
- When bending and pressing, pay attention to the avoidance measures during processing and the parallelism of the dies.
- If it involves expansion pressing, ensure that there are no cracks on the expanded edge and that it does not protrude above the surface of the workpiece.
