CNC press brakes are widely used in sheet metal processing to create precise bends. Bending is the process of applying pressure to sheet metal to produce a permanent deformation. CNC press brakes offer various bending methods to meet different processing needs.
Bending Methods
1. Air Bending
Air bending involves using the edges of the V-die opening and the tip of the punch to bend the sheet metal. The bending angle is determined by controlling the depth to which the punch enters the V-die. The advantages of air bending include lower bending force, even stress distribution on the die, and extended die life. However, variations in sheet metal thickness, mechanical properties, and rolling direction can affect the bending angle.
2. Bottom Bending (Coining)
Bottom bending, or coining, involves pressing the punch into the material until it is pressed against the die, creating a more precise bend with less springback. The blank angle is smaller than the die angle at the start of the stroke, generating negative springback. At the end of the stroke, the punch and die calibrate the material, ensuring the bend radius and angle match the punch exactly. Bottom bending effectively overcomes springback, providing high bending accuracy.
3. Three-Point Bending
Three-point bending uses a CNC press brake that can complete the bending process in one go without multiple clamping and repositioning. The hydraulic cushion evenly distributes pressure along the entire bending length, ensuring uniform bending force from the punch. Each punch is elastically supported, adapting to the straightness of the V-die, ensuring constant pressure distribution, and improving bending angle accuracy and the straightness of the bent edge. The bending angle error for three-point bending is ±15 degrees, which is comparable to bottom bending accuracy. When the sheet metal thickness exceeds 3mm, three-point bending becomes the method to achieve the desired bending precision.
R2 Punch and R4 Inner Arc Bending
Regarding whether an R2 punch can bend an inner radius of R4, the answer is no. An R2 punch can only create an R2 inner radius. To achieve an R4 inner radius, an R4 punch must be used.
Alternative Methods
If an R4 inner radius is required but only an R2 punch is available, the following alternative method can be used:
- Use the R2 punch to bend a 1mm thick iron plate, then attach it to the punch to serve as a new punch, capable of creating an R4 inner radius.
Understanding Air Bending
Air bending is the most versatile and widely used method. It allows for varying the bend angle by adjusting the punch's penetration depth into the die.
Advantages:
- Flexibility: Can achieve a wide range of angles.
- Reduced Tooling: Only one set of tools needed for multiple angles.
- Less Force Required: Lower tonnage required compared to bottom bending.
Disadvantages:
- Springback: Requires compensation for material springback.
- Accuracy: Slightly less accurate than bottom bending.
Example Calculation for Air Bending Force:
Bending Force (tons) = (Length (mm) x Thickness^2 (mm) x Material Factor) / (Die Opening (mm) x 1000)
For example, bending a 1000 mm length of 2 mm thick mild steel with a die opening of 20 mm:
Bending Force = (1000 x 2^2 x 1.5) / (20 x 1000)
Bending Force = 6 tons
In-Depth Look at Bottom Bending
Bottom bending provides higher precision bends by fully pressing the sheet metal into the die, forming the exact shape of the punch and die.
Advantages:
- High Precision: Less springback and more accurate bends.
- Consistency: Uniform bends with tight tolerances.
- Strength: Increased strength at the bend due to material compression.
Disadvantages:
- Tooling: Requires precise tools for each angle.
- Force: Higher tonnage required compared to air bending.
Example Calculation for Bottom Bending Force:
Bending Force (tons) = (Length (mm) x Thickness^2 (mm) x Material Factor) / (Die Width (mm) x 1000)
For bending a 1000 mm length of 2 mm thick mild steel with a die width of 20 mm:
Bending Force = (1000 x 2^2 x 1.5) / (20 x 1000)
Bending Force = 6 tons
Advantages of Three-Point Bending
Three-point bending offers precise control over the bend angle by utilizing a three-point support system, typically involving hydraulic actuation.
Advantages:
- Precision: High accuracy due to controlled force distribution.
- Uniformity: Consistent bends across the entire length.
- Reduced Springback: Minimizes material springback compared to other methods.
Disadvantages:
- Complexity: Requires sophisticated CNC control.
- Cost: Higher initial setup costs due to advanced machinery.
Example Calculation for Three-Point Bending Force:
Bending Force (tons) = (Length (mm) x Thickness^2 (mm) x Material Factor) / (Support Span (mm) x 1000)
For bending a 1000 mm length of 3 mm thick mild steel with a support span of 30 mm:
Bending Force = (1000 x 3^2 x 1.5) / (30 x 1000)
Bending Force = 13.5 tons
Conclusion
CNC press brakes provide various bending methods, including air bending, bottom bending, and three-point bending, to meet different bending needs. Understanding the principles and advantages of these methods helps in selecting the appropriate technique for specific processing applications. For specific arc bending requirements, choosing the correct punch is crucial to ensure accuracy and precision.
