The Function of Process Holes in Sheet Metal
Process holes in sheet metal are used to prevent material stretching during the bending process. When the bending line of the sheet metal intersects at a point or comes close to the edge of the sheet metal and is less than twice the thickness of the sheet, process holes are added to avoid material stretching during bending. Process holes are generally used in sheet metal parts that require edge bending, while they are not necessary for parts without edge bending.
Disadvantages of Not Having Process Holes: Without process holes, the corners would have less material thickness, requiring filler material during welding and being prone to welding deformation.
Advantages of Having Process Holes: In the case of edge bending, process holes allow the edge to bend smoothly without material stretching.
Techniques for Creating Process Holes in Sheet Metal Bending
How to Determine the Size of Process Holes?
The size of the process holes depends on their location. If the process holes are located at the intersection of two lines, they should be twice the thickness of the sheet. The minimum size should not be less than 1.5 times the thickness of the sheet. When bending thick sheets, considering the bending radius, the process holes should be appropriately enlarged.
Limitations of this Type of Process Holes: With this method, after bending, especially with thick sheets, there is significant material loss at the bending angle, which is not aesthetically pleasing and affects welding. Therefore, an improvement in the fabrication method of process holes is needed.
Optimization of Process Hole Design
The design of process holes can be determined based on the edge bending situation, taking into account the sheet thickness and the avoidance of bending radii. The example shown in the figure below illustrates this.
The figure above shows a square sheet metal box with a material thickness of 3 mm and a height of 15 mm for four-sided bending.
Improvement in Process Hole Design
Improvement in Cutting Method: There are two methods for cutting sheet metal, CNC punching and laser cutting. For CNC punching, circular holes are generally used for process holes, while square holes or elongated holes are limited by the tooling and are not easy to achieve. In the case of 3 mm sheet metal, laser cutting is used.
Improvement in Process Hole Shape: If elongated process holes are used, it can avoid aesthetic issues after bending.
How to Determine the Size of Elongated Process Holes: The width is generally set to 1 mm, which does not affect aesthetics or the release of sheet metal bending deformation. The depth is calculated as follows: 10 mm = bending height - 3 mm (bending coefficient 5); 4 mm = sheet metal thickness + 1.
Effect of Process Hole Improvement
After bending, the sheet metal part will have only a narrow 1 mm gap. With this type of process hole, the bending angle is well handled, and welding can be done without the need for filler material.
Why Avoid Material Stretching During Bending
Drawbacks of Material Stretching During Bending: 1. It affects the bending dimensions. During material stretching, a significant force is required to tear open the sheet metal thickness, and the direction of force is uncertain, causing workpiece movement and resulting in dimensional displacement.
2. It damages the bending dies. As mentioned earlier, there is a significant force at the sharp corners, which can exceed the die's load-bearing capacity, leading to corner collapse and damage.
Standard for Using Positioning Process Holes in Bending
1. Size and Form of Process Holes:
- For thin sheet bending positioning process holes, the opening size is 1 mm x 0.2 mm with a root radius of 0.1 mm, suitable for sheet thickness ≤3 mm.
- For thick sheet bending positioning process holes, the opening size is 1 mm x 0.4 mm with a root radius of 0.2 mm, suitable for sheet thickness from 3 mm to 6 mm.
Usage Scenarios:
1. Angle Type: For non-90-degree bending angles, all bending positions should have positioning holes, including edge folding.
2. Butt Joint Type: For appearance parts or high-precision requirements, all bending positions should have positioning holes.
3. Large Size Type: For bending dimensions larger than 200 mm, all bending positions should have positioning holes.
4. Complex Multiple Bends Type: For complex parts with multiple continuous bends, positioning holes should be added starting from the third bend.
5. Non-Standard Bending Sequence Type: For parts that cannot be processed in the normal bending sequence, all bending positions should have positioning holes.
6. Repeated Overbending Type: For parts that require repeated overbending due to equipment limitations, all bending positions should have positioning holes.
7. Non-Contact with Backgauge Type: For parts that cannot be in contact with the backgauge, all bending positions should have positioning holes.
8. Guide Rail Type: For parts with guide rails, all bending positions should have positioning holes.
9. Arc Positioning Holes: Positioning holes should be opened at the starting points of the arcs.
10. Pull Plate Type: For pull plate parts, all bending positions should have positioning holes.
11. Edge Folding Type: For edge folding parts, positioning holes should be opened at the folding positions and at the dimensions requiring further bending relying on the edge folding.
