1. Scope
This process is applicable to bending various angles of ferrous metals using press brakes in our company.
2. Equipment and Tools
2.1 Equipment:
CNC sheet metal press brakes, mechanical pressure bending machines
2.2 Tools:
Vernier caliper (0-150mm), steel tape measure (3.5m), universal angle ruler, and hex wrench.
3. Materials
3.1 Materials should meet the drawing requirements.
3.2 Materials should have good surface quality, with a smooth, even surface free of footprints, rust, and other defects. The thickness should comply with tolerance specifications.
4. General Bending Process
4.1 Preparation before bending
Check the length, width, and thickness of the material. The sheet metal should be cleaned, with burrs removed from the edges.
Choose the appropriate bending machine according to the bending plate specifications, types, and quality requirements.
Inspect the condition of the bending machine and perform necessary cleaning and maintenance.
Different bending radii require different top die radii (R), and different steel plate thicknesses require different bottom die depths.
Compare the workpiece with the drawing, verifying the unfolded shape, processing dimensions, material thickness, processing requirements, projection method, completion of previous operations, and the front and back sides of the workpiece.
Compare the actual cut length of the workpiece and determine the dimensions and methods of bending control based on the drawing dimensions.
Equipment adjustment: top and bottom die adjustment, template adjustment.
4.2 Selection of bending dies
Select the appropriate bending top die according to the workpiece’s bending radius size (R). If the top die does not meet the drawing’s R requirements, communicate with the technical staff and change the bending line.
The bottom die is usually determined based on the workpiece’s plate thickness, typically selecting the bending groove type with eight times the bending plate thickness.
Note: When bending at 90°, the minimum width of the “V” groove must not be less than 4t, otherwise, it may damage the die or cause the workpiece to be scrapped. If a smaller “V” groove (less than 4t) is required for small edge dimensions, first bend an appropriate obtuse angle, then use a larger “V” groove to reach 90°.
Fully consider possible abnormalities during the forming process, such as collisions between bent edges of the workpiece, etc.
4.3 First-piece bending
According to the drawing, follow the above process requirements, program the machining, and perform the first-piece inspection.
Inspect the piece strictly according to the drawing’s tolerance requirements. If the drawing has no requirements, inspect according to the following standard (see inspection).
After the first-piece inspection is passed, proceed with processing. Store blanks and workpieces on trays to avoid scratches.
For first-pieces that are not fully formed in one attempt, both operators and inspectors should calculate whether the remaining unfolded dimensions can meet the forming dimension tolerances.
4.4 Bending sequence
Template-based bending: First-piece bending → Inspection → Adjusting backgauge → Bending → Inspection (suitable for batch bending)
Line-based bending: Based on actual unfolded dimensions, controlled dimensions, bending order, and line drawing: First-piece trial bending along the line → Inspection → Adjustment → Line drawing → Bending along the line → Inspection;
Arrange the sequence according to “inner first, then outer, smaller before larger, complex before simple.” The most crucial factor is to consider the actual shape and adjust the sequence accordingly during operation.
4.5 Bending parameters
The minimum limit size of the bent edge depends on the depth of the die (h) and the setback dimension (c). For plate thicknesses of 1-3mm, the setback dimension c is 3mm, while for plate thicknesses above 3mm, the setback dimension c is equal to the plate thickness.
The accuracy of the bending dimensions depends on the precision of the calculated positioning dimensions and the appropriateness of the backgauge adjustment.
Generally, for bending “U” type workpieces, the upper surface of the backgauge should be about 1.5 times the plate thickness above the lower die, while for bending “Z” type workpieces, the bottom surface of the backgauge should be about 0.5 times the plate thickness below the lower die.
4.6 Process control
Surface quality control: After forming, workpieces should not have obvious indentations, die marks, or other defects. The surface should feel smooth to the touch.
For oxidized parts, electrolytic parts, or aluminum-zinc coated plates, a special protective film should be placed between the lower “V” groove and the workpiece. Workpieces should not be dragged or pulled to avoid scratches.
4.7 Bending defects and their elimination
In general, it is preferable to bend steel plates along the cross-grain direction. For components with high steel strength or those that can only be bent along the grain direction, the bending angle can be changed, and heat bending can be applied if necessary.
Common bending defects and their elimination methods are as follows:
| Item | Causes | Solutions |
| Cracks | Selection of too small top and bottom dies for steel plate bending along the grain | Alter top and bottom die cutting for vertical stripes |
| Inconsistent bending angles at both ends | Improperly adjusted top die and misaligned workpiece at the center of the machine | Adjust equipment or add shim iron |
| Inconsistent dimensions at both ends | Poorly adjusted back gauge and misaligned or unaligned line | Adjust the backstop, repair burrs, recheck bending line, and align correctly |
| Curving after bending | Caused by thermal stress from gas cutting | Cold calibration or hot calibration |
| Thick plate bending | Insufficient equipment pressure | Heat bending or adopting removal processing methods to suit equipment |
| Bending marks | Incorrect bottom die without proper shim placement | Change to appropriate lower die and add thin steel plate padding to reduce pressure marks |
5. Inspection
If the drawing has tolerance requirements, inspect strictly according to the drawing. If there are no requirements on the drawing, inspect according to the following standard. For unmarked bend angle tolerance size requirements:
| Bending short-side size | Angular tolerance | Bending short-side size | Angular tolerance |
| 6-10 | ±1° | 50-80 | ±20′ |
| 10-18 | ±50′ | 80-120 | ±20′ |
| 18-30 | ±40’ | 120-400 | ±15′ |
| 30-50 | ±30′ |
Unmarked bend angle length size requirements:
| Size | 1-6 | 6-30 | 30-120 | 120-400 | 400-800 |
| Tolerance | ±0.2 | ±0.4 | ±0.5 | ±0.8 | ±1 |
6. Precautions
- When changing the mold, the machine must be turned off. When leaving the equipment, the machine must be turned off. Two people working together must coordinate. During the bending process, operators should be quick, focused, and cooperate with the machine. People not involved in the operation should not stand near the machine.
- When loading materials, hold the workpiece with the correct method: the thumb on the top of the material and the other four fingers on the bottom. For smaller workpieces, use pliers to hold the workpiece instead of your hands.
- When bending longer workpieces, do not extend your head or chest over the workpiece to avoid injury from the workpiece springing up.
- When bending thicker materials (above 2.5mm) or wider and longer workpieces, the bending speed should not be too fast. Allow the upper and lower molds to slowly meet to avoid damaging the mold and affecting personal safety.
- For bending thinner and smaller workpieces, the speed should also be slow to avoid inconsistencies in human-machine cooperation, leading to workpiece deformation. When pressing the edge, the minimum angle should not be less than 40°.
- When processing samples, waste materials must be used for trial bending to meet the drawing requirements before forming. Any process issues affecting the current processing should be recorded and reported to the relevant project engineer.
- When the machine is in operation, it is strictly forbidden to extend your hand from the front of the machine under the mold to the back of the machine to adjust the rear positioning rule and repair the machine.
- Keep the working area clean and workshop passages unobstructed. Do not place any objects near firefighting equipment or within fire access routes.
