Choosing the wrong bending machine can lead to increased production costs and the inability to recoup the investment. Therefore, several factors need to be considered when making the decision.
Workpiece
The first important consideration is the parts you will be producing. The key is to purchase a machine that can complete the machining tasks with the shortest workbench and the smallest tonnage.
Carefully consider the material grade, maximum processing thickness, and length. If the majority of the work involves low-carbon steel with a thickness of 16 gauge and a maximum length of 10 feet (3.048 meters), a bending force of no more than 50 tons is necessary. However, if a significant amount of bottom-die forming is involved, a machine with a capacity of 160 tons should be considered.
Let's assume that the thickest material is 1/4 inch, and a 10-foot free bend requires 200 tons, while bottom-die bending (correction bending) requires at least 600 tons. If most of the workpieces are 5 feet or shorter, the tonnage can be halved, significantly reducing the acquisition cost. The length of the parts is crucial in determining the specifications of the new machine.
Deflection
Under the same load, the deflection of a 10-foot machine's workbench and slide is four times that of a 5-foot machine. This means that a shorter machine requires fewer shim adjustments to produce qualified parts. Reducing shim adjustments also shortens setup time.
Material grade is also a key factor. Compared to low-carbon steel, stainless steel typically requires an increase in load of around 50%, while most grades of soft aluminum require a decrease of around 50%. You can always obtain a tonnage chart from the bending machine manufacturer, which shows the estimated tonnage required per foot length for different thicknesses and materials.
Bend Radius of the Parts
When using free bending, the bend radius is 0.156 times the die opening distance. In the free bending process, the die opening distance should be eight times the thickness of the metal material. For example, when using a 1/2-inch (0.0127 meters) opening distance to form 16 gauge low-carbon steel, the bend radius of the part is approximately 0.078 inches. If the bend radius is almost as small as the material thickness, bottom-die forming is necessary. However, the pressure required for bottom-die forming is about four times that of free bending.
If the bend radius is smaller than the material thickness, a punch with a front-end radius smaller than the material thickness and the assistance of the stamping bending method are required. In this case, the pressure needed is 10 times that of free bending.
For free bending, the punch and die should be machined at 85° or less (slightly smaller is better). When using this set of tooling, attention should be paid to the clearance between the punch and die at the bottom of the stroke and enough overbend to compensate for springback and maintain a bend angle of around 90°.
Typically, the springback angle produced by the free bending die on a new bending machine is ≤2°, and the bend radius is 0.156 times the die opening distance.
For bottom-die bending, the die angle is generally between 86 and 90 degrees. At the bottom of the stroke, there should be a gap slightly larger than the material thickness between the punch and die. The forming angle is improved because bottom-die bending requires higher tonnage (about four times that of free bending), reducing the stress within the range of the bend radius that typically causes springback.
Stamping bending is similar to bottom-die bending, except that the front end of the punch is machined to the desired bend radius, and the clearance between the punch and die at the bottom of the stroke is smaller than the material thickness. By applying sufficient pressure (approximately 10 times that of free bending) to force the front end of the punch into contact with the material, springback is essentially eliminated.
To select the lowest tonnage specification, it is best to plan for a bend radius larger than the material thickness and use the free bending method as much as possible. Larger bend radii often do not affect the quality of the finished part and its future use.
Accuracy
Bend accuracy requirements are a factor that needs to be carefully considered and determines whether you need to consider a CNC bending machine or a manual bending machine. If the bend accuracy requirement is ±1 degree and cannot be varied, you must focus on a CNC machine.
The repeat accuracy of the CNC bending machine slide is ±0.0004 inches, and forming precise angles requires this level of accuracy and good tooling. The repeat accuracy of the manual bending machine slide is ±0.002 inches, and under the conditions of using appropriate tooling, it generally produces a deviation of ±2 to 3 degrees. In addition, CNC bending machines are prepared for rapid tooling changes, which is an undeniable consideration when you need to bend many small-batch parts.
Tooling
Even if you have a full rack of tooling, don't assume that these tools are suitable for your newly purchased machine. You must inspect the wear of each tool by measuring the length from the front end of the punch to the die shoulder and the length between the die shoulders.
For standard tooling, the deviation per foot should be around ±0.001 inches, and the total length deviation should not exceed ±0.005 inches. As for precision-ground tooling, the precision per foot should be ±0.0004 inches, and the total precision should not exceed ±0.002 inches. It is best to use precision-ground tooling for CNC bending machines and standard tooling for manual bending machines.
Length of Bent Parts
Suppose you bend a 90-degree angle along a 5x10 feet 10-gauge low-carbon steel sheet. The bending machine would need to apply an additional 7.5 tons of pressure to lift the steel sheet, and the operator must be prepared for the 280-pound straight edge to fall. Manufacturing such a part may require several strong workers or even a crane. Bending machine operators often need to bend long-edge parts without realizing how physically demanding their work can be.
Now there is a material handling device suitable for workshops engaged in this type of work, and this device can be improved according to the needs of both new and old machines. With the use of this device, bending long-edge parts only requires one person to operate.
Bending Machine Safety Operating Procedures
1. Strictly adhere to the machine tool safety operating procedures and wear appropriate personal protective equipment as required.
2. Before starting, carefully check if the motor, switches, circuits, and grounding are normal and secure. Check that all control components and buttons are in the correct positions.
3. Check the alignment and stability of the upper and lower dies. Check if all positioning devices meet the processing requirements.
4. When the upper slide and positioning axes are not in the home position, run the home position program.
5. After the equipment is started, let it run empty for 1-2 minutes. Move the upper slide through its full stroke 2-3 times. If any abnormal noise or malfunctions are detected, immediately stop the machine, eliminate the problem, and resume operation only when everything is normal.
6. During operation, one person should be in charge to ensure close coordination between the operator and the material feeding and pressing personnel. This ensures that all personnel are in safe positions before issuing bending signals.
7. When bending the sheet metal, it must be pressed down firmly to prevent it from lifting and causing injury during the bending process.
8. When adjusting the pressure die for the sheet metal, the power must be cut off and the machine must be stopped.
9. When changing the opening of the adjustable lower die, no material is allowed to come into contact with the lower die.
10. No one is allowed to stand at the rear of the machine while it is operating.
11. It is strictly prohibited to bend the sheet metal by pressing it from one end alone.
12. If any irregularities are noticed during operation, the machine must be stopped for correction. It is strictly forbidden to manually correct the irregularities while the machine is running to avoid hand injuries.
13. It is prohibited to bend iron plates that are excessively thick, quenched and tempered steel plates, high-grade alloy steel, square steel, or materials that exceed the performance capabilities of the bending machine, in order to prevent damage to the machine.
14. Regularly check the alignment of the upper and lower dies and the indications on the pressure gauge to ensure compliance with specifications.
15. In case of any abnormalities, immediately stop the machine, investigate the cause, and promptly rectify the issue.
16. Before shutting down the machine, place wooden blocks on the lower die below the oil cylinders on both sides to lower the upper slide onto the wooden blocks.
17. First exit the control system program and then cut off the power supply.
1. The machine adopts a fully welded steel structure, providing sufficient strength and rigidity.
2. Hydraulic transmission, with oil cylinders placed on both ends of the machine, directly driving the sliding operation.
3. The synchronization mechanism of the slide adopts a torsion shaft for forced synchronization.
4. It uses a mechanical block structure, which is stable and reliable.
5. The stroke of the slide can be adjusted quickly and manually, with a counter for display.
6. It features a wedge-type deflection compensation mechanism to ensure high bending accuracy.
