Bending machine is a type of machine used for bending metal sheets, and it is widely used in industrial production. With the gradual development and growth of modern industry, the demand for bending machines has also increased. However, along with the increasing demand, the industry has seen the emergence of bending machines with varying quality. The quality of these machines determines the smoothness or setbacks in the future production process.
So, how to correctly select a high-quality bending machine? It is best to start by carefully considering the machine's ultimate purpose, potential deflection variables, and the bending radius of the parts. When choosing a bending machine, as the decision-maker, you also need to have a detailed understanding of the machine's performance, processing range, processing capabilities, processing accuracy, and more. The selection process is not trivial because if chosen improperly, your production costs will increase, and the bending machine may not recoup its expected costs. Therefore, Haas CNC has compiled the following content to help you choose the bending machine that suits your needs:
Hydraulic Bending Machine
Workpiece - Specifications and Models
The first important consideration is the parts you want to produce. Make a full estimation of the standard dimensions, shapes, and structures of the parts you intend to produce. The key is to purchase a machine that can complete the processing tasks with the shortest workbench and the smallest tonnage, achieving practicality and cost-efficiency.
Carefully consider the metal sheet material, maximum processing thickness, and length. If the main material for production is low carbon steel with a thickness within 3mm and a maximum length not exceeding 2500mm, then a bending force of more than 80 tons is not necessary for free bending. However, if there is a significant amount of bottom bending during the operation, you may consider purchasing a bending machine of around 150 tons.
If the sheet material to be bent is low carbon steel, with a maximum thickness of 6mm and a length within 2500mm for free bending, then it is necessary to consider purchasing a bending machine of more than 100 tons. If bending with correction is required, a bending machine with a larger tonnage is needed.
If most of the bent workpieces are 1250mm or shorter, consider reducing the tonnage of the bending machine by about half, which can greatly reduce the purchasing cost. Therefore, the length of the processed parts is crucial in determining the specifications and models of the machine.
Deflection - Compensation Function
Bending machines, especially when bending long-sized workpieces, may experience deflection. The longer the workpiece, the greater the deflection. Under the same load, the deflection of a 2500mm machine's workbench and slide is four times that of a 1250mm machine.
Deflection - Compensation Function
This means that shorter machines require fewer shim adjustments to produce qualified parts, reducing preparation time. However, most CNC hydraulic bending machines currently have hydraulic deflection compensation functions in their production design. This reduces the need for operators to adjust the equipment and improves bending accuracy and production efficiency. The hydraulic deflection compensation function is controlled by the CNC system, and hydraulic oil enters the compensation cylinder through a solenoid valve to push the workbench upward. At the same time, the deflection compensation force increases with the increasing bending force, thus achieving the purpose of deflection compensation.
Electric Compensation and Manual Compensation
Material - Bending Force
The material of the processed sheet is also a key factor. Compared to low carbon steel, stainless steel typically requires an increase of about 50% in the required load, while most soft aluminum materials require a reduction of about 50%. The bending force parameters according to relevant standards can be obtained from the bending machine manufacturer. The table shows the required bending force per 1000mm length for different thicknesses and materials.
Material - Bending Force
Bending Radius - Tonnage Specification
The bending radius of the workpiece is also a factor to consider during the bending process. When using free bending, the bending radius is approximately 0.156 times the size of the V-groove opening.
In free bending, the size of the V-groove opening should be 8 times the thickness of the metal sheet. For example, when bending 1.5mm low carbon steel with a V-groove opening size of 12mm, the bending radius of the part is approximately R=1.9mm. If the bending radius is close to or smaller than the thickness of the sheet, bottom bending should be performed. However, the pressure required for bottom bending is about 4 times higher than that for free bending. During free bending, pay attention to the gap between the upper and lower dies at the bottom of the stroke, as well as the use of compensation and springback to maintain an approximate 90° bend of the sheet. Typically, the springback angle generated by the free bending die on a new bending machine is ≤2°, and the bending radius is 0.156 times the opening of the lower die. Therefore, the die angle for free bending with upper and lower dies is generally 86° to 90°. At the bottom of the stroke, there should be a gap slightly larger than the thickness of the sheet between the upper and lower dies.
Bending Radius - Tonnage Specification
The forming angle can be improved because the tonnage for bottom bending is larger (about 4 times that of free bending), reducing the stress that typically causes springback within the range of bending radii. Press bending is the same as bottom bending, except that the front end of the upper die is machined to the desired bending radius, and the gap between the upper and lower dies at the bottom of the stroke is smaller than the thickness of the sheet. By applying sufficient pressure (approximately 10 times that of free bending) to force the front end of the upper die to contact the sheet, springback is essentially avoided. To select the lowest tonnage specification, it is best to plan for a bending radius larger than the thickness of the sheet and use free bending as much as possible. In many cases, a larger bending radius does not affect the quality of the finished part or its future use.
Bending Radius - Tonnage Specification
Accuracy - Intelligent CNC
Bending accuracy is a factor that needs careful consideration, as it determines whether you should purchase a CNC bending machine or a conventional bending machine. The selection of a bending machine involves the precision used for the mechanical system's rigid indicators because, from a scientific perspective, errors are inevitable and can only be minimized as much as possible. Some workpieces and simple parts can tolerate errors within a small range, so there is no need to pursue high-precision bending machines in such cases. However, for workpieces that require high precision, a bending machine with high precision is necessary, and it is recommended to purchase a CNC bending machine.
Twist-axis Bending Machine vs. Hydraulic Bending Machine
If the bending accuracy requirement is ±0.5° and cannot be easily changed, you must focus on CNC bending machines. The repeat precision of the CNC bending machine's slide can generally be guaranteed within ±0.01mm, and precise forming angles require such precision and high-quality molds. Conventional bending machines have a repeat precision of ±0.5mm for the slide and can still produce deviations of ±2~3° even with suitable molds. In addition, CNC bending machines are equipped with quick clamping and fast tool-changing systems, making them an unquestionable choice when you need to bend many small parts.
Accuracy - Intelligent CNC
Mold - Wear Condition
The bending mold also directly affects the bending accuracy, so it is necessary to regularly check the wear condition of the molds by measuring the length from the front end of the upper die to the table shoulder and the length between the lower die and the table shoulder.
For standard molds, the deviation should be around ±0.01mm for every 10mm, and the total length deviation should not exceed ±0.15mm. As for precision-ground molds, the precision should be ±0.005mm per 100mm, and the total precision should not exceed ±0.05mm. It is best to use precision-ground molds for CNC bending machines and standard molds for conventional bending machines.
Mold - Wear Condition
Conclusion
As a user or manufacturer purchasing a bending machine, the most important thing is to acquire an economical and practical bending machine, considering many factors and based on your actual situation. The points mentioned above only briefly outline some aspects of the issue. The best approach is to choose a professional bending machine manufacturer and let them provide you with several alternative plans based on your specific needs, making the decision-making process more reasonable and effective.
