Things to Know When Selecting Hydraulic Bending Machines and CNC Bending Machines

　　Several factors to consider when choosing a bending machine

　　Hydraulic bending machines and CNC bending machines differ in their application fields and processing characteristics. Therefore, as a manufacturer, you should give careful consideration when purchasing a bending machine. It’s best to start by thoroughly evaluating the machine’s intended end use, the known deflection variations that may occur with the machine, and the bending radius of the parts you’ll be working with. As a decision-maker, it’s your responsibility to gain a detailed understanding of the equipment’s performance, processing range, processing capabilities, machining accuracy, and other relevant factors. This responsibility is not to be taken lightly—once you make the wrong choice, your production costs will soar, and you may fail to recoup the investment in the bending machine as expected. Thus, there are several key factors that you must carefully weigh when making your decision.

　　Workpiece

　　The first important consideration is the parts you plan to manufacture. The key point here is to purchase a machine that can handle your machining tasks while featuring the shortest possible worktable and the lowest tonnage requirement. Carefully take into account the material grade as well as the maximum thickness and length you’ll be processing. For instance, if most of your products are made from low-carbon steel with a thickness of no more than 3 mm and a maximum length of 2,500 mm, the free bending force required need not exceed 80 tons. However, if you’re involved in large volumes of bottom-die forming operations, you might want to consider a press brake with a tonnage rating around 150 tons. If the thickest material you’ll be bending during production is 6 mm thick and made from low-carbon steel, with a maximum length of 2,500 mm when using free bending, then you’ll need to factor in a press brake with a tonnage capacity of at least 100 tons. And if you’re bending materials using a bottom-die forming process (corrective bending), you’ll require an even higher-tonnage press brake. Furthermore, if most of the parts you bend are 1,250 mm or shorter, you can roughly halve the required press brake tonnage, significantly reducing your initial purchase costs. Therefore, the length of the parts you intend to process plays a crucial role in determining the specifications and model of the new machine.

　　Scratch change

　　In bending operations, especially when bending workpieces with long dimensions, deflection is inevitable—and the longer the workpiece, the greater the degree of deflection. Under the same load conditions, the deflection occurring on the worktable and slider of a 2,500-mm machine is four times that of a 1,250-mm machine. This means that shorter machines require fewer shim adjustments to produce qualified parts, and fewer shim adjustments also shorten the setup time. However, today’s CNC hydraulic bending machines are designed with an added hydraulic deflection compensation function, which reduces the need for operators to manually adjust the equipment, while simultaneously improving bending accuracy and production efficiency. The hydraulic deflection compensation function is controlled by the CNC system; hydraulic oil flows through a magnetic servo valve into a compensation cylinder, pushing the worktable upward. At the same time, the compensation force increases proportionally with the bending force, thereby effectively compensating for deflection. The material of the workpiece being processed is also a critical factor. Compared to low-carbon steel, stainless steel typically requires about 50% higher loading, whereas most soft aluminum materials require approximately 50% less loading. Relevant standard bending pressure parameters can be obtained from the bending machine manufacturer; this table shows the bending force required per 1,000 mm length under different thicknesses and materials.

　　Bending radius of the part

　　During the bending process, the radius of the bend angle of the workpiece is also a factor that needs to be considered. When using free bending, the bending radius is 0.156 times the opening dimension of the V-notch. In free bending, the V-notch opening dimension should be eight times the thickness of the metal material. For example, when bending 1.5 mm low-carbon steel using a V-notch opening dimension of 12 mm, the resulting bend radius will be approximately R = 1.9 mm. If the bend radius approaches or falls below the material thickness, it becomes necessary to use bottom-die forming. However, bottom-die forming requires a pressing force that is about four times greater than that needed for free bending. When performing free bending, pay attention to the clearance between the upper and lower dies at the bottom of the stroke, as well as to overbending slightly beyond 90° to compensate for springback and ensure the material maintains the desired angle. Typically, in new bending machines, the springback angle produced by free-bending dies is ≤2°, and the bending radius equals 0.156 times the opening distance of the lower die. Therefore, in general, the die angles used for free upper-and-lower die bending are between 86° and 90°. At the bottom of the stroke, there should be a gap between the upper and lower dies that is slightly larger than the material thickness. The forming angle can be improved because bottom-die bending involves significantly higher tonnage—about four times that required for free bending—thus reducing the stress within the bend radius that typically causes springback. Press brake bending is similar to bottom-die bending, except that the front end of the upper die is machined to the desired bend radius, and the clearance between the upper and lower dies at the bottom of the stroke is less than the material thickness. By applying sufficient pressure—approximately ten times that used in free bending—the front end of the upper die is forced into contact with the material, virtually eliminating springback altogether. To select the lowest possible tonnage specification, it is best to aim for a bend radius larger than the material thickness and, whenever feasible, to use the free-bending method. When the bend radius is relatively large, this usually does not affect the quality of the finished part or its subsequent performance.

　　Accuracy

　　Bending accuracy requirements are a factor that needs careful consideration—and it’s precisely this factor that determines whether you should opt for a CNC bending machine or a conventional NC bending machine. If your bending accuracy requirement is within ±0.5° and cannot be relaxed, you must choose a CNC bending machine. CNC bending machines typically offer slide repeatability accuracy of ±0.01 mm, and achieving highly precise forming angles requires such precision along with high-quality tooling. In contrast, the slide repeatability accuracy of an NC bending machine is ±0.5 mm, and under proper tooling conditions, it generally results in deviations of around ±2 to 3 degrees. Moreover, CNC bending machines are equipped with rapid die-changing CNC systems and die-clamping fixtures, making them an unquestionable choice when you need to bend many small-batch parts.

　　Mold

　　Bending dies also directly affect the accuracy of bending operations; therefore, it is essential to inspect the dies for wear. The method involves measuring the length from the front end of the upper die to the shoulder and the length between the shoulders of the lower die. For standard dies, the deviation per 10 mm should be around ±0.01 mm, and the total length deviation should not exceed ±0.15 mm. As for precision-ground dies, the accuracy per 100 mm should be ±0.005 mm, and the overall accuracy must not exceed ±0.05 mm. It is best to use precision-ground dies on CNC bending machines and standard dies on NC bending machines.

　　In summary, whether you’re a user or a manufacturer purchasing a bending machine, Jiangsu Liwei Machine Tool Company offers you an economical and practical bending machine—though of course, many factors need to be taken into account. At the same time, you should base your decision on your own specific circumstances; the points mentioned above merely provide a brief overview of certain partial aspects.