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In the field of sheet metal processing, press brake is the key equipment to mould sheet metal into various shapes. The bending radius, as one of the core elements, has a profound impact on product quality, performance and productivity. Mastering the knowledge of bending radius is an essential skill for every sheet metal processing practitioner, and it is also the key to ensure that the product meets high quality standards. Next, let’s delve into the mystery of bending radius of the bending machine.
In the complex process of sheet metal processing, bending radius is by no means an optional parameter, it occupies a pivotal position. Whether it is the manufacture of small and delicate electronic components shell, or to create a strong and durable large-scale mechanical structure parts, bending radius of the choice and control are directly related to the success or failure of the final product.
Correctly grasp the bending radius, can effectively improve the quality of the product to ensure that its structure is solid, beautiful appearance. On the contrary, if the bending radius is not handled properly, it may cause a series of problems, such as material cracking, part deformation, etc., which will not only increase the production cost, but also lead to the product can not meet the design requirements, delaying the production schedule. Therefore, an in-depth understanding of the bending radius of the bending machine is important for improving the quality and efficiency of sheet metal processing. Next, let us explore all aspects of the bending radius of the bending machine in a comprehensive and in-depth manner.
Bending radius, from a professional point of view, refers to the bending operation on sheet metal or bar, from the bending axis to the surface of the sheet, we usually call it the inner radius. It is like a ‘core data’ at the bend that determines the shape of the curve inside the bend. This inner radius is not a simple value; it is a critical parameter that affects the structural integrity and appearance of the final product.
The calculation of the outer bending radius, on the other hand, is not complicated; it is usually equal to the sum of the inner bending radius and the thickness of the sheet. The inner bending radius is the curvature of the inside of the bend, while the outer bending radius covers the thickness of the material itself and reflects the overall shape of the outside of the bend. By clearly defining and understanding the inner and outer bending radii, we can better grasp the geometric characteristics of sheet metal bending.
When a metal material is bent on a bending machine, the internal stress distribution is very complicated. In the bending region, the outer material is like a stretched rubber band, bearing tensile force; while the inner material is like a squeezed sponge, bearing compressive force.
It is important to note that the size of the inner bending radius has a significant effect on the material forces. The smaller the radius of the inner bend, the more intense the tensile and compressive forces on the material. Imagine trying to bend a piece of thick wire into a very small arc, doesn’t it feel like a lot of effort and the wire breaks easily? This is because too small an inner bending radius makes the tensile stress on the outer layer of the material too great. In actual sheet metal processing, if the tensile stress at the outer bend exceeds the ultimate strength of the material, it will lead to cracking and fracture of the material, which will undoubtedly have a serious impact on the quality and service life of the product.
Bending radius plays an irreplaceable role in determining the final shape and quality of a bent metal part. In terms of shape, the bending radius directly determines the curvature and angle of the bend, and different bending radii can make the same sheet metal take on very different shapes.
In terms of quality, it has a key impact on the strength of the material. Reasonable bending radius can ensure that the material still maintains good strength and stability after bending, while improper bending radius may weaken the strength of the material, so that the product is prone to deformation or even damage in the process of use. Bending radius also affects the appearance of the product, accurate bending radius can make the product of the bending part of the smooth lines, beautiful and generous, to enhance the overall texture of the product.
Once the bending radius calculation error, the consequences will be unimaginable. Part deformation is one of the most common problems, the wrong bending radius may lead to parts can not achieve the design requirements of the shape, affecting its cooperation with other parts. Material failure should not be overlooked. Too small a bending radius can cause the material to fracture during the bending process, resulting in wasted material and production stoppages.
Parts that are out of specification can cause problems throughout the production process and may need to be reworked or scrapped, which will undoubtedly increase production costs and time costs. Therefore, in order to achieve the desired bending accuracy and repeatability, the use of a professional bending machine bending radius calculator is essential. It can help us accurately calculate the bending radius, to avoid a variety of problems caused by calculation errors.
The mechanical properties of the material is one of the important factors affecting the minimum bending radius. The sheet metal to be bent needs to have sufficient plasticity, so that it will not easily crack in the bending process. At the same time, a relatively low yield strength and a high modulus of elasticity are desirable performance indicators.
High plasticity is like giving the material a ‘flexibility’ that ensures that it does not crack during bending, i.e. a smaller minimum bending radius can be achieved. A lower yield strength and a higher modulus of elasticity, on the other hand, make it easier for the material to deform as we expect it to when bending under force, so that a precise bending shape can be achieved. The thickness of the material also has a significant effect on the minimum bending radius; in general, the thicker the material, the larger the minimum bending radius. This is analogous to bending a thin sheet of paper versus a thick board, where the thicker board obviously requires a larger bending radius to bend successfully.
Effect of Bending Centre Angle (α)
During the bending process, the bending centre angle (α) also has an effect on the minimum bending radius. Theoretically, the degree of bending deformation is mainly related to r/t (ratio of bending radius to material thickness). However, in actual bending operations, the situation is more complicated.
When the bending centre angle (α) is small, the bending deformation is relatively small, however the adjacent parts of the material may be subjected to more tensile deformation. This results in the minimum permissible bending radius being correspondingly smaller in this case. This effect can be understood more intuitively by comparing specific data. With a bending centre angle (α) of 120° – 130°, the minimum bending radius rmin is 30% – 50% larger than with α of 90°, whereas with α less than 90°, rmin can be reduced by 20%. These data clearly show the influence of the bending centre angle on the minimum bending radius.
Plate width and shear hardening layer should not be neglected. As the width of the sheet increases, the minimum bending radius increases. This effect is not unlimited, however, and diminishes as the width of the sheet increases to about (8 – 10) t (t is the thickness of the sheet).
As we know, the blank before bending is usually obtained by shearing or punching, and a work-hardening layer is formed on the sheared surface. This hardened layer reduces the plasticity of the material and makes the minimum bending radius increase. Therefore, when we need a very small bending radius, annealing before bending is necessary to remove the hardened layer from the blank and restore the plasticity of the material.
Most sheet metal used for bending is rolled, which gives them a fibrous structure with anisotropic mechanical properties in the thickness direction and in the plane of the sheet. The direction of the bending line is closely related to this anisotropy.
When the bending line is perpendicular to the rolling (fibre) direction, the minimum bending radius of the plate is relatively small. And when the bending line is parallel to the rolling (fibre) direction, the minimum bending radius will be larger. For materials with significant anisotropy such as brass and phosphor bronze, it is even more important to pay special attention to the direction of the bending line during processing.
When punching layouts, we must give full consideration to the rolling direction. When the bending line has a different relationship with the rolling direction, the bending radius and possible problems are also different. When the bending line is perpendicular to the rolling direction, the bending radius is relatively small and not easy to crack; while the bending line is parallel to the rolling direction, it is easy to crack. Therefore, the layout should try to make the bending line perpendicular to the rolling direction. For the bending line perpendicular to each other for smaller parts, the layout of the bending line and the rolling direction of the plate angle (β) should be greater than 30 °, which can effectively reduce the bending line due to the direction of the problems caused by improper.
The surface and shear quality of the blank material will also have an effect on the minimum bending radius. When the blank material of the part is defective, or the shear cross-section is not smooth, has burrs, or is of poor quality, it will lead to stress concentration, which will trigger rupture during the bending process.
To avoid this, we can appropriately increase the minimum bending radius. Removing burrs before bending, or facing the burred side towards the pressure zone of the punch, can also effectively reduce the chance of rupture. Shear quality of the narrower blank material bending is particularly significant, but with the increase in the width of the blank material, this effect will gradually reduce.
In practice, it is a common mistake to select a bending radius that is too small. Doing so creates a weak point in the metal, making the metal susceptible to fracture under stress. It can also cause permanent deformation that changes the dimensions of the part, resulting in a part that does not meet the design requirements.
To avoid this mistake, we first need to understand the properties of different metals. Different metals have different minimum bending radius to thickness ratios. In general, the harder and thicker the metal, the larger the minimum bending radius. The minimum internal bending radius is greater when the direction of bending coincides with the grain direction (longitudinal) than when it crosses the grain direction (transverse). We can determine this ratio based on data from the material supplier, the direction of the bend, and application-specific information. Reference can also be made to the free ‘Air Bending Force Chart’ which can help us determine the minimum internal bending radius for mild steel. The yellow area in the chart is the ‘optimum area’ and following the data in this area will often give the best results.
Placing features such as holes, slots, notches, etc. too close to the bending radius or even at the edge of the mould opening is also a problem that tends to occur. Doing so can distort these features and the part may not be able to hold the necessary hardware near the bending radius, affecting the normal use of the part.
To avoid this, we need to be careful where features are placed. Features should be placed no less than three times the thickness plus the bend radius from the bend. If the feature must be closer to the bend than the recommended distance because of design needs, then consider extending the opening beyond the bend line, which reduces the impact on the feature and the part.
When the formed offsets (bending lines) are too close to each other, there is a situation where they cannot be produced with a standard bending machine tool. To produce such a part, a special tooling is required, and the cost of using a special tooling is too high for small orders and is not cost effective.
To avoid this situation, we can refer to a specific table to determine the standard offset dimensions. If the measurements are out of the table range, don’t worry, we can contact professional engineers who will help us to determine an alternative solution to make sure the production can run smoothly.
An overly narrow required flange is also a problem to be aware of. A flange that is too narrow will cause the equipment to be subjected to excessive pressure during machining, leading to deformation of the part, and may also damage the mould and increase production costs.
To avoid this, we need to ensure that the minimum inner flange width is at least four times the material thickness plus the bending radius, so that it can be adapted to the machining requirements of the mould. We can also refer to the free ‘Air Bending Force Chart’ to determine the minimum flange length for mild steel to ensure that the flange width meets the requirements.
It is important to keep these common bending radius errors and solutions in mind during the design engineering process. This will avoid time-consuming manufacturing revisions during production, increase productivity and reduce costs. If there are more questions about bending radius issues or bending machine related content, please feel free to consult a professional.
Selecting materials with appropriate properties is the first step in preventing cracking when bending sheet metal. Different materials have different mechanical properties, we have to choose materials with good plasticity, yield strength and elastic modulus according to the specific processing requirements and usage environment. Such a material can better withstand the stress during bending and reduce the risk of cracking.
Strict adherence to the recommended minimum bending radius is key. We have already described in detail the factors affecting the minimum bending radius, and in practice, we should accurately calculate and adopt the appropriate bending radius according to these factors. Avoid using too small a bending radius in pursuit of a special shape, which may lead to material cracking.
The selection of bending direction is also important. Due to the anisotropy of sheet metal material, when the bending direction crosses the grain, the force on the material is more uniform and can reduce the possibility of cracking. Therefore, in the processing, we should try to choose the bending direction and texture cross the way to bend.
For some blank materials that have been sheared or punched, there may be a hardened layer on the surface, which will reduce the plasticity of the material. Pre-bending and annealing before bending removes the hardened layer and restores the plasticity of the material, thus effectively reducing the risk of cracking.
The uniformity of the thickness of the material also has a significant impact on the prevention of cracking in bending. Uneven thickness will lead to uneven stress distribution in the bending process, which will easily trigger stress concentration, thus leading to cracking. Therefore, when selecting materials, it is necessary to ensure that the thickness of the material is uniform to avoid cracking caused by thickness problems.
6.6 Reasonable placement of holes and features
Reasonable placement of holes and features can avoid stress concentration due to improper location. Holes and features that are too close to the bending radius increase the risk of cracking, so we need to follow the previously mentioned method of placing the holes and features in the right place to reduce the possibility of stress concentrations.
Applying back tension during bending is an effective way to prevent cracking. Back tension cancels out some of the stresses within the material, allowing the material to be more uniformly stressed during bending, thus reducing the risk of cracking.
During some processes, the material may be subjected to heat, creating a heat-affected zone. The material properties in the heat-affected zone will change and become more prone to cracking. Therefore, we should try to avoid bending operations in heat-affected zones to ensure that the material properties are not affected.
Investing in good quality bending machine equipment and regular maintenance is the basis for ensuring the quality of the process. Quality equipment has higher precision and stability, and can better control the parameters of the bending process. Regular maintenance can identify and solve equipment problems in a timely manner to ensure that the equipment is always in good working condition and reduce cracking caused by equipment problems.
Bending radius plays an extremely important role in sheet metal processing. Through this paper, we have gained a comprehensive understanding of the definition of bending radius, a deep understanding of the importance of bending radius calculation, a detailed analysis of the various factors affecting the minimum bending radius, a clear method of preventing common errors in the bending machine, as well as mastering the skills to prevent cracking when bending sheet metal.
In the actual sheet metal processing operations, the correct use of these bending radius knowledge can effectively improve product quality, avoid the emergence of a variety of processing problems, improve production efficiency and reduce production costs. It is hoped that every practitioner engaged in sheet metal processing will keep this knowledge in mind and apply it flexibly to actual work, continuously improving their processing level and contributing to the development of the industry.
