Features of geometric profile tube bending
It is common to see fabricators bending round metal tube and box section tube, but as complex and geometric profiles become increasingly popular and the market more demanding, industrial applications have expanded to include more complex profile designs, which require fabricators to push the boundaries of traditional tube bending.
“We are seeing many more fabricators take on applications with geometric profiles,” said Lindley Searles, regional sales manager, BLM GROUP USA, Novi, Mich. “In the structural industry, it’s not just used for rollover protection for the tractor but now also for the doorframe. It’s more of a special shape that has to be produced using one piece of material. Designers are really pushing this trend, especially in the furniture industry where we are seeing a bigger focus on aesthetics. Everyone wants their products to look good and really differentiate themselves from competitors, so fabricators are now taking on more complex work and finding ways to effectively bend these profiles.”
The cost of the special profiles is greater due to the complexity of the shaping process, which makes ensuring effective bending practices even more important. There are many challenges with these applications, but one of the biggest considerations is guaranteeing the necessary support during bending.
“Geometric profiles present special challenges compared to round tube or a simple square tube, specifically, the stability of the part,” said Philipp Knobloch, sales director, Schwarze-Robitec, Cologne, Germany. “Round tube has great stability because of its symmetric cross section. This is why it behaves consistently and in a repeatable way during the bending process. Profiles, especially a profile with special geometries, act very differently. Because the cross section is not symmetric, the bending process can be unpredictable if the part is not properly stabilized.”
For a profile to be bent effectively, the material must be supported right across the entire length, at the very least on the outside of the material, but often on the inside of the profile tube as well. The outside support can be a challenge, but the interior support presents an even bigger challenge for fabricators.
“A box shape has four sides and a round tube has a consistent circumference,” said Searles. “With a simple profile, like a D-shaped tube, it could require combining bending practices of both a box and a round shape. However, any kind of complex profile needs additional support to prevent stretching and twisting of material that will lead to distortion and variations within the length of the part. There is a level of uncertainty in the bending process of complex profiles that there just isn’t with box or round tube.”
Knobloch noted that the key to stability and bending appropriately is developing the necessary tooling. For any bending process, the goal is to cover and stabilize the part to ensure the best possible bend, but this is especially true for geometric profiles. Specialized tooling for internal and external stability can make this possible.
Open Versus Closed
Whether the profile is open or closed will make a difference in its level of difficulty.
“This goes hand-in-hand with the stability challenge,” said Searles. “For a closed shape, a fabricator really only needs to worry about supporting the outside. There are some cases where the inside needs support, but mainly the outside is the focus. However, when fabricators move into bending open shapes, it’s important to ensure that the opening remains open and not deformed during the process. But creating a method of supporting the opening through the bend can be challenging.”
Closed profiles tend to offer a more natural stability than an open profile shape. There is a greater chance of deformity with an open profile.
“For open profiles, the part needs to be supported in more complex ways, which means the tooling is more complex and more expensive,” added Knobloch.
Beyond open versus closed profiles, the material type could add to the difficulties of bending profiles. “The challenges are going to be different, whether it’s steel or aluminum,” said Searles. “Aluminum typically offers a more consistent shape because it’s extruded, whereas steel typically is a low-form shape, which has a lot more variation to it.”
With steel profile tubes, there are a certain number of limitations as far as what can be produced. Aluminum tube, however, can have more complex profiles because it can be extruded. And although many complex shapes can be made, not all can be bent effectively, so feasibility comes into play as well.
“The material type, like it does for every process, definitely affects the level of springback that fabricators can expect to see,” said Knobloch. “Round tube tends to have quite consistent springback, but this isn’t the case for special profiles. Having lots of information and details about the semi-finished product and material specifications can help fabricators make better decisions in order to optimize the process and help design the tool.”
With profiles, special tooling is necessary. Having well-designed tools can be the difference between a good part versus a scrapped part.
“You have to figure out the best approach to support the material, to keep it in the shape that you want it,” said Searles. “Sometimes that’s done with hard tooling, where the tooling is just on the outside surface, or you’re using a mandrel to support it. Other times that’s not enough. In some cases, fabricators may need to use consumable materials in the gaps of the open shape in order to maintain it. Once bending is complete, these consumable materials can be stripped out. This can be an expensive process but necessary for these types of applications.”
Searles gives the example of a window frame. As the frame shape, which is usually made of aluminum, is bent, the open feature where the glass will go needs to be maintained, so using consumable materials in this gap will keep its structure through the bend.
“Special tooling is necessary,” said Knobloch. “Getting good tooling that is designed well will make a difference in the feasibility of bending the profiles. With round tubes or square tubes, the tooling is fairly easy to maintain using a turning process. It’s less expensive and less time-consuming to maintain these tools. But for special profiles, a shop may need to use a milling process to maintain the complex tooling that is required. It’s important to consider that this can be more cost-intensive and time-consuming.”
The tooling for these special profiles needs to be able to open and close, according to Searles. A split die will give fabricators the ability to open the tooling and insert the material to sit within the tool that closes around it. The tooling may need to open in more than one location within the section, depending on the profile design. Working with reputable tooling suppliers to develop the best design is a good starting point.
The best way to ensure feasibility is through test bending and trial and error. Working with bend experts and machine OEMs can help fabricators determine whether a project will be possible and cost-effective.
“Test bending using the necessary tooling is one of the best ways to see how the profile behaves during forming and bending,” said Knobloch. “We use the part drawings with basic information of the part parameters to create the right tool to bend the part. There can be a lot of support from CAD programs, which offer estimations, but to really see where this profile needs to be supported in a special way, test bending is the best option. The best option is found by working together with the client and testing out different options.”
The basic information for a profile tube is much more extensive than that of a round tube or box profile, which generally has only an outer shape, outer diameter, wall thickness, and radius. A special profile has many additional dimensions to consider, especially as the radius changes throughout the cross section. There needs to be clear communication between the supplier of the job and customer about the exact dimensions and features required.
“The more you can control on the machine, the better,” said Knobloch. “The better a machine can control and cover the springback effect of the materials, the twisting effect of the material while bending, the better the part will turn out. A bending machine that can repeatedly control many axes, which are affecting the result of the part, will be more beneficial than a machine that can only control one or two axes, which cannot work against the twisting of material during the process.”
This is why trial and error is so important. Getting a sense of the machine’s capabilities for these applications will give fabricators a better understanding of the nature of the variables during the bend process.
“Software techniques and proper sequencing in the equipment will allow fabricators to control the twist and variations in the bending process while managing material flow,” said Searles. “This is especially important if there are various bend radii within the same part, as we often see in the recreational vehicle industry. Accessories can be incorporated to make accommodations for this. Test bends and working with suppliers can really help reduce the learning curve.”
The experts agree that test bending and trial and error is a good place to start. But they also suggested that partnering with a supplier that has experience with bending profiles will give a fabricator the unique insights from both the tooling side and the machine side of the application.
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