Figure 1. In CNC bending, commonly known as panel bending, the metal is clamped in place and the top and bottom bending blades form positive and negative flanges.
A typical sheet metal shop may have a combination of bending systems. Of course, bending machines are the most common, but some stores are also investing in other forming systems such as bending and panel folding. All these systems facilitate the formation of various parts without the use of specialized tools.
Sheet metal forming in mass production is also developing. Such factories no longer need to rely on product-specific tools. They now have a modular line for every forming need, combining panel bending with a variety of automated shapes, from corner forming to pressing and roll bending. Nearly all of these modules use small, product-specific tools to carry out their operations.
Modern automatic sheet metal bending lines use the general concept of “bending”. This is because they offer different types of bending beyond what is commonly referred to as panel bending, also known as CNC bending.
CNC bending (see figures 1 and 2) remains one of the most common processes on automated production lines, mainly because of its flexibility. The panels are moved into place using a robotic arm (with characteristic “legs” that hold and move the panels) or a special conveyor belt. Conveyors tend to work well if the sheets have previously been cut with holes, making them difficult for the robot to move.
Two fingers stick out from the bottom to center the part before bending. After that, the sheet sits under the clamp, which lowers and fixes the workpiece in place. A blade that curves from below moves upward, creating a positive curve, and a blade that curves from above creates a negative curve.
Think of the bender as a big “C” with top and bottom blades at both ends. The maximum shelf length is determined by the neck behind the curved blade or the back of the “C”.
This process increases the bending speed. A typical flange, positive or negative, can be formed in half a second. The movement of the curved blade is infinitely variable, allowing you to create many shapes, from simple to incredibly complex. It also allows the CNC program to change the outside radius of the bend by changing the exact position of the bent plate. The closer the insert is to the clamping tool, the smaller the outer radius of the part is about twice the thickness of the material.
This variable control also provides flexibility when it comes to bending sequences. In some cases, if the final bend on one side is negative (downward), the bending blade can be removed and the conveyor mechanism lifts the workpiece and transports it downstream.
Traditional panel bending has disadvantages, especially when it comes to aesthetically important work. Curved blades tend to move in such a way that the tip of the blade does not stay in one place during the bending cycle. Instead, it tends to drag slightly, much in the same way that the sheet is dragged along the shoulder radius during a press brake’s bending cycle (although in panel bending, resistance only occurs when the bending blade and the point-to-point part contact the outer surface).
Enter a rotational bend, similar to folding on a separate machine (see fig. 3). During this process, the bending beam is rotated so that the tool remains in constant contact with one spot on the outer surface of the workpiece. Most modern automated swivel bending systems can be designed so that the swivel beam can bend up and down as required by the application. That is, they can be rotated upwards to form the positive flange, repositioned to rotate around the new axis, and then bent the negative flange (and vice versa).
Figure 2. Instead of a conventional robot arm, this panel bending cell uses a special conveyor belt to manipulate the workpiece.
Some rotational bending operations, known as double rotational bending, use two beams to create special shapes such as Z-shapes that include alternating positive and negative bends. Single-beam systems can fold these shapes using rotation, but access to all fold lines requires turning the sheet. The double beam pivot bending system allows access to all bend lines in a Z-bend without turning the sheet over.
Rotational bending has its limitations. If very complex geometries are required for an automated application, CNC bending with infinitely adjustable movement of the bending blades is the best choice.
The rotation kink problem also occurs when the last kink is negative. While the bending blades in CNC bending can move backwards and sideways, the turning bending beams cannot move in this way. The final negative bend requires someone to physically push it. While this is possible in systems requiring human intervention, it is often impractical on fully automated bending lines.
Automated lines are not limited to panel bending and folding – the so-called “horizontal bending” options, where the sheet remains flat and the shelves are folded up or down. Other molding processes expand the possibilities. These include specialized operations combining press braking and roll bending. This process was invented for the manufacture of products such as roller shutter boxes (see figures 4 and 5).
Imagine that a workpiece is being transported to a bending station. The fingers slide the workpiece laterally over the brush table and between the upper punch and the lower die. As with other automated bending processes, the workpiece is centered and the controller knows where the fold line is, so there is no need for a backgauge behind the die.
To perform a bend with a press brake, the punch is lowered into the die, the bend is made, and the fingers advance the sheet to the next bend line, just as an operator would do in front of the press brake. The operation can also perform impact bending (also known as step bending) along the radius, just like on a conventional bending machine.
Of course, just like a press brake, bending a lip on an automated production line leaves a trail of the bend line. For bends with large radii, using collision only can increase the cycle time.
This is where the roll bending feature comes into play. When the punch and die are in certain positions, the tool effectively turns into a three roll pipe bender. The tip of the top punch is the top “roller” and the tabs of the bottom V-die are the two bottom rollers. The fingers of the machine push the sheet, creating a radius. After bending and rolling, the top punch moves up and out of the way, leaving room for the fingers to push the molded part forward out of working range.
Bends on automated systems can quickly create large, wide curves. But for some applications there is a faster way. This is called flexible variable radius. This is a proprietary process originally developed for aluminum components in the lighting industry (see Figure 6).
To get an idea of the process, think about what happens to the tape when you slide it between the scissors blade and your thumb. He twists. The same basic idea applies to variable radius bends, it’s just a light, gentle touch of the tool and the radius is formed in a very controlled way.
Figure 3. When bending or folding with rotation, the bending beam is rotated so that the tool remains in contact with one place on the outer surface of the sheet.
Imagine a thin blank fixed in place with the material to be molded fully supported underneath. The bending tool is lowered, pressed against the material and advanced towards the gripper holding the workpiece. The movement of the tool creates tension and causes the metal to “twist” behind it by a certain radius. The force of the tool acting on the metal determines the amount of induced tension and the resulting radius. With this movement, the variable radius bending system can create large radius bends very quickly. And because a single tool can create any radius (again, the shape is determined by the pressure the tool applies, not the shape), the process doesn’t require special tools to bend the product.
Shaping corners in sheet metal presents a unique challenge. Invention of an automated process for the façade (cladding) panel market. This process eliminates the need for welding and produces beautifully curved edges, which is important for high cosmetic requirements such as facades (see fig. 7).
You start with an empty shape that is cut out so that the desired amount of material can be placed in each corner. A specialized bending module creates a combination of sharp corners and smooth radii in adjacent flanges, creating a “pre-bend” expansion for subsequent corner forming. Finally, a cornering tool (integrated into the same or another workstation) creates the corners.
Once an automated production line is installed, it will not become an immovable monument. It’s like building with Lego bricks. Sites can be added, rearranged, and redesigned. Assume that a part in an assembly previously required secondary welding at a corner. To improve manufacturability and reduce costs, engineers abandoned welds and redesigned parts with riveted joints. In this case, an automatic riveting station can be added to the fold line. And since the line is modular, it does not need to be completely dismantled. It’s like adding another LEGO piece to a larger whole.
All this makes automation less risky. Imagine a production line designed to produce dozens of different parts in sequence. If this line uses product-specific tools and the product line changes, tooling costs can be very high given the complexity of the line.
But with flexible tools, new products may simply require companies to rearrange Lego bricks. Add some blocks here, rearrange others there, and you can run again. Of course, it’s not that easy, but reconfiguring the production line is not a difficult task either.
Lego is an apt metaphor for autoflex lines in general, whether they’re dealing with lots or sets. They achieve production line casting performance levels with product-specific tools but without any product-specific tools.
Entire factories are geared towards mass production, and turning them into complete production is not easy. Rescheduling an entire plant can require long shutdowns, which is costly for a plant that produces hundreds of thousands or even millions of units per year.
However, for some large-scale sheet metal bending operations, especially for new plants using the new slate, it has become possible to form large volumes based on kits. For the right application, the rewards can be huge. In fact, one European manufacturer has reduced lead times from 12 weeks to one day.
This is not to say that batch-to-kit conversion does not make sense in existing plants. After all, reducing lead times from weeks to hours will deliver a huge return on investment. But for many businesses, the upfront cost may be too high to take this step. However, for new or completely new lines, kit-based production makes economic sense.
Rice. 4 In this combined bending machine and roll forming module, the sheet can be placed and bent between the punch and the die. In rolling mode, the punch and die are positioned so that the material can be pushed through to form a radius.
When designing a high-volume production line based on kits, carefully consider the feeding method. Bending lines can be designed to accept material directly from coils. The material will be unwound, flattened, cut to length and passed through a stamping module and then through various forming modules designed specifically for a single product or product family.
This all sounds very efficient – and it’s for batch processing. However, it is often impractical to convert a roll bending line to kit production. Sequentially forming a different set of parts will most likely require materials of different grades and thicknesses, requiring changing spools. This can result in downtime of up to 10 minutes – a short time for high/low batch production, but a lot of time for a high speed bending line.
A similar idea applies to traditional stackers, where a suction mechanism picks up individual workpieces and feeds them to the stamping and forming line. They usually only have room for one workpiece size or maybe several workpieces of different geometries.
For most kit-based flexible wires, a shelving system is best suited. The rack tower can store dozens of different sizes of workpieces, which can be fed into the production line one by one as needed.
Automated kit-based production also requires reliable processes, especially when it comes to molding. Anyone who has worked in the field of sheet metal bending knows that the properties of sheet metal are different. Thickness, as well as tensile strength and hardness, can vary from lot to lot, all of which change molding characteristics.
This is not a major problem with automatic grouping of fold lines. Products and their associated production lines are usually designed to allow for variations in materials, so the entire batch must be within specification. But then again, sometimes the material changes to such an extent that the line cannot compensate for it. In these cases, if you are cutting and shaping 100 parts and a few parts are out of specification, you can simply re-run five parts and in a few minutes you will have 100 parts for the next operation.
In a kit-based automated bending line, every part must be perfect. To maximize productivity, these kit-based production lines operate in a highly organized fashion. If a production line is designed to run in sequence, say seven different sections, then the automation will run in that sequence, from the beginning of the line to the end. If Part #7 is bad, you can’t just run Part #7 again because the automation isn’t programmed to handle that single part. Instead, you need to stop the line and start over with part number 1.
To prevent this, the automated fold line uses real-time laser angle measurement that quickly checks each fold angle, allowing the machine to correct inconsistencies.
This quality check is critical to ensure that the production line supports the kit based process. As the process improves, a kit-based production line can save a lot of time by reducing lead times from months and weeks to hours or days.
FABRICATOR is North America’s leading steel fabrication and forming magazine. The magazine publishes news, technical articles and success stories that enable manufacturers to do their job more efficiently. FABRICATOR has been in the industry since 1970.
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Post time: May-18-2023