Sheet metal Forming

Metal folding’s place in the forming department

Walk into most commercial or precision sheet metal operations with CNC metal folders on the floor and you’ll probably find a collection of CNC press brakes too. Sure, press brakes have a long history in U.S. metal fabrication than folders do, but that’s often not why fabricators keep both types of forming machines on the floor. They have both because in many ways metal folders and press brakes aren’t competitive technologies. In fact, they’re complementary.

Having both presents opportunities, but to take advantage of them, you need to understand how exactly folders work and what their inherent benefits and limitations are. While some limitations remain, folders can do so much more than they could even a decade ago. They also add much-needed forming capacity to fabrication operations that, thanks to the fiber laser, have more blanking capacity than ever.

Folding Basics

CNC folding occurs in a variety of machines, from those that require full operator involvement to fully automated, lights-out operation. It all depends on the task at hand.

Many shops take a hard look at metal folding for not only efficiency but also worker ergonomics. Can you imagine the operator not having to support the weight of the part as he moves through the bend sequences? Do some large or heavy parts require several operators to manipulate through bending? Eliminating these concerns is one of the folding’s most basic inherent advantages.

When you invest in a press brake, you consider bed length, tools, and tonnage capacity for the machine and tools you choose. When you invest in a folder, tonnage doesn’t enter the conversation, just material gauge. Machines are designed to handle up to specific material thickness, with carbon steel as the typical baseline. Material thickness capacity is a few gauges thicker (lower gauge) for softer aluminum and should be a few gauges thinner (higher gauge) for stainless.

For instance, when you are folding 14-ga. carbon steel on a machine rated for 14 ga., the machine should be able to fold that material along the entire length of the bed. Bend length is a factor, though. Folders can bend some material that is thicker than the rated capacity in shorter lengths as long as the tooling is rated for the heavier material. Most machine manufacturers will provide a performance graph reflecting the tooling’s capability.

In a typical metal folding setup, sheet metal is positioned on a gauging table behind the work envelope. This includes an upper beam with upper beam tools, a lower beam with lower beam tools, and, finally, the folding beam with folding beam tools.

During air bending on a press brake, the punch descends into the die space, the material drags over the die shoulders, and the inside radius forms as a percentage of the die width.

A metal folder also “air-bends”—in the sense that the material is not bottomed or coined—and because of this, there is little to no wear in tooling for most folding applications. But the machine’s approach to bending is entirely different. For the most part, folders incorporate servo technology to drive and position all axes. This results in the most accurate product.

With integrated sheet support and back gauging systems, the part is positioned flat on the table, and only the flange is bent. The operator doesn’t need to balance or support the part in any way during the forming cycle. Segmented upper beam tools are grouped to accommodate bend lengths and necessary clearances for previously formed flanges The upper beam tools descend, and the workpiece is clamped between the upper and lower beam tools. The folding beam tool then moves into position, contacts the material, and rotates to form the first flange. 

Entry-level up acting-only folders require operators to flip the part to achieve a negative bend. In semiautomated folders, the folding beam can bend both positive and negative, no flipping of the material required, greatly reducing run times. Specifically, these bidirectional folders can rotate the beam upward for positive bends, then reposition itself to a new pivot point before swinging downward for a negative bend. Hybrid gauging and suction cup gauging can reduce operator involvement and part positioning problems. 

The distance between two points of contact on the workpiece—the edge of the lower beam tool and the tip of the folding beam tool—determines the inside bend radius. Although a 1-to-1 inside-bend-radius-to-material-thickness relationship might be achievable, typical setups maintain a 1.25-to-1 relationship between the inside bend radius and material thickness depending on the machine’s clamping force and the workpiece’s material type and thickness. On some folding systems, the beam can move outward slightly; like having a larger die opening on a press brake, this allows for a larger inside radius on some material thicknesses. On some thicknesses, a larger than 1.25X thickness radius bend can be achieved. If a workpiece requires a larger radius, the folder usually turns to incremental bending or bumping. The lower beam is programmable to accommodate different material thicknesses automatically up to the rated capacity of the machine.

In fact, most folding machines automatically adjust for material thickness changes. After the program has been developed and proven via offline software, the machine is ready to run. And because programming occurs offline, the machine can produce other parts while the new program is being generated.

Levels of Folding Automation

CNC folding machines are sold with varying levels of automation. In a semiautomated machine, the operator’s involvement is limited to loading, positioning, rotating, and unloading the part.

In fully automated folding systems, once the material is loaded, the machine inspects the part to provide its identity. For instance, some systems use infrared light that focuses on specific areas of a sheet metal blank. With the piece identified, the machine automatically changes to the required tools, and manipulator positions and rotates the part during the bend cycle.

Automated folding systems can be integrated with material load towers, shuttle load systems, and robot load/unload. In essence, material handling is driven by application requirements.

Offline programming software has also helped automate and streamline folding even further. Today some software can import CAD-generated STEP files, along with a variety of other formats. It automatically generates a 3D model for viewing, creates a recommended bend sequence for review, and runs a simulation of the product being formed. From there it converts the job into machine tool language. All this can be done offline, effectively eliminating the need for on-machine programming.

Crowning on a Folder

Crowning systems also enhance overall accuracies. Depending on the level of folding technology you consider, manual, automated, and engineered crowning systems are all available. 

In a typical press brake, crowning occurs on the surface of the lower beam, just below where the dies are seated. In wedge-style crowning systems, the wedges slide against one another to provide a crown that counteracts deflection during bending.

On a folder, crowning occurs in the folding beam tooling or in the beam itself, just below where the folding beam tooling seats. In manual crowning systems, operators use a special folding beam tool that allows them to dial in a specific crown. This is common for architectural shops that might need to bump-form gutters or similar products.

Figure 3 Suction-based gauging systems help eliminate part positioning inconsistencies.

Commercial and precision sheet metal shops often use CNC folders with intelligent crowning integrated into the folding beam. The mechanism itself is similar to the wedge-style systems seen on press brakes, but the folder’s crowning system as a whole is distinctly different. The folding beam swings up 10 degrees, detects the material, engages the crowning where needed, then commences the folding cycle.

Tooling Factors

In metal folding, the folding beam tool moves upward in an arc during the form and, as such, maintains constant contact with the workpiece’s outside surface. This makes the process well-suited for surface-sensitive material.

The process is also well-suited for high-product-mix, even kit-based production because, again, it uses a toolset that’s universal for most applications. In fact, usually, one set of tools is enough for most formed parts. Tools are precision-ground and clamped in place, so no alignment is required except for positioning along the length of the bed of the machine to accommodate the size of the part. There’s no punch that needs to be centered with a bottom V die.

Over a typical shift, a standard folding beam tool is used for multiple material thicknesses, though various upper beam tool segments can be staged across the beam length to provide necessary clearances for the job mix at hand. The segmented upper beam tools might need to be rearranged to accommodate different bend lengths and required clearances. 

Some operations stage these tools across the length of the machine to accommodate all or most jobs over a shift, complete with tools designed to bend certain common forms, like an upper tool with rotating foot corners that provides clearance for adjacent return flanges. Many manual tool positioning systems, such as those in semiautomated machines, offer pneumatic clamping. Some advanced folding systems have automatic tool changing, which reduces setup times even further.

Flange Factors

A folder has no flange height limitation for open bends, but at 90 degrees or more the flange height limit depends on the upper beam, tooling shut height, at least on certain machines.

That said, many folders do have significant tool height for deep box bending. Moreover, some folders have slanted upper beams that give clearance for very tall flanges (see Figure 4). Material handling and integrity (bowing of large, flimsy parts, etc.) are the only practical limitations.

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