The tooling fundamentals of metal folding

The upper beam of a folding machine lifts the clamping tools, allowing suction grippers to manipulate the work between bends.

Metal folders can form one distinct part after another, each with different material thicknesses and grades, bend geometries, flange lengths, even radii. And they can do it all with a limited range of tools.

Still, not all folder tooling investments are the same. Every fabricator has unique forming needs, and they can use different tooling strategies to achieve them.

Why Folding Tools Last

Folding tools generally aren’t consumable items. They’re not indestructible, especially if inadvertently dropped or damaged. But properly handled, they can last decades.

The reason for this has to do with how metal folders interact with the sheet metal parts they’re bending. Every folder has an upper beam (or clamping beam), a lower beam, and a folding beam that swings to create the bend. On these three components are three different toolsets. As their name implies, the clamping beam tools clamp the material. Their shape and geometry provide the necessary clearance for previously formed flanges.

The clamping beam tool secures the workpiece against the bottom beam tool. Finally, the folding blade tool performs the bending, swinging upward and (for bidirectional folders) downward to create positive and negative flanges. Properly sized, folding machine tools should handle all material thicknesses, from the machine’s capacity all the way down to shim stock.

Once tools contact metal, that point of contact remains. The workpiece never rubs against the tools under pressure during forming, and the area of bending—where the inside bend radius is created—occurs without any direct contact with tooling.

To understand how this happens, imagine an operator standing behind a folder. He places a blank onto the gauging table and against the folding blade, which is positioned to act as a frontgauge. The clamping tools descend and secure the work against the bottom tools. Next, the folding blade contacts the bottom of the workpiece, then swings upward, pivoting around the theoretical center of bend.

The distance between the clamping point and the blade determines the radius (see Figure 1). The larger that distance, the larger the radius. When the radius changes, so does the metal elongation. If the part was previously air formed on a brake, its bend calculations to account for that elongation will be the same or similar on a folder.

When the radius is small, the theoretical center of bend is on or near the material surface, and the gap between the clamp and folding blade is tight. For a larger radius, the theoretical center of bend moves above the material, and the gap between the clamp and blade is larger.

As the folding beam swings upward, the blade remains in constant contact with the flange surface. The radius forms between the beam tools and the bending blade on the folding beam. No tools touch the actual area of bend.

FIGURE 1. The theoretical center of bend (marked by the diamond) represents the point around which the folding beam pivots. Changing this changes the position of the folding blade and, ultimately, the bend radius.

This all makes folding less sensitive to material variation. Metal folders do need to adjust for tensile strength and hardness characteristics in the metal they’re forming. Still, corrections are minimal, and this has a lot to do with how the metal is folded.

The folding blade forms the part on the outside plane of the metal. This means material thickness variation has no impact on the final bend angle.

Beam and Deflection Basics

Tooling choice first hinges on the machine design, including the shape and design of the upper beam (see Figure 2). An upper beam designed for bending bidirectionally is shaped like a right triangle—vertical in front and angled upward in back (the hypotenuse of the triangle) to give room for the operator to manipulate pieces and the necessary clearance for previously formed flanges.

An up-only folder is usually run from the front, and its upper beam angles back to provide clearance for long return flanges. These folders, most of which are used in the architectural market, often have very short clamping tools that give clearance for long “back bends” all the way to a 45-degree inside angle.

A few machines do have beams that give clearance both behind and in front. They’re often used for earth-moving buckets and other specialty applications.

But for the most part, the standard up-only and bidirectional clamping beam designs provide enough clearance for the vast majority of folding applications on the market. And for precision metal fabricators in the industrial space, the bidirectional design has become by far the most popular.

A properly designed folder experiences minimal deflection, thanks in part to the nature of the process. It’s clamping and folding up or down rather than applying pressure from above. Still, every forming machine deflects to some degree, and that deflection must be accounted for. Most deflection in a properly set up folder doesn’t occur in the upper and lower beams. It instead occurs where the “action” is, in the swinging folding beam, which is why the latest systems have crowning to compensate for that deflection.

Deflection governs rules of thumb about minimum flange lengths. In most cases, flanges need to be about six to seven times the material thickness in mild steel and eight to 10 times the material thickness in stainless steel. Shorter than this, and you risk excessive deflection and negatively impact bending consistency.

This is just a rule of thumb, of course. In truth, the minimum flange can depend on the part geometry, material thickness, and your machine’s folding capacity. If you’re folding very thin material on a high-capacity machine, you can achieve narrower flange lengths. For instance, if you bend 18 ga. on a 0.25-in.-capacity machine, you can likely achieve a flange length that’s just three times the material thickness.

Even the bending direction can be a factor. A down-bend will generally deflect less than an up-bend, simply because the lower beam’s immense rigidity compared to the upper beam. This in turn can allow for a narrower flange. Regardless, the minimum-flange rule of thumb helps establish a consistent setup that is sure to provide consistent parts.

FIGURE 2. Typical industrial folders have an upper beam that’s vertical in front and angled in the back (left). Architectural folders often have upper beams angled in the front (center), while some specialty upper beams are angled to give clearance in front and back (right).

Clamping Tool Heights

When fabricators invest in a folder, they usually invest in the tallest clamping tools that meet the machine’s capacity and provide good bending results. This again has to do with deflection.

The taller the tool, the greater clearance you have for vertical flanges and box bending. At the same time, clamping tool height needs to be balanced with the machine’s capacity. Very tall clamping tools holding material that approaches the machine capacity could cause deflection under full load.

Say you have a folding machine with a 3-m bed that can form up to 6.35-mm material. The optimal clamping tool height for that setup might be calculated to be 330 mm. The machine can accept 400-mm-tall tooling, but it will likely deflect when folding the machine’s specified maximum material thickness. This gives you suboptimal results, with wider-than-expected radii. You can “over-crown” the folding beam to make up for deflection, but doing so might not be ideal. The best approach would likely be to choose slightly shorter tooling, engineered to provide perfect bending results for the thickest material the machine is rated for, all the way down to shim stock.

Segmented Clamping Tools

Folder clamping tools come in segments. Each has a “foot” that extends toward the clamping point, giving clearance for return flanges (see Figure 3). Typical setups also involve a left and right corner, with wings that extend outward when needed. They serve the same function as a horned punch on a brake, providing clearance for return flanges perpendicular to the bend, like in a box bend.

Next, a setup will have basic clamping tool segments 200 mm wide, combined with thin adapter clamping tools, and each can be anywhere between 25 mm and up to 50 mm wide. If you need a precise bend length or optimal clearance for a side flange, you can space the tool segments with gaps in between—sometimes as large as six to 10 times the material thickness. This is possible because of how the folder forms. The clamping tool never contacts the material at the point of bend.

Such flexibility helps when dialing in the bend lengths. Say you have side flanges, as when bending boxes and enclosures. Similar to a punch that’s box bending on a brake, the edge of the end clamping tool should not extend into the radius of the previously formed adjacent bend. Doing this will push and “bulge” that radius out and distort the resulting formed part.

Fabricators aim to choose clamping tools that will accept the widest variety of their parts. If they have very deep return flanges to deal with, they can choose very deep “gooseneck-like” clamping tools to account for them. They also might choose tall, skinny clamping tools to give clearance to form a very deep and narrow U shape. Some have deep reliefs near the bottom to account for a very short and deep return flange. Still other specialty clamping tools come in shapes that streamline the creation of certain forms (see Figure 4). For instance, a round clamping tool allows a folder to essentially “press” a flange against it to create a specific radius, plus springback. For certain applications, special clamping tool shapes can make a world of sense. That said, you’ll rarely if ever find a folding setup with a massive library of special clamping tools. In most cases, standard clamping tools will accommodate a broad product mix.

Choosing the Right Folding Blade

Basic folding “straight” blades are rectangular. Others give clearance for tight return flanges, roughly analogous to a gooseneck punch on a brake. And like a gooseneck, you can install it front or backward to give you the clearances you need (see Figure 5).

Every blade is designed with a maximum capacity, and the rule of thumb here mirrors the one about minimum flange length. The folding blade thickness needs to be about six to seven times the thickness of mild steel and eight to 10 times the thickness for stainless.

The folding blade’s area of contact with the workpiece is nothing but a lever. The longer the lever, the more leverage you have to form with. A narrow 10-mm folding blade under 6.35-mm mild steel will never give you enough leverage to bend the material. In this case, a 35- or 40-mm-thick bending blade gives you a sufficiently long “lever” to fold the material accurately.

FIGURE 3. A standard clamping tool has a “foot” to account for return flanges.

For most parts, an operator will use the thickest-available folding blade, which can form the full range of material thicknesses. However, thick blade tools can’t access tight spaces like narrow offsets or Z forms. For this reason, folders in conventional setups have two different sets of folding blade tools: a narrow one for offsets and Z forms (and other “space-constrained” applications), and a thicker one for everything else.

Note that if the flange is less than the folding blade width, choosing a thicker folding blade won’t give you any more leverage to help form it accurately. When it comes to leverage and bending pressure, it’s the area of contact between the blade and material that matters. No matter how thick the blade might be, a really short flange has only so much area for the blade to contact.

Automatic Tool Change in Folding

An automatic tool change (ATC) system on a press brake might retrieve and set different toolsets dozens of times during a single shift. Folders, however, operate differently. On occasion, operators might need to choose a different bending blade. And they will need to “re-gap” clamping tools (rearranging them on the upper or clamping beam rail) to accommodate different part widths on four-sided profiles. Done manually, this only takes a minute or two, and it usually doesn’t justify automating the tool change.

In folding, moving heavy tools is the ATC’s sweet spot (see Figure 6). Modern folders have open heights (that is, the space between the lower and upper beam) of more than 40 in.—giving space for clamping tools more than 20 in. tall to fold boxes just as deep. How do you move them efficiently and safely? With the ATC.

The folder ATC can streamline tool changes, but rearranging and re-gapping short clamping tools doesn’t take long to begin with. The folder ATC makes the use of very tall tooling practical, changing the part envelope and improving the flexibility of the machine. The idea is to go with the clamping tool that can accommodate the tallest part in the mix.

About Flexibility

Many folding machines today process an extreme variety of parts, often sequenced not in batches but in kits, with one unique part after another entering the work envelope. The technology aims to increase forming flexibility, and tooling remains a big part of the equation. You don’t need different tools to form different radii. One toolset can effectively form different grades, thicknesses, bend angles, hems, and bump forms.

But you do need to think about part clearance, the folding sequence, and the potential part geometries to be folded. Done well, the right tool selection can extend the part envelope, maintain folding accuracy, and, if they’re well maintained, last the life of the machine.

Slide Show | 7 Images