Estimated reading time: 16 minutes

Many people think that the bending machine mold is a secondary accessory in metal forming, but the fact is just the opposite. Although the bending machine has developed into a multi-axis, high-precision machine with a self-stabilizing function, only the tool touches the part during the bending process.

The boundaries between RFA, new standards, European and American standard tools have been blurred. Many functions required for high-performance bending have been migrated to all various tool types. No matter which tool and clamping method you choose, make sure it meets at least some minimum requirements.

High Precision

The manufacturing tolerance of the tool should be within 0.0004 inches. This is essential for achieving part accuracy without shimming or other adjustments during the setup process.

Segmented Part

These allow you to build various lengths from several pre-cut pieces. Small pieces are also safer and easier to handle.

Self-sustaining Installation

You should be able to load tools with a push rod. The tool clamping system should hold multiple workpieces in place until clamping pressure is applied (see Figure 1).

When clamping pressure is applied, the punch is mechanically pulled into position. This eliminates the need to place the punch into the bottom of the mold during the set-up process.

Before Loading

You should be able to install the tool from the front of the machine. This shortens the setup time because you no longer need to spend time sliding the tool from the end of the press brake. In most cases, front-loading also eliminates the need for forklifts and overhead cranes.

Standard Size

The universal height tool can reduce the need for machine adjustments when changing jobs. The front support arm, the rear gauge height, and the safety device are all kept in a common position. Since the tools are of the same height, you can add ready-made parts and make sure they match your existing tools.

Many high-quality bending machine tools are manufactured by metric standards. So the nominal size is 0.250 inches. The V-shaped opening is 6 mm or 0.236 inches. In addition, the bends in sheet metal have a slightly elliptical corner radius, so you only need to get close to get the correct result. For simplicity, the imperial sizes in this article are rounded off.

Please note that the focus of the following discussion is on air bending, for good reason. The trend is to abandon bottoming or die casting as much as possible and use air bending as much as possible. Please note, however, that not all parts can be produced using classic air bending technology.

Operators across the industry use very different tools to make parts of similar or identical quality. Many operators use incorrect tools to make acceptable parts because they cannot use the correct tools. They make it work; but “make it work” is not efficient or repeatable, and it can severely hinder the workflow. The best practice of tool selection should indeed have an elegant and simple goal: to obtain the highest quality parts in the shortest possible time.

What Tools Do You Need And Why?

The bending tools needed and used in the repair shop are different from those of the custom manufacturer. Therefore, before delving into the details, please determine your needs and budget constraints.

For example, you may need other tools to reduce setup time. You may follow the principles of lean manufacturing and realize the benefits of having a separate tool library for each press brake-therefore, you are willing to invest in duplicate toolsets stored on the machine. You won’t waste precious setup time, going back and forth between the tool bench and other places to find the right tool. Another advantage here is that there is no longer a need for machine-to-machine tool style compatibility because tools tend to stay on their intended machine (see Figure 2).

If you need to purchase additional, repetitive tools to expand the dedicated toolbar for each actuator, selecting them is relatively simple. You will often find these tools inconvenient places if they are not already in the press brake. Look for the most worn tools-those with bright and brightwork surfaces. The main body of the tool may also be clean and bright. Rusty and dirty tools at the bottom of the rack are unlikely to be candidates.

Mold Selection

To get the maximum benefit, please choose the minimum number of lower molds to cover the entire metal thickness range of your workshop form. Shops that lack tribal knowledge, unforeseen applications, and limited budgets should try to use the 8×2 rule to choose lower molds.

First, determine the thickness range of the metal to be bent. For example, you may need to bend 0.030 inches to 0.250 inches thick material.

Second, evaluate the minimum V-chip required by multiplying the thinnest metal by 8. In this example, it is 0.030 inches. The material requires the smallest mold, so: 0.030 × 8 = 0.24, we will round to 0.25.

Third, evaluate the largest V-shaped mold required by multiplying the thickest metal by 8. In this case, the thickest material of 0.250 inches will require the largest mold: 0.250 × 8 = 2.

You have now determined the smallest and largest chips you need-0.25 and 2 inches.

To fill the gap between the two, you can start with the smallest V-chip and double its size. In this case, this gives you 0.5 inches. Dead (0.25 × 2 = 0.5). Next, double 0.5 inches. The mold gets 1.0 inches and then doubles it to get 2.0 inches. This gives you at least four different V-shaped mold openings to bend 0.030 to 0.250 inches. Materials: 0.25, 0.5, 1.0 and 2.0 inches.

Punch Selection

You can also use the material thickness to determine the minimum number of upper punches. For materials 0.187 inches or thinner, you can use a 0.04-inch sharp offset punch. radius. Sharp angles allow bending more than 90 degrees, and offsets allow you to form a J shape. To withstand more force when forming materials from 0.187 to 0.5 inches thick, consider using a straight punch of approximately 0.120 inches. radius.

Please note that for some applications, including those that use thicker and high-strength materials, when using common industry bending standards, the workpiece tends to wrinkle, crack, or even split in half. This boils down to physics. The narrower punch exerts more force on the bending line; combine it with the narrow V-shaped die opening and the force will rise even more. For challenging applications, especially when the material thickness exceeds 0.5 inches, it is best to consult your material supplier for the recommended punch tip radius.

The Rule Of 8

In a perfect world, you should be able to use what we call the rule of 8 to choose the opening of the V-shaped mold; that is, the opening of the V-shaped mold should be 8 times the thickness of the material. To determine this, multiply the material thickness by 8 and select the closest available mold. So if you have 0.060 inch thick material, you need a 0.5 inch chip (0.060 × 8 = 0.48; 0.50 inch is the closest chip width); for 0.125 inch. Material, you need a 1 inch. Die (0.125 × 8 = 1). This ratio provides the best angle performance, which is why many people call it the “sweet spot” for V-die selection. Most of the published bending charts are centered on this formula.

Is it simple enough? Well, this will be in that perfect world. If the sheet metal designer always follows the rule of 8, you can live in that perfect world. However, it is a pity that exceptions abound in the real world.

V-die Opening Determines The Radius

When air bends mild steel, the inner bend radius is formed at approximately 16% of the V-shaped die opening. Therefore, if you bend the material air more than 1 inch. For V-shaped molds, your inner bend radius is approximately 0.16 inches.

Assume that printing specifies 0.125 inches. Material. In a perfect world, you would multiply that thickness by 8 and use 1 inch. V is dead. It’s simple enough. But many sheet metal designers like to specify a bending radius equal to the thickness of the metal. What if the inner radius specified for printing is 0.125 inches?

Similarly, the inner radius of the material air bend is about 16% of the mold opening. This means your 1 inch. The mold can produce a radius of 0.160 inches. now what? Just use a narrower V-shaped chip.

0.75 inches. The die will give you an inner radius close to 0.125 inches (0.75 × 0.16 = 0.12).

A similar idea applies to prints that specify a larger bending radius. Suppose you need to shape mild steel 0.125 inches thick to 0.320 inches thick. Inner bending radius-more than twice the thickness of the material. In this case, you would choose 2 inches. For the mold, this will produce an internal bending radius of approximately 0.320 inches (2 x 0.16).

This has limitations. For example, if you find that to achieve the specified inner bending radius, you need a V-shaped mold opening that is less than five times the metal thickness, you will affect the angle accuracy, may damage the machine and its tools, and put you in a very dangerous situation. Unsafe conditions.

Minimum Flange Length

When choosing a V-shaped mold, keep the flange length in mind. The smallest flange that a given V-shaped mold can form is about 77% of its opening. So a part is formed on 1.-in. V die needs at least 0.77 inches. Flange.

Many sheet metal designers like to save metal and specify a flange that is too short, such as 0.5 inches. 0.125-inch flange material thickness (see Figure 3). According to the rule of 8, 0.125-inch thick material requires 1-inch thick material. V is dead-but that one inch. V-shaped molds require that the workpiece has a flange of at least 0.77 inches. What should we do now? Similarly, you can use a narrower V-chip. For example, 0.625 inches. The mold can form parts with flanges as short as 0.5 inches (0.625 × 0.77 = 0.48, rounded to 0.5).

This also has limitations. Just like the internal bending radius is very narrow, if the width of the mold required for the flange is less than five times the thickness of the material, you will encounter angular accuracy problems, which may damage the machine and its tools, and put yourself in danger.

Punch Selection Rules

For the L shape, the rule is…there is no rule. Almost any punch shape can be used. Therefore, when selecting punches for a set of parts, you should always consider these L-shaped parts last, because almost any punch shape can handle them.

When forming these L shapes, use punches that can also form other parts instead of adding unnecessary tools to the library. Remember, when specifying tools, less is always best—not only can you minimize tool costs, but you can also shorten setup time by reducing the number of tool shapes required in the workshop (see Figure 4).

Other shapes do require specific punch selection rules. For example, when forming a J shape, the rule is (see Figure 5):

When the upper leg is longer than the lower leg, you need a gooseneck punch.

When the upper leg is shorter than the lower leg, any punch shape is fine.

When the lower upper leg is equal to the lower leg, you need an offset sharp punch.

As you can see, the punch selection rules mainly deal with workpiece interference, and this is where bending simulation software can play an important role. If you cannot use the bending simulation software, you can use the drawing of the tool supplier with a grid background to manually check the stamping interference (see Figure 6).

Offset Rule

If you are using a traditional toolset, you need to use two punching cycles to form the offset or zigzag. For these shapes, the rules are (see Figure 7):

The center leg (web) must be greater than half the width of the V-shaped phantom; please note that this is the entire width of the phantom, not the V-shaped die opening.

The side legs must be shorter than the height of the V mold plus the height of the riser.

When the center leg (web) is less than half the width of the V-shaped phantom, you will need a special tool to form two bends in one punch stroke. The advantage of these form tools is that you don’t need to flip the tray. The disadvantage is that they require about three times the bending force of standard air.

Bending Rules Across Cuts And Miter Joints

Any unsupported material in the V-shaped mold will deform; in holes and other cuts, this deformation appears as a burst (see Figure 8). When the hole near the bend line is small, the associated blowout will also be small. In addition, most applications will accept some deformation, so when the cut is at or close to the bending line, there is no clear rule for choosing the best V-chip width.

When flanges, cuts, and miter joints are too close to the bending line of the metal thickness, you can specify a rocker mold. The rocker rotates and supports the material throughout the bending process, thereby eliminating bursts.

Figure 9 shows the same part with a cut near the bend line; the foreground—the informative blowout—is formed using a traditional V-shaped mold; the background is formed using a rocker-type mold. Also, note that the two ellipses on the left have the same width (from front to back) and the same distance from the bend line; they just have different lengths. You can see more blowouts on longer ellipses.

Punching Height For A Given Box Depth

When forming three-sided and four-sided boxes, the height of the punch becomes critical. In some cases, if the molded side can be hung on the side of the bending machine during the last (third) bending, the short punch can form a three-sided box. If you want to form a four-sided box, you need to choose a punch that is high enough to cross the height of the box diagonally (see Figure 9):

Minimum punch height for box bending = (box depth/0.7) + (plunger thickness/2)

If there is no top (return) flange, or the top flange protrudes outward, the part can be removed after bending without too much clearance between the upper punch and the lower die. However, if you have return flanges (top flanges protruding inward) on all four sides, you need enough clearance to twist and remove the box after bending.

Combination Of Bending And Hem

The hemming tool can form parts with hemmed edges in one setup, as shown in (see Figure 10). Please note that if you need to crimp thicknesses greater than 0.125 inches, you may need custom tools to accommodate the excessive force required.

The selection rules of V-die opening here are the same as those of standard bending tools. Due to the acute angle, the 30-degree pre-bending of the flange does require a longer minimum flange—at 115% of the selected V-die opening. For example, if you are molding more than 0.375 inches of material. For V-shaped molds, you need a flange of at least 0.431 inches (0.375 × 1.15).

Scratch-free Parts

Almost all typical V-shaped mold bending tools will leave some marks on the part, this is simply because the metal is pulled into the mold when it is bent. In most cases, the markings are the smallest and acceptable. Increasing the shoulder radius can reduce the markings.

For applications where even the smallest marks are unacceptable, such as when bending pre-coated or polished materials, you can use nylon inserts to eliminate scratches (see Figure 11). Scratch-free bending is especially important for manufacturing critical aircraft/aerospace components because it is difficult for inspectors to visually inspect parts and distinguish the difference between scratches and cracks.

Simplicity Is A Virtue

Today’s precision tools and bending machines can reach an unprecedented level of precision. Using the right tools and consistent materials, the bending machine operation can bend the flange to a specific angle with a specific inner bend radius. But again, air bending forms the inner bend radius as a percentage of the mold opening-it is important to have the right tools. Specifying many different radii with tight tolerances will increase tooling costs. And the more tools you need, the more conversions you will have, which will further increase costs.

In other words, if the sheet metal part designer follows some basic rules when designing the part, it can make tool selection and overall bending operations easier:

1. The inner bending radius should be 1.5 times the metal thickness.

2. The flange length should be at least six times the metal thickness. This also applies to holes in parts; in other words, the location of the holes should be far away from the bend line and the distance should be at least six times the thickness of the material.

3. The size of the offset (Z-shaped) web should be at least 10 times the metal thickness.

Exceptions to these rules abound, and each one comes with complexity. You can use a narrower V-shaped die opening to bend a smaller radius or shorter flange-but the bend radius is too sharp and you may bend the line and exceed the tonnage rating of the tool and press brake. You can bend narrower offsets, but also require special tools and a large amount of forming tonnage.

If the part does not require short flanges, narrow offsets, or small radii, why should it be complicated? By following these three simple rules, you will improve angular performance, shorten setup time, and reduce tool costs.

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