Bending machine dies are tools used on bending machines to process sheet metal. They mainly achieve the shaping of workpieces by changing the physical state of the sheet metal. Under the pressure of the bending machine, the sheet metal is formed into workpieces with specific shapes and dimensions. Bending machine dies are generally divided into upper dies and lower dies, which are selected and replaced based on the specific conditions of the processed sheet metal. To help you choose the appropriate bending machine dies, we have compiled the following key points.
Bending machine dies have a significant impact on the accuracy of the processed workpieces. It is commonly believed that bending machine dies are minor accessories on the bending machine, but in fact, it is quite the opposite. Although bending machines have evolved into multi-axis, high-precision machines with built-in stability functions, during the bending process, the dies are the only parts that come into contact with the workpieces.
Currently, the boundaries between standard dies, European-style dies, and American-style dies have become blurred. Many characteristics required for high-performance bending machines have been applied to various types of dies. Regardless of the type of die and clamping method chosen, it is essential to ensure that it meets at least the following minimum requirements.
Ultra-high precision: The precision of the dies during the bending process greatly affects the accuracy of the workpieces, making it crucial. Before use, the wear of the dies must be checked. The method of inspection involves measuring the length from the front end of the upper die to the die shoulder and the length between the die shoulder and the lower die shoulder. For standard dies, the deviation per mm should be around ±0.0083mm, and the total length deviation should not exceed ±0.127mm. As for precision ground dies, the accuracy should be ±0.0033mm per meter, and the total accuracy should not exceed ±0.0508mm. It is generally recommended to use precision ground dies for hydraulic or torsion bar bending machines and standard dies for manual bending machines.
Segmented dies: Segmenting the dies involves pre-cutting larger dies into smaller sections of different lengths based on frequent usage. This makes them more convenient to use, safer, and easier to handle.
Automatic fixed installation: When the slide reaches the top dead center, the upper die is installed, and the die clamping system can hold multiple dies in the proper position until clamping pressure is applied.
Hydraulic clamping system: The hydraulic clamping system is the most efficient clamping method. It can be used on both new and old machines, saving time and costs. If the load-bearing surface of an old bending machine is damaged, the hydraulic clamping system is the best choice to remedy the damage while improving clamping and installation efficiency.
Automatic extrusion seating: When clamping pressure is applied, the upper die is automatically pulled up and extruded into position. This eliminates the need to press the upper die into the bottom of the die during the bending process.
Front loading: The dies can be directly installed from the front of the machine. This reduces the loading and unloading time as there is no need to spend time loading or unloading the dies from the end of the bending machine. In most cases, front loading does not require a forklift or crane.
Standard sizes: Using standard dies simplifies machine adjustments when changing workpieces. The front feed, backstop, and safety devices can all remain in the same position. Since the height of the dies is the same, ready-made dies can be directly added, ensuring that the new dies match the processed workpieces. Many high-quality bending machine dies are manufactured according to metric standards, so V-opening dies are actually 6mm. Additionally, slight elliptical radii are generated during sheet metal bending, so it is possible to bend suitable workpieces with dies that are close in size. Operators use different dies to process similar or identical quality workpieces. If they cannot use the dies correctly, they will not be able to produce qualified workpieces. They rely on the dies to perform their work, but if the process is not efficient or continuously cyclical, it can severely hinder the workflow. The selection of dies follows a very simple principle: obtaining the best quality workpieces in the shortest possible time.
What kind of dies do we need? Why?
The needs and usage of bending machine dies in factories differ significantly from those of custom manufacturers. Therefore, before delving into the details of the dies, it is important to determine the requirements and budget. For example, if additional dies are needed to shorten processing time, it is advisable to follow lean production principles as much as possible and recognize the benefits of equipping each bending machine with dedicated dies. It is recommended to purchase two or more sets of compatible dies and install them on the respective bending machines. This eliminates the waste of valuable time searching for the correct dies in a die library or elsewhere. An additional benefit is that different dies for different bending machines do not necessarily have to be compatible since the dies can be installed on the machine that suits them best.
Die library
If you need to purchase additional duplicate dies to expand the dedicated die library for each bending machine, it is easy to select suitable ones. Look for the most severely worn dies or those with surface brightness but worn edges and corners. Additionally, dies with rust or excessive dirt on the bottom are not suitable for use.
Die selection
To maximize cost-effectiveness, choose fewer lower dies that cover the thickness of all the sheet metal to be processed. If you have limited knowledge or are unsure about the type of sheet metal to be processed in the future, or if you have a limited budget, it is recommended to use the 8×2 rule for die selection. First, determine the thickness of the sheet metal to be bent. For example, if you want to bend sheet metal with a thickness ranging from 0.75mm to 6.30mm. Then, multiply the thickness of the thinnest sheet metal by 8 to estimate the minimum required V-opening size for the lower die. In this example, the minimum required die is for the 0.75mm sheet metal, so 0.75×8=6. Third, multiply the thickness of the thickest sheet metal by 8 to estimate the maximum required V-opening size for the lower die. In this example, the maximum required die is for the 6.30mm sheet metal, so 6.30×8=50.4. Now we have determined the minimum and maximum required dies—6mm and 50.4mm. To meet the requirements, start with the smallest V-opening lower die and double its size, i.e., 6×2=24. Then, double 24mm to get 48mm. Double it again to get 96mm. This gives us at least four different V-opening dies, which are 6mm, 24mm, 48mm, and 96mm, for bending sheet metal with a thickness of 0.75mm to 6.30mm. When processing thicker or high-tensile sheet metal, using the usual bending standards can cause wrinkles, cracks, or even splitting of the workpiece. This can be attributed to a physical explanation. The narrower upper die applies greater force along the bending line, which, when combined with a narrow V-opening lower die, increases the force even more. Therefore, for challenging workpieces, especially when the sheet metal thickness exceeds 12.5mm, it is advisable to consult the manufacturer to purchase the appropriate upper die.
8× Rule
Here is a method for selecting dies based on the thickness of the sheet metal called the 8× rule, which means that the V-opening of the die should be eight times the thickness of the sheet metal. After understanding this method, multiply the thickness of the sheet metal by eight to select the closest die. For example, for a 1.5mm thick sheet metal, a 12mm die is needed (1.5×8=12mm); if it is a 3.0mm sheet metal, a 24.0mm die is needed (3.0×8=24.0). This ratio provides the optimal angle selection, which is why many people refer to it as the "best choice." Most published bending charts are also based on this formula.
V-opening Determines the Radius of the Lower Die
When bending low carbon steel, the internal bending radius is formed at approximately 16% of the V-opening of the lower die. So, if bending a 1mm thick sheet metal, the internal bending radius of the V-opening of the lower die is approximately 0.16mm. Suppose bending a 0.125mm thick sheet metal, multiply the thickness by eight, and use a 1mm die. However, many engineers prefer to specify a bending radius equal to the thickness of the metal. If specifying an internal radius of 0.125mm for bending, similarly, the internal bending radius for sheet metal bending is approximately 16% of the die opening. This means that a 1mm die will produce a 0.160mm radius. What to do in this case? A narrower V-opening die, specifically a 0.75mm die, needs to be used to achieve a closer internal radius of 0.125mm (0.75×0.16=0.12). The same principle applies to bending sheet metal with larger radii. For example, if the bending radius is 0.125mm and low carbon steel with a thickness of 0.320mm needs to be bent. In this case, the sheet metal thickness exceeds twice the internal bending radius. In such a situation, a 2mm die needs to be selected, which will produce an internal bending radius of approximately 0.320mm (2×0.16). Of course, this approach has its limits. For example, if a specified internal bending radius needs to be achieved, a V-opening lower die smaller than five times the metal thickness needs to be used, which can affect angle accuracy, potentially damage the machine and its dies, and even pose a safety risk to the operator.
Minimum Bending Length
When selecting V-opening dies, attention needs to be paid to the length of the bending line. The minimum bending length that a given V-opening lower die can form is approximately 77% of its opening, so the formed part should be greater than 1mm. A V-opening lower die requires a minimum bending length of 0.77mm. Many engineers prefer a shorter bending length for metal-saving reasons. For example, for a bending length of 0.5mm, with a 0.125mm thick sheet metal (see Figure 4), according to the 8× rule, a 0.125mm thick sheet metal requires a 1mm V-opening lower die, which gives the workpiece a minimum bending length of at least 0.77mm. What can be done in this case? A narrower V-opening lower die, such as 0.625mm, can be used. This allows the die to form a workpiece with a bending length of 0.5mm (0.625×0.77=0.48, rounded to 0.5).
Bending Length
Usually, a 1mm die is chosen for processing a 0.125mm thick sheet metal, but considering the specified bending length, a narrower die is required. Of course, this approach also has its limits. Just like when the internal bending radius is very small, if the die width that meets the bending length is less than five times the sheet metal thickness, it can affect angle accuracy, potentially damage the machine and its dies, and even pose a safety risk to the operator.
Upper Die Selection Rules
When bending L-shaped workpieces, there are no specific rules for upper die selection, as almost any upper die can be used. Therefore, when selecting upper dies for a set of workpieces, L-shaped workpieces can be considered last, as almost any upper die can bend them. When bending these L-shaped workpieces, it is recommended to use upper dies that can also bend other workpieces instead of purchasing unnecessary dies. Remember, it is better to minimize the number of dies when making purchases - not only to minimize die costs but also to reduce installation time by reducing the number of required die shapes.
Other workpiece shapes require specific rules for upper die selection. For example, when forming a J-shape, the rules are as follows:
Special Upper Dies
When the upper segment of the workpiece is longer than the lower segment, a goose-neck die is needed. When the upper segment is shorter than the lower segment, any upper die can be used. When the upper segment is equal in length to the lower segment, a sharp angle upper die is required. In summary, the selection of upper dies depends mainly on the interference of the workpiece, which is where bending simulation software can play an important role. If the system being used cannot simulate the bending situation, manual checking of workpiece interference with the upper die can be done using a grid background drawing, as shown in the following figure.
Manual Checking
Segment Difference Rule
If traditional dies are used, bending twice is required to form a segment difference or a Z-shape. The rules for forming these shapes are as follows:
Bending Twice
The middle segment (web) must be larger than half the width of the main body of the V-opening die; note that it is the entire body width, not the V-opening of the lower die. The side segments must be shorter than the height of the V-opening of the lower die plus the sprue height. When the middle segment (web) is smaller than half the width of the V-opening die, a special die is needed to form two bends in one upper die stroke. The advantage of these special dies is that they do not require flipping the sheet metal, but the disadvantage is that they require approximately three times the bending force.
Shearing and Bevel Bending Rules
In holes or other cutouts, any unsupported sheet metal inside the V-opening die will deform, which manifests as bursting, as shown in the following figure.
Bursting
When the holes near the bending line are small, the associated bursting is also small. Additionally, most applications tolerate some deformation. Therefore, when the cutouts are on or near the bending line, there is no specific rule for selecting the optimal width of the V-opening of the lower die. When the bending length, cutouts, and bevels are too close to the bending line in relation to the metal thickness, rocker-style dies can be used. The rocker rotates and supports the sheet metal throughout the bending process, eliminating deformation. The image above shows the same workpiece with cutouts near the bending line. The front piece shows signs of bursting - it was processed using a traditional V-opening die, while the back piece was processed using a rocker-style die. Also, note that the two ellipses on the left have the same width (from front to back) and the same distance from the bending line, only differing in length, clearly demonstrating that longer ellipses result in more severe bursting.
Upper Die Height for Given Box Depth
When bending three or four sides of a box, the height of the upper die becomes crucial. In some cases, if during the final (third) bend, a formed workpiece's side may hang on the side of the bending machine, a shorter upper die can only produce a three-sided box. To produce a four-sided box, a sufficiently tall upper die needs to be selected to span the height of the box diagonally, as shown in the following figure:
Box Depth Determines Upper Die Length
The minimum height of the upper die for box bending = (box depth/0.7) + (back gauge thickness/2). If there is no top flange or if the top flange protrudes outward after bending, there is no need for a large gap between the upper and lower dies to remove the workpiece. However, if all four sides have a top flange or if the top flange protrudes inward, then there must be enough space to twist and remove the box after bending.
Bending and Flanging Combination
Flanging dies can bend workpieces with flanges, as shown in the following figure. It is only necessary to know that the flange thickness is greater than 0.125mm, although custom dies may be required to withstand higher bending forces.
Flanging
The selection rules for the V-opening lower die in flanging are similar to those for standard bending dies. A 30° pre-bend for flanging requires a longer minimum flange length because the selected V-opening die has an acute angle of 115%. If bending a sheet metal above 0.375mm, using a V-opening die requires a minimum flange length of at least 0.431mm (0.375×1.15=0.431).
Mark-Free Dies
Almost all typical V-bending machine dies leave some marks on the workpiece because the metal is pressed into the die during bending. In most cases, the marks are small or acceptable, and increasing the radius can reduce the marks. However, for workpieces where even the smallest marks are unacceptable, such as painted or polished sheet metal before bending, nylon inserts can be used to eliminate the marks, as shown in the following figure. Mark-free bending is particularly important for manufacturing aircraft or aerospace components, as inspectors find it difficult to visually inspect a workpiece and differentiate between scratches and cracks.
Mark-Free Dies
Today's precision dies and bending machines can achieve unprecedented levels of accuracy. With the right dies and consistent sheet metal, bending machine operations can bend the sheet metal to a specific angle with a specific internal bending radius. However, bending also forms an internal bend radius approximately 1% the size of the die opening, so selecting the appropriate die is important. Specifying multiple different, tightly toleranced radii increases processing costs. Additionally, the more dies required, the more conversions are needed, which adds more costs. In other words, if sheet metal designers follow a few basic rules when processing workpieces, they can more easily select dies and perform overall bending operations:
1. The internal bending radius should be 1.5 times the thickness of the sheet metal.
2. The length of the bending line should be at least 6 times the thickness of the sheet metal. This also applies to holes in the workpiece, meaning the holes should be far from the bending line, at a distance of at least 6 times the thickness of the sheet metal.
3. The size of the Z-shaped web should be at least 10 times the thickness of the sheet metal.
Of course, there are numerous exceptions to these rules, and each comes with its own complexities. For example, we can use a narrower V-opening lower die to bend smaller radii or shorter flanges, but when the bending radius is too large, it can cause wrinkling of the bending line and exceed the rated tonnage of the dies and bending machine. We can bend a narrower segment difference, but this also requires a special die and a significant bending tonnage. If a workpiece does not require excessively short bending line lengths, narrow segment differences, or very small bending radii, why complicate matters? By following these three simple rules, angle accuracy can be improved, installation time can be reduced, and die costs can be lowered.
