1、Common Bending Dies

The common bending dies are shown in the figure below. To extend the life of the dies, rounded corners should be used in part design whenever possible.

For edges with a height that is too small, even with bending dies, it is not conducive to forming. Generally, the bending edge height L should be at least 3 times the thickness (including the wall thickness).

Processing of  Steps

For some sheet metal Z-shaped steps with a lower height, manufacturers often use simple dies on stamping presses or hydraulic presses for processing. For small batches, segment difference dies can also be used on bending machines, as shown in the figure below. However, the height H should not be too high, generally it should be within (0–1.0) times the thickness. If the height is between (1.0–4.0) times the thickness, consider using a die with an unloading structure according to the actual situation.

The step height of this die can be adjusted by adding shims, so the height H is arbitrarily adjustable. However, there is a disadvantage: the length L dimension is not easy to ensure, and the verticality of the vertical edge is not easy to guarantee. If the height H is large, consider bending on a bending machine.


Bending Machines

There are two types of bending machines: ordinary and CNC (numerical control). Due to high precision requirements and irregular bending shapes, sheet metal bending for communication equipment generally uses CNC bending machines. The basic principle is to use the bending die (upper die) and V-groove (lower die) of the bending machine to bend and shape the sheet metal parts.

Advantages: Easy clamping, accurate positioning, and fast processing speed.

Disadvantages: Low pressure, only capable of processing simple shapes, and lower efficiency.

Basic Principles of Shaping

The basic principle of shaping is shown in the figure below.

Bending Die (Upper Die)

The form of the bending die is shown in the figure below. During processing, it is mainly selected according to the shape required by the workpiece. Manufacturers usually have a variety of bending die shapes, especially those with a high degree of specialization, who customize many shapes and specifications of bending dies for various complex bends.

The lower die generally uses a V-groove with V=6t (t is the material thickness).

There are many factors that affect bending processing, including the upper die radius, material, thickness, lower die strength, and lower die mouth size. To meet product requirements and ensure the safe use of the bending machine, manufacturers have already standardized the bending die series. We need to have a general understanding of the existing bending dies during the structural design process. See the figure below, the left side is the upper die, and the right side is the lower die.

**Basic Principles of Bending Processing Order**:

  1. Bend from the inside out.
  2. Bend from small to large.
  3. Bend special shapes first, then general shapes.
  4. The previous process should not affect or interfere with subsequent processes.

The current bending forms are generally shown in the figure below.

2、Bending Radius

When bending sheet metal, there must be a bending radius at the bending point, which should not be too large or too small and should be appropriately selected. A too small bending radius can easily cause cracking at the bend, while a too large bending radius can cause the bend to rebound.

The preferred bending radius (inner radius) for various materials of different thicknesses is shown in the table below.

The data in the table above is preferred and for reference only. In fact, the fillet of the manufacturer’s bending die is usually 0.3, and a few bending dies have a fillet of 0.5.

For ordinary low-carbon steel plates, anti-rust aluminum plates, brass plates, and copper plates, an inner fillet of 0.2 is no problem, but for some high-carbon steel, hard aluminum, and super-hard aluminum, this bending fillet will cause bending fracture or outer fillet cracking.

3、Bending Springback

Springback angle Δα = b – a, where b is the actual angle of the workpiece after springback, and a is the die angle.

**Size of the Springback Angle**

The springback angle for a single 90-degree free bending is shown in the table below.


**Factors Affecting Springback and Measures to Reduce It**


  1. The mechanical properties of the material: The size of the springback angle is directly proportional to the yield point of the material and inversely proportional to the elastic modulus E. For sheet metal parts with high precision requirements, to reduce springback, materials should be chosen as low carbon steel as much as possible, and high carbon steel and stainless steel should not be chosen.
  2. The relative bending radius r/t: The larger the value, the smaller the degree of deformation, and the larger the springback angle Δα. This is an important concept. The fillet of sheet metal bending should be as small as possible within the allowable range of material performance, which is conducive to improving accuracy. It is especially important to avoid designing large radii, as shown in the figure below, which poses great difficulty for production and quality control.


4、Minimum Bending Edge Calculation for One-time Bending

The starting state of L-shaped bending is shown in the figure below.

The starting state of Z-shaped bending is shown in the figure below.


The minimum bending dimensions L for Z-shaped bending of sheet metal of different material thicknesses are shown in the table below.

Competition in the sheet metal processing industry is becoming increasingly fierce. If companies want to remain invincible, they must continue to improve their processing technology. In some bending applications such as precision stainless steel, aluminum alloy, aerospace parts and copper plates, users are no longer satisfied with the functionality of the product, and have higher requirements for its processability and aesthetics. Traditional bending processes are more likely to damage the surface of the work piece. The surface in contact with the mold will form an obvious indentation or scratch. Since the mark cannot be removed, the part will be scrapped. Therefore, users have an increasingly strong demand for no-mark bending.we also call it mark-free bending

The impact of press brake die parameters on bending indentation

  • The opening width of V

When coming to bend metal sheets of different thicknesses, the V opening width of lower die is also different.

Under the conditions of the same plate thickness and the same upper die, the larger the size of the V opening, according to the principle of three-force balance, the smaller the pressure between the metal plate and the shoulder of the V opening, the smaller the friction force, and the naturally smaller the indentation depth. Therefore, it is recommended to use a slightly larger V opening lower die.

  • The R-value of the rounded corner

The R value of the V-shaped shoulder radius is different, and the friction force on the plate during the bending process is different.

The larger R value of the V-opening shoulder, the smaller pressure between the plate and the V-opening shoulder, and the lighter the indentation, and vice versa.

In the figure 1,the WILA tools press Indentation on the same sheet metal but different R value(Ra<Rb),Obviously, the bending indentation of Ra is more significant than Rb.

Therefore, it is recommended to use a lower die with a slightly larger shoulder radius.

The solution for no-mark bending

1.Protective cloth or foil

From the perspective of friction, the upper and lower molds can be separated by a foil to avoid direct contact between the molds with the plate, and a non-marking pressing film foil can be used to separate the upper and lower molds. This mainly acts as a buffer between the workpiece and the shoulder of the lower mold, reduces the friction coefficient, and offsets the pressure between the mold and the plate, thereby preventing the workpiece from having indentations when it is bent.

It has the advantages of low cost and easy use. When using it, just put the no-mark pressing film on the lower mold.

Figure 2 The test results show that the use of non-marking pressing film can effectively avoid bending marks.

2.Tools with plastic inserts

From the perspective of friction pair, it is possible to consider changing the V-groove shoulder to no-metallic material which is softer than the plate, such as polyurethane, nylon, etc., while ensuring the original required extrusion effect.

high-strength nylon material combinate with an extended mold. It is usually used for bending aluminum plates with a thickness of 0.4~0.5mm in specific occasions, will have a better results.

3.Rotabend bottom tools

Rotabend bottom tools helps reduce marking.lts hardened rotor wingsreduce friction.which using rotating supports to enable high quality sheet metal forming,we called Unmarked bending of tension spring in China

Thanks to the tightest tolerances, CNC deep hardening of the contact surfaces and the rotatable V-neck, it is easy to produce parts with short flanges using rotary wing dies to maximally reduce the marks on the work piece when bending the sheet metals.

The advantage of Rotabend bottom tools:

(1) Holes and grooves are produced without deformation in the area close to the bend centerline.

(2) Indentations on the surface of the work piece are reduced when it is bent.

(3) Very small bend fillets can be obtained.

(4) The rotor and contact surfaces are hardened to ensure a long service life and precision.

(5) The rotor is easy to disassemble and assemble, making it easy to clean and maintain.

(6) The strict tolerance requirements during mold processing ensure the ultimate precision during bending operations.

(7) The compact mold size design ensures maximum bending freedom.

(8) The same mold can be used to bend plates of various thicknesses.

(9) It can be used in combination with certain customized lower molds to perform 90° bends

4.Ball mark free bending mold

Based on the principle of reducing the friction coefficient of the friction pair between the sheet and the V groove, the sliding friction pair of the sheet and the shoulder of the V groove can be transformed into a rolling friction pair, thereby greatly reducing the friction force to which the sheet is subjected.This can effectively avoid bending indentation.

Bending is a process of folding and bending flat panels, which follows the cutting process in the entire manufacturing chain. The workpiece is placed on a die with a V-shaped opening. A wedge-shaped tool (upper tool) presses the workpiece into the V-shaped opening and bends the sheet to the required angle in this manner.

Most bending parts are made using processes and methods such as free bending, die bending, and edge folding and pressing. The operation follows the same principle: the punch presses the workpiece into the lower die of the die. Therefore, the bending machine that performs the above processes and methods is called a die bending machine.

1. **Free Bending**

The punch presses the workpiece into the die without pressing it against the die wall. As the punch descends, the edge of the workpiece bends upwards and forms an angle. The deeper the punch presses the workpiece into the die, the smaller the angle becomes. At this time, there is a gap between the punch and the die.

Free bending is also known as a path-dependent process. Each angle requires a specific path. The machine tool control system calculates the path and the corresponding punch force simultaneously. The path and punch force depend on the mold, material, and product characteristics (angle, length).

2. **Die Bending**

The punch fully presses the workpiece into the die, leaving no gap between the die, workpiece, and punch. This process is called die closing.

The punch and die must fit together precisely. Therefore, each angle and shape requires the corresponding mold components. Once the workpiece is fully pressed in, the punch cannot continue to move downward. The machine tool control system continues to increase the punch force until it reaches the specified value. The pressure applied to the workpiece increases, thereby presenting the outline of the punch and die. The angle gradually solidifies under high pressure, almost completely eliminating the rebound problem.

3. **Flanging Bending**

The bending arm built into the machine is made of C-shaped profiles, with lower and upper bending molds installed on it. During bending, the C-shaped profile moves up and down or performs a small range of elliptical motion, that is, flipping. The arm bending machine operates semi-automatically, known for its speed and flexibility, even small batch production is completed without inferiority. In addition, through flanging bending technology, it is also possible to use the same mold to achieve efficient bending of various radius sizes on a single part.

4. **Edge Folding and Pressing**

The edge of the sheet is usually completely bent (such as the edge of a box), and then the edges are parallel to each other. As a result, the finished piece is more stable or forms an edge protection. Subsequently, the edges usually need to be hung with other parts. Edge folding and pressing are completed in two steps: first, the operator pre-bends a 30° angle, and then the workpiece is reinserted and pressed to close the angle. If there is a gap between the edges, it is called folding. In pressing, the bent edges are completely squeezed against each other. Folding is path-dependent, while pressing is force-dependent.

**Industry Applications**

Whether it’s a microwave oven, washing machine, or stove – most household appliances are made up of structures using very thin sheets and a large amount of forming processing, which must withstand many years of use. Therefore, white goods manufacturers need production partners who provide stamping and bending machines to flexibly process sheet metal parts. Since household appliances always have many visible parts, processing without surface damage or indentation is essential. The commonly used galvanized sheet is beneficial to stamping because the zinc layer is pulled to the top of the edge during the stamping process, preventing future corrosion. The washing machine drum in particular requires high-quality welding to prevent damage to clothing.

Laser welding is convincing with its high-quality results.

Switch cabinet housings, plug-ins, and the casings of electrical equipment itself – all of these are made of sheet metal structures. The laser cutting machines, stamping presses, and bending machines can process thin plate parts with many edges, often very complex. The flexible processing methods of Tongkuan machine tools can be used for pressing in components, forming processing, and profile processing. Laser welding technology also saves time and cost for customers in the electronics industry because it achieves high-quality seams, almost without any follow-up work.


Smbctools has a complete and scientific quality management system, and has been recognized in the industry for its integrity, strength, and product quality.

Determining the bending radius of sheet metal is crucial to ensure successful and accurate bending operations without causing damage to the material. The bending radius is the minimum radius at which a sheet metal part can be bent without cracking or deforming. To calculate the bending radius, follow these steps:

1.Material Properties:

Gather information about the material you’re working with, such as its type (aluminum, steel, etc.) and its mechanical properties, including tensile strength, yield strength, and elongation.To avoid crack formation or part deformation while bending, it’s essential to adhere to minimum bend radius guidelines. These guidelines can vary based on the material and its properties:

  • Mild Steel: For materials with a thickness up to 1.2 mm, a minimum bend radius of 0.8T (T = material thickness) is recommended.
  • Aluminum: A minimum bend radius of 2T is typically suggested for materials below 4 mm thickness.
  • Stainless Steel: For thicknesses up to 3 mm, a minimum bend radius of 1.5T is proposed.

These are general guidelines, and it’s crucial to consult material-specific recommendations or experiment with your specific sheet metal and tooling combinations to achieve the desired outcome. By adhering to appropriate bend radius guidelines, you can ensure a high-quality end product with fewer defects, less waste, and increased strength.

There is a minimum bending radius that should always be maintained in sheet metal design practices, and this minimum BR is displayed as a multiple of the specific raw material’s thickness. This value changes based on the type of material being bent as well.

See below for details:

2.Sheet Thickness:

Measure the thickness of the sheet metal accurately. This is an important factor in calculating the bending radius.

A simple and rough method to determine the bending radius is:

  • If the plate thickness is less than 6mm, the bending radius can be equal to the plate thickness.
  • If the plate thickness is between 6mm and 12mm, the bending radius is typically 1.25 to 1.5 times the plate thickness.
  • If the plate thickness is greater than or equal to 12mm, the bending radius is typically 2 to 3 times the plate thickness.

Experience in actual sheet metal processing shows that when the plate thickness is generally no more than 6mm, the inner radius of sheet metal bending can directly use the plate thickness as the radius.

When the bending radius is r = 0.5, the general sheet metal thickness t is equal to 0.5mm.

If a bending radius different from the plate thickness is required, a special die must be used for processing.

When the sheet metal drawing calls for a 90-degree bend with a particularly small bending radius, the sheet metal should first be grooved and then bent.

Special press brake tooling, such as punches and dies, can also be used.


The relationship between the bending radius of sheet metal and the width of the lower die groove of the bending die has been established through numerous experiments in sheet metal processing.

For example, when a 1.0mm plate is bent with an 8mm groove width, the ideal bending radius is R1.

If the groove width is increased to 20mm, the depth of the stretched plate increases, resulting in a larger tensile area and a larger R angle.

To avoid damaging the press brake die and to maintain the desired bending radius, it is recommended to bend with a narrow groove, following the standard ratio of 1:8 between plate thickness and groove width.

The minimum recommended ratio is 1:6 and bending with a ratio of less than 1:4 is not recommended.

Suggestion: If the strength allows, it is preferable to groove first and then bend in order to achieve a small sheet metal bending radius.

The following figure is a table provided by the press brake manufacturer, which shows the corresponding relationship between bending radius, pressure, and minimum bending height.

  • V – bend notch width
  • R – bend radius
  • B – minimum bending height
  • S – sheet thickness

Note: The data with gray scale in the table represents the required bending pressure P (KN/m), and the maximum bending force of the press brake machine is 1700KN. There are five available bending knife edges: V = 12, 16, 25, 40, and 50.

Please refer to your available knife edge and bending length to determine the bending radius, which will help you calculate the accurate length of the material to be unfolded.

The above information pertains to the pressure parameters and bending die width of a single press brake.

The actual calculations should be based on the pressure and bending die of your own sheet metal processing facility.


3.Bend Angle:

Determine the desired angle of the bend. This will influence the bending radius as well.

4.Bend Radius Calculation:

There are a few different methods you can use to calculate the bending radius, depending on the specific circumstances. Here are two common approaches:

a. Formula Method:

Using the formula for the minimum bend radius based on the material’s properties:

Minimum Bend Radius (R) = (K * Thickness) / (Tensile Strength / Yield Strength)


  • K is a constant typically ranging from 0.33 to 0.5 (it varies based on factors like material type, hardness, and temper).
  • Thickness is the thickness of the sheet metal.
  • Tensile Strength is the maximum amount of tensile stress the material can endure before failing.
  • Yield Strength is the stress at which the material starts to deform plastically.

b. Rule of Thumb Method:

Use a rule of thumb that’s commonly applied in the industry. For example, a common rule might be to use a bending radius of approximately 1.5 times the sheet thickness for most materials.

5.Safety Factors:

Depending on the precision and criticality of your application, you might want to apply a safety factor to the calculated bending radius. This helps account for uncertainties and potential variations in material properties and process conditions.

6.Bend Allowance:

Keep in mind that the bending process also involves a bend allowance, which is the extra length required on the neutral axis of the bend to accommodate the material’s elongation during bending. The bend allowance varies based on the material’s properties and the bending angle.

7.Test Bends:

Before carrying out a production run, it’s a good practice to perform test bends using scrap material to fine-tune the bending radius and other process parameters.

8.Consult Guidelines:

Some material suppliers and bending machine manufacturers provide guidelines and charts for determining bending radii based on material type, thickness, and other factors. These resources can be valuable references.

Remember that the actual bending process involves various other factors such as tooling selection, machine capabilities, and springback, which should also be considered for successful sheet metal bending operations.

What Is Press Brake Crowning

To guarantee accurate, repeatable forming results, it’s essential to compensate for the deflection that inherently occurs in the beam (ram) and table of the press brake when load is applied. Without deflection compensation, it’s likely that a workpiece will have some form of deformation at its center when it’s bent along the full length of the press brake. This is especially so for press brakes 8 feet or longer, 80 tons or more, and when bending long or large parts, but it also can be the case when forming shorter workpieces. To keep the bend angle consistent over the full length of the part, Therefore, it is necessary to take corresponding measures to reduce or eliminate the deflection caused by the deformation.The so-called deflection compensation device we call it crowning has preset a deformation in the direction of the opposite force-deformation in the ram and the upper die or the worktable and the lower die working table, and the deformation has the same amount of deformation generated in the actual work.a press brake needs a crowning system, either in the beam, in the table, or in both.

In this age of short batches, complex parts, and quick turnaround, crowning ensures efficiency, part accuracy, and repeatability. In high-tonnage applications, crowning helps produce straight bends in challenging, high-tensile-strength materials.

Types of press brake crowning

Hydraulic Crowning

The hydraulic automatic deflection Crowning mechanism of the workbench is composed of a group of oil cylinders installed in the lower workbench. The position and size of each Crowning oil cylinder are designed according to the deflection ram of the ram and the workbench finite element analysis. The hydraulic crowning realizes the bulge Crowning of the neutral version through the relative displacement between the front, middle and back three vertical plates. The principle is that the bulge is realized through the elastic deformation of the steel plate itself, so the Crowning amount can be adjusted within the elastic range of the workbench.

When the ram is down, the auxiliary cylinder is filled with liquid oil and goes downward.

During the bending process, hydraulic oil inlet into the auxiliary cylinder, so that the ram generate downward deflection for compensation.

Install the auxiliary hydraulic cylinder in the lower part of the worktable.

During the bending process, it generates an upward force on the worktable, which forms the automatic crowning system.

The pressure compensation device is composed of several small oil cylinders, comprising an oil cylinder, a motherboard, an auxiliary plate and a pin shaft, and a compensating cylinder is placed on the worktable.

The pressure compensation system is formed with a proportional relief valve.

During working, the auxiliary plate supports the oil cylinder, the oil cylinder holds up the motherboard up, just overcomes the deformation of the ram and the worktable.

The convex device is controlled by a numerical control system, so that the preload can be determined according to the thickness of the plate, the opening of the die and the tensile strength of the material when bending different sheet materials.

The advantage of hydraulic crowning is that it can realize the deflection compensation for continuous variable deformation with large compensation flexibility, but there are also some disadvantages of complex structure and in relatively high cost.

Mechanical Crowning

Mechanical crowning is a kind of new deflection compensation method, which generally uses a triangular oblique wedge structure.

The principle is that the two-triangle wedge block with α angles, the upper wedge moving is fixed at X-direction and can only move in Y-direction.

When the wedge moves the △x distance along the X-direction, the upper wedge moves up the H distance under the lower wedge force.

Regarding the existing mechanical compensation structure, two bolster plates are placed in full length on the worktable, the upper and lower plates are connected through the disc spring and bolts.

The upper and lower plates are consist of a number of oblique wedges with different slopes, through the motor drive to make them relatively moving and forming an ideal curve for a set of convex positions.

When we talk about press brake bending,we are familiar with three bending methods.Air Bending,Bottom Bending,Coin Bending.in this article we will explain all three bending methods.

Bending Method V-width IR Angle Accuracy Features
Air Bending 12T—15T 2t~2.5t >±45’ Can achieve a wider range of bend angle.
Bottoming 6T—12T 1t~2t ±15’—30’ The higher bending precision is obtained with the smaller press force.
Coining 4T—6T 0t~0.5t ±10’ It can achieve high bending precision, but the bending force is very large.

Air bending

Air bending means only part of the material is in contact with the toolings for bending.

From the above image, we can see that the toolings only touch A, B and C points of the metal during the bending process (the punch tip and the die shoulders). The rest position is not.

Because of the above reason, the actual angle of the toolings becomes unimportant.The factor that determines the bending angle is how far the punch descends into the die.The further the punch descends, the acuter the bend angle.Therefore, the fabricator can get a wide range of bending angles with only one set of tooling since the depth of the stroke (not the tooling) determines the bend angle.Besides, there will have a certain amount of spring back in air bending, so you need to bend a slightly more acute angle so as to get the desired bend angle.

Features of air bending:

  • Wide bending angle with one set of tooling. The angle can’t be smaller than the punch tip angle. If using a 30° punch, 180°-30° bending angle can be obtained.
  • The bending need less press force.
  • The bending angle is not in high accuracy.
  • The material has more spring back.

SEE ALSO: Air Bending Force Chart: The Most Authoritative Data From Amada

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Bottoming means the punch will descend to the bottom of the die so that the material makes contact with the punch tip and the sidewalls of the V-opening.

Bottoming is a method to obtain good bending precision with less pressure and is also a commonly used bending method.

V-opening width of the die can refer to below table:

T 0.5-2.6 3-8 9-10 ≥12
V 6T 8T 10T 12T

IR of workpiece

The interior radius of the workpiece is usually represented by IR.

During the bottom bending process, the IR is about 1/6 of the die’s V-opening (IR=v/6).

However, for different materials, the IR is also different, like SUS and Al has different IR.

Tooling accuracy of bottom bending

The angle after bottom bending will be affected by the spring back, so the bending spring back will be considered when choosing bottom bending.

The usual solution to obtain the target angle is by overbending.

  • Material, shape and thickness with small spring back – 90° tooling
  • Material, shape and thickness with big spring back – 88° tooling
  • Material, shape and thickness with bigger spring back – 84° tooling

When adopting bottom bending, the principle of using the same angle for both punches and dies should be abided by.

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The term “coining” is derived from the stamping method of the coin, which also means get very high accuracy.

For the coining process, enough tonnage of the press brake will be used to conform the sheet metal to the exact angle of the punch and die.

In coining, the sheet metal is not just bent, it’s actually tinned by the compress between the punch and die.The coining not only featured high accuracy, but also very small IR of the workpiece.The tonnage required by coining is 5-8 times higher than bottom bending.

V-opening width

The V-opening width required by coining is smaller than bottom bending, generally is 5X the thickness of sheet metal.

This is mainly for the purpose of reducing the IR of the workpiece so as to reduce the stamping into IR position of the workpiece by the punch tip.

Reducing the area of V-opening can obtain higher surface pressure.

Pressure limit

Because the pressure of bending is very large, the thickness of the SPCC should not exceed 2mm, and the thickness of SUS should not exceed 1.5mm.

The reason is that 2mm SPCC material need 1100KN pressure for bending which exceeds the allowable pressure of tooling 1000KN.

Note: different toolings have different allowable pressure, so not all toolings can be used to bend 2mm SPCC material.

Coining problem

The tonnage of the press brake needs to be increased due to the big bending force, and the abrasion of tooling will also become serious.

Therefore, only toolings with high allowable pressure can be used.


  1. Press brake toolings are made of high-quality steel products made by special heat treatment, with high hardness, not easy to wear and tear, able to bear great pressure, but each mold has its limits pressure: ton/meter, so choose the length of the mold when using the press brake toolings correctlyThat is, how much pressure per meter to bear, must not exceed the marked limit pressure.
  2. In order not to damage the die, we stipulate that align with the original point, it is necessary to use the upper and lower modes with the length of 300mm to do the alignment.The upper mold lower mold of the same height only can be used after the alignment.It is strictly forbidden to use the split small mold for the alignment, and the alignment must be based on the regulated pressure inside the press brake machine.
  3. In the use of the mould, due to the various height of the molds, the mold can only be used in a press brake machine with the same height, which can not be used in different heights.
  4. When using the press brake toolings, choose the right upper die and lower die according to the sheet metal hardness, thickness, and length.Generally, the slot width of the lower die should be 5-6 times the thickness of the metal plate, while the length should be longer than the sheet.The harder and the thickness of the sheet metal material, the wider of the lower die slot.
  5. When bending an acute angle or a dead angle, 30° punch should be chosen. Bending the acute angle first, then flattening.When bending R angle, R punch and R die should be chosen.
  6. In bending the long piece of work, it is better not to use a segmented mold in order to reduce the indentation at the mold connection position, and it is better to choose a single slot die, because the exterior angle R of lower die with single slot V is big, not easy to generate bending indentation.
  7. When selecting the top punch, we should know the type of die and the parameters well, and then decide which punch should be used based on the shape of the product that needs to be formed.
  8. When bending hard or too thick product, do not use press brake die to bend steel bar or other cylinder products.
  9. When using the press brake mold, the operators should have a clear mind.The upper die and lower die of press brake machine should be locked after alignment, in case the punch fall down to hurt the workers or damage the die.Attention should be paid when adding pressure during the operation process, do not add too much pressure and pay attention to the data changes on the display screen.
  10. After finish using the press brake toolings, put the die back in the press brake die cabinet (press brake store) according to the marking, clean the dust on the mold regularly, and apply the anti-rust oil to prevent rust which will reduce the precision of the toolings

Before discussing aluminum alloys, we should cover some background about the factors that affect their bendability. As you can imagine, products like aluminum foil, gutters, traffic signs, and automotive body parts, which are all made from aluminum alloys, have different bendability.

Three factors control the bendability of the various alloys in these products:

  • The formability of the aluminum alloy
  • Thickness and bend radius
  • Percent elongation

Let’s have a deeper look at these factors!

1st Factor – Formability

Formability is the ability of a given material to experience permanent deformation without the forming process cracking or tearing it. Permanent deformation is also known as plastic deformation in the materials science world.

Generally speaking, formability is a relative term and not a specific value. For example, the applied force necessary to shape a product depends on more than just the strength and ductility of a material. It also depends on factors such as the shape of the part and the thickness of the starting material.

In other words, we can measure the forming force to produce a specific part from a particular starting material. However, changing the shape of the part or the physical properties of the starting material will change the amount of force that needs to be applied.

2nd Factor – Thickness and Bend Radius

If you’ve handled regular aluminum foil, you’ll know that it is effortless to bend. However, if you had to bend a sheet of aluminum that was one-thousand times thicker than aluminum foil, it would be much harder! That is because the thicker a material is, the more difficult it is to bend.

You can also bend an aluminum gutter with your bare hands. But if you try to bend it to a tight angle without breaking it, you will have a hard time! Bending metal to a small bend radius has the potential to cause tearing or cracking.

The Fabricator offers certain key tables and general rules which are helpful for understanding the limits to bendability for specific aluminum alloys. You can use these to determine the minimum allowable bend radius for particular thicknesses of aluminum sheet.

3rd Factor – Percent Elongation

Percent elongation represents the ability of the material to be plastically deformed under tension. It is also known as plastic strain or stain applied beyond the yield strength limit of a material.
The more ductile aluminum alloys can experience more significant plastic deformation with small increases in applied stress. This results in better overall aluminum bendability.

Like the other properties, the percent elongation varies for each alloy. Take a look at the stress-strain curve above. You’ll see that annealed aluminum alloy 3003 (shown as AA3003-O) has a very high percent elongation (strain %) of roughly 35%. It has very high bendability relative to other alloys.


3 of the Best Aluminum Alloys for Bending

Numerous metal alloying agents can be combined with aluminum to produce different aluminum alloys. The system for naming them uses four digits, with the first digit representing their chemical composition. We explain this in our article on aluminum alloy designations and tempers.

Generally speaking, aluminum alloys from the 1XXX, 3XXX, and 5XXX series demonstrate better bendability than other aluminum alloys. Some 6XXX series alloys are fairly bendable as well.

However, the different properties offered by each may make some more desirable than others. For example, 1XXX series aluminum generally has poor mechanical properties and is not suited to structural applications.

Now let’s discuss which alloys offer the best bendability and when you should use them.

#1 – Aluminum Alloy 3003

This alloy is primarily alloyed with manganese and is one of the most commonly used aluminum alloys for bending applications. It has excellent formability properties and does not require heat to be bent or molded.

Companies often make gutters, roofing, siding, chemical equipment, and storage tanks from 3003 aluminum.

#2 – Aluminum Alloy 5052

With magnesium as the primary alloying element, AA5052 demonstrates moderate-to-high strength characteristics. At the same time, it retains good bendability, and designers can use it for more intensive applications than AA3003. The corrosion resistance of this alloy is also excellent against seawater, meaning it is excellent for applications in marine equipment.

Manufacturers often produce hydraulic tubes, traffic and hardware signs, medical equipment, marine equipment, and electronics (chassis and enclosures).

#3 – Aluminum Alloy 6061

You will find this is an extremely common alloy in your day-to-day life. Even though it is not as bendable or formable as the two alloys above, it is the strongest among all three. It has magnesium and silicon as alloying elements, and you can further enhance its strength with heat treatment.

Alloy 6061 is widely referred to as “structural aluminum” because it is so commonly used in structural (construction) applications. Nevertheless, due to its outstanding properties, it is also used in food and beverage containers, ladders, aircraft and automotive parts, scuba tanks, bicycle frames, and more.

Why Are These 3 Alloys Important?

Despite their different properties, these alloys are excellent examples of bendability in aluminum alloys. They demonstrate that even though some aluminum alloys feature better formability and percent elongation for a given bend radius and thickness, they each serve a unique purpose and a wide variety of applications.

Even with slightly lower bendability, the strength of alloy 6061 makes it one of the most widely used aluminum alloys. In the same way, alloy 3003 has multiple uses in applications that require superior bendability. Meanwhile, alloy 5052 is commonly used thanks to its balance in terms of bendability and strength.

Explanation of aluminum plate grade

1000 series representative

1050, 1060, 1070, 1000 series aluminum plate is also called pure aluminum plate.Among all series, 1000 series belongs to the series with the most aluminum content.The purity can reach more than 99.00%.

Because it does not contain other technical elements, the production process is relatively single and the price is relatively cheap.It is the most commonly used series in conventional industry at present.At present, most of the products in circulation in the market are 1050 and 1060 series.

The minimum aluminum content of the 1000 series aluminum plate is determined according to the last two Arabic numerals. For example, the last two Arabic numerals of the 1050 series are 50.According to the international brand naming principle, the aluminum content must reach 99.5%, and the above is regarded as qualified products.China’s aluminum alloy technical standard (GB / t3880-2006) also clearly stipulates that the aluminum content of 1050 must reach 99.5%.Similarly, the aluminum content of 1060 series aluminum plates must reach more than 99.6%.

1000 series typical brand and application:Pure aluminum plate 1060 is mainly used in occasions requiring high corrosion resistance and formability, but for parts with low strength, such as chemical equipment, marine equipment, railway oil tank car, conductor materials, instrument materials, welding rods, etc.

2000 series aluminum plate representative

2A16 (LY16), 2A06 (LY6) and 2000 series aluminum plates are characterized by high hardness, of which the original content of copper is the highest, about 3-5%.

2A12 aluminum alloy is a kind of high strength hard aluminum, which can be strengthened by heat treatment; 2A12 aluminum alloy has good spot weldability and tends to form intergranular cracks when gas welding and argon arc welding are used;

2A12 aluminum alloy has good machinability after cold work hardening.

The corrosion resistance is not high. Anodizing and painting methods or aluminum coating on the surface are often used to improve corrosion resistance.

2000 series aluminum plate belongs to aviation aluminum material, which is not often used in conventional industry at present.

At present, there are few manufacturers producing 2000 series aluminum plates in China.The quality cannot be compared with that of foreign countries.

At present, imported aluminum plates are mainly provided by Korean and German manufacturers.With the development of China’s aerospace industry, the production technology of 2000 series aluminum plate will be further improved.

3000 series aluminum plate representative

3000 series aluminum plates are mainly represented by 30033003 and 3A21. It can also be called antirust aluminum plate.The production process of 3000 series aluminum plate in China is excellent. 3000 series aluminum plate is mainly composed of manganese.The content is between 1.0-1.5. It is a series with good antirust function.It is commonly used in humid environments such as air conditioner, refrigerator and underbody. The price is higher than 1000 series. It is a more commonly used alloy series.

3000 series typical grade and its application:

3003 aluminum is mainly used for processing parts requiring good molding performance, high corrosion resistance, or good weldability, or workpieces requiring higher strength than 1000 series alloy, such as tanks and tanks for transporting liquid, pressure tanks, storage devices, heat exchangers, chemical equipment, aircraft fuel tanks, oil duct, reflective plates, kitchen equipment, washing machine cylinder block, rivet, welding wire.

3004 aluminum requires higher parts than 3003 alloy, chemical product production and storage devices, sheet processing parts, building baffles, cable ducts, sewers and various lamp parts.

4000 series aluminum plate representative

The 4000 series aluminum plate represents the 4a014000 series aluminum plate, which belongs to the series with high silicon content.

Generally, the silicon content is between 45-6.0%.It belongs to building materials, mechanical parts, forging materials and welding materials; Low melting point and good corrosion resistance.Product features: heat resistance and wear resistance.

5000 series representative

The 5000 series represents 5052, 5005, 5083 and 5A05 series.

5000 series aluminum plate belongs to the commonly used alloy aluminum plate series.The main element is magnesium, and the magnesium content is between 3-5%. It can also be called aluminum magnesium alloy.The main features are low density, high tensile strength and high elongation.

Under the same area, the weight of aluminum-magnesium alloy is lower than that of other series, so it is often used in aviation, such as aircraft fuel tank.

It is also widely used in conventional industry. The processing technology is continuous casting and rolling, which belongs to the hot-rolled aluminum plate series, so it can do deep oxidation processing.

In China, the 5000 series aluminum plate is one of the more mature aluminum plate series.

5000 Series typical grade and its application:

5052 aluminum has good forming and processing performance, corrosion resistance, weldability, fatigue strength and medium static strength. It is used to manufacture aircraft fuel tank, oil pipe, sheet metal parts, instruments, street lamp supports, rivets, wires, etc.

6000 series representative

6061 mainly contains magnesium and silicon, so it integrates the advantages of 4000 series and 5000 series.

6061 is a cold-treated aluminum forging product, which is suitable for applications requiring high corrosion resistance and oxidation resistance.

Good usability, excellent interface characteristics, easy coating and good processability.

It can be used for low-pressure weapons and aircraft joints.

General features of 6061: excellent interface features, easy coating, high strength, good usability and corrosion resistance.

Typical uses of 6061 aluminum: aircraft parts, camera parts, couplers, ship accessories and hardware, electronic accessories and connectors, decorative or various hardware, hinge head, magnetic head, brake piston, hydraulic piston, electrical accessories, valves and valve parts.

7000 series representative

7075 mainly contains zinc. It also belongs to Aviation series. It is aluminum magnesium zinc copper alloy, heat treatable alloy, superhard aluminum alloy and has good wear resistance.

7075 aluminum plate is stress relieved and will not deform and warp after processing.

All super large and super thick 7075 aluminum plates are detected by ultrasonic, which can ensure that there are no sand holes and impurities.

7075 aluminum plate has high thermal conductivity, which can shorten the forming time and improve the work efficiency.

7075 is mainly characterized by hardness.

7075 is a high hardness and high strength aluminum alloy, which is commonly used in the manufacture of aircraft structure and futures.

It requires the manufacture of high stress structural parts and molds with high strength and strong magic corrosion resistance.

At present, it basically depends on imports, and China’s production technology needs to be improved.

8000 Series

The more commonly used 8011 belongs to other series.

In my memory, it is an aluminum plate whose main function is to make a bottle cap. It is also used in radiators. Most applications are aluminum foil, which is not commonly used.

Figure 1 

To get the most out of your press brake dies tools, the tangent of the bend, where the radius starts, ideally should be halfway down the die face. In this situation, half the die face is equal to the outside setback (OSSB), the distance from the outside mold line (planes that run parallel to the workpiece) to the tangent point of the bend.



Our fabrication department is documenting standard processes to select the correct die and punch combination to produce the desired results when air bending. We want to achieve a 90-degree bend for a 0.0751-in.-thick piece of 304 stainless steel with an inside bend radius that’s also 0.0751 in. There’s the 20 percent rule, and then there’s the 8x material thickness rule. How should I apply these rules to select a die opening?


The 8x rule is an age-old rule of thumb based on 60,000-PSI-tensile cold-rolled steel that states it’s best practice to choose a die-opening width that’s eight times the material thickness. You generally get the best working results when working with the 8x rule. You ease forming and attain bend angle stability while working within tonnage requirements. You will find you can produce an inside radius approximately equal to the material thickness.

Still, “8x” is only a label, and the factor can increase or decrease with the material thickness. Sometimes the die-opening width equals 6x material thickness, other times 10x or 12x. The 8x rule is a good rule of thumb that keeps the tonnages low and the parts stable, at least to a point. But, unfortunately, it really doesn’t take different material types into account.

The 20 percent rule defines the floated inside radius in an air form over a given die. Unlike the 8x rule, the 20 percent rule can be factored for material type. In 304 stainless steel, the inside radius will be 20 to 22 percent of the die-opening width; for cold-rolled steel, the inside bend radius will be 15 to 17 percent; and for 5052 H32 aluminum, the inside bend radius will be 9 to 11 percent. You start with the median value (in the case of 304 stainless, this is 21 percent), then adjust if needed.

The 20 percent rule simply describes the resulting inside radius when air bending, and it’s used to calculate bend deductions. However, it usually isn’t a means of developing a die opening, because it doesn’t take into account springback or tonnage limits.

For your stainless steel job, you could rewrite the 20 percent rule formula—Width of die opening × 21 percent = Inside bend radius—to read Inside bend radius/21 percent = Width of die opening. This would give you: 0.075 in./0.21 = 0.357-in. die-opening width. But again, this does not take into account springback or tonnage limits, and it could seriously overload the press or the tooling. This is small for a die opening, and tonnage should be a consideration.

To achieve a certain radius, you need the right tooling and press brake. Ultimately, available die-opening widths in your tooling library, as well as tonnage capabilities of your tooling and press brakes, will determine the inside bend radii you can achieve when air bending a given material type and thickness. A tooling selection procedure for air bending should include the following:

1. Ensure that the specified inside bend radius is not less than the minimum sharp bend radius. If it isn’t, the inside bend radius can’t be achieved physically, short of stamping or bottoming. That’s because, when the bend turns sharp, the punch starts to dig a ditch into the material. For mild steel, a bend usually turns sharp when the inside radius reaches about 63 percent of material thickness. (For more on sharp bends, see How a bend turns sharp.) In your case, of course, you’re looking to achieve a ratio of 1-to-1 for material thickness to inside bend radius, which is certainly achievable, as long as your tooling and machines can handle the tonnage requirements.

2. Select the die opening. When it comes to any kind of machine, you generally do not want to overuse or underuse it. You get the most out of the machine at half of the maximum working value. That being said, isn’t the combination of the die, punch, and material really a “machine”? Of course it is. So what is half the working value of a die? Under perfect conditions, that point occurs halfway down the die face, as shown in Figure 1.

To find the geometrically perfect die opening—one in which the bend occurs halfway down the die face—use the following formula: (Outside bend radius × 0.7071) × Factor = Perfect die opening. (Editor’s note: For more detail behind this formula, visit www.thefabricator.com and type “Finding the perfect die opening” in the search bar.)

To calculate your outside bend radius, add the desired inside bend radius to the material thickness. So in your example, you would add 0.075-in. inside radius to the 0.075-in. material thickness and get an outside bend radius of 0.150 in.

The factor in the formula is a multiplier, and a multiplier of 4.0 would give you a value as close to geometrically perfect as possible, practically speaking, but with no allowance for springback. To account for springback, increase the multiplier slightly. In material thicknesses less than 0.125 in., a realistic working multiplier is 4.85. In material between 0.125 and 0.250 in., the multiplier is 5.85 in. (material more than 0.250 in. thick is calculated differently). This die selection method keeps the relationships consistent whether the radius is large and the material thin, or the material is thick and the radius small.

In your situation, you would calculate the following: (Outside bend radius × 0.7071) × Factor = Perfect die opening; or (0.150 in. × 0.7071) × 4.85 = 0.514 in. Of course, your shop probably doesn’t have a die-opening width of 0.514 in., so you’ll likely need to choose the closest width available, between a 0.472- in. or a 0.551 in. die. Choosing the closest available die opening will keep your inside bend radius as close as possible to the called value. This assumes that excess tonnage is not an issue if a smaller die is used.

(Note that using a factor of 4.0, the die width value would be 0.424 in., which, at least in the theoretical sense, is geometrically perfect for the job, but again does not take springback into consideration.)

3. Calculate the tonnage requirements. Now that you’ve determined the ideal die opening, you need to make sure it doesn’t exceed available tonnage of your press or tooling. To calculate this, use the following: [(575 × Material thickness2)/Die opening] × Material factor =Tonnage per foot.

We use the 60,000-PSI-tensile AISI 1035 (the most common type of cold-rolled steel used) as a baseline and so give it a material factor value of 1. To obtain a factor for a specific material, you can perform a simple comparison of tensile strengths, working with 60,000-PSI tensile as the baseline. If your 304 stainless is specified as having 85,000-PSI tensile, then you divide that tensile by 60,000 to get 1.4. So your tonnage calculation would be: [(575 × 0.005625) / 0.551] × 1.4 = 8.22 tons per foot. You will need to consider the length of the bend as well. If you’re within the tonnage limits of your tooling and press brake, then it’s on to the next step.

4. If the die width is acceptable, calculate the bend radius using the 20 percent rule. Start with the median value. Back to our 304 stainless example, the median percentage is 21. Multiply this percentage by the actual die opening you’ll be using, and you get the resulting inside bend radius: 0.551 in. × 0.21 = 0.1157-in. inside bend radius.

The actual radius will be approximately 0.116 in., which is as close to 1-to-1 as you can get in an air form. Yes, the radius is larger than a 1-to-1 ratio of bend radius to material thickness, but the die also is larger than perfect. Even a geometrically perfect die width would yield an inside radius slightly larger than the material thickness. Short of stamping, an exact 1-to-1 ratio is not possible without custom tooling.

5. Use this inside bend radius value to calculate the bend deduction. You now insert this inside bend radius value into your bend deduction formulas. Software has automated these calculations nowadays, but for a review of the math, check out “How the inside bend radius forms,” available at www.thefabricator.com.

6. Use the selected tool set to achieve the calculated bend deduction. You’ve determined that the inside bend radius is physically possible; you’ve selected a die-opening width that will get you as close as possible to the desired inside bend radius; you calculated your bend deductions based on the 20 percent rule; and you’ve taken available tonnage and springback into account. Considering all this, you’re well on your way to building perfect parts.


The press brake toolings are divided into two parts.

The toolings installed on the top of the ram are called punches, and the bottom toolings on the workbench are called dies.

The two parts work together on the metal plate to complete the bending of the workpiece.

The process that which the punch of the press brake exerts a force on the metal plate on the die is the bending process.

The upper die presses the metal plate through different power sources driving the ram.

The driving sources include mechanical, hydraulic, and servo motors.

Standards of Press Brake Toolings

There are different types of punches and dies.

The accuracy and efficiency of bending can be improved only by selecting the correct matching punches and dies for bending.

The standard size toolings are more convenient to replace. Because the design of tooling parts with standard dimensions is consistent.

In this way, there is no need to make too many adjustments when replacing the punches and dies. These toolings are kept in the same position to replace conveniently.

The upper parts of the ram of the press brake need a clamping device for fixing the punches.

The clamping can fix the punches in the required position. The punches bend the metal plate with the motion of the ram.

Die segmentation can facilitate the bending process of various sized workpieces. Press brake toolings require very high accuracy.

Especially the accuracy of punch tip and die shoulders. Cause these parts will directly contact the sheet metal during bending.

Moreover, the punches and dies with high precision can reduce the adjustment in the installation process.


Types of Punches and Dies

Press brake dies include V-die, U-die, and Z-die, and the most common is V-die. The minimum flange length shall be at least 4 times the material thickness.

Otherwise, the exact bending angle cannot be obtained. V-die sets with different opening widths match the corresponding punches.

In this way, the press brake can bend at different angles and different materials.

Press brake punches are mainly classified into three types: straight punch, gooseneck punch, and acute punch.

The die of the press brake can be divided into single V die, double V dies, and Multi-V die.


Consideration for Tooling Selection

Bending Material

First, the type of metal you want to bend is an important factor.

The thickness of the metal determines the die opening, bending radius, and bending angle.

For example, some steels have greater strength and higher resistance than other steels.

This resistance is the tensile strength(UTS) of the metal. The tensile strength of metals is different, which requires different strength molds.

In addition, it needs to be considered that the length of the metal plate determines how many toolings are required.

The V Opening and Radius of Material

When bending sheet metal, if the thickness and metal type are the same, there is not only one V-die opening size.

The sheet metal will not be lost during bending. If the internal radius is less than the thickness of the metal plate, the plate will be stretched, which will lead to the deformation of the workpiece. While a radius greater than the thickness of the sheet will not cause deformation.

When choosing the perfect V-die opening, we should not only avoid radius deformation but also choose a smaller radius.

The rule of 8

There is a rule of thumb applicable to the V-opening of press brake dies: the rule of 8.

The rule of 8 is based on 60000 PSI tensile cold-rolled steel. It stipulates that the V opening die shall be eight times the thickness of the bending material.

The rule of 8 applies to the most bending processes. Within the specified tonnage range, an internal radius approximately equal to the thickness of the material can be produced.

However, this is not a perfect law, because the factor will increase or decrease with the variation of the material thickness.

As a result, the width of some V-die openings is 6 times, 10 times, or even 12 times the thickness of the material.

The thicker plate is usually 10 times. Because thicker plates tend to lose some ductility.

So we use a larger V opening to distribute the force in a larger area and avoid cracks in the plate. Before determining the press brake dies, first determine the thickest and thinnest metal sheet to be bent.

Use the rule of 8 to determine the correct size of the V dies.

Select the smallest V to die and double its size to determine the next V die until the maximum mold is reached. If an exact match cannot be found, the dimensions should be rounded to the nearest available mold.

V opening affects the radius of bending material.

It is ideal on most bending materials when the internal radius equals 1 thickness. If the inner radius is less than 1 thickness, it means that the material extracted from the radius disappears.

In plate bending, if the inner radius is less than 1 thickness, we can see how the “side bulge” appears at the bending.


The larger the V-die opening, the larger the radius of the metal plate.

However, the tensile strength of the material will also affect the radius.

On a given V-die opening, the stronger the material, the greater the radius.

On mild steel, the bending radius (R) is usually 1 / 8 of the V-die opening, resulting in the following formula: R=V/8

This rule will vary for different metal types.

Minimum Length of the Flange or Leg

When selecting V-shape dies, pay attention to the flange length or leg required by the workpiece.

When bending, the sheet metal must always be in direct contact with the shoulder of the die.

Less than the specified flange length will fall into the V-shape opening.

If so, the bending result will be inaccurate.

Therefore, the larger the V-shape opening, the larger the minimum flange or leg on the metal plate.

The minimum flange formed by V-die is about 70% of the opening of the standard V-die.

The acute angle die can reach 110% or more of the V-die opening.

Before determining the minimum flange length, the sheet metal should be placed on the die.

In this way, the material contact on the die shoulder is equal to 20% of the V-die opening.


Correct tooling selection and material use can improve the accuracy of bending workpieces.

They will affect the bending angle, internal radius, minimum flange length, and the appearance of the whole workpiece.

Using the correct tooling can improve bending efficiency, reduce cost, prevent profile deformation, and protect the safety of press brake operators.