Press Brake Bending Basics (A Guide to Sheet Metal Bending)

Looking to learn the basics of press brake bending? Look no further!

In this article, we will guide you through the fundamental principles of press brake bending, including the bending process, the most commonly used bending methods, punch and die selection, and the calculation of bending force.

Whether you are a press brake operator or simply interested in the manufacturing process, this article will provide you with all the information you need to know to get started.

From the working principle of the press brake to the different types of bending methods, we have got you covered. We will also delve into the importance of choosing the right press brake bending axis and how to calculate bending springback.

So, if you want to expand your knowledge on press brake bending and improve your skills, keep reading!

The information provided here can also be utilized for press brake operator training. Let’s get started.

What is press brake bending?

Press brake bending involves the elastic deformation of metal sheeting under the pressure of the upper die or lower die of a press brake machine, followed by plastic deformation.

At the start of plastic bending, the sheet is free to bend. With the pressure of the upper or lower die applied to the plate, it gradually conforms to the inner surface of the V-groove of the lower die, with the radius of curvature and bending force arm decreasing.

This continues until the upper and lower dies are in full contact at the end of the stroke, forming a V-shape, known as bending.

In general, press brake bending is a process technology that modifies the plate or angle of the plate by exerting pressure on it.

Working principle of the press brake

The upper and lower dies are mounted on the upper and lower worktables of the press brake machine, respectively. The relative movement of the worktables is driven by hydraulic transmission, and their shape, in combination with the upper and lower dies, enables the bending formation of the plate.

Common bending methods

Free bending, three-point bending, and correction bending are some examples. The distinction between these three methods can be seen in the diagram below.

Free Bending

Free bending, also known as air bending, is less complex than other methods. The bending angle is controlled by the depth of the upper die into the V-groove of the lower die.

The accuracy of the bent parts depends on various factors, such as Y1, Y2, and the V-axis upper and lower molds and plates.

However, it is widely used due to its versatility and broad processing range. It is applied to structures with a simple structure, large volume, or not too large output.

Three-Point Bending

Three-point bending, also referred to as die bending (bottoming), has a bending angle that is determined by the wedge height in the lower die.

The upper die only provides enough bending force and eliminates non-parallelism between the dies through the hydraulic pad on the ram.

This method can produce parts with high precision, meaning small angle and straightness errors. It is used for structures with complex structures, small volume, and mass processing.

Correction Bending

Correction bending is formed in the cavity composed of upper and lower dies, allowing for an ideal section shape to be obtained. However, it requires a large bending force and repeated mold repairs, and has poor mold versatility.

This bending method is often used when there are special requirements or special section shapes that cannot be achieved by free bending.

How to choose press brake bending axis

Y1 and Y2 Axis: Controls the movement of the ram up and down

V Axis: Controls the deflection compensation of the press brake

X, R, Z1, Z2, and X’ Axis: These are the control axis of the rear positioning system, responsible for controlling the positioning position of the rear stop (refer to the definition of each axis in the illustration)

T1 and T2 Axis: Servo material support (sheet follower). During the bending process, the processed plate follows the support, and the sheet followers provide support for the material.

The following axes are necessary for each press brake machine: Y1, Y2, and V. Users can select the rear stop and servo follow-up material support axes based on the needs of the processed parts.

It’s important to note that the X’ axis cannot be selected separately and must be used in conjunction with the Z1 and Z2 axes to have any practical significance.

V axis is the deflection compensation axis, and there are two implementation methods.

One is position control, which compensates for the elastic deflection deformation of the machine during bending by giving an equal amount of reverse deformation at its corresponding points based on the deflection deformation curve of the worktable during bending.

The other method is pressure control, which adjusts the pressure of multiple deflection compensation cylinders to generate a reaction force against the bending force at multiple points on the vertical plate of the workbench to prevent deflection deformation.

It’s generally agreed that position control results in higher bending accuracy and is used in 500T+ press brake machines. A schematic diagram of the worktable convex principle can be seen in the following figure.

The accuracy of the Y1, Y2, and V axes is crucial for the angle and straightness of the bent parts. It is important to note that for thin plates (<3mm), the accuracy of the bent parts is directly determined by the quality of the plate itself, such as the size of the thickness error, material uniformity, and rolling texture direction.

Bending principle of sheet metal

After V-shaped bending, the inner surface of the bending part of the workpiece experiences compression deformation, while the outer surface experiences tensile deformation.

The greatest deformation occurs at the surface of the material, and it decreases as the plate thickness deepens.

There is a neutral line, called the X-X line, that is neither compressed nor stretched.

To determine the position of the neutral layer, consider the following:

If the IR (inside radius) of the workpiece is 5 times greater than the plate thickness, the neutral layer is positioned in the center of the plate thickness.

If the IR of the workpiece is 5 times less than the plate thickness, the neutral layer position shifts towards the interior as the IR decreases, with the thickness of the bending position turning into t.

The relation between the radius of the neutral layer (represented by P) and IR can be described as follows:

• R≥5t, P-IR=0.5t
• R＜5t, P-IR=(0.25-0.4)t

The neutral layer has the characteristic of being neither stretched nor compressed, so its length is used as the expanded length of the bent piece.

The reasons for springback in sheet metal bending

Bending is the deformation of a plate caused by both tensile and compressive stress on its front and back.

After being bent to the desired angle, the material will tend to return to its original shape once the pressure is released, resulting in a phenomenon known as bending springback.

This springback is usually expressed in terms of the angle it causes and is influenced by various factors such as the material, plate thickness, pressure, and bending radius.

Accurately calculating the amount of bending springback is challenging.

The force applied during bending and the counterforce it creates are different, and once the pressing force is removed, the angle will decrease due to the restoring rebound.

1) When using the same punch with the same thickness of material, the resilience value for SPCC is lower than AL and AL is lower than SUS.

2) When using the same punch with the same material, a thinner plate has more resilience.

3) When using the same material, the one with a larger IR has more resilience.

4) The greater the pressing force, the less resilience.

Three most commonly used bending methods

Air bending

Air bending refers to a bending process where only a portion of the material comes into contact with the tooling.

As depicted in the image, the tooling only touches points A, B, and C on the metal during bending (the punch tip and die shoulders), while the rest of the material remains untouched.

As a result, the actual angle of the tooling becomes inconsequential. The bend angle is instead determined by the depth to which the punch descends into the die; the deeper the punch descends, the sharper the bend angle.

This means that fabricators can achieve a wide range of bend angles with just one set of tooling since the bend angle is controlled by the depth of the stroke rather than the tooling itself.

However, it is important to note that there will be some degree of spring back in air bending, so the desired bend angle can be achieved by bending the metal at a slightly sharper 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

Bottoming

Bottoming” refers to a bending method in which the punch is pressed down to the bottom of the die, causing the material to make contact with both the punch tip and the sidewalls of the V-shaped opening.

This method allows for the production of parts with good bending precision while using less pressure, and is widely used in the industry.

V-opening width

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

IR of workpiece

The interior radius of the workpiece is commonly denoted as IR.

In the bottom bending process, the IR is approximately 1/6 of the V-opening of the die (IR = v/6).

However, IR can vary for different materials such as SUS and Al, which have distinct IR values.

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.

Coining

The term “coining” is derived from the coin-making process, which is known for its high accuracy.

In the coining process, a sufficient tonnage of the press brake is used to shape the sheet metal to the precise angle of the punch and die. The sheet metal is not only bent, but it is also compressed between the punch and die.

Coining is not only accurate, but it also results in a very small interior radius (IR) of the workpiece. The tonnage required for coining is 5-8 times higher than that required for bottom bending.

V-Opening Width

The V-opening width required for coining is smaller than that required for bottom bending and is typically 5 times the thickness of the sheet metal. This is done to reduce the IR of the workpiece and minimize the stamping of the workpiece’s IR position by the punch tip. By reducing the size of the V-opening, a higher surface pressure can be achieved.

Pressure Limit

Due to the high pressure involved in bending, the thickness of SPCC should not exceed 2mm and the thickness of SUS should not exceed 1.5mm. For example, 2mm SPCC material requires 1100KN of pressure for bending, which exceeds the 1000KN allowable pressure of some tooling. Note that different tooling has different allowable pressures, so not all tooling can be used to bend 2mm SPCC material.

Coining Problems

Due to the large bending force involved in coining, the tonnage of the press brake must be increased, which can lead to serious wear and tear on the tooling. Thus, only tooling with high allowable pressure can be used for coining.

Top punch selection

1. The selection of the top punch is determined by the workpiece shape.

In simpler terms, there should be no collision between the punch and the workpiece during the bending process.

To ensure that the punch and the workpiece do not interfere with each other, determining the proper bending sequence is crucial.

When selecting the top punch shape, a 1:1 figure or cross-section illustration of the top punch can be used.

2. The selection of punch tip R

The interior radius (IR) of the workpiece is determined by the V-opening of the lower die (IR = V/6), and the selection of the punch tip radius (R) is influenced by several factors.

The IR of the workpiece can be calculated using the formula IR = V/6, and the punch tip radius can be slightly smaller than the IR. However, in recent years, a 0.6R punch tip has been recommended for bending thin sheet metal because:

• Able to center the punch and die correctly
• The abrasion of the punch tip

3. The selection of punch tip angle

For the coining process, a 90° punch is used.

However, if the spring back of the workpiece is minimal when bending soft steel plate less than 2mm, a 90° punch can also be utilized.

For materials with a significant amount of spring back (such as SUS, Al or medium plate), the 88° punch, then the 84° punch, and finally the 82° punch can be selected based on the material’s level of spring back.

It is important to note that the angle of the die should match the angle of the punch tip.

Common Punch Tip Radii (R):

• 0.2R
• 0.6R
• 0.8R
• 1.5R
• 3.0R

Standard Punch Tip Angles include: 90°, 88°, 86°, 60°, 45°, 30°, etc.

For 90° bending, the commonly used punch tip angle is 88°.

4. The segmentation of punch and die

• A-type segmentation: 100(left horn),10,15,20,40,50,200,300,100(right horn) = 835mm
• B-type segmentation: 100(left horn),10,15,20,40,50,165,300,100(right horn) = 800mm

Selection principle of 88° die and 90° die

The tensile strength of the material

• High tensile strength – choose 88° die
• Low tensile strength – choose 90° die

The bending spring back amount

• Large amount of spring back – choose 88° die
• Small amount of spring back – choose 90° die

Coining method

• Choose 90° die

V-opening width selection

1. If using coining, please refer to the following table:

Confirm the minimum bending width (b) of the product and ensure that the selected V-opening meets this requirement (b=0.7V).

Note:

The smaller the V-opening, the higher the pressure required for bending will be.

If ir is not specified in the drawings, use the standard R value (R=thickness).

If ir is specified, select the V-opening strictly based on the specified ir (ir=V/6).

The selected V-opening may need to be larger or smaller than the target V-opening width depending on the conditions.

After determining the V-opening width, perform a bending force calculation.

Confirm the following for the calculated bending force:

• Whether it can meet the tonnage requirements of the press brake for bending fabrication?
• Whether to meet the tooling’s allowable tonnage?

The elongation of the material

In the bending process, due to the compression on the inside and stretching on the outside of the material, there is a partial extension of the material, referred to as the elongation rate.

The formula for determining the elongation rate is A + B – expansion length.

The elongation rate of the material is not constant and is affected by various factors, including:

• Properties of materials (texture, plate thickness)
• Properties of toolings (V-opening width, punch tip R)
• Processing properties (bending angle)

The elongation rate of the material is now calculated by computers, with each manufacturer’s method being protected as patented technology and therefore not disclosed.

However, during actual processing, there may be deviations in the calculation of the elongation rate, so the most accurate measurement must be obtained through actual testing.

5 properties affect bend fabrication

• Mechanical properties: what machine tools are used
• Material properties: what materials are used
• Toolings properties: what toolings are used
• Fabrication properties: what size and angles
• Environment properties: under what circumstances

V-shape bending force calculation

• Ｐ: bending force (KN/M)
• Ｖ: lower die V-opening width (mm)
• Ｌ: bending length (mm)
• Ｔ: plate thickness (mm)
• σｂ: tensile strength of the material (N/mm²)
• Ｃ: correction coefficient

Ｃ correction coefficient list:

﹡The above calculation formula of bending force is obtained through experiments.

You can also check this article to learn all the 3 ways to calculate the required bending force.

The allowable tonnage of toolings

Each tooling has a corresponding maximum allowable tonnage value. If the pressure applied during processing exceeds the tooling’s allowable value, it can result in deformation, bending, or even breakage of the tooling.

The allowable tonnage of the tooling is measured in kiloNewtons per meter and is calculated based on the length of the bend parts.

For example, if the product length is 200mm and the marking on the tooling is 1000KN/M, then the maximum bending force is calculated as follows:

1000KN/M x 0.2M = 200KN/M (20 ton)

Therefore, the maximum bending force should not exceed 20 tons.

Allowable tonnage calculation of punch

Let’s take the HRC47 material as an example:

The calculated maximum allowable tonnage (KN/M) can be determined using the formula: 9.42 x H^2/L x 10.

For example, if H = 15 and L = 30, the maximum allowable tonnage can be calculated as:

9.42 x (225/30) x 10 = 9.42 x 7.5 x 10 = 706.5 KN/M = 70 TON/M.

The allowable tonnage of punch will decrease under the following conditions

① Open avoidance slot, hole punch or some other additional works

Open hole and slot at the horn

② When heating and hardness decrease

When using the grinding wheel cutting machine to make the horn, the hardness of the punch is decreased due to heat.

③ There’s a little bit of cracking

Continue to be used even there are tiny cracks

Selection of the punch height

The stroke is calculated as follows:

Stroke = opening height – intermediate plate height – punch height – die base height – (die height – 0.5V+t)

For example:

opening height: 370mm

Max stroke: 100mm

Stroke (above fig.) = 370-120-70-75-(26-0.5*8+t) = (83-t) mm

Attention should be paid when selecting the tooling’s height:

0.5V＜ stroke ＜ max stroke

Theoretical calculation of bending expansion（90°）

During bending, the outer layer is subjected to tensile stress, while the inner layer is subjected to compressive stress. There is a transition layer known as the neutral layer that is neither subjected to tensile nor compressive stress.

This neutral layer remains the same length before and after the bending and serves as the benchmark for calculating the length of the bent part.

Common factors affecting bending coefficient:

• thickness
• material
• die width
• die tip R
• punch tip R
• material’s rolling
• others

Material’s properties

1. The impact of plate thickness on the stroke

• If the thickness of the plate increases, the stroke of the bending angle will be decreased. (The thicker the plate, the smaller the V/t)
• The influence of plate thickness change on stroke change, SUS<SPCC<AL
• The impact of plate thickness on stroke increased:

(average plate thickness difference)< (nominal thickness) < (plate thickness changes)

2. The influence of material coefficient changes on the stroke

• The greater the V-opening width and thickness of the plate, the greater the influence of the material coefficient on the change of stroke.

(The larger the bending angle, the more susceptible to the change of coefficient)

• The influence of material coefficient change on the change of stroke, generally speaking.

AL  <   SPCC  <  SUS   gradually increasing.

• The change reasons of the material coefficient are as follows:

Not the same coil < Material differences within the same manufacturer < Different manufacturers < Material handling is different， gradually increasing based on the condition.

How to adjust the parallelism of bending workpiece

Regardless of whether you are a press brake operator or the head of the production department, it is important to understand the significance of parallelism in bending workpieces. I will outline 4 steps for adjusting the parallelism of bending workpieces for you.

1) Return the press brake ram to the starting position and reduce the pressure gauge value to the lowest value that just moves the ram.

2) Place two blocks of equal height on the table, ideally under the left and right cylinders.

3) Change the hydraulic sheet bending machine to the “jog adjustment” mode, remove the upper and lower molds and any other attachments, raise the mechanical block to its highest position, and disconnect the coupling on the mechanical block drive shaft gear.

4) Carefully place the ram onto the two blocks (the bottom face of the ram mold should touch the blocks).

Related security strategy

Press brake is a type of press machine.

When only producing one product, it is easy to maintain safety. However, when producing multiple products, even in small quantities, it becomes more difficult to control safety.

There are also safety measures to be taken during the bending process and when installing the die.

The same safety issues that arise in other tasks are also present in the bending process. For example, fingers can get caught in the punch and die or become sandwiched between the punch and the workpiece.

To prevent accidents, relying solely on light safety devices or fence-type safety devices is not enough. It is crucial to establish correct operating methods and raise operators’ safety awareness.

Safe operation

Confirm the toolings’ allowable tonnage

Confirm that the toolings’ center is consistent before the punch and die closure

Proper use of 2V die

Select the correct punch

When taking apart the toolings, try to insert the punch into the lower die to prevent the punch from falling and hurting the finger.

Do not hang items on the emergency stop button

Incorrect tooling installation