In this post, we will discuss every detail about press brake bending basics, including the bending principle, springback analysis, most commonly used bending method, punch and die selection, bending force calculation etc.
Let’s dive in.
The application of bending processing in various fields
- Automatic financial cabinet
- Architectural decoration
- Office equipment
- Electric telecommunication industry
Bending principle of sheet metal
After v-shaped bending, compression deformation occurred on the inner surface of the bending part of the workpiece, and tensile deformation occurred on the outer surface.
The deformation of these compressions and stretches is greatest on the surface of the material. With the deepening of the plate thickness, the deformation is gradually reduced. It can also be said that there is a middle surface (neutral line) that is neither compressed nor stretched. Here we call it X—X line.
How to determine this neutral layer position?
— If the IR of the workpiece is 5X more than the thickness of the plate, then it’s position is in the center of the plate thickness.
— If the IR of the workpiece is 5X less than the thickness of the plate, the thickness of the bending position turns into t’, the position of the neutral layer is gradually shifted to the interior with the decrease of the IR of the workpiece.
— If the radius of the neutral layer is represented by P, then P and IR have the following relation:
- R≥5t, P-IR=0.5t
- R＜5t, P-IR=(0.25-0.4)t
The neutral layer has the characteristics of neither stretching nor compression, so the length of the neutral layer is used as the expansion length of the bending piece.
The reasons for spring back for sheet metal bending
The so-called bending is the deformation of tensile stress and compressive stress on the front and back of the same plate.
Once the plate is bent to the target angle, the material will bounce back to its original shape once the pressure is removed due to tensile stress and compressive stress. We call this kind of bounce as bending spring back.
The amount of bending spring back is generally expressed in terms of angle. Bending angle will be affected by material, plate thickness, pressure, bending radius etc.
It is very difficult to calculate the bending spring back accurately.
The force pressed on the sheet metal during bending is different, the counterforce is also different. Once eliminate the pressing force, the angle will also bounce back in downsizing direction. We call this “restore rebound”.
1) When using the same punch with the same thickness of the material, the resilience value SPCC＜AL ＜ SUS
2) When using the same punch with the same material, the thinner plate has more resilience.
3) When using the same materials, the one with bigger IR has more resilience.
4) The greater the pressing force, the less the resilience.
The 3 most commonly used bending method
|Bending Method||V-width||IR||Angle Accuracy||Features|
|Air Bending||12T—15T||2t~2.5t||＞±45’||Can achieve a wider range of bend angle.|
|Bottom Bending||6T—12T||1t~2t||±15’—30’||The higher bending precision is obtained with the smaller press force.|
|Coining||5T（4T—6T）||0t~0.5t||±10’||It can achieve high bending precision, but the bending force is very large.|
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.
Bottom bending (the most commonly used bending method)
Bottom bending 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.
Bottom bending is a method to obtain good bending precision with less pressure and is also a commonly used bending method.
The V-opening width of the die can refer to below table:
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.
The term “coining” is derived from the stamping method of the coin, which also means get very high accuracy. For 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 been 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.
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.
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 exceed 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.
The tonnage of press brake need 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.
Upper punch selection
1. The selection of upper punch is determined by workpiece shape. In other words, there can be no shape interference between the punch and the workpiece when bending the workpiece.
To realize the non-interference between the punch and the workpiece, the determination of the bending sequence will play an important role.
When choosing upper punch shape, the 1:1 figure or cross-section illustration of upper punch can be adopted.
2. The selection of punch tip R
The IR of the workpiece is determined by the V-opening of the lower die (IR=V/6), while the selection of the punch tip R is also determined by many different factors.
The IR of the workpiece can be obtained by the formula IR=V/6, the punch tip R can be slightly smaller than IR.
However, the 0.6R punch tip for thin sheet metal bending is been recommended in recent years due to:
- Able to center the punch and die correctly
- The abrasion of the punch tip
3. The selection of punch tip angle
For coining, the 90° punch will be used. However, when bending soft steel plate less than 2mm, the 90° punch also can be used if there is small spring back of the workpiece.
For the material with a large amount of spring back (like SUS, Al or medium plate), 88° punch→84° punch→82° punch can be selected according to different spring back of materials.
Besides, the angle of the die should be the same as the punch tip angle.
The most commonly used R angle of punch tip：
Standard punch tip angle of punch include: 90°,88°,86°,60°,45°,30° etc. Among them, the 90° bending generally use 88° punch.
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 use coining, please refer to the following table:
2. Confirm the minimum bending width (b) of the product. Check whether the selected V-opening meets the minimum bending width (b) of the workpiece. (b=0.7V)
Note: the smaller the V-opening, the more pressure of the bending will be needed.
3. If the drawings are not specified in ir, please use standard R (R=thickness).
If the ir is specified, V-opening must be selected strictly according to the specified ir (ir=V/6).
The selected V-opening should be bigger or smaller than the target V-opening width based on different conditions.
＊After determining the V-opening width, it is necessary to perform the bending force calculation.
For the calculated bending force, please confirm:
- 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, because the inside produces compression, and the outside produces stretching, there is a partial extension of the material, we call it elongation rate.
A+B-expansion length= elongation rate
The elongation rate of the material is not fixed. The main factors affecting the elongation rate are as follows:
- Properties of materials (texture, plate thickness)
- Properties of toolings (V-opening width, punch tip R)
- Processing properties (bending angle)
Now the extension rate of material is calculated by the computer. Each manufacturer’s method of calculation is patent technology and is not to be disclosed.
However, in the process of actual processing, there will be some deviation in the calculation of the extension rate, so the most accurate extension rate needs to be measured by the actual test.
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 used during processing exceeds the allowable value of the tooling, the tooling will deform, bend or burst.
The allowable tonnage on the tooling is measured in meters. It is calculated according to the length of the bending parts.
product length – 200mm, marks on the tooling: 1000KN/M
The max bending force cannot exceed 20 ton.
Allowable tonnage calculation of punch
Let’s take HRC47 material for example:
The ensured allowable tonnage (KN/M) = 9.42×H2/L×10
If H=15 L=30, allowable tonnage=9.42 ×(225/30) ×10=9.42 × 7.5×10=706.5KN/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
Stroke = opening height – intermediate plate height – punch height – die base height – (die height – 0.5V+t)
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°）
The outer layer is subjected to tensile stress and the inner layer is subjected to compressive stress during bending. There is a transition layer which subjected to neither tensile stress nor compressive stress is been called neutral layer.
The neutral layer stays the same length before and after the bending, so the neutral layer is the benchmark for calculating the length of the bending part.
Common factors affecting bending coefficient:
- die width
- die tip R
- punch tip R
- material’s rolling
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 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.
Related security strategy
Press brake is one of the press machines. If making only one type of product, it’s easy to control safety. However, if there are many kinds of products even with a small amount, the safety will not be controlled easily.
There are also safety countermeasures in the process of bending and the installation of the die.
Security problems that occur frequently in other assignments are also present in the bending process.
In the bending process, the fingers are often caught in the punch and die, which are also sandwiched between the punch and the workpiece.
For the safety countermeasures of accidents, it is not enough to rely on some light safety devices and fence type safety devices, and the correct operation methods and safety awareness of operators must be established.
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.
Incorrect tooling installation