The tonnage of a bending machine is mainly determined by factors such as material thickness, length, material type, and bending angle. The commonly used formula is to calculate the required bending force based on the bending moment and the tensile strength of the material, and then determine the tonnage of the bending machine.
1. Calculation of bending machine tonnage
In general, the tonnage calculation of a CNC bending machine can be performed according to the following table and set in the processing parameters.
1) How to calculate the tonnage of a bending machine?
The values in the table are the bending pressure when the plate length is 1m.
For example: S=4mm, L=1000mm, V=32mm, and P=330KN is obtained from the table.
This table is calculated based on the material strength σb＝450N/mm2.
When bending other different materials, the bending pressure is the product of the data in the table and the following coefficients:
- Bronze (soft): 0.5
- Stainless steel: 1.5
- Aluminum (soft): 0.5
- Chromium-molybdenum steel: 2.0
“Accurate calculation of bending force is the primary condition for achieving efficient bending processing. For different materials, the difference in tensile strength will cause the bending force to change by more than 200%.” —- “CNC Bending Machine Technical Manual”, China Machinery Industry Press, 2023, page 37
2) Approximate calculation method of bending pressure
Bending machine tonnage calculation formula:
Pressure = 650S2L/V (σb = 450MPa)
- P: Bending force (Kn)
- S: Plate thickness (mm)
- L: Plate width (mm)
- V: Lower die V-shaped opening (mm)
The actual tonnage is affected by many factors. This formula calculates the bending machine force required for air bending. The actual force of our machine is preferably allowed to be up to 20% Additional capacity.
Comparison table of bending force calculation standards
| Material | ISO 14692 standard coefficient | DIN 6935 standard coefficient | JIS B 7501 standard coefficient |
|---|---|---|---|
| Low carbon steel | 1.0 | 1.0 | 1.0 |
| Stainless steel 304 | 1.5 | 1.45 | 1.5 |
| Stainless steel 316 | 1.65 | 1.7 | 1.65 |
| Aluminum alloy | 0.5 | 0.4-0.5 | 0.45 |
| Copper alloy | 0.7 | 0.65-0.75 | 0.7 |
| Titanium alloy | 1.8 | 1.75-1.9 | 1.85 |
Data source: International Organization for Standardization (ISO), German Institute for Standardization (DIN), Japanese Industrial Standard (JIS), 2024 edition
2. What factors affect machine tonnage?
1) Material tensile strength
The tensile strength of metal sheet refers to the maximum stress that the material can withstand when subjected to tension, and the unit is usually megapascal (MPa).
It is one of the important indicators of the mechanical properties of materials, which directly affects the performance and processability of materials. Tensile strength can be measured by the stress-strain curve of the material, usually in a standard tensile test.
Definition of tensile strength
Tensile strength refers to the maximum stress value that a metal material can withstand during a tensile test, from the application of external force to the fracture process.
The formula is:
σb=Fmax/A0
- σb is the tensile strength (MPa)
- Fmax is the maximum tensile force that the specimen can withstand when it breaks (N).
- A0 is the original cross-sectional area of the sample (mm²)
“According to the American Society for Testing and Materials (ASTM), the tensile strength of the same material with different thicknesses may differ by 5-15%, which will directly affect the calculation of bending force.” — American Society for Testing and Materials (ASTM), Material Performance Test Report E8/E8M-21, 2022
Factors affecting the tensile strength of metal plates
The tensile strength of different types of metal materials varies greatly, and is affected by the following factors:
Material type:
The atomic structure, alloy composition, grain size, etc. of the metal will affect the tensile strength.
Heat treatment:
Process treatments such as annealing, quenching, and tempering will change the crystal structure of the metal, thereby affecting the tensile strength.
Cold working:
Processes such as cold rolling and cold drawing will increase the tensile strength of the material through work hardening.
Thickness:
The thickness of the metal sheet will affect the tensile strength to a certain extent, especially for thin plate materials, which are more affected by deformation.
Tensile strength of common metal materials
The following are the tensile strength ranges of several common metal materials:
| Material | Tensile strength range (MPa) | Description |
|---|---|---|
| Low carbon steel (mild steel) | 300 – 500 | Common building and structural materials |
| Medium carbon steel | 600 – 800 | Used for mechanical parts |
| High carbon steel | 800 – 1200 | High hardness and strength |
| Stainless steel | 520 – 1250 | Corrosion resistant, widely used |
| Aluminum alloy | 200 – 600 | Light weight, easy to form |
| Brass | 300 – 900 | Good corrosion resistance |
| Titanium alloy | 700 – 1200 | Lightweight, high strength, good corrosion resistance |
| Magnesium alloy | 200 – 350 | Extremely light, but low tensile strength |
Importance of tensile strength in applications
Structural design:
Tensile strength is an important safety design indicator in buildings, bridges, and large mechanical structures. Materials must be reasonably selected based on their tensile strength to ensure the strength and safety of the structure.
Processing technology:
Metal forming processes such as bending, stamping, and deep drawing all need to consider the tensile strength of the material. Materials with high tensile strength are generally more difficult to form, but can provide higher strength.
Fatigue strength:
Tensile strength also affects the fatigue properties of materials in high-load, repeated stress environments.
Determination of tensile strength
Tensile strength is determined by tensile testing. On a tensile testing machine, a standard specimen is stretched until the specimen breaks, and its maximum stress is recorded. Typically, the test process is as follows:
- Fix the standard specimen to the fixture.
- Apply the tensile force step by step.
- Record the stress-strain curve until the material breaks, and use the maximum stress as the tensile strength.
2) Material thickness and length
Assuming the material thickness is 1/4 inch and the length is 10 feet, the empty bending force requires 165 tons according to the bending machine tonnage formula, while some formed workpieces require at least 600 tons of bending force.
If most of the workpieces are 5 feet or shorter, the length can be reduced by almost half, greatly reducing the procurement cost.
Therefore, the length of the workpiece is very important in determining the specifications of the new machine. Similarly, the thickness of the workpiece also plays an important role in the calculation of the tonnage of the hydraulic metal bending machine.
3) Upper punch (lower die) width
The punch (also called the upper die) of the bending machine is an important part of the bending machine die, responsible for transferring force to the material to complete the bending operation. The design and selection of the upper die directly affect the quality, precision and appearance of the workpiece. According to different bending requirements, the upper die has various shapes and types, each with its own unique purpose. Now various bending machines are designed to bend according to a certain proportion.
From the tonnage table of the bending machine, the force is designed according to the ratio of the bottom die to the thickness of the plate: 8 to 1. Usually, bending at this ratio can get a better bending effect.
But this ratio is not fixed.
- For thin materials, we can use a ratio of 6 to 1.
- But when forming thick materials, the ratio of the bottom die V groove to the material thickness can be selected to be 10 to 1 or 12 to 1, which can reduce the bearing capacity of the bending machine and get a perfect bending effect.
The basic structure of the upper die of the CNC bending machine
The upper die of the bending machine is usually a metal bar or strip tool installed on the upper beam of the bending machine. When bending, the upper die moves downward and cooperates with the lower die (usually a V-groove) to force the sheet to bend to the desired angle and shape.
The basic characteristics of a typical upper die include:
Die shape:
The shape of the upper die determines the shape of the bend. Common upper die shapes include straight knives, sharp-angle knives, rounded-angle knives, etc.
Die angle:
The upper die angle is generally 88° or 90°, which is used for standard 90-degree bending. There are also special upper dies for non-90-degree bending operations.
Material:
The upper die is usually made of hardened steel to withstand heavy loads and long-term use.
Common upper punch die types
Depending on the bending requirements, there are many types of upper dies for bending machines, mainly the following:
Straight knife (flat head upper die)
Features: The straight knife upper die has a straight blade, which is suitable for standard 90-degree bending or greater bending operations on materials.
Use: It is used for conventional 90-degree bending and some specific larger angle bending, and is widely used in daily production.
Acute angle punch
Features: The sharp angle knife upper die has a sharp blade (less than 90°), which is used for sharp bending operations less than 90 degrees.
Use: It is suitable for precision bending of thin plates, especially for occasions where sharp bending is required, such as making V-grooves, U-grooves or some special structural parts.
Round upper die
Features: The rounded upper die has a rounded edge, which is suitable for bending operations with a certain bending radius.
Use: It is used for workpieces that require a larger bending radius, such as aluminum and copper parts, to avoid cracks or fatigue damage in the material during the bending process.
Gooseneck punch
Features: The shape of the gooseneck punch is like an inverted “S” shape, which is designed to prevent the punch from interfering with the already formed parts.
Use: It is suitable for complex workpieces or operations that require multi-step bending, especially for secondary or multiple bending of formed parts.
Multi-function punching machine
Features: The punch has an adjustable angle and replaceable cutter head to adapt to bending at various angles.
Use: It is suitable for small batch and multi-variety bending operations, which can reduce the frequency of die changes and improve production efficiency.
Special design of the punch
For some specific processing requirements, the punch may have some special designs to meet more complex bending requirements:
Segmented punching
The punch can be designed as a segmented structure so that multiple small segments of the punch can be combined and used together. This design is easy to adjust and combine according to the specific shape and size of the workpiece, and is particularly suitable for the production of small batches or variable parts.
Upper and lower die matching accuracy
The precision matching of the upper die and the lower die is very important, especially in high-precision bending operations. The upper die head and the V-groove of the lower die must be accurately aligned to ensure the accuracy of the bending angle and shape.
Anti-indentation upper die
In some workpiece processing with higher requirements, in order to avoid indentations on the material surface, you can choose an upper die with a special coating or a smooth surface, which can reduce the surface damage of the workpiece while maintaining the bending strength.
Relationship between upper die and bending force
In addition to material thickness and tensile strength, the bending force is also related to the shape of the upper die. Generally, the sharper the upper die head, the smaller the contact area, and the greater the force required for bending. The blunter or rounder the upper die head, the smaller the required bending force. Therefore, choosing a suitable upper die shape helps to reduce the bending force and improve bending efficiency.
Maintenance and care of the upper die
After long-term use of the bending machine, the upper die may be worn due to high-intensity bending operations. In order to extend the service life of the upper die, regular maintenance and care are very important.
Regular cleaning
The upper die is prone to accumulate metal chips or oil during use, affecting the accuracy of bending. Therefore, it is very important to clean the surface of the upper die regularly and keep it smooth.
Anti-rust treatment
Especially for steel upper dies, anti-rust treatment can effectively extend its service life. Usually, a layer of anti-rust oil can be applied before storage to prevent moisture in the air from corroding the surface of the upper die.
Regularly check the wear and tear
During the use of the upper die, the blade may become blunt or produce tiny cracks due to frequent use. Regularly check the wear and tear of the upper die, and repair or replace it when necessary to ensure the stability of the bending quality.
The upper die of the bending machine punch is a key component in the bending process. Reasonable selection of the upper die can not only improve the bending efficiency, but also ensure the accuracy and surface quality of the finished product. Factors such as the type, angle, bending radius, and material thickness of the upper die need to be considered comprehensively to meet different bending requirements. Regular maintenance of the upper die and maintaining its accuracy are also important steps to ensure the smooth progress of the bending process.
4) V-die (lower die) opening size
The opening size of the lower die of the bending machine (the opening width of the V-groove) is a very important parameter in the bending process, which directly affects the size of the bending force, the bending accuracy and the quality of the finished product. Selecting the appropriate lower die opening size can ensure the forming quality of the workpiece during the bending process and reduce the force required for bending.
Definition of the lower die opening size of the bending machine
The lower die opening size refers to the width of the V-groove opening of the bending machine, usually represented by the letters V or W. Selecting the correct lower die opening size is one of the key links in the bending process because it directly affects the contact area and bending radius during the bending process.
Selection criteria for the lower die opening size
The lower die opening size is usually determined according to the plate thickness t. In actual production, the basic principle for selecting the lower die opening size is: the lower die opening width is 6 to 12 times the plate thickness.
The common selection range is as follows:
- For thin plates (thickness 1-3mm), the lower die opening size of 8 times the plate thickness is usually selected.
- For medium and thick plates (3-12mm), the lower die opening size is usually 10 times the plate thickness.
- For thick plates (above 12mm), the lower die opening size is usually 12 times the plate thickness.
Recommended die opening size table
| Plate thickness (mm) | Lower die opening size (mm) |
|---|---|
| 1.0 | 6 – 8 |
| 2.0 | 12 – 16 |
| 3.0 | 16 – 20 |
| 4.0 | 24 – 32 |
| 6.0 | 32 – 48 |
| 8.0 | 40 – 64 |
| 10.0 | 50 – 80 |
| 12.0 | 60 – 96 |
| 16.0 | 80 – 128 |
