During the bending process, the force between the upper and lower dies is applied to the material, causing the material to plastically deform. Working tonnage refers to the bending pressure during bending.
The influencing factors for determining the working tonnage are: bending radius, bending method, mold ratio, elbow length, thickness and strength of the bending material, etc., as shown in the figure below.
Generally, the working tonnage can be selected according to the following table and set in the processing parameters.
1. The value in the table is the bending pressure when the sheet length is one meter:
Example: S=4mm L=1000mm V=32mm Check the table and get P=330kN
2. This table is calculated based on the material with strength σb=450N/mm2. When bending other 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.
3. Approximate calculation formula of bending pressure: P=650s2L/1000v The unit of each parameter P——kN S——mm L——mm V——mm
Bending pressure comparison table
B. Commonly encountered problems in hydraulic sheet metal brakes
Commonly used bending mold
Commonly used bending mold, as shown below. In order to extend the life of the mold, fillets are used as much as possible in the design of the parts.
If the height of the flange is too small, even using a bending die is not conducive to forming. Generally, the flange height L≥3t (including wall thickness)
Step processing method
For some low-profile sheet metal Z-shaped step bending, processing manufacturers often use simple molds to process on punches or hydraulic metal metal press, and the small batches can also be processed on the sheet metal press brake for sale with stepped die, as shown in the following figure. However, the height H should not be too high, generally it should be between (0～1.0)t, if the height is (1.0～4.0)t, it is necessary to consider the use of the mold form of the loading and unloading structure according to the actual situation.
The height of this mold step can be adjusted by adding shims, so the height H can be adjusted arbitrarily, but there is also a disadvantage that the length L size is not easy to guarantee, and the verticality of the vertical side is not easy to guarantee. If the height H size is large, it is necessary to consider bending on the bending machine.
Z-shaped step bending
There are two types of Hydraulic press for sheet metal bending : ordinary bending machine and CNC bending machine. Due to the high accuracy requirements and irregular bending shapes, the sheet metal bending of communication equipment is generally bent with a CNC bending machine. The basic principle is to use the bending knife (upper die) and V-groove (lower) of the Hydraulic sheet metal brake . Die), bending and forming sheet metal parts.
Advantages: convenient clamping, accurate positioning and fast processing speed;
Disadvantages: low pressure, can only process simple forming, and low efficiency.
Basic principles of forming
The basic principle of forming is shown in the following figure:
Basic principles of forming
Bending knife (upper die) of metal shear and brake
The form of the bending knife is as shown in the figure below. The processing is mainly based on the shape of the workpiece. Generally, the bending blades of the processing manufacturers have many shapes, especially the highly specialized manufacturers. In order to process various complex bending , Custom-made bending knife of many shapes and specifications.
The lower die generally uses V=6t (t is the material thickness) die.
There are many factors that affect the bending process, mainly including the radius of the upper die arc, material, material thickness, lower die strength, die size of the lower die and other factors. In order to meet the needs of the product, the manufacturer has already serialized the bending knife mold under the condition of ensuring the safety of the bending machine. We need to have a general understanding of the existing bending knife mold during the structural design process. See the figure below for the upper die on the left and the lower die on the right.
Schematic diagram of CNC Press Brake Mold
The basic principles of the bending process sequence for Sheet Metal Shear brake :
(1) Bending from inside to outside;
(2) Bending from small to large;
(3) Bend the special shape first, and then bend the general shape;
(4) After the former process is formed, it does not affect or interfere with the subsequent process.
The current Hydraulic bending brake form is generally shown in the following figure:
Press Brake Machine Bending Form
When bending sheet metal, a bending radius is required at the bending place. The bending radius should not be too large or too small, and should be selected appropriately. If the bending radius is too small, it is easy to cause cracking at the bending point, and if the bending radius is too large, the bending is easy to rebound.
The preferred bending radius (inner bending radius) of various materials with different thicknesses is shown in the table below
The data in the table above is the preferred data, for reference only. In fact, the corners of manufacturers' bending knives are usually 0.3, and the corners of a small number of bending knives are 0.5.
For ordinary low-carbon steel plates, rust-proof aluminum plates, brass plates, copper plates, etc., the inner fillet of 0.2 is no problem, but for some high-carbon steel, hard aluminum, super hard aluminum, this bending fillet It will cause bending and breaking, or cracking of the outer corners.
Bend and rebound
Bending rebound diagram
Rebound angle Δα=b-a
Where b——the actual angle of the part after rebound;
a—The angle of the mold.
The size of the rebound angle
See the table below for the springback angle when a single angle is 90° free bending.
Single angle 90° rebound angle when free bending
Factors affecting rebound and measures to reduce rebound
(1) Mechanical properties of the material The rebound 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, in order to reduce springback, the material should be selected as low-carbon steel as possible, not high-carbon steel and stainless steel.
(2) The greater the relative bending radius r/t, the smaller the degree of deformation and the greater the rebound angle Δα. This is a relatively important concept. The rounded corners of sheet metal bending should be as small as possible when the material properties allow it to improve accuracy. In particular, pay attention to avoid designing large arcs as much as possible, as shown in the following figure, such large arcs have greater difficulty in production and quality control:
The arc of the sheet metal is too large
Calculation of the minimum bending edge in one sheet metal bending press
The initial state of the L-shaped bend at the time of metal sheet press brake is shown below:
The initial state of the Z-shaped bending at the time of bending is shown in the figure below
The minimum bending dimension L corresponding to the Z-bending of sheet metal for large press brake forming with different material thickness is shown in the following table:
Z shaped Bending Min.Height