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Sheet Metal Forming——Sheet Metal Stamping And Bending

Views:425     Author:Site Editor     Publish Time: 2021-04-13      Origin:Site Inquire

The process of bending a blank into a certain angle or a certain shape with mechanical equipment and tools is called mechanical bending. According to the different types of bending equipment and processed materials, mechanical bending can be divided into sheet metal stamping and bending, sheet metal rolling, sheet metal bending, and so on. In the bending process, according to whether the blank is heated, the bending process can be divided into cold bending and hot bending.


The stamping and bending of sheet metal use presses and other pressure processing equipment to make the to-be-processed blank undergo plastic deformation under the action of a bending moment through a special bending die or a general bending die and the bending of the workpiece is completed in the working cavity of the die. The stamping and bending of sheet metal use presses and other pressure processing equipment to make the to-be-processed blank undergo plastic deformation under the action of a bending moment through a special bending die or a general bending die and the bending of the workpiece is completed in the working cavity of the die. Stamping and bending of sheet metal is an important part of mechanical bending, and it is also one of the main methods of sheet metal bending. It can bend curved parts with more complex shapes and relatively high dimensional accuracy.


Bending process

The following figure shows the bending deformation of the sheet metal. For the convenience of observation, before bending, mark the bending start line, bending middle line, and bending end line on the bending part of the sheet metal. The following figure (a) and the following figure (b) are bending Parts after forming.

Sheet metal forming

Deformation when the sheet metal is bent

It can be seen from the above figure (a) that before bending, the three lines ab=a'b'=a"b" on the section of the sheet material, after bending, the inner layer is shortened, and the outer layer is elongated, namely: ab<a'b' <a"b", see picture (b) above. This shows that when the sheet is bent, the material of the inner layer is shortened due to compression, and the material of the outer layer is elongated due to tension. Then, between the stretching of the outer layer and the compression of the inner layer, there must be a layer of material that does not change in length. This layer is called the neutral layer. Since the length of the neutral layer does not change before and after bending, the calculation of the expanded material should be based on this layer. The length is determined. The position of the neutral layer is related to the bending radius. In general, the change is not large, and it is usually approximately taken at the position of one-half of the material thickness.


After the sheet is bent, the thickness in the bending zone generally becomes thinner, and cold work hardening occurs, so the rigidity increases and the material in the bending zone appears hard and brittle. Therefore, if the bend is repeated or the rounded corner is too small, it will easily break due to tension, compression, and cold work hardening. Therefore, when bending, the number of bending and corner radius should be limited.


On the other hand, the bending of the sheet is the same as other deformation methods. When bending, the outer surface of the sheet is stretched and the inner surface is compressed. While plastic deformation occurs, there is also elastic deformation. Therefore, when the external force is removed, the bending Produce angle and radius rebound. The angle of rebound is called the rebound angle.


Minimum bending radius and bending spring back

Controlling or reducing the spring back of the bending angle and the bending radius of the bending part is an important content for obtaining the accuracy of the bending part and ensuring the quality of the bending part. In production processing, the control of bending angle and bending radius spring back is usually achieved by the minimum bending radius and bending spring back value.


⒈Minimum bending radius The minimum bending radius generally refers to the minimum value of the inner radius of the part that can be obtained by the press bending method. When bending, the minimum bending is limited by the maximum allowable tensile deformation of the outer layer of the sheet. If the deformation exceeds this degree, the sheet will crack. 


During the bending process, the bending radius is too small to cause bending cracks, but the bending radius is too large, the sheet will be completely restored to the original straight state due to spring back, at this time, the bending radius cannot be greater than the maximum bending radius Rmax:

Sheet metal forming

⒉The determination of the bending spring back value is generally determined according to the relative bending radius r/t (r is the inner fillet radius of the bending part, t is the thickness of the blank).

●When rlt<(5~8), the rebound value of the bending radius is not large, so only the angle rebound is considered.

●When r/t≥10, due to the relatively large bending radius, not only the angle of the workpiece rebounds, but the bending radius also has a larger rebound.


Process requirements for stamping and bending

The stamping and bending process can complete the processing of more complex shape parts, and the produced parts have the advantages of higher precision and good product consistency. To improve the bending quality and simplify the mold manufacturing, there are specific requirements in the following aspects for the processed bending parts.


⒈The fillet radius of the bent part should not be too large or too small. If the fillet radius is too large, it is not easy to guarantee the bending angle and fillet radius of the part due to the influence of spring back. If the fillet radius is too small, because it is easy to bend and crack, it needs to be bent twice or more, that is, bend into a corner with a larger fillet radius in advance, and then bend to the required bending radius, thereby prolonging the production cycle. It also brings disadvantages to bending work.


⒉When the relative bending radius r/t<0.5~1, the bending line should be perpendicular to the direction of the rolled fiber of the material. If the parts have different bending directions, the angle between the bending line and the direction of the rolled fiber should be maintained at 45°.


⒊The bending height of the bending part should not be too small, and its value is h>r+2t (see the figure below). Otherwise, because the supporting surface of the flange is not enough on the mold, it is not easy to form a sufficient bending moment, and it is difficult to obtain a part with an accurate shape. If the height of the flange does not meet the above-specified range, generally technical measures should be taken, that is, first lengthen the flange, and then cut off the excess part after bending.

Sheet metal forming

⒋For parts with a curved stepped shape, because they are easy to tear at the root of the rounded corners, the length B of the unbent part should be reduced to make it exit outside the bending line. If the length of the part is not allowed to be reduced, a groove must be cut between the bent part and the unbent part, as shown in the figure.


⒌For parts with notches on the curved edges, the notches should not be made in advance, and they will be cut off after they are formed. In this way, it can avoid the phenomenon of forks or forming difficulties during the bending process.

Sheet metal forming

⒍When the sheet with holes is bent, the distance I from the edge of the hole to the center of the bending radius should be ensured: when t<2mm; l≥t, when t≥2mm, l≥2t. If the hole is located in the bending deformation zone, the shape of the hole will be distorted.


⒎The shape and size of the bent parts should be as symmetrical as possible. To ensure that the material is balanced during bending and prevent slippage, the bending parts should be r=r2, r3=r4.

Sheet metal forming

Determination of the position of the eyelet of the bending part

Symmetrical bending parts

⒏The section of the blank obtained by shearing or punching often has burrs, so it is easy to cause stress concentration during bending. Therefore, the burr should be filed before bending, and at the same time, the side of the burr should be close to the punch in the compression zone and then bend to prevent cracks on the outer edge of the part.


Types and structure of bending die

There are many types of bending dies. According to the different shapes of the processed bending parts, the bending dies can be divided into V-shaped bending dies, U-shaped bending dies, and several-shaped bending dies. According to whether the mold uses a pressing device and its working characteristics, the bending dies can be divided into open type, with pressing device type, pendulum type, pendulum shaft type, etc. The common types and structures of bending molds are as follows.


⒈V. U-shaped part open bending molds that complete a bending process in one punching stroke of the press are called single-process bending molds. The open bending die structure can complete the processing of simple bending parts with low requirements for bending shape and dimensional accuracy. The figure below shows the open bending die structure of V and U-shaped parts, which is the simplest form of die structure.

Sheet metal forming

Open bending die for U and V-shaped parts

The upper and lower molds of the whole set of molds are open-type, convenient to manufacture, and have strong versatility. However, when the mold is used for bending, the sheet material is easy to slide, the side length of the bending part is not easy to control, and the bending accuracy of the workpiece is not easy. The bottom of the U-shaped piece is high and uneven.


⒉To improve the bending accuracy of the bent parts and prevent the sliding of the bent blank, the bending die structure with the pressing device can be used as shown in the

figure.


In figure (a), the spring ejector rod 3 is a pressing device used to prevent the blank from deviating during bending. In figure (b), a pressing device is set. When stamping, the blank is pressed on punch 1 and the pressing plate. 3. Gradually descend, the unpressed material at both ends slides and bends along the round corners of the female mold, enters the gap between the male mold and the female mold, and bends the parts into a U shape. Since the sheet material is always under the pressure between the punch 1 and the pressing plate 3 during the bending process, the flatness of the bottom of the U-shaped piece can be better controlled, and the bending accuracy can be better guaranteed.

Sheet metal forming

Bending of V and U-shaped parts with pressing device

⒊The semicircular bending mold diagram shows the structure of the semicircular bending mold. When working, put the blank between the positioning plates so that it cannot move freely. When the press is down, the punch will drop to a certain position to contact the surface of the material. When the punch continues to drop, the blank begins to bend, and the fillet rg slides. At the same time, the ejector 8 moves downward and compresses the spring. As the punch advances, the blank is bent and formed, and the spring is compressed to store energy. When the punch rises, the ejector pin uses the elastic force of the spring to hold the part Eject.

Sheet metal forming

Bending die for semicircular parts

To ensure the balance of the force when the blank is bent, the fillet radius r on both sides of the die 5 should be equal. The die is fixed on the lower die base 7 with two positioning pins and four screws. The die has two U-shaped positioning plates 4.


⒋Dumpling chain bending mold Figure 7-35 shows the dumpling chain bending mold. Among them: Picture (a) is the pre-bending mold of the dumpling chain, that is, the straight blank end is pre-bent into an arc, and then the subsequent rounding process is carried out; Picture (b) is the vertical dumpling chain bending mold, It has the advantages of simple structure and easy manufacturing. It is mainly used for the rolling of thicker and short-length parts with low forming quality requirements; Figure (c) shows the horizontal dumpling chain bending mold, which uses the inclined wedge 3 to push the rolling The concave mold 4 is bent and rolled in the horizontal direction, and the convex mold 1 also plays a role of pressing materials. The forming quality of the parts is better, but the mold structure is more complicated. For the two mold structures, if there are strict requirements on the quality of the rounding, the rounding with a mandrel should be used.

Sheet metal forming

Bending die for hinge parts

Generally speaking, when r/t>0.5 (r is the radius of the coil) and the quality of the coil is high, two pre-bending procedures should be used, and then the coil; when r/t=0.5~2.2, but the coil When the round quality requirements are general, the round can be rolled with one pre-bending; when rlt ≥ 4 or there are more stringent requirements on the round, the round with a mandrel should be used.


⒌Bending molds for closed and semi-closed bending parts The bending molds for closed and semi-closed bending parts are more complicated, and pendulum blocks and inclined wedge structures are mostly used in bending molds. Figure (b) is a one-time directly bent into the pendulum block type bending die structure of the clamp type cylindrical part shown in Figure (a), because the bending process is completed by the swing of the movable die 12 around the mandrel 11, so It is called swing bending die. The pendulum block bending mold structure can complete the processing of bending semi-closed and closed bending parts.

Sheet metal forming

Pendulum Bending Die

One-time direct bending into the pendulum bending dies structure of the clamp type cylindrical part as shown in Figure (a). Since the bending process is completed by the swing of the movable die 12 around the mandrel 11, it is called swing bending mold. The pendulum block bending mold structure can complete the processing of bending semi-closed and closed bending parts.


When the mold is working, the blank is positioned by the positioning groove on the movable die 12. When the upper mold moves downward, core 5 first bends the blank into a U shape, and then core 5 presses the movable die 12 to swing it toward the center to bend the workpiece. After the upper mold rises, the movable concave mold 12 is lifted and separated by the top post 10 under the action of spring 9. The workpiece remains on core 5 and is taken out longitudinally.


The following figure shows the structure of the bending die with an oblique wedge for closed and semi-closed bending parts with a bending angle of less than 90°.

Sheet metal forming

Bending die with an oblique wedge with bending angle less than 90 degrees

When the mold is working, the blank part is first pressed into a U-shaped part under the action of the punch 8. As the upper template 4 continues to move downwards, the spring 3 is compressed, and the two oblique wedges ⒉ mounted on the upper template 4 press against the roller 1, causing the movable concave modules 5 and 6 with the roller 1 to move to the middle respectively. , Bend both sides of the U-shaped piece inward to an angle less than 90°. When the upper mold returns, the spring 7 resets the female module. Since the mold structure relies on the elastic force of the spring 3 to press the blank into a U-shaped piece, limited by the spring force, it is only suitable for bending thin materials.


Determination of the main process parameters of bending

To ensure the quality of bending parts, the following process parameters should be determined when formulating the bending process and the design of related bending dies.


⒈Calculation of bending force: The bending force refers to the pressure applied by the press when the workpiece completes the predetermined bending. The bending force includes free bending force and correcting bending force.


●Calculation of free bending force: The bending force F during free bending refers to the bending force required for bending deformation of the sheet metal.

sheet metal shaping

Where F free bending force-free bending force at the end of the stamping stroke, N;

K——safety factor, generally take K=1.3;

b——the width of the bent part, mm;

t——the thickness of the bending material, mm;

r——the inner bending half of the bending part, mm;

The strength limit of the material, MPa.


●Calculation of correcting bending force: Since the correcting bending force is much larger than the pressing bending force when correcting bending, and the two forces act one after another, only the correcting force needs to be calculated. The correction force F of V-shaped parts and U-shaped parts is calculated by the following formula F correction bending force = Ap

Where F——the bending force when correcting the bending, N;

A——The vertical projection area of the correction part, mm2;

p——correction force per unit area, MPa, select according to the table.

Material Thickness t/mm
≤3 3~10
Al 30~40 50~60
Brass 60~80 80~100
10~20 Steel 80~100 100~120
25~35 Steel 100~120 120~150
Titanium alloy TA2 160~180 180~210
Titanium alloy TA3 160~200 200~260

●Calculation of ejector force or discharge force: When the bending die is equipped with an ejector device or discharge device, the ejector force F or discharge force F can be approximately 30% of the free bending force~ 80%.


●Determination of the tonnage of the press: the tonnage of the press is determined separately according to the two conditions of free bending and correcting bending.


When free bending, considering the influence of the ejector force or unloading force during the bending process, the tonnage F of the press is F press tonnage ≥ (1.3~1.8) F free bending force.


When correcting the bending, the correcting force is much larger than the ejector force and the unloading force. The weight of F top or F unloading is insignificant, so the tonnage of the press is F press tonnage ≥ F correcting bending force.


⒉Determination of the Bending Die Gap The size of the gap Z between the punch and the die has a great influence on the pressure required for bending and the quality of the parts.


When bending a V-shaped workpiece, the gap between the convex and concave molds is controlled by adjusting the closing height of the press, so there is no need to determine the gap on the mold structure.


When bending U-shaped workpieces, an appropriate gap must be selected. The size of the gap has a great relationship with the quality of the workpiece and the bending force. For general bending parts, the gap can be obtained from the table or directly obtained by the following approximate calculation formula.


When bending non-ferrous metals (red copper, brass), Z=(1~1.1)t

When bending steel=(1.05~~1.15)t


When the precision of the workpiece is high, the gap value should be appropriately reduced, taking Z=t. In production, when the material thickness is not required to be thinner, to reduce springback, etc., also take the negative gap, take Z=(0.85 ~0.95)t.


⒊Calculating the size of the working part of the bending die The design of the working part of the bending die is mainly to determine the convex and concave mold fillet radius and the size and manufacturing tolerance of the convex and concave molds.


The corner radius of the punch is generally slightly smaller than the radius of the inner corner of the curved part. The corner radius at the entrance of the die should not be too small, otherwise, the surface of the material will be scratched. The depth of the die should be appropriate. If it is too small, there will be too many free parts at both ends of the workpiece, and the bent part will rebound greatly, and it will not be straight, which will affect the quality of the part; if it is too large, it will consume more die steel and require a longer press stroke.


The size of the die thickness H and groove depth his determined for the bending of V-shaped parts. The structure of the die is shown in the figure. The size of the die thickness H and groove depth his determined in the table.

sheet metal shaping

Schematic diagram of the mold structure of the curved V-shaped part

The determination of the dimensions H and h of the curved V-shaped part.

Thickness 1 1~2 2~3 3~4 4~5 5~6 6~7 7~8
h 3.5 7 11 14.5 18 21.5 25 28.5
H 20 30 40 45 55 65 70 80

Note: 

1. When the bending angle is 85°~95°, L1=8t, r convex=r1=t. 

2. When k (little end) ≥ 2t, the value of his calculated according to the formula h=L1/2-0.4t.


●The determination of the radius and depth of the bend fillet The determination of the fillet radius r concave and the depth L0 of the V-shaped and U-shaped bends are shown in the figure and the table below.

sheet metal shaping

Bending die structure size

●Calculation of working size of bending punch and die.

When the workpiece needs to ensure the external dimensions, take the concave mold as the reference, and the gap is taken on the punch; if the workpiece is marked with the internal dimensions, take the punch as the reference, and the gap is taken on the concave mold.


When the workpiece needs to ensure the external dimensions, the size of the concave mold L and the size of the punch L convex are calculated according to the following formulas:

sheet metal shaping

When the inner dimension of the workpiece is to be guaranteed, the punch size L convex and the concave die size L concave are calculated according to the following formulas:

sheet metal shaping


Essentials of Bending Die Design and Application

The use of bending molds can complete the processing of various relatively complex shapes. Among them, the design of the bending mold is the key to ensuring the shape, size, and accuracy of the bending parts. For this reason, the following essentials must be paid attention to when designing and applying the bending mold.


⒈To produce qualified bending parts economically and reasonably, it is usually required that the dimensional tolerance level of the bending part should be better than IT13, and the angle tolerance should be greater than 15'. The following table shows the tolerance levels that can be achieved for various dimensions of stamping and bending parts.


The angle tolerances of general bending parts are shown in the table. The precision-level angle tolerances in the table can only be achieved by adding shaping procedures.

sheet metal bending

Thickness t/mm A B C A B C
Economical Precision
≤1 IT13 IT15 IT16 IT11 IT13 IT13
1~4 IT14 IT16 IT17 IT12 IT13~14 IT13~14

Tolerance class of bent parts

Short side of bending part 1~6 6~10 10~25 25~63 63~160 160~400
Economical ±1°30‘~±3° ±1°30‘~±3° ±50‘~±2° ±50‘~±2° ±25‘~±1° ±15‘~±30’
Precision ±1° ±1° ±30‘ ±30‘ ±20‘ ±10‘


⒉Formulating a correct and reasonable bending process plan is a prerequisite for ensuring the quality of the bent parts. Generally, when formulating a bending process plan, for simple-shaped bent parts, one-time forming is mainly considered. At this time, the main consideration should be whether the process arrangement can guarantee the workpiece The shape, size, and tolerance level are required; for curved parts with more complex shapes, two or more bendings are generally used. For particularly small workpieces, a set of complex molds should be used as much as possible to form, which is helpful to solve the safety problems of the positioning and operation of the bent parts. It is also possible to use strips, coils, etc. to use progressive molds. For multiple bending parts, generally, bend the corners of the two ends first, and then bend the corners of the middle part, and the previous bending must consider the reliable positioning of the subsequent bending. The latter bending does not affect the previously formed part. For stamping parts with a large number of corners and bending times and stamping parts with asymmetrical shapes, attention should be paid to the reliability of the process used. For punching parts with holes or cuts, pay attention to the size errors that are particularly likely to be caused or appear due to the effect of bending. At this time, it is best to punch and cut after bending. Also, the bending forming of large thick plates is often done on a press concerning molds or tires. At this time, the bending process should mainly consider the economy, reasonableness, and good operability and maintainability.


⒊When designing the bending die, it is necessary to combine the processing technology of the bending parts, carefully analyze the problems that are likely to occur in the bending process of the structure of the processed parts, and take corresponding measures during the mold design, so that the designed mold structure can meet the processing requirements needs. For example: In single-angle bending, due to the unbalanced bending force during the bending process, the sheet material is prone to slippage. Therefore, in the mold structure, there should be anti-skid measures. The following figure shows the measures often used in the processing of acute-angle bent parts: Figure (a) is a commonly used positioning using the existing holes on the board or adding process holes; Figure (b) is using the positioning block of the mold to prevent side movement, and cooperate with the strong press edge The force controls the possible slippage caused by the bending of the part; and Figure (c) uses the strong pressing force of the mold, and at the same time, uses the inclined wedge to bend. Because the bending process is smooth and gentle, the accuracy of the bent part is better and It can better control the bending rebound.

sheet metal shaping

Anti-slip structure of bending die

The anti-skid structure of the above bending die is suitable for all single-angle bending. To increase the obstructive effect of the press plate on the sheet material, in addition to increasing the spring force, if the part does not require high surface quality, the following measures can often be taken. Figure (a) shows the installation of sharp pain in the discharge block of the lower mold. The sharp angle of 60° protrudes from the plane of the pressing block by 0.1 to 0.25mm, and the sheet material is pressed on the sharp corner by the punch. The protruding height of the pointed pin is adjusted by a bolt with a thread on the head, and is locked with a nut with an external thread; Figure (b) is to add a pointed pin on the spring pressure plate of the upper mold, and when the material is bent and pressed, it is wedged into the plate Without sliding the board.

sheet metal shaping

Ways to increase the pressing force

The commonly used press pin form is shown in the figure:

sheet metal shaping

Common form of pressing pin

Picture (a) is to wedge the outer edge of the sharp edge into the board surface, and the wedge depth is below 0.12mm; Picture (b) is the stop pin with blade b, the effect is better, to prevent the rotation of the round pin, you can use another The round pin is prevented from rotating by the long groove c. Picture (c) is a pin with an embossed pattern on the head. It is used for occasions where the sheet material does not move too much, but after use, there is no obvious pit on the sheet; picture (d) is used In the case of a large movement of the sheet material, the sharp wedge e is 8°-12°, the relief angle is 25°-30°, and the long groove f is also used to prevent the rotation of the bolt.


Another example is when bending asymmetrical polygonal bending parts if the bending die shown in the following figure (a) is used for bending when the punch is pressed down, point B first contacts the material, which is caused by uneven force on the blank. Offset, and then C-point contact causes the blank to be bent by bidirectional pressure. When the punch continues to fall, because point B is affected by the frictional resistance of points A and C, the material at corner B will be strongly stretched and break, so the dimensional accuracy of the part cannot be guaranteed. If the bending method shown in the following figure (b) is adopted, that is, the working parts of the convex and concave molds are made into an inclined state, the above-mentioned defects can be overcome. This is because the material force point B is located on the vertical centerline, and the pressure center point D exactly divides AC (that is, AD=DC). Therefore, when the punch is pressed down, the forces on points A and C are uniform and equal, which prevents The blank is shifted, and at the same time, the stretched condition of the material at the B corner is changed, thereby ensuring the quality of the part.

sheet metal shaping

Bending method of asymmetric polygonal bending parts

⒋It is necessary to carefully analyze the processing material and surface quality requirements of the bending parts. For non-ferrous metals with high surface quality requirements and vulnerable to damage, to ensure the quality of the parts and the service life of the mold, the appropriate processing method should be determined and the corresponding mold structure should be designed. Generally, the available mold structure is as follows.


The following figure (a) is the mold structure with rollers added to the concave mold to reduce friction and protect the curved surface; the following figure (b) is the mold structure with only rollers; the following figure (c).

sheet metal shaping

Bending die structure to protect the curved surface

It is a bending die with a lever. Because friction is eliminated, it helps to protect the curved surface. It can be used for bending workpieces with or without flanges.


When bending thick and hard plates, the bending die should adopt the oblique angle form shown in Figure (a). The concave die mouth is inclined about 30°, and the gap between the die and the convex die is 3t, and then the rounded corner and the straight plane are smoothly transitioned, where: rd=(0.5~2)t, rd2=(2~4)t. If necessary, the transition part of the mold can also be made into geometric shapes such as parabola that are easy to slide into the cavity, so that the material flow resistance is small, the flow is stable, the contact area with the cavity is increased, and the compression stress of the cavity is reduced. The rounded corners of the die are not prone to agglomeration, and no strain is formed on the workpiece, which improves the forming quality of the bent part and the life of the die. For thick non-ferrous metal bending, to prevent the workpiece and the die mouth from grinding grooves during bending, and cause the deflection of the plate, the roller dies shown in Figure (b) can be used for bending. When working, after the workpiece blank is positioned between the positioning pins, the punch moves down, and the blank is smoothly bent to the bottom block between the rollers. The depth of the concave mold is ((8~12)t and a negative gap (0.9~0.95)t can be used. Large impact method to reduce rebound.

sheet metal shaping

Bending die for protecting thick plate bending

For metal bending, to prevent the workpiece and the die mouth from grinding out grooves during bending, and cause the deflection of the sheet material, the roller dies shown in Figure (b) can be used for bending. When working, after the workpiece blank is positioned between the positioning pins, the punch moves down, and the blank is smoothly bent to the bottom block between the rollers. The depth of the concave mold is ((8~12)t and the negative gap (0.9~0.95)t can be used. Large impact method to reduce rebound.


Besides, for the bending processing of non-ferrous metals, the round corners of the die should be kept smooth and clean at all times, and heat-treated to 58-62HRC. For bending processing of stainless steel, the working part of the die is best designed as an insert structure and made of aluminum bronze.  

sheet metal shaping

Bending of non-ferrous metals

⒌For V-shaped, U-shaped, Z-shaped, and other bent parts with simple shapes, multiple varieties, and small production batches that appear in production, to shorten the mold manufacturing cycle and reduce product manufacturing costs, general bending molds can generally be used to complete the processing of the parts.


⒍The general bending die structure for bending V and U-shaped parts is used on the press. The characteristic of this kind of mold is that the two concave molds 7 can be matched to make four angles, and they can be matched with four kinds of convex molds with different angles to bend V and U-shaped parts with different angles.


When working, the blank is positioned by the positioning plate 4, and the positioning plate can be adjusted back and forth and left and right according to the size of the blank. The concave mold 7 is installed in the mold base 1 and fastened by screws 8. The concave mold and the template are processed into an H7/m6 transitional fit, to ensure the bending quality and accuracy of the workpiece. After the workpiece is bent, it can be ejected by the ejector rod ⒉ through the buffer to prevent the bottom surface of the workpiece from bending.


The figure below shows the general bending die structure for bending U-shaped parts. 


The working parts of the whole set of molds adopt a movable structure to adapt to the processing of parts with different widths, different thicknesses, and different shapes (U, several shapes). A pair of movable concave molds 14 are installed in the mold sleeve 12, and the working width of the two concave molds can be adjusted to an appropriate size by adjusting bolt 8 according to the width of different bending parts. A pair of ejector blocks 13 are always close to the concave die under the action of spring 11, and play the role of material pressing and ejecting through the backing plate 10 and the ejector rod 9. A pair of main punches 3 are installed in a special mold handle 1, and the working width of the punches can be adjusted by bolts 2.

sheet metal shaping

When bending parts, a secondary punch 7 is also needed, and the height of the secondary punch can be adjusted by bolts 4, 6, and inclined top block 5. When bending the U-shaped piece, it can be adjusted to the highest position.


Installation and adjustment of bending die

Bending processing on the press with a bending die is the most important form of bending processing. The processing should be carried out in strict accordance with the stamping operation rules to prevent misoperation. To complete the bending process of the parts, the installation and adjustment of the bending die should be done first.



⒈The installation method of the bending dies The installation method of the bending die is divided into two types: the non-guided bending die and the guided bending die. The installation method is the same as that of the punching die. The installation of the bending die is the same as the gap between the convex and concave dies. In addition to the adjustment of the adjustment, discharge device, etc., the two bending dies should also complete the adjustment of the upper and lower positions of the upper bending die on the press at the same time. Generally, it can be carried out according to the following methods.

Installation and adjustment of bending die

Universal bending die suitable for U-shaped and square-shaped parts

First, when bending the upper die, the rough adjustment should be made on the press slider, and then a gasket or sample that is slightly thicker than the blank should be placed between the lower plane of the upper punch and the discharge plate of the lower die, and then use the adjusting link The method of length is to pull the flywheel or jog by hand again and again until the slider can pass through the bottom dead center normally without blockage or stop. In this way, the flywheel can be pulled for several weeks to finally fix the lower die for trial punching. Before the test punching, the gaskets placed in the mold should be taken out. After the test punching is qualified, the fastening parts can be tightened again and checked again before they can be officially put into production.


⒉The adjustment points of the bending die When the bending die is used for processing, to ensure the quality of the bending part, the bending die must be carefully adjusted. The adjustment and precautions mainly include the following aspects.


●Adjustment of the gap between the convex and concave molds. Generally speaking, after the upper and lower positions of the upper bending die on the press are determined according to the installation method of the above bending die, the gap between the upper and lower bending dies is also guaranteed at the same time. The relative position on the press is all determined by the guide parts, so the lateral clearance of the upper and lower molds is also guaranteed; for the bending mold without a guiding device, the lateral clearance of the upper and lower molds can be cushioned Use cardboard or standard samples to adjust. Only after the gap adjustment is completed, can the lower template be fixed and tested.


●Adjustment of the positioning device. The positioning shape of the positioning parts of the bending die should be consistent with the blank. During the adjustment, the reliability and stability of its positioning should be fully guaranteed. Using the bending die of the positioning block and positioning nail, if the position and positioning are found to be inaccurate after trial punching, the positioning position should be adjusted in time or the positioning parts should be replaced.


●Adjustment of unloading and returning devices. The discharge system of the bending die should be large enough, and the spring or rubber used for the discharge should have sufficient elasticity; the ejector and the discharge system should be adjusted to be flexible in action, and the product parts can be discharged smoothly, and there should be no jams and Astringent phenomenon. The force of the unloading system on the product should be adjusted and balanced to ensure that the surface of the product after unloading is smooth and will not cause deformation and warpage.


⒊Precautions for adjusting the bending die When adjusting the bending die, if the position of the upper die is lowered, or you forget to clean out the gasket and other debris from the die, the upper die, and the lower die will be under the stroke during the stamping process. Violent impact at the dead center position may damage the mold or punch in severe cases. Therefore, if there are ready-made bent parts at the production site, the test piece can be directly placed on the working position of the mold for mold installation and adjustment, to avoid accidents.


Methods to improve the quality of press-bent parts

The main factors affecting the quality of press-bent parts are spring back, offset, fracturing, and changes in the cross-section of the deformed area. The measures and methods adopted mainly include the following aspects.


⒈Factors influencing the rebound value and prevention methods The forming process of the bent part goes through two stages from the elastic deformation of the material to the plastic deformation. Therefore, after the plastic deformation of the metal, elastic deformation is inevitable, resulting in bending spring back and tending to bend The direction of the front, so that the angle and fillet radius of the part after bending, the bending angle and fillet radius of the part and the die have a certain difference, that is, the bending spring back. According to the factors caused by bending spring back, the following measures can be taken.


●Take measures from the selection of materials. The rebound angle of the bending rebound is proportional to the yield limit of the material and inversely proportional to the elastic modulus E. Therefore, on the premise of meeting the requirements of the use of the bending parts, materials with a large elastic modulus E and a small yield strength os should be selected as much as possible to reduce the spring back during bending. Besides, according to experiments, when the relative bending radius r/t is 1 to 1.5, the rebound angle is the smallest.


●Improve the structural design of bending parts. Under the premise of not affecting the use of the bending parts, some structures can be improved in the design of the bending parts, and the rigidity of the bending parts can be enhanced to reduce the spring back. For example, reinforcing ribs can be set in the bending deformation zone, as shown in Figures (a) and (b). ), or adopt a U-shaped side wing structure, as shown in Figure (c), by increasing the section moment of inertia of the bending part, reducing the bending spring back.

Installation and adjustment of bending die

Bending structure to reduce springback

●Rebound compensation. For materials with the large elastic rebound, the punch and the top plate can be made to compensate for the rebound of the convex and concave surfaces, so that the bottom of the bent part will bend. When the bent part is taken out of the concave mold, the curved part will rebound and stretch. Straight, so that both sides produce inward deformation, thereby compensating for the outward rebound of the rounded corners, as shown in the figure.

Installation and adjustment of bending die

Springback compensation

For harder materials, the shape and size of the working part of the mold can be corrected according to the rebound value.


●Take corrective bending instead of free bending or add corrective procedures. The following figure shows the mold structure where the corners of the bending punch are made into a partially protruding shape to correct the bending deformation zone. The principle of controlling the bending resilience is: when the bending deformation is over, the punch force will be concentrated on the bending deformation zone, forcing the inner metal to be squeezed to produce elongation deformation, and the bending resilience will be reduced after unloading. It is generally believed that a better effect can be obtained when the corrective compression of the metal in the bending deformation zone is 2% to 5% of the plate thickness.

Installation and adjustment of bending die

Correction method of mold structure

⒉The main reasons for the deviation and preventive measures are the main reasons for the deviation of the bent part. One is the incorrect positioning of the blank in the die or the unstable placement, so that the force and the blank surface are not vertical, which results in a horizontal component of force. The second is that when the blank moves along the edge of the die during the bending process, due to the asymmetry of the workpiece, the frictional resistance on each side is not-so the blank always shifts to the side with the greater resistance, so that the side with the smaller resistance is very large. Easy to pull into the die. The amount of offset is mainly related to factors such as die fillet radius, mold gap, slippage conditions, etc., especially for asymmetrically curved parts, the offset phenomenon is more serious. To overcome the deviation of the part in the bending process, the following methods can be used.


●Press the sheet tightly. The blanking device is used to gradually bend and shape the blank in a compacted state, to prevent the blank from sliding and obtain a flat workpiece, as shown in Figures (a) and (b).

Installation and adjustment of bending die

●Choose a reliable positioning form. Use the hole on the blank or design process hole, insert the positioning pin into the hole and then bend it so that the blank cannot move, as shown in Figure (c).


●Make the billet force uniformly and symmetrically. When bending asymmetrically shaped parts, it is often encountered that the blanks move due to uneven forces. To ensure uniform force on the part during bending, the asymmetric shape can be combined into an asymmetrical shape, which is then cut after bending, as shown in Figure (d).


⒊Limit the bending radius to prevent bending cracks. Because the outer fiber of the bending part is stretched, the deformation is the largest. When the limit deformation value of the material is exceeded, it is easy to bend and crack. However, the tensile deformation of the outer fiber of the part is mainly determined by the critical bending radius that causes the material to crack. The minimum bending radius is related to factors such as the mechanical properties of the material, heat treatment state, surface quality, the size of the bending angle, and the direction of the bending line. According to the factors that cause bending cracks, the main measures that can be taken are as follows.


●Choose materials with good surface quality and no defects as the blank. Defective blanks should be cleaned up before bending. To prevent bending cracks, large burrs on the sheet should be removed, and small burrs should be placed on the inner side of the curved fillet.


●Take measures from the craft. For relatively brittle materials, thick materials, and cold-work hardened materials, heating, and bending are used, or annealing is used to increase the plasticity of the material before bending.


●Control the value of the inner bending angle. Under normal circumstances, the bending inner angle of bending should not be less than the minimum allowable bending radius in the design, otherwise, the deformation of the outer layer of metal during bending may easily exceed the deformation limit and break. If the bending radius of the workpiece is less than the allowable value, it should be bent two or more times, that is, first bend into a larger fillet radius, after intermediate annealing, then bend to the required bending radius by the correction process, so that it can Enlarge the deformation area and reduce the elongation of the outer layer material.


●Control the bending direction. When bending processing and parts layout, the bending line and the rolling direction of the sheet metal are specified in the following process. For unidirectional V-shaped bending, the bending line should be perpendicular to the rolling direction. For bidirectional bending, the bending line should preferably be at 45° to the rolling direction, as shown in the figure.

Installation and adjustment of bending die

Control of bending direction

●Improve the manufacturability of product structure. Choose a reasonable fillet radius. For small bend fillets and thick materials, process incisions and grooving can be added to the local bending parts to avoid any geometric shapes that can cause stress concentration on the outside of the bending zone, such as Clear corners, notches, etc. to avoid root breakage. As shown in figure (a), slot the inner side of the corner of the curved part with a small fillet radius to ensure that no cracks will occur in the curved part with a small fillet radius. The clear angle of the crack is moved out of the bending zone. It is recommended to move the distance b≥r to ensure that no cracks occur during bending.

Installation and adjustment of bending die

Improve the manufacturability of product structure

●Avoid hot bending in the blue brittle zone and hot brittle zone. When using the hot bending process, when selecting the hot pressing temperature, avoid bending in the blue brittle zone and hot brittle zone. This is because: in certain temperature ranges of the heating process, brittleness often occurs due to the precipitation or phase change of the excess phase, which reduces the plasticity of the metal and increases the deformation resistance, such as when carbon steel is heated to between 200 and 400 ℃ because the aging effect reduces the plasticity and increases the deformation resistance. This temperature range is called the blue brittle zone. At this time, the performance of the steel deteriorates, and the fracture is easy to be brittle, and the fracture is blue. In the range of 800 to 950°C, plasticity will decrease again, and fracture will also occur during bending. This temperature is called the hot brittle zone.


⒋Change the size and structure of the working part of the mold to suppress the deflection. To prevent the bending and distortion of the bending part in the width direction, the deformation f measured in advance can be added to the mold structure. This can avoid deflection and distortion due to the influence of stress and deformation in the width direction after the part is formed.

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