Essential Guide to Sheet Metal Press Bending

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Bending is to place metal materials (sheets, profiles, pipes, etc.). A bending mold (or bending mold) uses a press or special equipment pressure to bend to a certain angle and shape along its bending line. Gold forming process. Press-bending processing is mainly used for the processing of parts with larger production batches, more complex shapes, and higher precision requirements. Figure 1-1 is the basic principle of using a V-shaped bending die to bend a V-shaped part.

Punch 1 and punch 2 are basically the same as the inner and outer contours of the bending workpiece. During bending, when the punch is pushed down by external force (such as the movement of the press slider), it will be placed between the convex and concave molds. The sheet material is bent into the required workpiece.

Bending Equipment And Tools

Press bending is one of the sheet metal forming methods that plastically deform the material to form a certain angle shape. According to the basic principle of press bending, press bending is mainly done by bending die and press machine to complete bending. In production, the equipment used for bending is usually the same as the punching equipment, that is, the crank press is used, and the bending die is the most important tool to complete the bending process. Generally speaking, parts of different shapes often need different bending. In addition, the machining accuracy of the bending diet also directly affects the machining accuracy of the bending parts. Generally, the economic tolerance level of the bending parts is best to be below IT13.

Common Structure of Bending Die

The shape of the bending part is ever-changing, so the structure of the bending die is also diverse. The shape of the commonly used bending mold for sheet metal parts is a single-process mold, that is, a bending mold that only completes one bending process in each stroke of the press. According to the different bending forming methods, it can be divided into free bending and corrective bending. According to whether it has a guide during bending, it can be divided into open bending mold, mold base guiding bending mold, and so on.

• Free bending die. Figure 1-2 (a) and (b) are the free bending dies for V and U-shaped parts, respectively, and the die structure is open.

This kind of mold is convenient to manufacture and has 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, the bending accuracy of the workpiece is not high, and the bottom of the U-shaped part is not flat.

• Correct the bending mold. In order to improve the bending accuracy of the bent parts and prevent the sliding of the bent blank, the corrective bending die shown in Figures 1-3 can be used. Among them: Figure-1-3(a) The spring top rod 3 is a pressing device used to prevent the blank from deflection during bending. Figure 1-3(b) is equipped with a pressing device. When stamping, the blank is pressed between the punch 1 and the pressing plate 3 and gradually descends, and the material that is not pressed at both ends slides and bends along the round corners of the female mold. Enter the gap between the male mold and the female mold, and bend the part 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.

Figure 1-4 is a U-shaped piece bending die with a movable load cell. The movable side pressure block has a corrective effect on the curved part, and the rebound is small. When working, the punch moves downwards, and first contacts the blank to bend into a U shape, and then the shoulder of the punch presses the movable die side pressing block downward. Due to the effect of the inclined surface, the movable die pressure measuring block slides toward the center, exerting pressure on both sides of the curved part, which has a corrective effect, and the curved part can meet the requirements of shaping accuracy.

Bending Die Structure Composition

According to the above introduction, it can be seen that the structure and composition of the bending die are basically similar to that of the punching die, and it is also composed of a punch and a die, a positioning device, and a discharge device. Among them: the convex mold and the concave mold are the parts that directly produce the bending effect on the material, and are the core part of the bending mold. Usually, the convex mold is fixed on the upper mold plate and the concave mold is fixed on the lower mold plate.

Determination of Bending Process Parameters

Calculation of Length of The Bent Blank

The calculation of the length of the bent blank is the same as the calculation of the unfolded length during manual bending.

Calculation of Bending Force

The bending force refers to the pressure applied by the press when the workpiece is bent to complete the predetermined bending. The bending force is an important basis for designing the bending die and selecting the tonnage of the press. When calculating, first distinguish the bending type and use the empirical formula respectively.

• Bending force F₁ during free bending. According to the shape of the workpiece to be bent, the bending force F₁ during free bending is calculated by the following formula.

a. V-shaped piece

b. U-shaped piece

Where F₁——the 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 radius of the bending part, mm;

• Bending force F₂ when correcting the bending. Since the correcting bending force when correcting the bending is much larger than the compression bending force, and the two forces act one after the other, only the correcting force needs to be calculated. The corrective force of the V-shaped piece and U-shaped piece are calculated as follows: F₂=Ap

F2——bending force when correcting bending, N;

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

P——Corrective force per unit area, MPa, selected according to Table 1-5.

• Ejector force and unloading force FQ. No matter what form of bending is used, ejector force and unloading force are required during bending. Ejector force and unloading force FQ can be approximately 30% to 80% of the free bending force. which is: F_Q=(0.3~0.8)F₁

• When the press tonnage F₃ is free to bend, taking into account the influence of the ejector force and the discharge force during the bending process, the press tonnage is: F₃≥F₁+F_Q=(1.3~1.8)F₁

When the press tonnage F₃ is free to bend, taking into account the influence of the ejector force and the discharge force during the bending process, the press tonnage is: F₃≥F₂

Determination of Bending Die Gap

The size of the gap between the male mold and the female mold is the same as the fillet radius, which 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 press closing instructions, and there is no need to determine the gap on the mold structure.

For general bending parts, the gap can be found in Table 1-6 or can be obtained directly from the following approximate calculation formula.

Non-ferrous metals (red copper, brass): z=(1~1.1)t

Steel: z=(1.05~1.15)t

When the workpiece progress requirement is high, the gap value should be appropriately reduced, taking z=t. In production, when the requirement for thinning of the material thickness is not high, to reduce spring back, etc., also take the negative gap, take z=(0.85 ~0.95) t.

Calculation of Size of The Working Part of The Bending Die

The design of the working part of the bending mold is mainly to determine the radius of the convex and concave mold corners, as well as 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 bending part will rebound greatly, and the uneven value will affect the quality of the part; if it is too large, it will consume more die steel and require a longer press stroke.

• Bending mold structure for V-shaped parts. For the bending of V-shaped parts, the structure of the mold is shown in Figure 1-7, and the dimensions of the die thickness H and groove depth h are determined in Table 1-8.

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

     2. When K (small end) ≥ 2t, the value of h is calculated according to the formula h=L₁/2-0.4t.

• Determination of the fillet radius r₁ and depth L₀ of V-shaped and U-shaped bends. The round corner radius r₁ and depth L₀ of V-shaped and U-shaped bends are determined as shown in Figure 1-9 and Table 1-10.

• Calculation of the width of the bending punch and die. General principle: When the workpiece has an external dimension, the mold is based on the concave mold (that is, the concave mold is made into a nominal size), and the gap is taken on the convex mold; if the workpiece is marked with internal dimensions, the mold is based on the convex mold ( That is, the convex mold is made into a nominal size), and the gap is taken on the concave mold.

When the workpiece needs to ensure the external dimensions, the width dimension of the concave die L₁ and the width dimension L₂ of the convex die is calculated according to the following formula:

Where, ——the maximum dimension of the width of the bending part, mm;

——The minimum size of the width of the bending part, mm;

L₂——the width of the punch, mm;

L₁——The width of the cavity, mm;

z——The gap between the convex mold and the concave mold on one side, mm;

∆——the dimensional tolerance of the width of the bent part, mm;

——The manufacturing deviation of punch and die, mm, is generally selected according to IT9 level.

Installation And Adjustment of Bending Die

Installation Method of Bending Die

The installation of bending dies is divided into two types: non-guided bending dies and guided bending dies. The installation method is basically the same as that of punching dies. Same as punching dies, the installation of bending dies should be carried out except for adjusting the gap between convex and concave dies and unloading. In addition to the debugging of the device and other aspects, the two bending dies should also complete the adjustment of the upper and lower positions of the bending upper die on the press at the same time. Generally, it can be carried out according to the following method.

First of all, the upper mold should be roughly adjusted on the press slider first, and then a gasket slightly thicker than the blank is placed between the lower plane of the upper punch and the discharge plate of the lower mold (the gasket is generally 1~1.2 of the thickness of the blank) Times) or use the sample, and then use the method of adjusting the length of the connecting rod to move the flywheel (press machine with rigid clutch) or jog (press machine with friction clutch) by hand again and again until the slider can be normal The ground passes the bottom dead center without blocking or motionless (the so-called “holding” and “biting”). After moving the flywheel for several weeks in this way, the lower die can be finally fixed 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.

The Adjustment Points of Bending Die

• Adjust 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 above-mentioned installation method of the 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 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 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 unloading system of the bending die should be large enough, and the spring or rubber used for unloading should have sufficient elasticity; the ejector and the unloading system should be adjusted to move flexibly, and the product parts can be unloaded 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.
• Precautions for adjusting the bending die. During the adjustment of the bending die, if 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 violently collide at the bottom dead center position of the stroke during the stamping process. , In severe cases, the mold or punch may be damaged. 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, so as to avoid the possibility of accidents.

Operation of Typical Sheet Metal Bending Parts

Suppression of Orbicular Sky And Rectangular Earth Pipe

Orbicular sky and rectangular earth pipes are often made of radial molds for manual bending. However, for the large-scale, thick-material orbicular sky and rectangular earth pipes, they can often only be pressed by a press with a radial mold. Figure 1-11 shows the pressing method of 1/4 sheet (the production of radial mold is the same as manual bending).

First, divide the circle (sky circle) on the upper end of the workpiece into equal parts, and connect the corresponding square (place) on the lower end of the workpiece to send out a number of radiation rays, and press them in line. Figure 1-11(a) is the positional relationship between the punch and the round steel of the die when the first knife is pressed down. The end of the material square is placed in the middle of the small end of the mold, and the first transition line of the round end, that is, the arc and the plane intersect The ridgeline is placed on the edge of the round steel. When the first knife is pressed down, it should fall on the secondary line, and the tire will not move before pressing the second knife. Figure 1-11(b) shows the pressing method of each knife of the proximal line. At this time, the position of the concave mold can be adjusted at any time to make the convex mold basically fall in the middle of the concave mold. The two ends first, then the middle part, the template should be clamped once when pressing a short section to grasp the degree of pressing.

When the template is stuck, the template vertical to the sheet material is wrong, and it should be inclined at an angle toward the small end of the material. Suppression should be carried out in accordance with the principle of surpassing debts. Because under-arc correction is much easier than over-arc correction, when the arc is under-corrected, a few transition lines are properly pressed until they are in agreement with the template.

Of course, the above-mentioned operation is also applicable to the pressing of the orbicular sky and rectangular earth pipes.

Suppression of Positive Frustum

The production of the positive frustum can be manually bent or rolled by a plate rolling machine. However, for the positive frustum shown in Figure 1-12, because the diameter of the lower port is too small, it is inconvenient to make manual grooves and cannot be rolled by a plate rolling machine. The system can only be molded with a mold.

Using molds to press, the design of molds is the key. The molds are generally made according to 1/n of the fan-shaped sheet material, and the corresponding 1/n sheet material is pressed at a time during pressing; it can also be made into any small wrap angle. The sheet is fed in section by section and pressed section by section. Both methods can achieve the purpose of forming.

Figure 1-13 shows a mold made by 1/3 blanking and larger than 1/3. Figure 1-13 (a) is a convex mold, and Figure 1-13 (b) is a concave mold. The corner radius r of the die is designed to be twice the thickness of the plate, and r takes 45mm.

• Calculation of each vertical board RN. The R_N. of each vertical board is calculated according to the following formula (see Figure 1-12, 1-13).

In the formula, R₁₁——the inner radius of the large end of the frustum;

      h——The height of the equal division of the frustum, in this case, each equal division is 100mm;

      a——Design the given bottom angle of the frustum.

Like.

• Calculation of the width of convex and concave molds. The width of the convex and concave molds are calculated according to the following formulas (see 1-14, 1-15).

1. Punch width. . B₆=2*361*=678(mm).
2. Die width. B’₆=678+60=738(mm).
3. The lowest height of the die. h₆=730-361=369(mm).

The manufacturing method of the convex and concave molds In order to more accurately explain the manufacturing of the convex and concave molds, now take Figure 1-15 as an example to illustrate the sixth convex and concave mold.

On the premise of ensuring 730mm below the axis of the frustum and 80mm above the axis, draw an arc with O₆ as the center and O₆=361mm as the radius, and the intersection with the width of the convex and concave die calculated above, the two contours formed are the sixth Block convex and concave molds. According to practical experience, “regardless of hot pressing or cold pressing, due to the small arching height of 1/3 tiles and the strong pressure applied to the press, the shrinkage and spring back are not considered, nor the plate thickness. , Draw an arc with the radius of the inner skin where the frustum is located…” is changed to “Regardless of hot pressing or cold pressing, because the arching height of 1/3 of the tiles is small and under the strong pressure of the press, it is not considered The amount of shrinkage and rebound, regardless of the thickness of the plate, draw an arc with the inner skin radius of the frustum (marking the radius of the inner shape of the frustum), and cut out with one cut, which is the actual arc of the convex and concave mold at the corresponding position, which can be guaranteed after pressing Design curvature”.

Suppression of Small Frustum

The shaping of small vertebrae is generally thin due to their small size. Of course, there are also thick small vertebrae. The shaping method can be carried out on the radial tire with a sledgehammer and a grooved arc hammer. However, in order to save labor and improve efficiency, it can also be used Press on a small press. For the convenience of pressing, most of them are lowered into two halves and rarely are lowered into monoliths (when the height is low, they can also be lowered into monoliths).

• The shaping of small vertebrae is generally thin due to its small size. Of course, there are also thick small vertebrae. The shaping method can be carried out on the radial tire with a sledgehammer and a grooved arc hammer. However, in order to save labor and improve efficiency, it can also be used Press on a small press. For the convenience of pressing, most of them are lowered into two halves, and rarely are lowered into monoliths (when the height is low, they can also be lowered into monoliths).

• Suppression of the whole fan-shaped material. When the unfolded material is a monolithic material, the mold design is shown in Figure 1-18, and the space dimension of the cavity should be larger than the maximum dimension of the frustum (the maximum dimension refers to the height of the frustum and the diameter of the large end). The design of the frame punch The purpose is to accommodate the formed vertebral body, and the pressing method is basically the same as that of manual bending.

Suppression of Angle Steel Ring

There are many forming methods for angle steel ring: it can be bent on a section steel bending machine; it can be rolled on a plate bending machine; it can also be manually bent; it can also be used as a mold to be pressed on a press. Which method is better depends on the equipment situation of the unit and the requirements of the angle steel ring? Figure 1-19 is the construction drawing of an outer bend angle steel ring, which is made of 63mm*63mm*6mm equilateral angle steel. After forming, the inner diameter is 4800mm. It is decided to use a mold to press it on a press.

• The design of the mould. Figure 1-20 shows the mould designed for pressing outwardly bent diagonal steel. This mould can also be pressed by a single piece, but the distortion is larger after pressing. This is because the corners of the contact punch are squeezed and embedded in the concave The vertical surface in the groove is stretched to form a bend, and at the same time, another kind of deformation occurs, that is, the plane of the convex and concave molds is squeezed and becomes thinner and elongated, and the root shrinks and the wings extend, so that the opposing plane produces a sharp edge. In the vertical bending direction, the two deformations are superimposed to produce distortion.