How to control sheet metal bending accuracy?

Sheet metal bending is a key process of forming most parts, the quality of the bending directly affects the final shape and performance of the product.


High-precision CNC bending machine


There are many factors that affect the bending accuracy of sheet metal, such as the accuracy of the expansion size of parts, the rationality of the mold selection, and the bending order, so the research on the bending accuracy of sheet metal must be analyzed from these factors one by one, and how to control the bending quality, in order to achieve the overall improvement of the bending quality.


1. Sheet metal expansion size calculation


(1) accessories bending radius design material bending, the outer corner area is stretched, and the inner layer is compressed. When the thickness of the material is certain, the smaller the inner fillet, the greater the tensile and compression ratio of the material, when the tensile stress of the outer fillet exceeds the ultimate strength of the material, it will crack or break, therefore, the structural design of the bending part should avoid too small bending fillet radius.


The minimum bending fillet radius is related to mechanical properties, surface quality, hardening degree, and fiber direction. The minimum bending fillet radius is only used when the product design is required. Generally, the bending inner fillet is equal to or slightly less than the thickness of the sheet material.


(2) Bending coefficient calculation products to ensure the accurate bending size, and to determine the expansion length of sheet metal is the primary factor. When bending, the outer layer of sheet material is stretched longer and the inner layer is compressed shorter. Only the length of the neutral layer is unchanged. In theory, the length of the neutral layer is equal to the length of the material. In fact, the same thickness of the sheet material, due to the difference in material and hardness, in bending hardness of the material tensile deformation is a small, neutral layer on the outside; The material with small hardness has large tensile deformation, and the neutral layer is on the inside, so the bending coefficient is needed to correct the material length when calculating the expansion.


In addition to the sheet material, sheet thickness, bending Angle, and mold shape have an impact on the bending coefficient. Due to the influence of the above factors, the calculation of the bending coefficient is difficult. At present, PRO/E and other 3D software are mainly used to calculate the bending factor of sheet metal, which is consistent with the bending coefficient.


2, bend the hole side distance


If the blank material with pre-processed holes is located in the bending deformation zone when bending, the shape of the holes will be stretched and deformed after bending, and it will also affect the size of the parts after bending. In order to avoid the distribution of holes in the bending deformation zone, it is generally necessary to ensure that the hole edge distance b(the closest distance between the outside and the hole edge after bending) is not less than 3 times the plate thickness. For elliptic holes parallel to the bending line, to ensure the bending accuracy and prevent the deformation of the hole position, the distance between the hole edges should be not less than 4 times the thickness of the plate.


If the holes must be distributed in the deformation zone, in order to ensure accuracy, the method of machining the holes first and then reaming the holes after bending is generally adopted to meet the requirements. The deformation zone can also be transferred by punching the process holes or notching at the bending position.


3, bending straight edge height


For 90° bending, in order to facilitate forming, the height of the right-angle side of the workpiece h should not be less than 2 times the thickness of the plate t. If the design needs to bend the straight edge height h<2t, the first to increase the height of the bending edge, to be bent forming after processing to the required size; Or in the bending deformation zone after processing shallow groove bending.


For the bending part with the bevel side, that is, when the bending deformation area is on the bevel, the workpiece will be deformed after bending due to the low straight line height at the end of the bevel, so the minimum height of the bending side should meet h>2t, otherwise the height of the straight side of the bending part should be increased or the structure of the part should be changed.


4, the bending direction of the bending piece


When determining the bending direction, the blanking fault zone of the blank should be placed on the inside of the bending part as far as possible to avoid the microcracks in the fault zone expanding into cracks under the action of the lateral tensile stress. If the structure of the parts is limited, it must be bent in both directions, the bending radius should be increased as far as possible or other technological measures should be adopted.


The anisotropy of the sheet also has a certain influence on the bending deformation, especially for the material with poor plasticity. Under permitted circumstances, the bending line of the workpiece should be as vertical as possible to the direction of the sheet fiber, otherwise, when the bending line is parallel to the direction of the fiber, the outer side of the bending line is easy to form a crack. If it is necessary to bend in multiple directions, the bending line should be at an Angle to the direction of the fiber.


5. Rebound of bending pieces


The spring back of the bending part refers to the phenomenon that the shape and size of the bending part change after the plastic deformation of the sheet material makes it leave the die. The degree of spring back is usually expressed by the difference between the actual bending Angle of the workpiece and the bending Angle of the die, namely the size of the spring back Angle.


The factors that affect springback include the mechanical properties of the material, the relative bending radius, the shape of the workpiece, the die clearance, and the pressure during bending. As there are many factors affecting springback, the theoretical analysis and calculation are complex. Generally speaking, the greater the ratio of the inner fillet radius of the bending part to the thickness of the plate, the greater the springback. At present, the springback of bending parts is mainly solved by taking certain measures to reduce the springback when mold manufacturers design molds, such as reserving the springback Angle of the lower die, adopting the V-shaped Angle of 88° or 86°, or increasing the correct positive pressure when bending.


6. Selection of sheet metal bending


(1) The selection of the type of upper die is determined by the shape of the workpiece because the bending process between the die and the workpiece shall not interfere, for example, in the U-shaped bending, should choose the appropriate upper die according to the size ratio of the three sides. In general, if the size of the bottom edge is greater than or equal to the other two right angles, the frame can be selected to use the mold; If the bottom edge is smaller than the other two sides, the gooseneck upper die should be used.


(2) The selection of upper die fillet radius R workpiece fillet radius is mainly determined by the V groove width of the lower die, and the upper die fillet radius R also has a certain influence. The fillet radius R of the upper die is generally the same or slightly smaller than the thickness of the plate. In the folding of duralumin and other parts with poor plasticity, in order to prevent fracture or crack, the upper and lower die with a larger fillet radius and V-shaped groove size should be selected. At the same time, the two ends of the bending line of the accessories are designed to stop the crack groove.


(3) In addition to the 90° upper die, the 86° or 88° upper die can be selected according to the size of the material rebound when the SUS stainless steel plate, aluminum plate, or medium thick plate has a large amount of spring, and the lower die with the same Angle should be selected to match.


7. Selection of lower die of sheet metal bending


(1) The selection of V-groove width of the lower die The selection of V-groove width is mainly based on the thickness of the plate, the larger the width of the V-groove, the smaller the bending pressure required. In general, compared with the sheet material, V=6t is usually taken, where V is the width of the V-groove of the lower die; t is the plate thickness.


In addition, the bending size of parts should be considered. When the size is small, if the width of the V-shaped groove of the lower die is large, the top of the sheet can not contact the shoulder of the V-shaped groove at the same time when bending, it will slide into the V-shaped groove, resulting in unable to form.


(2) The selection of the shape of the lower die is generally divided into single slots and double slots. The single-slot lower die is flexible and convenient to use, and the double-slot lower die has good stability. The lower die to use should be decided according to the actual situation. In addition, there are some special shapes of the lower die, such as section differential die, edge beat flat die, and bend the arc of the elastic rubber die.


(3) The V-shaped groove Angle of the lower die according to the Angle of the V-shaped groove is divided into the right Angle and acute Angle of the lower die, the common Angle of the acute Angle of the lower die is 30° and 45°, the common Angle of the right Angle of the lower die is 88° and 90°, the standard Angle of the lower die is 88°, the choice is based on the nature of the material and the amount of springback to determine. When the tensile strength of the material is larger and the amount of rebound is larger, such as stainless steel or thin sheet material, should choose 88° lower die; Ordinary low carbon steel and copper, and other soft materials can be selected 90° lower die.


The factors influencing rebound are analyzed as follows:


1) It is related to material properties. Under the condition of the same mold and the same material thickness, the comparison of springback quantity is SUS>Al>SPCC.


2) Under the condition of the same mold and the same material, the springback of a thin plate is greater than that of a thick plate.


3) If the inner arc radius R of the same material is larger, the springback will be larger.


4) The greater the bending pressure, the smaller the springback.


8. About bias bending


If feasible, the workpiece should be placed symmetrically on the central axis of the machine for bending as far as possible, so that the operation is more accurate than the workpiece deflection bending, and can avoid the adverse impact of the machine due to the deflection load. If deflection bending is necessary, it is recommended that the bending tonnage should not exceed 30% of the total tonnage.

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