Sheet metal bending accuracy is a key process in the forming of most parts, and the quality of the bending directly affects the final shape and performance of the product.
There are many factors that affect the sheet metal bending accuracy of sheet metal, such as the accuracy of the expansion size of the accessories, the selection of the mold and the rationality of the bending sequence, etc. Therefore, the research on the bending accuracy of sheet metal must be analyzed from these factors one by one, and how to control it. Bending quality can achieve a comprehensive improvement in bending quality.
Sheet Metal Unfolded Size Calculation
- Bending radius design of fittings When the material is bent, the outer layer of the fillet area is stretched and the inner layer is compressed. When the thickness of the material is constant, the smaller the inner corner of the bend, the greater the tensile and compression ratio of the material. When the tensile stress of the outer corner exceeds the ultimate strength of the material, cracks or breaks will occur. Therefore, the structural design of the bending part should avoid bending the fillet radius too small.
The minimum bending fillet radius of the bending part is related to the mechanical properties, surface quality, degree of hardening, and fiber direction of the material. The minimum bending fillet radius is only used when the product design requires it. Generally, the bending fillet is equal to or slightly smaller than the thickness of the sheet.
- The bending coefficient calculation product must ensure the accurate bending size, and determining the unfolded length of the sheet is the primary factor. Since the outer layer of the sheet is stretched and lengthened during bending, and the inner layer is compressed and shortened, only the length of the neutral layer remains unchanged. In theory, the length of the neutral layer is equal to the length of the material. In fact, for sheets of the same thickness, due to the difference in material and hardness, the material with high hardness has a small tensile deformation during bending, and the neutral layer is on the outside; the material with small hardness has large tensile deformation, and the neutral layer is inward, so when calculating the length of the unfolded material, the bending coefficient needs to be corrected.
In addition to the material of the sheet, the thickness of the sheet, the angle of bending, and the shape of the mold all have an impact on the sheet metal bending accuracy. Due to the influence of the above factors, the calculation of the bending coefficient is difficult.
Bending Part Hole Edge Distance
For the blank with pre-machined holes, if the hole is in the bending deformation zone during bending, the shape of the hole will be stretched and deformed after bending, and it will also affect the size of the accessories after bending. In order to avoid the distribution of holes in the bending deformation area, generally, ensure that the hole edge distance b (the shortest distance from the outer edge to the hole edge after bending) is ≥ 3 times the thickness of the plate. For the elliptical hole parallel to the bending line, in order to ensure the bending accuracy and prevent the deformation of the hole position, the distance from the hole edge should generally be ≥ 4 times the thickness of the plate.
If the holes must be distributed in the deformation area, in order to ensure the sheet metal bending accuracy, the method of processing small holes first, and then expanding the holes after bending can meet the requirements, or punch holes or gaps at the bending position to transfer the deformation area.
Straight Edge Height of Bending Part
For 90° bending, in order to facilitate forming, the height h of the right-angle side of the workpiece cannot be < 2 times the plate thickness t. If the design requires the straight side height h<2t of the bending part, the height of the flange should be increased first, and then processed to the required size after bending; or a shallow groove should be processed in the bending deformation area before bending.
For a bending part with a level on the side of the bending, that is, when the bending deformation area is on the inclined line, the workpiece will be deformed after bending due to the low height of the straight line at the end of the inclined line. Therefore, the minimum height of the bending side should meet the h>2t, otherwise, the height of the straight edge of the bending part should be increased or the structure of the part should be changed.
Bending Direction of Bent Part
When determining the bending direction, the blanking fracture zone of the blank should be placed on the inner side of the bending part as much as possible to avoid the micro-cracks in the fracture zone from expanding into cracks under the action of the outer tensile stress. If it is limited by the structure of the part when the front and back sides must be bent in both directions, the bending radius should be increased as much as possible or other technological measures should be adopted.
The anisotropy of the sheet also has a certain influence on the sheet metal bending accuracy, especially for materials with poor plasticity. If permitted, the bending line of the workpiece should be made perpendicular to the direction of the fiber of the sheet, otherwise, the bending line should be perpendicular to the fiber direction. When parallel, cracks are easily formed on the outside of the bending line. If it is necessary to bend in multiple directions, the bend line should be made at an angle to the fiber direction.
Re-bound of Bent Parts
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 causes the bending part to leave the mold. The degree of spring-back is usually expressed by the difference between the actual bending angle of the workpiece after bending and the bending angle of the mold, that is, the size of the spring-back angle.
Factors that affect spring-back include the mechanical properties of the material, the relative bending radius, the shape of the workpiece, the die gap, and the pressure during bending. Because there are many factors that affect the spring-back, the theoretical analysis and calculation are complicated. Generally speaking, the larger the ratio of the radius of the inner fillet of the bending part to the thickness of the plate, the greater the spring-back. The spring-back of the bending part is mainly through the mold manufacturer to take certain measures to reduce the spring-back when designing the mold. It can be solved by increasing the corrective pressure when bending.
Selection of Press Brake Machine Upper Punch
- Choice of upper punch type
The choice of upper punch is determined by the shape of the workpiece. Because there must be no interference between the die and the workpiece during the bending process. For example, during U-shaped bending, the appropriate upper punch should be selected according to the size ratio of the three sides mold. In general, if the size of the bottom edge is greater than or equal to the other two right-angle edges, the upper punch for the optional frame should be used; if the bottom edge is smaller than the other two sides, the gooseneck upper punch should be used.
- Selection of the corner radius R of the upper punch
The outer corner radius of the workpiece is mainly determined by the width of the V-shaped groove of the lower die, and the corner radius R of the upper punch also has a certain influence. The fillet radius R of the upper punch is generally the same as or slightly smaller than the plate thickness. When folding parts with poor plasticity such as hard aluminum, in order to prevent fracture or cracks, a larger fillet radius and V-shaped groove should be selected. Upper and lower tooling, and crack relief grooves are designed at both ends of the bending line of the accessories.
- Choice of tip angle of the upper punch
In addition to the 90° upper punch, when folding the stainless steel plate, aluminum plate, or medium-thick plate with a large amount of spring-back, 86° and 88° upper punch can be selected according to the size of the material spring-back and should be selected at the same time. A lower die of the same angle matches it.
Selection of Down Die of Press Brake Tooling
- The choice of the width of the V-shaped groove of the lower die V-shaped groove is mainly based on the thickness of the plate. The width of the V-shaped groove of the lower die is the thickness of the plate.
Consider the part bend size. When the size is small, if the width of the V-shaped groove of the lower die is large, the top of the sheet cannot be in contact with the shoulders of the V-shaped groove at the same time during bending, and it will slide into the V-shaped groove, resulting in failure to form.
- The choice of the shape of the lower die is generally divided into a single-slot lower die and a double-slot lower die. The single-slot lower die is flexible and convenient to use, and the double-slot lower die has a better stability. The lower die to be used should be determined according to the actual situation. In addition, there are also some special-shaped lower dies, such as segment difference dies, edge blanking flat dies, and elastic rubber lower dies with arc bending.
- The V-shaped groove angle of the lower die is divided into the right-angled lower die and acute-angled lower die according to the angle. The die angle is 88°, which is selected according to the properties of the material and the amount of spring-back. When the material has large tensile strength and a large amount of spring-back, such as stainless steel or thin sheet materials, an 88° lower die should be used; for softer materials such as ordinary low carbon steel and copper, a 90° lower die can be used.
The factors affecting the rebound are analyzed as follows
- It is related to the properties of materials.
- Under the conditions of the same mold and the same material, the spring-back of the thin plate > the spring-back of the thick plate.
- The larger the radius R of the inner arc of the same material bending, the larger the spring-back.
- The greater the bending pressure, the smaller the spring-back.
About Offset Bends
If feasible, the workpiece should be bent as symmetrically as possible on the central axis of the machine. This operation is more accurate than the workpiece offset bending, and it can avoid the adverse effect on the machine due to eccentric load. If it is really necessary to offset bending, it is recommended that the bending tonnage should not exceed 30% of the total tonnage.
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