We conduct in-depth research on the bending process of sheet metal parts, analyze the objective factors affecting the bending quality in detail, analyze the causes of quality problems in the operation process, and put forward solutions, so as to guide and summarize the bending process of sheet metal parts.
Sheet metal processing is widely used in aerospace, automotive electronics, railway locomotives, engineering machinery and other fields. Sheet metal bending is a key process of forming most parts. The bending quality directly affects the final shape and performance of products.
There are many factors affecting the bending accuracy of sheet metal, such as the accuracy of the unfolded size of accessories, the rationality of die selection and bending sequence. Therefore, the research on the bending accuracy of sheet metal must analyze these factors one by one and study how to control the bending quality in order to achieve the overall improvement of the bending quality.
1. Sheet metal unfolding dimension calculation
(1) Fitting bending radius design
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 bending 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, cracks or breaks will occur. Therefore, the structural design of the bending parts should avoid too small bending fillet radius.
The minimum bending fillet radius of bending parts is related to the mechanical properties, surface quality, hardening degree and fiber direction of materials. The minimum bending fillet radius can only be used when required by product design. Generally, the bending fillet is equal to or slightly less than the sheet thickness.
(2) Calculation of bending coefficient
the accurate bending size of the product shall be ensured, and the determination of the unfolding length of the sheet metal is the primary factor. During bending, the outer layer of the sheet is stretched 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 material length. In fact, for plates of the same thickness, due to different materials and hardness, the material with high hardness has less tensile deformation during bending, and the neutral layer is close to the outside; The material with small hardness has large tensile deformation, and the neutral layer is close to the inside, so the bending coefficient is needed to correct when calculating the unfolded material length.
In addition to sheet material, sheet thickness, bending angle and die shape all affect the bending coefficient. Due to the influence of the above factors, it is difficult to calculate the bending coefficient. At present, three-dimensional software such as Pro / E is mainly used to calculate the bending factor of sheet metal, which is consistent with the bending factor.
2. Distance from hole edge of bending part
For the blank with pre processed holes, if the holes are located in the bending deformation area during bending, the shape of the holes will be stretched and deformed after bending, and will also affect the size of the parts after bending. In order to avoid the hole position distributed in the bending deformation area, generally ensure that the hole edge distance b (the nearest distance from the outer edge to the hole edge after bending) ≥ 3 times the plate thickness. For the elliptical hole parallel to the bending curve, in order to ensure the bending accuracy and prevent the hole position deformation, the hole edge distance shall generally be ≥ 4 times the plate thickness.
If the hole position must be distributed in the deformation area, in order to ensure the accuracy, the method of machining small holes first and then expanding the hole after bending is generally adopted to meet the requirements. The deformation area can also be transferred by punching process holes or notches at the bending position.
3. Straight edge height of bending part
For 90 ° bending, in order to facilitate forming, the height h of the right angle edge of the workpiece shall not be less than twice the plate thickness t. If the straight edge height h ＜ 2T of the bending part is required in the design, the bending height shall be increased first, and then processed to the required size after bending; Or bend after machining a shallow groove in the bending deformation area.
For the bending part with oblique angle on the bending side, that is, when the bending deformation area is on the oblique line, due to the low linear height at the end of the oblique line, the workpiece will deform after bending, so the minimum height of the bending side shall meet H > 2T, otherwise the straight edge height of the bending part shall be increased or the part structure shall be changed.
4. Bending direction of bending part
When determining the bending direction, the blanking fracture zone of the blank shall be located on the inner side of the bending part as far as possible to avoid the microcrack in the fracture zone from expanding into a crack under the action of the outer tensile stress. If it is limited by the part structure and must be bent in both front and back directions, the bending radius shall be increased as much as possible or other process measures shall be adopted.
The anisotropy of the sheet also has a certain impact on the bending deformation, especially for the materials with poor plasticity, the bending curve of the workpiece shall be perpendicular to the fiber direction of the sheet as far as possible, otherwise when the bending curve is parallel to the fiber direction, cracks are easy to form on the outside of the bending part. If it is necessary to bend in multiple directions, the bending curve shall be at an angle to the fiber direction.
5. Springback of bent parts
The springback of bending parts refers to the phenomenon that the shape and size of bending parts change after they leave the die due to the plastic deformation of sheet metal. The degree of springback is usually expressed by the difference between the actual bending angle of the workpiece after bending and the bending angle of the die, that is, the size of the springback angle.
The factors affecting springback include material mechanical properties, relative bending radius, workpiece shape, die clearance and bending pressure. Because there are many factors affecting springback and the theoretical analysis and calculation are complex, generally speaking, the greater the ratio of fillet radius to plate thickness, the greater the springback. At present, the springback of bending parts is mainly solved by taking certain measures to reduce the springback when the mold manufacturer designs the mold, such as reserving the springback angle in the lower mold, adopting the V-angle of 88 ° or 86 °, or increasing the correction pressure during bending.
6. Selection of upper die for sheet metal bending die
(1) Selection of upper die type
Which upper punch is selected is determined by the shape of the workpiece, because there shall be no interference between the die and the workpiece in the bending process. For example, during U-shaped bending, the appropriate upper die shall be selected according to the size proportion of the three sides. Generally, if the size of the bottom edge is greater than or equal to the other two right angle edges, the upper die can be used for the frame; If the bottom edge is smaller than the other two sides, the gooseneck upper formwork shall be selected. In order to prevent the part from interfering with the die during bending, resulting in deformation or scrapping of the part, the display screen of Amada bending machine can be used to simulate the bending of the fitting to detect whether the die is suitable for the bending forming of the fitting.
(2) Selection of upper die fillet radius R
the fillet radius of the workpiece is mainly determined by the V-groove width of the lower die, and the fillet radius r of the upper die also has a certain influence. The fillet radius r of the upper die is generally the same as or slightly smaller than the plate thickness. When folding hard aluminum and other parts with poor plasticity, in order to prevent fracture or crack, the upper and lower dies with large fillet radius and V-groove size shall be selected, and crack arrest grooves shall be designed at both ends of the bending line of the accessories.
(3) Selection of the tip angle of the upper die
in addition to the 90 ° upper die, when folding back the SUS stainless steel plate, aluminum plate or medium plate with large elasticity, the 86 ° and 88 ° upper die can be selected according to the size of material rebound, and the lower die with the same angle shall be selected to match it.
7. Selection of lower die for sheet metal bending
(1) Selection of V-groove width of lower die T
he selection of V-groove width is mainly based on the plate thickness. The larger the V-groove width is, the smaller the bending pressure is required. Generally, v = 6T is often taken for thin plate materials, where V is the V-groove width of lower die; t is the plate thickness; 8 times the plate thickness is taken for 3mm carbon steel plate, and 10 times the plate thickness is taken for plates above 10mm.
In addition, the bending size of the part shall 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 cannot contact with the two shoulders of the V-shaped groove at the same time during bending, and the force will slide into the V-shaped groove, resulting in failure to form.
(2) Selection of lower die shape
lower die is generally divided into single groove lower die and double groove lower die. The single slot lower die is flexible and convenient to use, and the double slot lower die has good stability. The applicable lower die shall be determined according to the actual situation. In addition, there are some lower dies with special shapes, such as segment difference die, blank holder flattening die and elastic rubber lower die with arc folding, etc.
(3) V-groove angle of lower die
V-groove is divided into right angle lower die and acute angle lower die according to the angle. The common angles of acute angle lower die are 30 ° and 45 °, the common angles of right angle lower die are 88 ° and 90 °, and the standard lower die angle is 88 °. Its selection is determined according to the properties of materials and rebound amount. When the material has high tensile strength and large rebound, such as stainless steel or thin plate, 88 ° lower die shall be selected; 90 ° lower die can be selected for soft materials such as ordinary low carbon steel and copper.
The factors affecting springback are analyzed as follows.
1) Related to material properties. Under the condition of the same die and the same material thickness, the comparison of springback is as follows: sus > al > SPCC. 2) Under the condition of the same die and the same material, the springback of thin plate is greater than that of thick plate. 3) The larger the inner arc radius r in the bending of the same material, the greater the springback. 4) The greater the bending pressure, the smaller the springback.
8. About offset bending
When feasible, the workpiece shall be placed symmetrically on the axis of the machine for bending as far as possible, so that the operation is more accurate than the offset bending of the workpiece, and the adverse impact of the offset load on the machine can be avoided. If the offset bending is really necessary, it is recommended that the bending tonnage shall not exceed 30% of the total tonnage.
We mainly analyzes the sheet metal bending process and related process parameters from the perspective of bending process, and focuses on how to calculate the unfolding size and bending compensation value, how to reasonably select the die, determine the bending force and the common problems in the bending process. For other types of bending, such as bar bending Hinge bending is not mentioned. If necessary, please refer to relevant materials or contact us for professional solutions.
