How to improve the accuracy of workpiece bending in sheet metal processing

Due to the structural principle of the sheet bending machine, the manufacturing accuracy of the bending machine itself, the manufacturing accuracy of the tooling, the performance of the sheet being processed and the unevenness of the performance, there are inevitable angular errors, straightness errors and edge length errors in the sheet processing. The existence of sheet metal processing errors affects the subsequent assembly, increases the workload of die repair and post-forming calibration, prolongs the product development cycle, and restricts the further promotion and application of progressive bending, especially for the application of large workpieces formed with high-strength and high-resilience steel plates.

1. Analysis of sheet metal workpiece bending process

In order to meet different design requirements, sheet metal parts are often bent into a variety of shapes. Here, the bending process of sheet metal is analyzed by bending the sheet material at 90°. When the sheet is bent, the inner fibers of the sheet are shortened by compression and the outer fibers are elongated by tension in the rounded deformation zone. This is called the neutral layer, and the calculation of the unfolding dimension of the sheet is to calculate the length of the neutral layer. When the type of material, thickness, bending radius and bending method, the location of the neutral layer also varies, bending the neutral layer is generally moved to the side of the pressed fiber. In the approximate calculation can be considered neutral layer in the material thickness of 1/2 position.

2. Factors affecting the accuracy of bending

2.1 Machine deflection deformation

Deflection deformation is mainly caused by uneven loading. For example, the working length of the bending machine is 3200mm, two cylinders distributed on the left and right sides, tonnage of 100t. In the case of load, the upper and lower slider will produce deflection deformation, so that the actual displacement of the middle part of the slider is reduced, causing the workpiece in the direction of the entire length of the angle inconsistency, resulting in the phenomenon of the middle angle is larger than the two ends, the angle inconsistency directly affects the dimensional accuracy of the workpiece.

2.2 Unevenness of the plate

Due to the error in the thickness direction of the processed material, the inconsistency of the actual displacement of the upper slider in the length direction during the gap bending will cause the angle error. And the plate is rolled, the uneven distribution of stress causes different rebound, and these different characteristics make the angular accuracy of the workpiece affected.

2.3 Rebound

In gap bending, the sheet will spring back due to the change of bending stress, and the maximum bending stress is generated on the outer surface of the bent part of the material. To get a satisfactory bending angle need to implement "excess bending", but in the actual bending, there may be various directions of bending at the same time, resulting in different amounts of rebound in each part, resulting in inconsistent bending angle accuracy.

2.4 Calculation accuracy of workpiece unfolding material

The accuracy of the unfolded material directly affects the dimensional accuracy of the workpiece, the unfolded size of the workpiece is calculated according to the size of the workpiece, and the method of bending coefficient is used in the calculation. This bending coefficient is an empirical data, for different materials of the plate, different lengths, different shapes of bending, there are some differences. But the actual calculation is using the same value. There is another reason that can cause unfolded material error is the undercutting processing error, when the shear undercutting, it will produce an actual value that is different from the theoretical value, which also makes the dimensional accuracy is affected.

3. Precision bending workpiece path

3.1 Bending process parameters design

(1) the minimum bending radius. 

Factors affecting the minimum bending radius are the mechanical properties of the material, the quality of the surface and sides of the material, the direction of the bending line, the bending center angle, etc. In the premise of the plate material does not occur damage, choose to complete the minimum round radius of the inner surface of the part, but if the bending radius is too small, bending the outer layer of material tensile deformation is too large, more than the tensile strength of the tensile stress, it will lead to cracks in the outer layer of the plate material, resulting in the scrapping of the workpiece, so it is not advisable to choose too small bending radius.

(2) bend the edge of the hole distance. Pre

Processed hole blank material, in bending if the hole is located in the bending deformation zone, then the shape of the hole in the bending will stretch deformation. In order to avoid hole distribution in the bending deformation zone, hole edge distance should generally ensure that L ≥ 3 times the material thickness. For the long waist-shaped hole parallel to the bending line, in order to ensure bending accuracy and prevent hole deformation, hole edge distance should generally L ≥ 4 times the material thickness

(3) the height of the straight edge of the bent parts. 

For 90 ° bending, in order to facilitate the formation, must make the workpiece straight edge height h ≥ 2t. If the design needs to bend the straight edge height h < 2t, we must increase the height of the bending edge, so that the bending shape and then processed to the appropriate size; or in the bending deformation zone processing shallow groove before bending. Bending side with beveled bending parts that bending deformation area on the slant, due to the slant end of the straight line height is low, the workpiece will be deformed after bending.

3.2 Try to eliminate the impact of rebound on the bending accuracy

In the actual bending workpiece, due to the different shapes of the workpiece, and a variety of different kinds of bending, rebound can produce a comprehensive effect. One way to eliminate the rebound is to appropriately increase the bending pressure and extend the holding time. The second is the higher requirements of the workpiece, when the material can be removed from the edge of the whole plate part of the material (100mm that can be ). We have found that the edge of the material is not consistent with the middle of the rolling, this inconsistency directly caused by the inconsistency of the rebound

and affects the angular accuracy of the workpiece. There is another way to reduce the springback is also more effective, both in one side of the workpiece pre-fold a small edge, this small edge can be 90 ° bend, can also be 180 ° bend, and then fold the rest of the edges. In this way, when the workpiece size accuracy within a certain range, can also play a role in improving the accuracy. In the bending to choose the appropriate width of the lower die opening. The lower die opening is too wide to cause the workpiece inside the bending angle increases, the pressure decreases, the rebound increases. Too small will make too much pressure caused by overload load. For general workpieces, we choose the same inner bending radius as the material thickness, so as to determine the width of the lower die opening.

3.3 Eliminate the deflection of the machine itself

The starting point is the upper and lower sliding block deformation requirements consistent. Only then can the workpiece in the full length of the consistent angle. The main means of achieving this purpose is the upper die of the diagonal wedge structure. This structure can be the machine upper slider deflection deformation in advance compensation. The method is bending before the upper slide is adjusted to the middle of the projection of an arc, after the application of load, the upper slide tends to a straight line, so as to achieve the actual displacement of the upper slide consistent. In each bending workpiece with high accuracy, the upper slider should be pre-adjusted, and the adjustment steps are carried out step by step, with continuous trial bending, debugging and measurement until the final satisfactory results are achieved.

Conclusion

To sum up, processing manufacturing industry plays an irreplaceable role in the process of economic construction, sheet metal processing as an important part of the processing manufacturing field, its increasing prosperity with the market economy also has more development areas for processing a higher quality workpiece, bending should be specific analysis of specific problems, through continuous trial bending, bending, reasonable distribution of size, and finally can achieve satisfactory results.