Bending is one of the most important and widely used techniques in sheet metal fabrication. The basic principle of the sheet metal bending process is to utilize the plastic deformation of metal to produce parts or assemblies of the required shape and size.
In many sheet metal structural products, bending can replace complex components such as welding, riveting, and bolting.
Compared to welding, riveting, and bolting, bending makes it easier to control the shape and size of the plate precisely, and it also has higher accuracy and repeatability, which makes it suitable for the manufacture of various types of small, complex parts.
The bending process requires relatively simple tooling and equipment, mainly including bending machines, molds, etc., equipment operation is simple and easy to start, reducing processing costs and technical barrel.
Relative to welding, low requirements for the production environment, the quality of personnel and skills requirements are also lower, with lower production costs, can be in a short period for mass production characteristics, is widely used in various fields of mechanical industry in bulk.
Bending processing has a certain degree of flexibility, is not affected by differences in plate thickness, and is suitable for processing various plate thicknesses.
Comprehensively speaking, bending is more aesthetic than welding, riveting, and fixing two separate parts together. It also has the advantages of short cycle time and low cost-effectiveness of the product process.
Because of the increasing complexity of bending workpieces, product bending is becoming more and more difficult. Without good process guidance, it will not achieve the desired results.
Due to the increased bending complexity, interference problems are likely to occur, resulting in bending and forming not being fully completed. Therefore, a reasonable bending process plan, bending tooling selection, and tooling design are critical to solving the bending interference problem.
Bending interference problems
Bending conflict mainly exists in two or more bending products, specifically in the bending edge and mold, equipment, or the product itself collision, resulting in regular bending.
Bending interference is mainly affected by the shape of the part, size, mold, equipment structure, and bending order.
(1) bending edge and mold interference is mainly in the bending process, the bending edge of the bending rotation, and the upper or lower mold collision interference, resulting in the product not continue to bend or deformation.
This type of bending problem is the most common one, and it is also the first thing that all sheet metal craftsmen need to consider when carrying out bending analysis.
The upper die interference is shown in Figure 1, and the lower die interference is shown in Figure 2.
Figure 1 Upper die interference
Fig 2 Lower die interference
(2) Product and equipment interference This is often due to several sides of the closed product bending edge being too high, the product being too long, or in the middle of the wider sheet metal folding a ‘Z’ structure.
Three-sided closed bending, as shown in Figure 3, when the length direction of the two parallel sides of the completion of the bending, because the product itself bending edge is too high, the parallel side of the bending edge of the collision with the upper die interference, so the bending will also be interrupted.
The product can not be bent in the order of adjacent sides because after the completion of the first side of the bending and then folding the adjacent side, the first bending edge will interfere with the upper die if the first bending edge is considered to be placed on the end of the bending machine, the product in the positioning of the left and right sides of the bed of the bending machine and the positioning of the edge of the workpiece will interfere with the rear stopper device can not be positioned in this bending machine.
Figure 3 Three-sided closed bending
‘Z-shaped bending, shown in Figure 4, seems to be a simple ‘Z’ bending structure, but no matter which bending edge is completed first, the second bending edge cannot continue bending.
After the completion of the first bending edge, the product is presented as a large ‘L’ structure because the two ‘L’ sides of the height are too large, so according to the direction of the bending, when folding the second bending edge, the first bending edge will be facing downward, and will produce a bending machine table with the plane of the interference problem. Interference problems.
Figure 4 ‘Z’ bending
(3) Interference between the bending edge and other structural features of the product This is mainly found in products with a strict fit.
This is mainly found in products that have a strict fit relationship. Due to the accumulation of errors in the bending process and factors such as bending spring back, the product bending interference and angle bending is not in place.
Containing the assembly structure of the bending shown in Figure 5, the position marked by the red circle is bending with the position, if the width of the bending of the negative tolerance is too large, then the bending edge 1 and the bending edge 2 and the width of the two bending edges of the collision of the interference and can not be closed, the position of the 3 must be left a particular gap, otherwise the bending edge of the 2 can not be guaranteed that the angle of the 90 °.
Bending interference problems are complex and varied, all cases can not be listed, the above mainly lists some common cases for reference.
Figure 5 Bending with an assembly structure
Solutions
(1) Reasonable choice of mold and reshaping the mold structure Bending hook mold is the most commonly used mold to prevent bending interference.
This mold structure can well avoid the interference problem of a ‘U’ shaped bending structure, according to the bending edge’s size. The bottom edge of the ‘U’ shaped product can be selected or designed accordingly to hook the die (see Figure 6).
In addition to the hook die, reshaped bending dies are a common way to prevent bending interference.
Fig 6 Hook bending die
There are various forms of mold reshaping, which can be analyzed according to the interference of the product, which can be the upper or lower mold, manifested in the processing of notches, holes, etc. on the mold.
When reshaping the bending mold, we should fully consider the strength of the mold after reshaping to meet the use of requirements, if the mold notch, hor ole is too large, the mold will be easily deformed, which will significantly reduce the service life of the mold, and at the same time can not guarantee the accuracy of the product requirements. The reshaped mold is shown in Figure 7.
Fig 7 Conforming mold
Sometimes, the bending hook mold can not avoid bending interference, so it can be taken to increase the pressure line (or planing groove bending) process:
First, in the bending line pressure a mark (or in the bending line position processing a ‘V’ groove, the depth is generally controlled in 80% of the plate thickness, planing ‘V’ groove angle than the actual forming angle of 2 °, this is due to the bending and forming of the material will have a rebound, the gap of 2 ° to facilitate the control of the angle of processing). (Reserve a gap of 2 ° to facilitate processing control angle). The crimping process is shown in Figure 8.
Figure 8 Press line process
The first bending line can be folded to a certain angle, the purpose is to avoid subsequent bending touch knife interference, and then finally press the line bending edge pressure to 90 °, planing groove bending shown in Figure 9.
Figure 9 Slotted bending
In general, the pressure line process can be applied to the bending can not be completed once the structure, when the second bending, the pressure line position can be better formed following the position of the original bending line. Otherwise, it is prone to the second bending bias problems, or bending angle control is not a good phenomenon.
The disadvantage of the planing groove bending process is that it is prone to bending fracture, so when considering this process, the a need to judge and select according to the specific processing needs.
(2) Reasonable arrangement of bending process route In many cases, a reasonable bending process sequence can greatly reduce the bending interference and the need for special mold requirements.
For example, in the bending shown in Figure 4, you can first pre-bend one of the 90 ° bending edges to about 135 ° (see Figure 10), and then the second bending edge to 90 °, and finally, the 135 ° bending edge to continue to bend to 90 °, to complete the bending of the product.
Figure 10 Pre-bending schematic
In the bending interference shown in Figure 5, a reasonable bending sequence is a key factor in determining whether the workpiece can be processed.
For the analysis of the bending order of more bending edges and complex workpieces, you can use the reverse thinking method, from the point of view of ensuring that the bending does not interfere with the first to determine the last bending edge, and then up the reasoning analysis of the penultimate bending edge, and so on.
Figure 5 in bending edge 1, bending edge 2 can only be the last two bending edges, otherwise, bending the upper die can not reach the position of the red marking 4 corners, which results in the workpiece bending order as shown in Figure 11.
Figure 11 Bending sequence
Increasing the pre-bending process is also an effective solution to the common measures of bending interference. The method is to add bending interference in the location of the bending of the reverse bend so the general angle is not too large, depending on the degree of bending interference. The pre-bending process is shown in Figure 12.
Figure 12 process pre-bending process
(3) Select the appropriate bending equipment With the rapid development of equipment, bending equipment has become a variety.
According to the different processing methods, the bending machine can be divided into upper and lower bending.
Upper bending type bending machine pressure plate on the top, suitable for thin plate bending;
The downward bending type bending machine pressure plate below suits thick plates and large workpiece bending.
Bending product process analysis must be established based on familiarity with the performance of the equipment, you must understand the structural characteristics of the equipment, be familiar with the length of the bending machine stroke, the height of the stroke, the rear of the blocking stroke and the width of the working table, etc., these parameters are the influence of the bending interference factors.
(4) Design of special bending mold structure
For the complex structure of the bending workpiece, often need to make special non-standard molds according to product design. Non-standard mold structures in various forms can be single-shaped (see Figure 13) and assembled from many parts of the motion mechanism.
Figure 13 non-standard single-shaped mold
(5) Optimise bending structure design
Some sheet metal structure product designs often focus more on the performance, function, and appearance of the product, its structure does not necessarily have good craftsmanship.
For this structure of sheet metal bending, it is necessary to ensure that the product’s original structure, size, and performance under the premise of the product bending edges, weld structures, etc., process optimization to achieve good product manufacturability.
Figure 14 shows the structure of a locomotive lampshade, which is a first bending, then welded three-sided structure, bending edge ① and bending edge ②, and the bottom plate at an acute angle in the absence of a unique mold, it is challenging to achieve bending.
Figure 14 A locomotive lampshade structure
The weld edge and bending edge ② are structurally optimized; the original weld edge is designed as a bending edge, and the bending edge ② is designed as a weld, which immediately improves the manufacturability.
Firstly, the bending difficulty is instantly reduced, and the bending interference problem is solved;
Secondly, the weld length is shortened to 55% of the original length, reducing the cost and improving efficiency.
Summary
This paper lists the bending product structure as simple, from various aspects to reveal the basic principles of bending and bending interference, put forward the corresponding process solutions, and can be used in the application process.
The bending process is an efficient study, and manufacturing will continue to encounter various new problems. Many measures to solve the bending interference need to be considered from various aspects, combined with production, manufacturing, cost, and cycle time, to select the appropriate program to improve product quality and production efficiency.
