At present, the global economy is in the doldrums, and the manufacturing industry has been seriously injured;
Sheet metal industry as a part of the manufacturing industry, in this economic situation, the market competition is particularly fierce.
Quality is the key for manufacturing enterprises to survive in the fierce competition.
Therefore, it is of great significance to control the bending accuracy and stability to improve the quality of sheet metal parts.
2. Failure forms of parts
In the production process, such phenomena will appear:
When the new die is used to bend the sheet metal parts on the press brake, the parts fully meet the requirements of the drawing,
However, after a period of production, it is found that the parts bent out of the same die can not meet the requirements of the drawing, which are mainly shown in two forms of figure 1a and figure 1b.
Fig. 1 Bending failure forms
a: Buckling deformation b: Size increase
3. Failure cause analysis
3.1 Causes of upper die wear
In the final analysis, the reason why the phenomenon shown in Fig. 1 is caused by the wear of the upper die of the press brake.
The upper die of the general press brake is a general mold, and a group of general press brake upper dies can meet the bending of a series of sheet metal parts;
In other words, the upper die of the press brake is replaced less, and the same group of bending upper die is used for general bending;
Even in some small factories, they use single parts and the press brake die never change.
Any tool or die will wear out.
However, the upper die use frequency of the press brake is high.
The R angle of the general bending upper die is small, which is usually below 0.5mm.
Therefore, when bending, the pressure is completely concentrated on the R angle of the upper die, and the stress at the R angle is very large, so the upper die is easy to wear.
3.2. “Size increase” analysis
As shown in Fig. 2, the wear of the upper die results in a larger R angle.
In the unfolding calculation of parts, the size of the R angle in bending is one of the factors affecting the unfolding coefficient (related information has been introduced, but not explained in detail here).
For the same part, the larger the bending angle R is, the shorter the unfolding size is;
Generally, there are two ways to select the bending expansion coefficient in factories:
1) According to the R angle of the new upper die, it is selected from the empirical table of expansion coefficient;
2) Through the trial bending of the new upper die, the real data are obtained to determine the expansion coefficient.
The first method is more common, which is fast and convenient for general parts bending;
The second method is generally for the parts with high bending accuracy and many bending angles, and the data is accurate.
But in most factories, no matter what method is used to obtain the expansion coefficient, it will be solidified after the determination.
For example, when the new upper die is used, the expansion coefficient selected by SPCC with material thickness t = 1.0 mm is 0.4, so long as all materials with T = 1.0mm material are bent with this group of the upper die, the expansion coefficient of SPCC selected is 0.4.
When the R-angle wear of the upper die increases, the size of the part which has been expanded by the expansion coefficient before wear will inevitably become larger after bending, as shown in Fig. 1b.
It is not very obvious for single angle bending, if a part has n times bending in the same direction, then the difference will be n times of single angle bending.
For example, according to the development coefficient selected in the “experience table of development coefficient”, when calculating the single angle bending angle of SPCC made of t = 1.0 mm, the difference between the unfolding dimensions of R0.5mm and R1mm is 0.2mm;
If a part is bent six times in the same direction, the difference of unfolding is 1.2 mm, and the 2 mm will accumulate into a dimension segment after bending.
In order to save cost, many factories use medium carbon steel to make bending upper die, which has poor wear resistance.
After several dies are used, R0.5mm almost becomes R1mm.
3.3 “Buckling” analysis
The standard length of the single upper die of the press brake is 835 mm, which is generally used in a group depending on the type of the press brake.
As shown in Fig. 3, there are three bending upper dies in a group.
Many sheet metal processing factories make miscellaneous parts of different sizes, and the bending width is very different.
Generally, sheet metal parts with narrow bending width account for the majority;
Therefore, the middle section of the die is often used for bending, as shown in Fig. 3, resulting in great wear of the middle section.
When this group of dies is used to bend sheet metal parts with a large width, the pressure on both ends of the bending inner angle is greater than that on the middle wear section, and the inner angle R of the middle section is also greater than that of the two ends.
Increasing the pressure per unit area and reducing the bending angle R are effective measures to reduce springback.
On the contrary, the middle section occupies two favorable factors for springback.
Since the springback of the middle section is greater than that of both ends, the middle “buckling” phenomenon as shown in Fig. 1 appears.
Fig. 2 Wear diagram of upper die
Fig. 3 Bending upper die
4. Control method
Die wear can not be eliminated, but by analyzing the causes and taking appropriate measures, the two failure phenomena in Fig. 1a and Fig. 1b can be completely controlled.
Combined with production practice, the following five methods are summarized:
1) High hardness of heat treatment can be obtained by selecting materials with good wear resistance, such as Cr12MoV and SKD-11.
2) Carburizing or nitriding (for materials with low wear resistance) is used to improve the wear resistance.
3) It is necessary to repair and grind the R angle part of the die regularly, which is determined as half a year or once a year depending on the wear condition of the die.
4) It is necessary to exchange and balance the use of the combined upper die to make the same group of upper die R angle balanced wear.
5) The development coefficient needs to be revised regularly, and the cycle is half a year or one year depending on the wear of the die.
The above five methods can be selected according to the actual situation of the factory, and the effect of different methods is not the same. The best method is to maximize the benefits of the factory.
The sheet metal failure phenomenon discussed in this article occurs from time to time in the sheet metal manufacturing industry. I hope that through this article, more sheet metal manufacturing enterprises can do a good job to prevent and avoid unnecessary losses.
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