F old loss
For mechanical bending, due to the differences in various process parameters such as the curved surface and curved edge of the bent sheet, its dimensional accuracy will directly affect the assembly clearance of the vehicle body.
Boundary indentation is the most common, and its indentation value is also known as bending loss (FV), as shown in Figure 1.
Circular arc pre – folding knife
At present, most die benders adopt a pre-folding planar knife.
There is a mature research base on the influence of planar pre-folding knives on dimensional accuracy.
In the actual process of technology formation and development, the folding equipment supplier began to use the arc-shaped pre-folding knife in a small number of folding equipment, that is, a work surface is no longer a plane, but a curved surface with a certain curvature.
The model diagram is shown in Figure 2.
Description of the problematic object
According to the actual situation of the door panel, folding objects are divided into 9 types: curved surface edge, straight surface edge, straight face curved edge, straight face straight edge, etc.
For the bending process parameters, after comparing with the actual production, determine the value range of different parameters and explore the influence of different parameters on the bending results.
See Table 1 for parameter range.
Table 1 bending parameters
| Parameter | Parameter Range Sheet |
| Arc pre-folding knife type | Circular Arc Pre-Folding Knife |
| Flange length / mm | 5.5-7.5 |
| Flange angle /° | 90-110 |
| Clearance between inner and outer plates/mm | 0.5-1.5 |
| Die arc size/mm | 1-1.3 |
| Surface curvature radius / mm | 100 |
| Radius of curvature of the part contour / mm | 100 |
| Pre-folding knife arc radius / mm | 6-8 |
S imitation try
Sensitivity analysis of process parameters
After investigation, four process parameters are selected for sensitivity analysis:
bending size H, inner and outer plate clearance G, outer plate fillet radius R and bending opening angle θ.
Assess your range of influence on sheath loss.
In this post, the uniform test design table is used to judge by the range method.
Uniform design is adopted. In the experimental design, the selection range of four parameters is shown in Table 2.
Table 2 process parameter selection range
| Parameter | Value range | |
| Minimum | Max. | |
| Flange Size H/mm | 5.5 | 7.5 |
| Clearance between inner and outer plates G/mm | 0.5 | 1.5 |
| External plate radius R/mm | 0.35 | 0.75 |
| Flange open angle θ/° | 90 | 110 |
The 4-factor 5-level uniform test design table is adopted and AutoForm is used for simulation.
Under the condition that other conditions remain unchanged, the bending type is selected as concave straight edge, the same arc pre-bending knife is used for bending, and the FV bending loss value is read.
It can be seen that the first is the bending opening angle, which has the greatest impact on the bending loss, the second is the fillet radius of the outer plate, and the third is the gap between the inner and outer plates.
Fold size has the least impact on fold loss.
The relationship between changing the four process parameters and bending loss is shown in Figure 3.
As can be seen in the figure:
(1) The bending size has no significant effect on the bending loss value, and the bending loss value always fluctuates around 0.19 mm.
(2) As the gap between the inner and outer plates increases, the bending loss first increases and then decreases, and the decreasing trend is slow;
(3) With the increase of the fillet radius of the outer plate, the bending loss first increases and then decreases, but the increasing trend is obvious, and the maximum value will be reached between 0.55 ~ 0.75 mm.
(4) With the increase of the folding opening angle, the bending loss gradually decreases.
It should be noted that the type of sensitivity analysis is straight concave.
For other sheath types, the sensitivity analysis results may change and the influence law of various parameters may also change.
Response surface results
Test data is processed.
When response surface analysis is performed using specialized design software, the results of the analysis are that the primary parameters that have a significant impact on bending loss are: part contour radius of curvature, pre-blade arc radius, Bent in an arc shape, foldable opening angle.
And the cross-term parameters of the second term are the radius of curvature of the part contour and the folding opening angle.
The arc radius of the arc-shaped pre-folding knife is multiplied by the folding opening angle.
First, the parameters of the primary term are analyzed.
When the other four parameters are fixed, the value range of each parameter is selected as the abscissa and the bending loss value is selected as the ordinate, and the variable relationship between the bending loss and each parameter is drawn, as shown in Figure 4.
As can be seen in Figure 4:
(1) as the radius of curvature of the part contour increases, the bending loss decreases;
(2) with the increase of the arc curvature radius of the arc-shaped pre-folding blade, the sheath loss increases;
(3) with the increase of the bending opening angle, the bending loss decreases, which is the same as the sensitivity analysis result.
In the response surface nephogram, the bending loss value is nephogrammed in relation to the bending opening angle, the radius of curvature of the arc-shaped pre-bend blade and the radius of curvature of the part contour, and the results are shown in Figure 5.
As can be seen in Figure 5:
(1) when the bending opening angle remains unchanged, the bending loss decreases with the increase in the curvature radius of the workpiece contour and increases with the increase in the curvature radius of the circular arc pre-bending knife;
(2) when the radius of curvature of the part contour remains unchanged, the bending loss decreases with the increase of the bending opening angle;
(3) When the radius of curvature of the circular arc pre-folding blade remains unchanged, the bending loss decreases with the increase of the folding opening angle.
The above conclusion is the same as the influence trend of the previous significance parameters.
But in the cloud map of secondary significance parameters, in the face of selecting different parameter values, the folding loss can be obtained more accurately and conveniently.
Folding Loss Mechanism Analysis
Comparison of the difference between circular arc pre-folding knife and flat knife
Using AutoForm software, read the bending loss value after the pre-bending process and the bending loss value after the final bend in the bending process of the arc pre-bending knife and the flat knife.
As shown in Figure 6 and Figure 7, it can be seen that after the completion of the pre-bending process, that is, when the bending surface is at the same angle, the bending loss value of the circular arc pre-bending knife is 0.15mm, and the height of the outer plate flange is 6.2mm.
The bending loss value of the flat knife is 0.32mm, and the height of the outer plate flange is 5.9mm.
After the final bending process is completed, the bending loss value of the arc-shaped pre-bending knife is 0.33mm, the height of the outer plate flange is 2.41mm, the bending loss value of bending of the flat knife is 0.39 mm and the height of the outer plate flange is 2.39 mm.
Fig. 6 Circular arc pre-folding knife bending process
Fig. 7 Flat knife bending process
Compared with the flat pre-folding knife, the reason why the circular arc pre-folding knife can reduce the bending loss is that when the circular arc pre-folding knife is used, the outer plate is always in contact with the working surface of the circular arc point.
And the contact point will move upward along the working surface of the circular arc with pre-bending.
As the direction of force on the sheet metal by the working face of the pre-folding knife is always perpendicular to the working face of the pre-folding knife, and the contact point points to the center of the circle, when the arc-shaped knife is adopted molded pre-bend, the direction of the force on the sheet is always parallel to the normal direction of the arc of the contact point.
With the change of the contact point on the working surface of the pre-folding knife, the direction of the force of the outer plate changes all the time.
The direction of force change contributes to slowing down the tendency of the inward approach of the bending face of the outer plate and ultimately contributes to reducing the bending loss value.
Analysis of the influence of the radius of the blade pre-bent in a circular arc on the bending loss
From the analysis of the response surface equation, it can be obtained that the bending loss decreases with the increase in the radius of curvature of the pre-bending blade arc of the circular arc.
The reason for this changing trend is explored.
Change the arc curvature radius of the arc-shaped pre-folding knife to 6mm, 6.5mm, 7mm, 7.5mm and 8mm, and draw the corresponding arc-shaped pre-folding knife shape in UG, as shown in Figure 8.
It can be seen that when the starting and ending positions of the arc-shaped pre-folding blade are fixed, the center of the circle corresponding to the arc segment moves from the inside to the outside with the increase in the arc radius.
In the stress analysis of the pre-bending process of the circular arc pre-folding knife in the previous summary, the force on the bending surface of the outer plate always points to the center of the circular arc section from the contact point, i.e. , the upper end of the folding surface.
Therefore, when the force in the pre-bending process is constant, with the increase in arc radius, the horizontal component of the force on the outer plate will increase.
And increasing the component in this direction will increase the tendency for the bending face of the outer plate to approach inward, resulting in the bending loss value increasing.
To verify the accuracy of the above analysis, under the process parameter values, five arc-shaped pre-bending knives are used for bending simulation, and the bending loss after the pre-bending process and the bending process end are measured. See Table 3.
Table 3 Pre-bending and end-bending bending loss
| Group no. | Circular arc pre-folding knife bending radius (mm) | Prebending FV(mm) | End hem FV(mm) |
| 1 | 6 | 0.281 | 0.46 |
| two | 6.5 | 0.3 | 0.475 |
| 3 | 7 | 0.311 | 0.481 |
| 4 | 7.5 | 0.313 | 0.477 |
| 5 | 8 | 0.318 | 0.484 |
It can be seen that when the arc radius of the arc-shaped pre-bending knife increases, the bending loss after the pre-bending process also increases.
The final bending process increases the bending loss by almost the same amount.
Therefore, it can be inferred that the influence of the radius of curvature of the arc of the arc-shaped pre-folding knife on the bending loss value occurs in the pre-bending stage, so as to change the direction of the force on the bending surface of the outer plate and affects the final bending loss value.
Conclusion
In this post, taking the circular arc pre-bending sheet as the research object, the factors that may affect the loss of bending are analyzed using AutoForm software, and the following conclusions are obtained:
- The fillet radius of the outer plate, the bend opening angle, the arc radius of the workpiece contour and the arc radius of the arc-shaped pre-bent blade have a significant impact on the bend loss;
- when the arc radius of the arc-shaped pre-bending blade increases, the bending loss of the workpiece increases;
- when the radius of curvature of the part contour increases, the bending loss of the part increases;
- when the bending opening angle increases, the bending loss of parts increases;
- When the arc-shaped pre-folding knife works, with the change of the contact point on the working surface of the pre-folding knife, the direction of force on the outer plate changes all the time. The direction of force change contributes to slowing down the tendency of the inward approach of the outer plate's bending face and ultimately reducing the value of the bending loss.
