EP0341211B1 - Method of bending sheet metal - Google Patents

Abstract

For the purpose of bending pieces of sheet metal with the aid of a bending device which has a bending punch (1) and a die (2) with an adjustable plug (3), the actual bending characteristic (8) of a sample piece of sheet metal of the same thickness and the same quality of material is first of all determined and stored as a reference curve (8). In the course of the bending operation, the actual bending characteristic (9) is recorded and compared to the corresponding stored value. The position of the bottom plug (3) is then corrected on the basis of the difference established. <IMAGE>

Description

The present invention relates to a method for bending sheet metal with the aid of a bending device which has a bending punch and a die with an adjustable bottom, into which die the bending punch penetrates, depending on the desired bending angle, until the sheet is driven onto the die bottom, the theoretical Bending angle with constant die opening is determined by the relative position of the die bottom to the die opening. Such a method is described in EP-A-0 096 278.

It is known that the bending angle during sheet metal bending with a punch and a die, for a given width of the die, can theoretically be determined approximately by determining the depth of penetration of the punch into the die. Practice has shown, however, that the actual bending angle, depending on the material quality and thickness tolerance of the piece of sheet metal to be bent, has smaller or larger deviations from the theoretical value.

By adjusting the height of the die base, the desired angle could theoretically be set simply and repeatably. When repeating the bending process on different, qualitatively equivalent sheet metal pieces, however, deviations in the bending angle occur. On the one hand, this is due to the fact that during the bending process in the sheet metal, the theoretical, sharp edge is never created, which is the working edge of the punch corresponds, but there are roundings during bending, which significantly influence the bending angle. On the other hand, the two sheet metal legs spring back somewhat as soon as the sheet metal is released from the bending pressure. The size of the springback also depends on the sheet thickness and the material quality, so that the actual final bending angle can never be predicted theoretically with precision.

In practice, this means that two sheets of the same quality and thickness, manufactured by different manufacturers or originating from different production series, can have different bending angles after processing on the same machine with the same setting, since the material behavior with regard to the resulting rounding of the bow and the size the springback can be slightly different.

To improve the accuracy during the bending process, the applicant has already proposed a method according to which the forces occurring during the deformation of the sheet metal piece are continuously determined and the determined values are fed to a control device which compares them with stored target values and influences the stamp feed as a function of the deviations . (EP-PS 0 096 278).

Good results were achieved with this method. However, the aim of the invention is such a sheet metal bending process easier to design and still achieve a further increased accuracy with regard to the resulting final bending angle.

In order to achieve this goal, the proposed procedure has the features summarized in claim 1. In this way, angle measurements during the bending process are used to determine the correct height of the die base. It is possible with the aid of the proposed method to set the floor to the correct penetration depth on the basis of the determined angle differences during the bending process, the actual setpoint angle being exactly maintained at the end of the bending process. This process is significantly simpler than the known process and represents a major step forward.

Preferred developments of the method are defined in the dependent claims. For example, in addition to the comparison curve of the bending curve, it is also possible to determine the size and the curve of the bending force required to deform the sheet metal piece. This is then stored as a function of the depth of penetration of the punch or the angle of deformation, whereupon the actual course of forces when bending the sheet is compared with the stored course of forces and the differences found are used for additional correction of the depth of penetration.

Fig. 1-3

schematische Querschnittsskizzen durch eine Biegeeinrichtung;

Fig. 4

ein Diagramm des Biegewinkelverlaufs; und

Fig. 5

ein Diagramm des Kräfteverlaufs.

The proposed method is explained in more detail with reference to the drawings. Show it:

Fig. 1-3

schematic cross-sectional sketches through a bending device;

Fig. 4

a diagram of the bending angle curve; and

Fig. 5

a diagram of the course of forces.

For bending sheet metal pieces, a bending device is used, which is known per se and consists of a bending punch and a die with an adjustable bottom, into which the bending punch penetrates more or less depending on the bending angle. The theoretical bending angle is given by the position of the die bottom relative to the die opening while the die opening remains the same. According to the schematic sketch in FIG. 1, a bending punch 1 is provided, which works together with the fixed bending die 2, which has an adjustable base 3. To bend the sheet 4, the same is pressed against the adjustable base 3 along an edge 5 with the aid of the bending die 1. The angle W formed is determined by the position of the edge 5 with respect to the contact edges 6 of the die 2. The depth of penetration of the bending die 1 is designated E in FIG. 1.

As already mentioned at the beginning, the bending angle W is from the rounding radius depending on the bending edge. 2 schematically shows how the angle W changes. The sheet metal piece 4a has (theoretically) a very sharp edge in the form of a line, while the sheet metal piece 4b has a rounded bending edge. It can be clearly seen that the angle which the two legs of the sheet metal piece 4a enclose is somewhat larger than the angle enclosed by the legs of the sheet metal piece 4b. The following generally applies (with unchanged die setting): The larger the rounding radius of the bending edge, the smaller the resulting bending angle.

In Fig. 3, the springback behavior of a bent plate 4 is shown, greatly exaggerated. It is clearly evident that the sheet metal, relieved of the bending force exerted by the punch 1, springs back somewhat with its two legs, so that the bending angle when the sheet is relieved (punch 1 withdrawn) is somewhat larger than the theoretical bending angle in the situation, when the punch presses the sheet 4 completely against the die bottom. The springback rate depends on the sheet thickness and the material of the sheet and can hardly be predicted with the required accuracy.

The practical bending curve of a piece of sheet metal is shown in FIG. 4 by curve 8, which indicates the relationship between the penetration depth E and the angle W. In addition to the thickness tolerance of the curve 8 exert an influence on the course of the curve the yield strength, the modulus of elasticity and the hardening behavior of the piece of sheet metal.

According to the method according to the invention, this practical bending curve 8 of the piece of sheet metal is first determined by bending a sample sheet piece of a certain thickness and with a certain material quality in a series of tests. A large number of value pairs E / W are recorded, displayed in the form of a curve and saved as a reference curve for the above-mentioned, specific material quality and thickness.

When the actual bending process is carried out using sheets of equivalent quality and thickness, the position of the adjustable die base 3 and thus the depth of penetration is set to a value E _s which, according to the stored comparison curve 8, should result in the desired angle W _s . Then the bending process is started and continued continuously. When the punch has reached a first penetration depth E₅, the effective bending angle W₅ is measured and compared with the bending angle W₆, which results from the comparison curve 8. The difference W₅-W₆ shows that the effective bending angle W₅ is larger than the theoretically expected bending angle W₆.

The same procedure is used for the penetration depths E₃ and E₁: The effective bending angle W₃ or W₁ is measured and in each case with the assigned bending angle W₄ or W₂ the curve 8 compared. Based on the determined angle differences W₅-W₆, W₃-W₄ and W₁-W₂ it can be seen that there is a deviating from the comparison curve 8 bending curve 9 for the sheet metal just processed, which is shown in dashed lines in Fig. 4. The course of this curve 9 shows that the actual angle is greater than the expected angle with the same penetration depth. For this reason, the position of the adjustable bottom 3 of the die 2 must be corrected because the target angle will not be reached at the penetration depth E _s according to the comparison curve 8, but only at the corrected penetration depth E _k .

On the basis of the determined angle differences, the corrected penetration depth E _k , ie the corrected position of the bottom 3 of the die, can be determined and set immediately. Figuratively speaking, this can be done, for example, by adding a piece of curve 8 from angle W 1 to the previous, practically determined bending curve 9. This results in an intersection K of this (shown in FIG. 4 more strongly attached) curve piece 9a with the straight line assigned to the final target angle W _s , so that the associated corrected penetration depth E _k can be determined. Finally, the die bottom 3 is set to the new value E _{k of} the penetration depth; all of this takes place while the bending process continues. By the end of the bending process, the actual target angle W _{s will be} reached exactly.

In practical implementation, this extrapolation of the bending curve 9 takes place on the basis of the known bending curve 8, of course, in a computer-aided control device in which the curve 8 is also stored. The extrapolation is readily permissible, since in practice the deviation of curve 9 from curve 8 is very small and is greatly exaggerated in the drawing for reasons of clarity. The theoretically resulting inaccuracy due to extrapolation of curve 9 on the basis of curve 8 is so small that it can easily be neglected.

In the practical implementation of the method, it is advantageous to carry out the angle determinations as a function of the penetration depth in each case with the sheet loaded with the bending pressure, both in the determination of the reference curve 8 and in the actual control measurement at the penetration depth E 1. This allows continuous work without having to interrupt the bending process to measure the bending angle.

Furthermore, it is advantageous to carry out the last control measurement of the bending angle in sufficient time before the (expected) target angle is reached that there is still enough time to adjust the die base to the corrected height value. On the other hand, however, the last control measurement should take place as late as possible, so that only a relatively small area of curve 9 is extrapolated from reference curve 8 must, which further increases the accuracy.

A further embodiment of the proposed method can consist in that, in addition to the theoretical bending curve, the size and the curve of the force practically required for the deformation of the sheet metal piece are determined on a sample sheet of the same thickness and the same quality and stored as a function of the penetration depth or the deformation angle. It has been shown that with the same depth of penetration of the punch 1, even when using the same die with the same support edges, the angle does not remain the same, but changes as a result of manufacturing tolerances of the sheet, since one sheet has less force and the other sheet has more force needed for bending.

There is therefore a function between the bending angle and the penetration depth, which depends on the respective course of forces. In the perfection of the method, the magnitude of the bending force in the punch is measured along the path of the punch and a computer is fed with the measured values. The result is a curve 11 (FIG. 5), which represents the actual course of forces as a function of the path of the bending die during bending. This course of forces is compared with the stored reference course of forces, the differences determined being used for the additional correction of the penetration depth, ie for the additional correction of the position of the adjustable base 3.

In the practical embodiment, the bending punch 1 is made in two parts and consists of an upper part 1a and a lower part 1b, a measuring device 10 being accommodated between the two parts. This measuring device can be designed, for example, as an electrical load cell and is used to measure the pressure exerted on the sheet metal piece 4 by the stamp 1. The measured values are registered and processed in a computer which can act on the adjusting device of the die base.

Claims (5)

1. Method of bending metal sheets by means of a bending device, which comprises a bending bar (1) and a bottom die (2) with an adjustable base (3), in which bottom die (2) the bending bar (1) penetrates to a greater or lesser extent according to the desired bending angle (W), the theoretical bending angle being determined, in the case of a constant bottom die opening, by the relative position of the bottom die base (3) with respect to the bottom die opening (6-6), charcterised in that the first of all, in a series of trials with a pre-determined sheet metal quality, the effective bending angle (W) is ascertained as a function of the penetration depth (E) of the beding bar (1) in the bottom die (2) and is memorised as a comparison curve (8), whereupon in the actual process of bending further equivalent metal sheets the angle (W₁) of the loaded metal sheet (4) is measured in the course of the bending operation at at least one selected penetration depth (E₁) and compared with the corresponding angle (W₂) resulting from the comparison curve (8), and that the position of the bottom die base (3) is corrected on the basis of the angular difference ascertained (W₁-W₂), whereupon the bending operation is continued until the corrected position of the base (3) of the bottom die (2) is reached.
2. Method according to Claim 1, characterised in that the measurement of the bending angle (W₁) of the loaded metal sheet (4) and the comparison with the corresponding bending angles (W₂, W₄, W₆) resulting from the comparison curve (8) takes place with at least two, preferably three different penetration depths (E₁, E₃, E₅).
3. Method according to Claim 1 or 2, characterised in that the last measurement of the bending angle (W₁) and the comparison thereof with the angular value (W₂) from the comparison curve (8) takes place in the course of the actual bending process so far before reaching the desired bending angle (W_s) that sufficient time remains for correcting the position of the bottom die base (3).
4. Method according to one of the Claims 1 to 3, characterised in that the correction value for the adjustment of the bottom die base (3) is ascertained by extrapolation of the measured values of the angular difference on the basis of the memorised comparison curve (8).
5. Method according to one of Claims 1 to 4, characterised in that besides the comparison curve (8) of the bending behaviour, the magnitude and behaviour of the bending force necessary for the deformation of the specimen metal sheet (4) is also ascertained and memorised as a function of the penetration depth of the bending bar (1) or of the bending angle (W) as a further comparison curve (11), whereupon the actual force distribution at the time of bending further equivalent metal sheets (4) is compared with the memorised force distribution and the differences ascertained are used for the additional correction of the position of the bottom die (3).

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