EP0340167A2 - Method of bending work pieces - Google Patents

Abstract

In the method for bending a workpiece according to a given bending angle, the workpiece is first bent at a somewhat larger angle than the given final bending angle, whereupon the relief of the pre-bent workpiece (14) takes place by relieving the pressure on the bending punch (5). The amount of springback is then determined by measuring the effective bending angle after the first bending operation. Now the die base (22) is adjusted in such a way that the subsequent, second bending operation would result in a bending angle which corresponds to the given final bending angle minus the amount of springback. After relieving the load on the workpiece after the second bending operation, the workpiece will spring back in such a way that it has exactly the desired final bending angle.

Description

The invention relates to a method for bending a workpiece according to a given desired angle. The method is carried out with the aid of a bending device which has a bending punch and a bending die which has a die base which is adjustable in accordance with the angle to be formed.

When bending sheet metal with a punch and a die, exact compliance with a given bending angle is very difficult. The bending angle can be theoretically calculated precisely by determining the depth of penetration of the bending punch into the die for a given width of the die, but the theoretical values cannot be achieved practically, since the actual bending angle, depending on the accuracy of the punch feed into the die, and Quality and thickness of the sheet metal piece to be bent, minor or major deviations from the target value.

The use of a bending device, in which the bending angle can be set and changed very easily and precisely, brings a certain improvement. Such bending devices expediently have a bending die which has an adjustable die bottom. The bending angle can be exactly determined and changed without changing the die by adjusting or changing the height of the die bottom. The bending die points to it, the bending punch facing side on a longitudinal groove, which determines the current bending angle by the relative position of the two fixed contact edges delimiting the groove opening and the movable groove base. In this case, too, a springback occurs when the workpiece is unloaded, so that the actual bending angle does not match the theoretical, calculated value.

For this reason, it is essential to determine the deviations of the bending angle from the target value in a series of tests with a certain sheet quality and thickness and to carry out a corresponding correction of the punch feed into the die or a correction of the height of the adjustable die base before the actual production of the bent sheet Sheet metal can begin.

It is the object of the present invention to provide an improvement here and to propose a method for bending workpieces according to a given desired angle according to the preamble of claim 1, which avoids the need for time-consuming and material-intensive series of tests before production and with which workpieces are independent of the material quality and thickness can be bent very precisely.

According to the invention, this is achieved with the features specified in the characterizing part of patent claim 1.

This requires a device with which the sheet metal angle can be measured continuously and very precisely during the bending process. For this purpose, the applicant has already proposed an angle measuring device which has a support element that can be placed against the workpiece to be measured at a first point, and a pivoting head that can be pivotally connected to the support element and can be placed against the piece of sheet metal to be measured at a point away from the first contact point . The connection of the contact points of the support element and the probe with the workpiece define the current angular position of the workpiece. An angle measuring element is provided for detecting the pivoting movement of the probe.

With such an angle measuring device, the actual sheet metal angle can be determined continuously and precisely, so that a deviation from the target value can be precisely determined at each angular position.

Another measure that contributes to improving the accuracy during the bending process consists in preventing or compensating for deformation of the elements of the bending machine that are primarily involved in the pressing process in order to avoid inaccurate machining of the workpiece.

For this purpose, the applicant has already proposed to use a bending stamp, the stamp holder rests on a support element which ver vertically in a holding part is slidable and supported on an oil cushion. It is advantageous if the oil cushion is covered with a membrane on which the support element then rests. In order to avoid overcompensation of the deflection in the case of bending dies of different lengths, the applicant has already further proposed to use a bending die in which the support beam is divided several times along its length, so that beam parts which are guided vertically displaceably are present independently of one another.

In a preferred embodiment of the proposed method for bending a workpiece according to a given desired angle, a bending device is used which has an elastically supported bending punch and a bending die with a die base which can be adjusted according to the angle to be formed.

The work piece can be relieved in the second operation in an advantageous manner by reducing the pressure in the elastic support of the punch. The full load of the ram in the last step, however, takes place by increasing the fluid pressure in the elastic support. In this way, a simple, reliable and precise procedure is made possible, which allows the exact maintenance of a given sheet angle.

1 of the accompanying drawings is a suitable bend Device for performing the method shown schematically in vertical section. 2-7 show schematic cross-sectional sketches through punch, sheet metal and die during different phases of the bending process according to the invention.

The bending device basically consists of a vertically movable upper tool A and a fixed lower tool B. The upper tool A is provided with a carrier 1, which is arranged on the movable bar of a press brake in a known manner. A connecting member 2 is fastened to the beam 1 of the bar by a welded connection 3, the connecting member 2 having a slot-like recess which extends essentially over the entire width of the connecting member and into which a punch holder 4 projects. The punch holder 4 is provided with a bending punch 5 which is used directly for bending a workpiece, e.g. a piece of sheet metal 14 is used. The stamp 5 is fastened in the stamp holder 4 by means of an extension 6 which engages in a corresponding groove 7 in the stamp holder 4 and is held there with clamping members 8.

It is essential that there is a channel-like recess 9 in the connecting member 2, which extends over part of the width of the recess and is covered by a membrane 11. A membrane holder 12, which is supported against a holding flange 15, serves to hold the membrane 11 in place. The channel-like depression is filled with liquid 13, for example filled with pressure oil, the arrangement being such that the liquid pressure can be increased and decreased by known means, for example by a corresponding pump, not shown in the drawing.

The bending punch 5 has a working edge 17 where the two working surfaces 16 of the punch 5 collide at an angle that is smaller than the smallest angle that can be produced with this tool.

It is possible to insert an intermediate piece between the stamp holder 4 and the membrane 11, which acts as a support beam and is divided several times along its length, so that only part of the beam is exposed to the load, which part of the effective length of the stamp holder used 4 corresponds. Overcompensation of the deflection cannot occur in this way even if the punch holder 4 is relatively short.

The lower tool B has a die body 18 which is equipped with a longitudinal groove 19 on its surface facing the punch 5. This longitudinal groove determines the bending angle by the relative position of the two contact edges 20 delimiting the groove opening and by the depth of the groove base 21.

The flat longitudinal groove 19 is arranged in the vertical longitudinal center plane of the die body 18 and is adjustable Floor 21 equipped. For this purpose, the groove base 21 is formed, for example, by pins 22 acting as supports, which are inserted in vertical openings of the die body 18 arranged next to one another and are guided displaceably in these openings. The upper end faces of the pins 22 form the groove bottom 21.

To adjust the groove base 21, the pin ends facing away from the groove base 21 rest in a known manner on a stop bar, not shown, which is adjustable on an inclined surface and thus enables the groove base 21 to be raised or lowered precisely.

As can be seen from FIG. 1, the piece of sheet metal 14 to be deformed rests on the die body 18. To carry out the bending operation, the groove base 21 is first brought into a position in which the base together with the support edges 20 determines an angle which is greater than the setpoint angle. By lowering the punch 5, the sheet metal piece 14 is pressed against the groove bottom 21 with the help of the working edge 17. In this position, the piece of sheet metal is under a certain prestress, which is due to the material. This preload is released by relieving the sheet metal piece 14, the relieving being carried out by withdrawing the bending punch 5. In a preferred manner, however, it is also possible to relieve the piece of sheet 14 by reducing the liquid pressure in the channel-like recess 9. So that springs It should be pointed out that the representations are drawn in a greatly exaggerated manner in order to be able to represent the conditions occurring during the bending process more clearly.

2, the punch 5 has been raised and a piece of sheet metal 14 to be bent rests on the surface of the die 18. The adjustable groove base 22 is in a first position, in which the surface 21 of the groove base 22 is at a distance a below the surface of the die body 18. This position a corresponds to a specific bending angle α, which is somewhat larger than the predetermined bending angle β to be actually achieved.

Now the punch 5 is moved against the die 18, hits the surface of the sheet 14 and begins to bend it. The feed movement of the punch 5 is continued until the leading edge of the sheet metal bump hits the surface of the die bottom 22 and then stops. Now the two legs 14a and 14b of the sheet 14 enclose the calculated angle α. This situation is shown in Fig. 3.

The pre-bent sheet 14 is then relieved by, for example, pulling back the bending punch 5 somewhat. As can be seen from the illustration in FIG. 4, the bent sheet springs back somewhat, so that the angle α ′ enclosed by the two legs 14a and 14b is somewhat larger than the theoretical bending angle α. This springback can be observed in practically all types of sheet and the size of the springback depends on the material and quality of the sheet, its thickness, etc.

Next, the effective angle α 'is measured, which is present when the sheet 14 is relieved. This can be done with the angle measuring device mentioned at the beginning. The size of the angle is known and the difference (α ′ -α) is formed. Furthermore, the position b of the die base 22 is known, in which the sheet would be bent exactly at an angle β. Finally, the springback rate (α ′ -α) is known. From this, the definitive position (b + Δb) of the die base 21 can be calculated, in which the sheet 14 will ultimately have exactly the desired bend.

5, the die base 22 is shifted into a second position, in which the surface 21 is at a distance (b + Δb) from the surface of the die body 18. The amount of (b + Δb) is slightly larger than the amount of a, corresponding to a smaller, resulting bending angle. Then the punch 5 is again pushed against the die 18 until the leading edge of the sheet metal bend touches the surface 21 of the die bottom 22, and then stopped. This situation is shown in Fig. 6. Here, the two legs 14a and 14b of the sheet 14 include an angle β ', which is slightly smaller than the desired bending angle β.

After the sheet 14 has been relieved, e.g. by retracting the punch 5 (FIG. 7), the sheet 14 springs back somewhat in order to finally have the desired bending angle β.

In practice, the two angles alpha and β differ very little; The following example can serve as a guide:

If a sheet is to be bent at an angle of exactly 90 °, the first bending takes place at an angle of approximately 91 °, that is to say an angle which is certainly somewhat larger than the ultimately desired angle. After the sheet metal, which was bent for the first time, has been relieved, the two legs spring back somewhat so that they now enclose an angle of 93 °. The difference is therefore 2 °.

It can be assumed that the springback will take place after the subsequent, second bending operation by the same angular amount, since the conditions remain essentially the same. After the sheet has been relieved, the die base is adjusted so much that after the second bend a bending angle of 88 ° results with the sheet loaded. If the sheet is relieved after the second bending operation, the two legs spring back by 1 ° each, with the result that the bending angle is exactly the desired 90 °.

All processes can be carried out automatically and preprogrammed, since the already existing facilities described are suitable for this purpose in every respect.

Claims (3)

1. Method for bending a workpiece according to a given desired angle with the aid of a bending device which has a bending punch and a bending die with a die bottom which can be adjusted in accordance with the bending angle to be formed,
characterized in that
in a first operation, the height of the die base (21) is adjusted according to a first angle, which is greater than the given nominal angle, whereupon the workpiece (14) by lowering the punch (5) to the bottom (21) of the die (18) the first angle is bent accordingly;
in a second operation, the bent workpiece (14) is relieved, so that the same springs back into a relaxed position;
in a third operation, the angle formed by the spring-back, relaxed workpiece (14) is measured, compared with the first angle and the position of the die base (21) is corrected to a value that corresponds to the given target angle minus the difference between that measured on the relaxed workpiece Angle and the first angle, and what
in a fourth operation the pre-bent workpiece (14) is pressed against the die base (21) with the again fully loaded punch (5), which takes on the corrected definite height position. 2. The method as claimed in claim 1, in which the bending device has a bending stamp which is mounted elastically under liquid pressure,
characterized in that
the work piece is relieved in the second operation by reducing the fluid pressure in the elastic support (13) of the punch (5). 3. The method according to claim 2,
characterized in that
the full load of the stamp (5) in the last step is carried out by increasing the fluid pressure in the elastic support (13) of the stamp (5).

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