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The present invention describes a method for reducing the bending angle errors when bending a sheet in a die bending press consisting of a fixed lower tool (die) and a bending beam driven by linear axes with the upper tools (bending punch), the lower reversal point of the bending punch being based on the preset setpoint of the bending angle and the force-displacement curve measured during the bending process are calculated in advance.
Die bending is a widely used process in sheet metal working. Generally known bending machines consist of a C-frame on the lower tool carrier of which the lower tools are attached in the form of mostly V-shaped dies. The matching mostly knife-shaped upper tools are mounted on a movable press beam. This press beam is moved in the vertical direction by two linear axes arranged at its ends, which are usually driven hydraulically.
Bending in the die is usually carried out as free bending, i.e. the sheet lies only at two points on the V-die and at one point on the punch (viewed in cross-section). The bending angle results from the lower reversing position of the bending punch. This process is in contrast to stamping bending, in which the punch is moved into the die until a positive connection is established between the die, sheet metal and bending punch. The embossing process enables very precise bending angles to be produced, however a separate tool set and 4 - 6 times the pressing force (compared to free bending) are required for each bending angle and sheet thickness, which is why this process is rarely used.
In contrast, many different bending angles can be produced with a tool set when bending freely. A limitation with free bending is, however, the reduced accuracy of the bending angle achieved, which is a consequence of the springback of the sheet after the bending punch has been withdrawn from the lower reversal point. The press controls of the bending presses mainly used according to the state of the art today determine the springback to be expected based on simple, mostly empirical formulas and the material parameters or workpiece dimensions entered by the operator, and then move to a somewhat lower position with the bending punch, which is too a "bending over" that leads sheet metal. Ideally, the desired bending angle is then set after the load has been removed.
In practice, however, there are some uncertainties when predicting the springback angle, which can sometimes lead to considerable errors in the bending angle. The reasons for this are scatter or uncertainties in the material parameters, such as the tensile strength or the yield strength of the material (up to several 10%). Because of the manufacturing-related anisotropy of the material properties of a sheet, these values also fluctuate depending on the position of the bending direction in relation to the rolling direction. In practice, the sheet thickness is also subject to fluctuations of up to several%.
If a high degree of accuracy and reproducibility of the bending angle is required, the springback angle must be measured and taken into account individually - at least for each sheet type and workpiece geometry - which is done in a two-stage process according to the state of the art. In a first bending step, the desired end angle is not yet bent. The bending punch then withdraws so far that the springback can be measured using an angle measuring system. In the subsequent bending step, the actual spring-back angle is then bent so far that the desired end angle is set with an accuracy of a few 0.1.
Bending with the aid of such systems means a reduction in productivity due to the longer duration of the bending process. In addition, the angle measuring systems used are often expensive, complicated to use or can only be used in special cases.
There is therefore a need for an uncomplicated and cost-effective system that determines all the parameters required to compensate for springback in a single bending step and immediately determines the required lower reversing position of the bending punch.
In US4408471 (Gossard et al., Press brake having spring-back compensating adaptive control) and US4511976 (Press brake having spring back compensation stroke reversal control, Raymond J. Graf) this is achieved according to the prior art in that the by two Force applied to linear axes is measured as a function of the position of the press beam. Out
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The material properties of the sheet are derived from the relationship thus recorded, which then allow the calculation of the optimal lower end position of the bending die. Since the force-displacement relationship is recorded, the material properties are calculated and the optimum end position is determined in real time, there is no loss of time with this method compared to methods without taking the individual material properties into account.
To absorb the bending force, two force transducers in the area of the hydraulic axes used in the present case or two pressure transducers in the hydraulic system are used, which represents a certain additional effort compared to presses without force-displacement detection.
On the other hand, the measurement of the stamp path is implemented as standard in almost all bending presses of modern design, since the corresponding data are also required for precise control of the movement of the press beam. According to the prior art, two displacement transducers are usually used for this purpose, which are fastened in the area of the linear axes of the press beam and deliver displacement signals with a resolution of the order of 20 μm.
A similar procedure for checking the springback is claimed in DE19738955 (Haldenwanger et al., Method for controlling a forming process), but there for stretch bending. During the stretching phase during stretch bending, the force-displacement curve is recorded in order to characterize the material from it. The prestressing force required to set the desired springback is then determined from the data obtained in this way.
According to the state of the art, the force during bending in the die is recorded at two points, i.e. essentially only the total force composed of two components, which is introduced along the entire bending length, is available for the calculation of the material properties. Since the force transducers are permanently connected to the press, they must cover a large measuring range; have to work with a thin sheet with a small bending length as well as with a thick sheet with a large bending length. For this reason, poor accuracy can only be achieved at the lower limit of the measuring range.
As there is a flat shape change state during bending, apart from edge defects, the force introduced per length is primarily responsible for the characterization of the bending process. When measuring the total force, the bending length must also be known in order to be able to determine the relevant quantity "force per unit length". Since this value is not measured automatically in accordance with the prior art, the press control must be informed of the bending length before each bending operation, which means an additional complication. In addition, the material properties often vary over the bending length, which is why essential information is lost when measuring the total force.
An unevenness of the sheet metal or a bending punch that is not exactly parallel to the die on the sheet metal can lead to a distortion of the force-displacement relationship.
With larger bending lengths or with high bending forces, according to the prior art, the die is often pretensioned in such a way that it bends slightly upwards in the unloaded state, so that it does not have any bending under load due to the inevitable superimposed bending in the opposite direction , This deformation of the die, which is referred to as "crowning", likewise produces a distortion of the force-displacement curve,
To avoid the described disadvantages of measuring the total force when bending in the die, according to the invention the device for force measurement is integrated directly into the upper tool.
Since bending punches in the generally customary embodiment according to the prior art are composed of segments of a length of approximately 50 to 400 mm, the length of such a segment - especially in the case of narrow segments - is almost homogeneous. The shorter the length of the segment is chosen, the more precisely the actual force per length can be absorbed. Piezoelectric force transducers of a known type are preferably used to measure the force. Since the force range in which a bending punch of a certain design can be used is precisely defined, the force sensor can be easily adapted to the measuring range that occurs, which guarantees high-resolution measuring results.
As long as it is ensured that the tool segment with the force sensor is fully seated on the sheet, the measurement result is also independent of the total bending length.
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Together with the travel signals, which are supplied by the travel sensors designed according to the state of the art on the linear axes, the force-travel relationship is recorded on one or more segments of the upper tool and fed to the press control according to the claimed method. The press control system uses this relationship or these relationships to determine the lower reversal points for the linear axes of the bending press in real time.
According to the invention, the length-resolved force measurement can also resolve fluctuations in the sheet properties over the bending length. In particular, this is a fluctuating sheet thickness or varying material properties, which is particularly relevant when processing hot-rolled sheets. In one embodiment of the invention, different lower reversal points are calculated for the two linear axes of the bending press in order to best compensate for this variation over the length and to obtain a constant bending angle over the entire length.
In addition to the two linear axes, the adjustment of the crown of the lower tool, which is carried out according to known methods, also offers an additional degree of freedom in order to compensate for fluctuations in the sheet properties over the bending length. In one embodiment of the invention, three bending punches equipped with force transducers are therefore used, two in the edge regions of the sheet and one in the middle. The three recorded force-travel relationships are used to determine suitable ones under reversal points of the linear axes and a suitable setting of the crowning in the press control in order to obtain the desired bending angle.
The force-displacement relationship during the bending process contains a lot of information about the material used. The modulus of elasticity, the plug-in limit and the tensile strength are included. In addition, the position of the punch at the time of the force increase provides the actual sheet thickness at the respective measuring position. The characteristic values of the material derived from this connection are used in one embodiment of the invention to characterize the material without a priori information and then to control the bending process accordingly. To increase efficiency, additional information on the recognized material stored in a database can also be included in the control of the bending process.
In a further embodiment of the method according to the invention, the linkage of the input data in the form of the force-travel relationship with the output data, that is, the control signals for the linear axes and the crowning, is carried out via a neural network implemented in the press control. By evaluating each individual or selected bending process, in one embodiment of the invention, for example by measuring the bending angle achieved, this network learns from bend to bend, so that the bending results improve over time.
In another implementation of the invention, the force-displacement relationship recorded is used to adapt a numerical model to the respective material and the actual geometry. Using a suitable material model, the model calculates the bending line of the sheet, as well as the forces and moments that occur. By adapting the parameters of the model to the measured values, precise information about the springback and the required bending is made possible.
The method according to the invention is able to ensure reproducible bending angles despite fluctuating material properties. However, since it is not immediately possible to measure the bending angle directly, there is a risk that a systematic error in the bending angle will occur. In one preferred embodiment of the invention, this error is excluded by using an angle measuring method according to the state of the art in order to receive feedback about the efficiency of the process control via the force-travel relationship. The angle measured in this way is used to modify the control algorithm of the press control for better results.
FIG. 1 shows an example of an arrangement for carrying out the method according to the invention.
FIG. 2 shows a detail of a segment of the upper tool with an integrated device for force
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1 shows a preferred embodiment of the method according to the present invention. A hydraulic die bending press of known design is used, consisting of the C-frame (1) on which the lower tool holder (2) and two hydraulic linear axes (3) are mounted. The two hydraulic linear axes (3) support the movable press beam (drawn before bending (4b) and during bending (4a), on which the bending punches are attached in the form of several segments (5). The lower tool holder is also (2) equipped with several bending dies (6) Two position sensors (7) are used in a known manner to measure the position of the bending dies relative to the bending dies.
One or more bending punches of the die bending press shown is or are now equipped according to the invention with a device for force measurement (8) which supplies an electrical signal as a function of the acting force. The electrical signals of the displacement transducers (7), the force transducers (8) and the control signals for the linear axes (3) are brought together in the controller (9). The control generates the signals for controlling the two linear axes from the position and force signals as well as the user inputs.
FIG. 2 shows in detail a preferred embodiment of the force transducer on a bending punch. Two fastening pins (10) are screwed onto the bending punch (5), between which a piezoelectric force transducer (8) of known design is clamped. The cylindrical force transducer releases an electric charge proportional to the axial change in length, which is fed to a charge amplifier in a known manner by means of a cable (11). During the bending process, a uniaxial stress state is created in a good approximation in the bending punch, whereby only elastic stresses occur, so that the change in length is proportional to the acting force. The disturbance in the voltage distribution introduced by the force sensor is negligibly small.
As a result, the piezoelectric sensor produces an electric charge proportional to the force.
1. Method for reducing the bending angle errors when bending a sheet metal in a press brake consisting of a fixed lower tool (V-die) (6) and a bending beam driven by linear axes (3) with the upper tools (bending punch) (5), the lower reversal point of the bending punch being calculated in advance on the basis of the preset setpoint of the bending angle and the force-path relationship measured during the bending process, characterized in that a force measurement is carried out by one or more force sensors (8), the or the is or are integrated into the upper tool, which is of multi-part design, so that each force sensor only detects the force acting on the respective segment of the upper tool.