DE102022131427A1 - Method and device for detecting a trailing edge on a sheet metal part after a deep-drawing process - Google Patents

Abstract

The present invention relates to a method for detecting a fault in a sheet metal part (10) after a deep-drawing process, wherein a starting position (21) of a drawing edge indentation (20) is determined for a measuring position (71) before the start of the deep-drawing process for the sheet metal part (10) fixed by a deep-drawing tool (2), from the start of the deep-drawing process until the end of the deep-drawing process the drawing edge indentation (20) is continuously determined in relation to the starting position (21) of the drawing edge indentation (20) and recorded as an actual curve, after the end of the deep-drawing process the recorded actual curve is evaluated, during the evaluation of the actual curve a slope of the actual curve is determined, it is checked whether the determined slope deviates from a target slope of a predetermined target curve, and if a deviation of the determined slope from the target slope by a predetermined tolerance value is detected, the formation of a trailing edge is detected as a fault in the sheet metal part (10). The invention further relates to a trailing edge determination device (6) which is designed to carry out an above method, a deep-drawing tool (2) with such a trailing edge determination device (6) and a drawing press (1) with such a deep-drawing tool (2).

Description

The invention relates to a method for detecting a fault in a sheet metal part after a deep-drawing process, wherein a starting position of a drawing edge indentation is determined for at least one measuring position before the start of the deep-drawing process for the sheet metal part fixed by a deep-drawing tool, from the start of the deep-drawing process until the end of the deep-drawing process the drawing edge indentation is continuously determined in relation to the starting position of the drawing edge indentation and recorded as an actual curve, and after the end of the deep-drawing process the recorded actual curve is evaluated. Furthermore, the invention relates to a device for detecting a fault in a sheet metal part after a deep-drawing process comprising a sensor unit and a computing unit, wherein the sensor unit is designed to detect a drawing edge indentation, and wherein the computing unit is designed to store drawing edge indentations detected by the sensor unit and to process the stored drawing edge indentations. Furthermore, the invention relates to a deep-drawing tool with a device for detecting a fault in a sheet metal part after a deep-drawing process.

In the state of the art, it is known to detect the drawing edge indentation, also called sheet metal indentation, in order to detect faulty sheet metal parts, in particular tears or folds. In the EN 10 2019 205 464 B3 To determine a sheet metal run-in, it is proposed to apply a marking to the sheet metal forming tool to be formed, whereby a change in the position of the marking during a forming process is recorded and evaluated in real time. From the DE 43 38 828 C2 It is known to record the actual infeed path during the deep-drawing process as a function of time or press stroke and to compare it with a target infeed path. If there are deviations, the hold-down force is adjusted in order to achieve a production that is as tear- and wrinkle-free as possible.

In addition, the DE 102 05 393 A1 It is known to provide a first sheet metal hold-down device and a spring-loaded second sheet metal hold-down device to prevent trailing edges, which act on the opposite sides of the subsequent edge. It has been shown that trailing edges occur particularly near contact radii between the deep-drawing tool and the sheet metal part to be formed if there is a relative movement between the sheet metal part and the deep-drawing tool during the deep-drawing process. In the automotive sector, in order to prevent trailing edges on a body sheet metal part after a deep-drawing process, efforts are also made to design the dies of deep-drawing tools in such a way that trailing edges are avoided as best as possible, particularly in areas of the outer skin that will later be visible. With so-called design edge stamps, which create an elongated continuous edge in a sheet metal part, such as in the case of rear lid outer skin parts for notchback bodies or in the case of door and fender outer skin parts, in particular an elongated continuous edge in the sense of a "toronado line", it is already possible to produce a large proportion of the formed sheet metal parts without a trailing edge, so-called good parts. For the remaining sheet metal parts with a trailing edge, so-called bad parts, post-processing is possible and necessary in order to prevent trailing edges from becoming visible. One reason for the formation of trailing edges is the variation in external parameters, such as material quality, sheet metal lubrication, machine properties, production environment and/or contact surface abrasion. In the course of series production, this parameter variation can lead to drawing process fluctuations that affect the component quality of the sheet metal part, in particular with regard to integrity, dimensional accuracy and surface cleanliness. In particular, surface cleanliness can be negatively affected by trailing edges. If defective parts are not identified in time and processed as supposedly good parts, the amount of post-processing required increases significantly. State-of-the-art processes in which material surfaces are recorded with cameras in order to detect defects are complex. Such a process is used, for example, in the EP 3 450 965 A1 described.

Against this background, it is an object of the present invention to detect trailing edges after a deep-drawing process with high reliability and with as little effort as possible, especially in the case of sheet metal body parts for the outer skin of a vehicle body with elongated, continuous design edges.

To solve this problem, a method for detecting a defect in a sheet metal part after a deep-drawing process according to claim 1, as well as a device for determining a defect in a sheet metal part after a deep-drawing process, a deep-drawing tool and a deep-drawing press according to the independent claims are proposed. Further advantageous embodiments of the invention are described in the dependent claims and the description and shown in the figures.

The proposed solution provides a method for detecting a defect in a sheet metal part after a deep-drawing process, wherein for at least one measuring position, i.e. in particular for exactly one measuring position or in particular for one measuring position from a plurality of Measuring positions, before the start of the deep-drawing process for the sheet metal part fixed by a deep-drawing tool, a starting position of a drawing edge indentation is determined, from the start of the deep-drawing process until the end of the deep-drawing process, the drawing edge indentation is continuously determined in relation to the starting position of the drawing edge indentation and recorded as an actual curve, after the end of the deep-drawing process, the recorded actual curve is evaluated, when evaluating the actual curve, a slope of the actual curve is determined, in particular a slope of the actual curve is determined at predetermined points, it is checked whether the determined slope deviates from a target slope of a predetermined target curve, and if a deviation of the determined slope from the target slope by a predetermined tolerance value is detected, the formation of a trailing edge is recognized as a defect in the sheet metal part. The method is intended in particular for testing deep-drawn bodywork sheet metal parts that are intended to form parts of the outer skin of an automobile and which therefore have particularly high requirements for surface cleanliness, whereby the proposed method advantageously enables a high detection rate of bad parts. In addition, the method is intended in particular for detecting trailing edges when using design edge stamps that lead to an elongated, in particular an elongated and uninterrupted edge formation when the workpiece is formed, and in particular only form a convex contact radius. In contrast, trailing edges that form on the edges of a workpiece during deep drawing are advantageously excluded from detection because these trailing edges on the edges are hardly avoidable and are also perceived as not very disturbing. These trailing edges on the edges are therefore not meant when trailing edges are mentioned in the description of the invention, unless explicit reference is made to the trailing edges on the edges.

The at least one measuring position is advantageously determined by the arrangement of a sensor unit with which the drawing edge indentation is detected. Sensor units known in the prior art are advantageously used for this purpose, in particular optically operating sensor units. The sensor units are advantageously arranged in such a way that a measuring direction for measuring the drawing edge indentation is preferably aligned vertically to the punch edge of the deep-drawing tool. If several measuring positions and thus several sensor units are provided, the measuring positions are advantageously located on a common line parallel to the punch edge of the deep-drawing tool and/or opposite measuring positions are determined so that the workpiece is arranged between the sensor units, wherein the measuring direction of the sensors is advantageously aligned vertically to the main edge of the punch of the deep-drawing tool.

In the proposed method, the actual curve is advantageously formed from a plurality of recorded values for the drawing edge indentation. In particular, a correspondingly designed computing unit is provided for this purpose. Advantageously, the actual curve is not simply composed of the recorded values for the drawing edge indentation, but rather these values are advantageously smoothed. In particular, appropriately designed filters can be used for this purpose. It is also advantageous to define at least two measuring positions so that the actual curve can advantageously be formed with greater accuracy by combining the recorded values for the drawing edge indentation. In particular, it is provided that between 35 and 100 measured values, in particular 50 to 85 measured values, are recorded for a measuring position for a deep-drawing process. The number of measured values is advantageously adjustable, in particular also depending on the absolute value of the entire drawing edge indentation from the beginning to the end of the deep-drawing process, with the number of values advantageously being set to be greater the larger the entire drawing edge indentation is.

According to an advantageous embodiment, it is provided that the target curve is formed experimentally from a large number of recorded actual curves that were recorded for a good part, i.e. a deep-drawn sheet metal part without trailing edges. In particular, the target curve can be formed as a regression curve from the large number of actual curves. As a variant, it can also be provided that the target curve is specified by simulated or expected values. In a further advantageous embodiment, the target curve is further adapted when the proposed method is repeatedly carried out, taking into account the result of the error detection during a deep-drawing process and the recorded actual curve for this deep-drawing process, in particular in the sense of a self-learning adaptation. The tolerance values for the slope can also advantageously be learned. According to a variant, the tolerance values can also be set to zero. In particular, however, a relative deviation from the slope of the target curve, in particular a deviation of up to 10% as a deviation from the slope of the target curve, can also be set as a tolerance value.

A further advantageous embodiment of the method provides that an envelope curve is specified for a target group of curves. The envelope curve is in particular a band that is formed by an upper limiting curve and a lower limiting curve. The envelope curve is advantageously defined in such a way that all target curves, i.e. in particular all recorded actual curves that can be assigned to a sheet metal part without trailing edges, lie in the envelope curve. This large number of target curves advantageously forms the target curve family. When evaluating the recorded actual curve, it is advantageously also checked whether the actual curve lies at least partially outside the envelope curve.

According to a further advantageous embodiment of the method, during the evaluation it is first checked whether the actual curve lies at least partially outside the envelope curve, and then, if the actual curve lies at least partially outside the envelope curve, the slope of the actual curve is determined. It is then further checked whether the determined slope deviates from a target slope of a predetermined target curve. If a deviation of the determined slope from the target slope by a predetermined tolerance value is detected, the formation of a trailing edge is then recognized as a defect in the sheet metal part. In this respect, a proposed method for detecting a defect in a sheet metal part after a deep-drawing process advantageously provides that for at least one measuring position before the start of the deep-drawing process for the sheet metal part fixed by a deep-drawing tool, a starting position of a drawing edge indentation is determined, from the start of the deep-drawing process until the end of the deep-drawing process the drawing edge indentation is continuously determined in relation to the starting position of the drawing edge indentation and recorded as an actual curve, after the end of the deep-drawing process the recorded actual curve is evaluated, an envelope curve is specified for a set of target curves, wherein when evaluating the actual curve it is checked whether the actual curve lies at least partially outside the envelope curve, if the actual curve lies at least partially outside the envelope curve, a slope of the actual curve is determined, in particular a slope of the actual curve is determined at predetermined points of the actual curve, it is further checked whether the determined slope deviates from a target slope of a predetermined target curve deviates, and if the determined gradient deviates from the target gradient by a predetermined tolerance value, the formation of a trailing edge is detected as a defect in the sheet metal part. It has been shown that the formation of a trailing edge during a deep-drawing process can be detected with a high degree of reliability if it is checked whether the recorded actual curve lies at least partially outside the envelope curve and also whether the gradient of the recorded actual curve deviates by a predetermined tolerance value from the gradient of the specified target curve. Advantageously, according to a further embodiment, the formation of the trailing edge is only detected as a defect in the sheet metal part if, in addition to the detected deviation of the determined gradient from the target gradient, it is also detected that the actual curve lies at least partially outside the envelope curve, or if, in addition to detecting that the actual curve lies at least partially outside the envelope curve, it is also detected that the determined gradient deviates from the target gradient.

The method is also particularly advantageously designed to recognize the direction in which a trailing edge has formed. In particular, an elongated design edge stamp of a deep-drawing tool is used as the starting point, whereby a trailing edge can form to the left or right of the direction of extension of the stamp. In particular, it is provided that a trailing edge is recognized starting from the measuring position in the measuring direction if the actual curve exceeds the envelope curve. The measuring direction starts from the measuring position and goes in the drawing direction. If, for example, the measuring position is to the right of a longitudinal extension direction of a design edge stamp, the measuring direction points to the left. If, on the other hand, the measuring position is to the left of a longitudinal extension direction of a design edge stamp, the measuring direction points to the right. According to a further advantageous embodiment, a trailing edge is recognized starting from the measuring position against the measuring direction if the actual curve falls below the envelope curve. It is also advantageous to detect a trailing edge starting from the measuring position in the measuring direction if the determined deviation of the gradient from the target gradient is at least greater by the tolerance value. It is advantageous to detect a trailing edge starting from the measuring position against the measuring direction if the determined deviation of the gradient from the target gradient is at least smaller by the tolerance value.

A further advantageous embodiment of the method provides pairs of opposite measuring positions, in particular two opposite measuring positions, as the at least one measuring position, in particular such that one measuring position is arranged to the left and the other measuring position to the right of a longitudinal extension direction of a design edge stamp of a deep-drawing tool, further in particular centrally in relation to the design edge stamp. Advantageously, the measurement result from one measuring position is used to validate the measurement result from the second measuring position, in particular when teaching the target curve. Advantageously, this further increases the rate of sheet metal parts correctly identified as faulty. For the later detection and sorting out of sheet metal parts with a trailing edge, it is provided in particular that it is sufficient if from one measurement position has been identified as faulty. In such rare cases, a further check can be carried out downstream, for example a visual check by specialist personnel, in order to assess whether the part is good or bad. The result is then advantageously used to adjust the target curve and/or the envelope curve. If several measuring positions are provided next to one another, in particular on one side in relation to a longitudinal extension direction of a punch of a deep-drawing tool, the measuring positions are advantageously arranged on a common line which advantageously extends parallel to the edge of the punch. The measuring positions are advantageously not on an axis of symmetry of the sheet metal part to be formed. This advantageously further improves the error detection rate.

With the help of the proposed method, the drawing edge data are advantageously evaluated after each drawing stroke within an ongoing series production in such a way that a distinction can be made between three quality states: "without trailing edge", "trailing edge in the measuring direction" and "trailing edge against the measuring direction". The recorded measured values can also be visualized so that each drawing stroke can be additionally checked in a graphical user interface. In this way, the method can advantageously be used to detect reworked parts due to trailing edges in good time and to separate them from good parts before the reworked parts are further processed in subsequent processes.

The device for detecting a defect in a sheet metal part after a deep-drawing process, which is further proposed to solve the problem mentioned at the beginning, comprises at least one sensor unit and a computing unit, wherein the at least one sensor unit is designed to detect a drawing edge indentation, wherein the computing unit is designed to store drawing edge indentations detected by the at least one sensor unit and to process the stored drawing edge indentations, and wherein the device is designed to carry out a method according to the invention using the sensor unit and the computing unit. The device is thus designed to detect the formation of a trailing edge as a defect in a deep-drawn sheet metal part. The proposed device is therefore in particular a trailing edge detection device for detecting a trailing edge in a sheet metal part after a deep-drawing process. The device advantageously comprises arrangement means for arranging the device on a deep-drawing tool. Advantageously, the computing unit, which can be designed in particular as a microcontroller unit, comprises an interface via which detected sensor data can be output on an image display device. Further input and output interfaces are advantageously provided, in particular in order to be able to configure the computing unit with data, in particular with regard to a target curve and/or an envelope curve, and advantageously in order to be able to output a result of an evaluation with regard to a component, in particular to a control unit.

Advantageously, the at least one measuring position is determined by the arrangement of the at least one sensor unit. The sensor unit can in particular be an optical sensor unit, which is designed in particular to determine a drawing edge indentation using a laser. However, the sensor unit can also be designed as a touch sensor, wherein a touch element is in contact with the outer edge of the sheet metal part and is guided along it to determine the drawing edge indentation.

In particular, the trailing edge determination device is designed to determine a starting position of a drawing edge indentation for each measuring position determined by a sensor unit of the trailing edge determination device before the start of the deep-drawing process for the sheet metal part fixed by a deep-drawing tool, to continuously determine the drawing edge indentation in relation to the starting position of the drawing edge indentation from the start of the deep-drawing process until the end of the deep-drawing process and to record it as an actual curve, to evaluate the recorded actual curve after the end of the deep-drawing process, and to determine a slope of the actual curve when evaluating the actual curve, in particular at predetermined points. The trailing edge determination device is also designed to check whether the determined slope deviates from a target slope of a predetermined target curve and, if the determined slope deviates from the target slope by a predetermined tolerance value, to recognize the formation of a trailing edge as a defect in the sheet metal part.

The deep-drawing tool further proposed comprises a sheet metal holder, a die, a punch, in particular a design edge punch, and a device designed according to the invention for determining a defect in a sheet metal part after a deep-drawing process. The device is advantageously arranged in such a way, in particular arranged on the deep-drawing tool, that its at least one sensor unit can detect a drawing edge indentation of a sheet metal part fixed between the sheet metal holder and the die. The deep-drawing tool is also designed in particular to be used in a press and, when the press is actuated, to generate a Deep-drawing tool to reshape a sheet metal part. In particular, it is provided that the stamp comprises at least one elongated edge for reshaping a sheet metal part, in particular an elongated and uninterrupted edge for reshaping a sheet metal part.

According to a further advantageous embodiment, the deep-drawing tool is assigned a control device which is designed to specify control signals for controlling actuators of the deep-drawing tool and/or a drawing press, wherein the trailing edge detection device of the deep-drawing tool, i.e. the device for detecting a defect in a sheet metal part after a deep-drawing process, is designed to provide the control device with a result relating to a determined trailing edge. The control device is advantageously also designed to adapt the control signals taking into account the result relating to a determined trailing edge in order to avoid the formation of a trailing edge in sheet metal parts to be subsequently formed. In particular, it is provided that the deep-drawing tool comprises the control device. However, according to a further embodiment, the control device can also be comprised by a drawing press which uses the deep-drawing tool designed according to the invention.

Furthermore, a drawing press for deep-drawing sheet metal parts with a deep-drawing tool designed according to the invention is therefore proposed. The drawing press then advantageously comprises a sheet metal holder, a die, a punch, in particular a design edge punch, a press and a trailing edge detection device designed according to the invention. The trailing edge detection device is advantageously arranged such that its at least one sensor unit can detect a drawing edge indentation of a sheet metal part fixed between the sheet metal holder and the die.

• 1a in einer schematischen Seitenansicht ein Ausführungsbeispiel für ein erfindungsgemäß ausgebildetes Tiefziehwerkzeug in einer Ziehpresse zu Beginn eines Tiefziehvorgangs;
• 1b in einer Diagrammdarstellung den zu 1a zugehörigen Ziehrandeinzug als Ist-Kurve (Weg/Zeit);
• 2a in einer schematischen Seitenansicht das Ausführungsbeispiel für ein erfindungsgemäß ausgebildetes Tiefziehwerkzeug in einer Ziehpresse aus 1a zum Ende eines Tiefziehvorgangs;
• 2b in einer Diagrammdarstellung den zu 2a zugehörigen Ziehrandeinzug als Ist-Kurve (Weg/Zeit);
• 3a in einer schematischen Darstellung ein Ausführungsbeispiel für ein umzuformendes Blechteil und einen Kontaktradius eines Stempels eines Tiefziehwerkzeugs zu Beginn eines Tiefziehvorgangs;
• 3b in einer schematischen Darstellung das Ausführungsbeispiel gemäß 3a zum Ende eines Tiefziehvorgangs mit Darstellung einer Nachlaufkante in Messrichtung und einer Nachlaufkante entgegen der Messrichtung; und
• 4 in einer Diagrammdarstellung die Steigung einer erfassten Ist-Kurve und die Steigung der Soll-Kurve zu dem Zeitpunkt, zu dem die Ist-Kurve die Hüllkurve schneidet.

Further advantageous details, features and design details of the invention are explained in more detail in connection with the embodiments shown in the figures (Fig.: Figure).

• 1a in a schematic side view an embodiment of a deep-drawing tool designed according to the invention in a drawing press at the beginning of a deep-drawing process;
• 1b in a diagram representation the 1a associated drawing edge retraction as actual curve (distance/time);
• 2a in a schematic side view the embodiment of a deep-drawing tool designed according to the invention in a drawing press from 1a at the end of a deep drawing process;
• 2 B in a diagram representation the 2a associated drawing edge retraction as actual curve (distance/time);
• 3a in a schematic representation an embodiment of a sheet metal part to be formed and a contact radius of a punch of a deep-drawing tool at the beginning of a deep-drawing process;
• 3b in a schematic representation the embodiment according to 3a at the end of a deep drawing process with representation of a trailing edge in the measuring direction and a trailing edge against the measuring direction; and
• 4 in a diagrammatic representation, the slope of a recorded actual curve and the slope of the target curve at the time at which the actual curve intersects the envelope curve.

In the various figures, identical parts are generally provided with the same reference symbols and are therefore sometimes only explained in connection with one of the figures.

With reference to 1a until 2 B An exemplary embodiment of a method designed according to the invention for detecting a trailing edge 31, 32 in a sheet metal part 10 after a deep-drawing process and an exemplary embodiment of a drawing press 1 with a deep-drawing tool 2 designed according to the invention are explained below. In 1a and 2a In each case, a deep-drawing press 1 is shown with a deep-drawing tool 2 arranged between the pressing jaws 17, 18 and a sheet metal part 10. 1a shows the deep-drawing press 1 at the beginning 41 of a deep-drawing process, wherein the sheet metal part 10 is fixed by the deep-drawing tool 2. 2a shows the deep-drawing press 1 at the end 42 of a deep-drawing process, in which the sheet metal part 10 has already been formed. In the 1b and 2 B In the diagrams shown, the drawing edge indentation 20 of the sheet metal part 10 during the deep drawing process is shown as a path over time from the beginning 41 to the end 42 of the deep drawing process. Since the representation in 1b the deep drawing process is just beginning, the drawing edge indentation 20 at time t ₀ is only “0”. In 2 B Two examples of recorded actual curves 22, 23 for drawn edge indentations 20 are shown, whereby these two actual curves 22, 23 are each assigned to deep-drawn sheet metal parts in which a trailing edge 31, 32 has formed.

In the embodiment, the deep-drawing tool 2 comprises a sheet holder 3, a die 4 and a design edge stamp 5, which extends uninterruptedly into the image plane. The deep-drawing tool 2 also comprises a trailing edge detection device 6, which is designed to detect a trailing edge 31, 32 as a defect in a sheet metal part 10 formed using the deep-drawing tool 2. The trailing edge detection device 6 comprises a sensor unit 7 at a measuring position 71 fixed in relation to the deep-drawing tool 2, which is designed to detect a drawn edge indentation 20 of the sheet metal part 10 using a laser beam 75. The arrangement of the sensor unit 7 results in a measuring direction 72. To create redundancy or to further increase the detection rate of sheet metal parts with a trailing edge, the arrangement of a further sensor unit for detecting a drawn edge indentation can be provided at the further measuring position 73, in which case the measuring direction of this sensor unit would run counter to the measuring direction 72 of the sensor unit 7. In this exemplary embodiment, the trailing edge detection device 6 further comprises a computing unit 8. The computing unit 8 can in particular be designed as a correspondingly configured microcontroller unit. The computing unit 8 of the trailing edge detection device 6 is designed to receive, store and process sensor signals provided by the sensor unit 8 relating to the drawing edge indentation 20. Using the sensor unit 7 and the computing unit 8, the trailing edge detection device 6 is designed to detect whether a sheet metal part 10 formed by means of the deep-drawing tool 2 has a trailing edge 31, 32 and whether this trailing edge 31 is formed in the measuring direction 72 or whether the trailing edge 32 is formed against the measuring direction 72.

For this purpose, a starting position 21 of a drawing edge indentation 20 is first determined for the measuring position 71 for a sheet metal part 10 fixed between the sheet metal holder 3 and the die 4 before the start 41 of the actual deep-drawing process. This starting position 21 is then assigned the value "0" at a time t ₀ , which marks the start 41 of the deep-drawing process. When the deep-drawing process starts, the punch 5 and die 4 are moved towards each other and in the process reshape the sheet metal part 10, whereby the drawing edge indentation 20 changes. The changing drawing edge indentation 20 is determined by the sensor unit 7 from the start 41 of the deep-drawing process at time t ₀ to the end 42 of the deep-drawing process at time t _E continuously in relation to the starting position 21 of the drawing edge indentation 20 by determining the value for the drawing edge indentation 20 at discrete points in time, in the exemplary embodiment at 60 points in time, i.e. one value at time intervals of t _E /60. From these recorded values for the drawing edge indentation 20, the computing unit 8 then determines an actual curve 22, 23 for the changing drawing edge indentation 20, wherein the recorded values are smoothed to form the actual curve 22, 23 in order to avoid outliers in the values for the deep edge indentation and thus unwanted fluctuations in the actual curve. For the smoothing of the actual curve 22, 23, a number of values preceding and following a recorded value are advantageously taken into account.

The actual curve 22, 23 recorded in this way is evaluated by the computing unit 8 after the end 42 of the deep-drawing process. As part of the evaluation, the computing unit 8 first checks whether the recorded actual curve 22, 23 lies at least partially outside a predetermined envelope curve 27 stored in the computing unit 8. To do this, it is checked whether the recorded actual curve 22, 23 falls below a lower limit 28 of the envelope curve 27 or whether the recorded actual curve 22, 23 exceeds an upper limit 29 of the envelope curve. In 2 B For example, at time t ₁ it is apparent that the actual curves 22, 23 deviate from the target curve 25, but are still within the envelope curve 27. The envelope curve 27 is formed as an envelope for a family of curves from recorded actual curves that led to a formed sheet metal part without a trailing edge 31, 32. Since these actual curves led to good parts, the family of curves is also referred to as the target curve family. If the test by the computing unit 8 shows that the recorded actual curve 22, 23 is within the envelope curve 27, i.e. neither the upper limit 29 is exceeded nor the lower limit 28 is undershot, the computing unit 8 does not detect any formation of a trailing edge 31, 32. If, however, the test of the computing unit 8 shows that the recorded actual curve 22, 23 has fallen below the lower limit 28 of the envelope curve 27 or has exceeded the upper limit 29 of the envelope curve 27, then in a further step the computing unit 8 determines a slope of the recorded actual curve 22, 23 for points specified by the computing unit 8. In this exemplary embodiment, the predetermined points are determined by the computing unit 8 at the times at which the actual curve 22, 23 intersects the envelope curve 27 and advantageously at points in time adjacent thereto, in this specific exemplary embodiment for the actual curve 22 at the time t ₂ and the points in time adjacent thereto to the right t ₂ +t _E /60+ t _E /60+ ..., and for the actual curve 23 at the time t ₃ and the points in time adjacent thereto to the right t ₃ +t _E /60+ t _E /60+ ..., in particular at at least three adjacent points in time in each case. The points for determining the slope of the recorded actual curve can also be determined in another suitable manner.

The computing unit 8 then further checks whether the determined gradient deviates from a target gradient of a predefined target curve 25. In this embodiment, this target curve 25 is determined from the target family of curves, in particular as an average of this target family of curves. If the computing unit 8 does not detect a deviation of the determined gradient from the target gradient by a predefined tolerance value, which is set at 7% in this specific embodiment, the computing unit 8 detects no formation of a trailing edge 31, 32 and thus a good part. If, on the other hand, the computing unit 8 detects a deviation of the determined gradient from the target gradient that exceeds the tolerance value, the computing unit 8 detects the formation of a trailing edge 31, 32 as a defect in the sheet metal part 10 formed by the deep-drawing tool 2.

In this embodiment, the computing unit 8 is also designed to detect whether the trailing edge 31, 32 is formed starting from the measuring position 71 in the measuring direction 72 or against the measuring direction 72 with respect to the contact radius 50. The contact radius 50 is the surface profile of the stamp 5 which leads to at least a local maximum on the stamp surface, which in 3a and 3b is shown as an example. There, the contact radius 50 forms an absolute maximum on the stamp surface and is responsible for the formation of a design edge with a corresponding radius during the deep drawing of the sheet metal part 10, as in 3b shown. If the evaluation of the computing unit 8 shows that the recorded actual curve 22 exceeds the envelope curve 27, i.e. intersects the upper limit 29, and additionally that the determined gradient in and around the intersection point is greater than the target gradient by more than the tolerance value, the computing unit 8 detects a trailing edge 31 in the measuring direction 72. If, on the other hand, the evaluation of the computing unit 8 shows that the recorded actual curve 23 falls below the envelope curve 27, i.e. intersects the lower limit 28, and additionally that the determined gradient in and around the intersection point is smaller than the target gradient by more than the tolerance value, the computing unit 8 detects a trailing edge 32 opposite the measuring direction 72.

In 4 is for a recorded actual curve 22, as in 2 B shown by way of example, at the time t ₂ at which the actual curve exceeds the upper limit 29 of the envelope curve 27, the slope with the slope angle β of the actual curve 22 and, for the same time t _{2 ,} the slope with the slope angle α of the target curve 25 are shown. Since the slope of the actual curve 22 here exceeds the slope of the target curve 25 by more than the tolerance value, the computing unit 8 detects a trailing edge 31 in the measuring direction 72.

In the case of 1a until 2 B In the embodiment explained, the computing unit 8 also provides the result of the evaluation to a control device 9, which is used to control actuators (in 1a and 2a not explicitly shown) of the deep-drawing tool 2 or the drawing press 1. The spring force between the lower press jaw 18 and the sheet metal holder 3 can thus advantageously be adjusted via the actuators. The spacing between the sheet metal holder 3 and the die 4, in particular the drawing gap, can also advantageously be adjusted by means of at least one actuator that can be controlled by the control device 9. The control device 9 is designed, when trailing edges 31, 32 are repeatedly detected in the formed sheet metal parts by the computing unit 8, to adapt the control signals for controlling the actuators, taking into account the result of the evaluation of the computing unit 8 with regard to determined trailing edges 31, 32 in order to avoid the formation of a trailing edge 31, 32 in sheet metal parts 10 to be subsequently formed, and to control the actuators with the adapted control signals.

It is pointed out that the draw edge indentation 20, which is in 2 B As shown by way of example, the absolute path can also differ from one another for sheet metal parts 10 of a margin in the same deep-drawing process, so that the recorded actual curves can also end in a value S_E±S_X that differs from the value S_E, where S_X represents a deviation from the value S_E. However, this deviation is not evaluated by the computing unit 8 and does not in itself provide any reliable indication of the formation of a trailing edge 31, 32.

The embodiments shown in the figures and explained in connection with them serve to explain the invention and are not limitative of it.

List of reference symbols

1

Drawing press

2

Deep drawing tool

3

Sheet metal holder

4

die

5

Rubber stamp

6

Trailing edge detection device

7

Sensor unit

71

Measuring position

72

Measuring direction

73

Measuring position

75

laser beam

8th

Computing unit

9

Control device

10

Sheet metal part

17

Press jaw

18

Press jaw

20

Draw-edge indentation

21

Starting position of a draw edge indentation

22

Actual curve

23

Actual curve

25

Target curve

27

Envelope

28

lower limit of the envelope (27)

29

upper limit of the envelope (27)

31

Trailing edge (in measuring direction 72)

32

Trailing edge (opposite the measuring direction 72)

33

no trailing edge

41

Start of a deep drawing process

42

End of a deep drawing process

50

Contact radius

t0

Start of a deep drawing process

tE

End of a deep drawing process

t1

Time during the deep drawing process (actual curves 22, 23 still within the envelope curve 27)

t2

Time during the deep drawing process (actual curve 22 exceeds the upper limit 29 of the envelope curve 27)

t3

Time during the deep drawing process (actual curve 23 falls below the lower limit 28 of the envelope curve 27)

S_E

maximum drawing edge retraction (20) at time t _E

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• DE 102019205464 B3 [0002]
• DE 4338828 C2 [0002]
• DE 10205393 A1 [0003]
• EP 3450965 A1 [0003]

Claims (13)

Method for detecting a fault in a sheet metal part (10) after a deep-drawing process, wherein a starting position (21) of a drawing edge indentation (20) is determined for at least one measuring position (71) before the start (41) of the deep-drawing process for the sheet metal part (10) fixed by a deep-drawing tool (2), with the start (41) of the deep-drawing process until the end (42) of the deep-drawing process, the drawing edge indentation (20) is continuously determined in relation to the starting position (21) of the drawing edge indentation (20) and recorded as an actual curve (22, 23), and after the end (42) of the deep-drawing process, the recorded actual curve (22, 23) is evaluated, characterized in that when evaluating the actual curve (22, 23), a slope of the actual curve (22, 23) is determined, it is checked whether the determined slope differs from a target slope of a predetermined Target curve (25), and if the determined gradient deviates from the target gradient by a predetermined tolerance value, the formation of a trailing edge (31, 32) is recognized as a defect in the sheet metal part (10). Procedure according to Claim 1 , characterized in that an envelope curve (27) is specified for a target family of curves, wherein when evaluating the actual curve (22, 23) it is additionally checked whether the actual curve (22, 23) lies at least partially outside the envelope curve (27). Procedure according to Claim 2 , characterized in that during the evaluation it is first checked whether the actual curve (22, 23) lies at least partially outside the envelope curve (27), and then, if the actual curve (22, 23) lies at least partially outside the envelope curve (27), the slope of the actual curve (22, 23) is determined and it is checked whether the determined slope deviates from a target slope of a predetermined target curve (25), and if a deviation of the determined slope from the target slope by a predetermined tolerance value is detected, the formation of a trailing edge (31, 32) is recognized as a defect in the sheet metal part (10). Procedure according to Claim 2 or Claim 3 , characterized in that the formation of the trailing edge (31, 32) is only recognized as a defect in the sheet metal part (10) if, in addition to the recognized deviation of the determined gradient from the desired gradient, it is additionally recognized that the actual curve (22, 23) lies at least partially outside the envelope curve (27). Method according to one of the Claims 2 until 4 , characterized in that a trailing edge (31) starting from the measuring position (71) in the measuring direction (72) is detected when the actual curve (22) exceeds the envelope curve (27). Method according to one of the Claims 2 until 5 , characterized in that a trailing edge (32) starting from the measuring position (71) against the measuring direction (72) is detected when the actual curve (23) falls below the envelope curve (27). Method according to one of the preceding claims, characterized in that a trailing edge (31) starting from the measuring position (71) in the measuring direction (72) is detected if the determined deviation of the gradient from the desired gradient is greater by at least the tolerance value. Method according to one of the preceding claims, characterized in that a trailing edge (32) is detected starting from the measuring position (71) against the measuring direction (72) if the determined deviation of the gradient from the desired gradient is smaller by at least the tolerance value. Method according to one of the preceding claims, characterized by two mutually opposite measuring positions (71, 73) as the at least one measuring position. Device (6) for determining a defect in a sheet metal part (10) after a deep-drawing process, comprising at least one sensor unit (7) and a computing unit (8), wherein the at least one sensor unit (7) is designed to detect a drawing edge indentation (20), and wherein the computing unit (8) is designed to store drawing edge indentations detected by the at least one sensor unit (7) and to process the stored drawing edge indentations, characterized in that the device (6) is designed to carry out a method according to one of the preceding claims using the at least one sensor unit (7) and the computing unit (8). Deep drawing tool (2) comprising a sheet holder (3), a die (4) and a punch (5), characterized by a device (6) according to Claim 10 for determining a defect in a sheet metal part (10) after a deep-drawing process, wherein the device (6) is arranged on the deep-drawing tool (2) in such a way that its sensor unit (7) can detect a drawing edge indentation (20) of a sheet metal part (10) fixed between the sheet metal holder (3) and the die (4). Deep drawing tool (2) after Claim 11 , characterized by a control device (9) which is designed to specify control signals for controlling actuators of a drawing press (1), wherein the device (6) for determining a defect in a sheet metal part (10) after a deep-drawing process is designed to provide the control device (9) with a result relating to a determined trailing edge (31, 32), and wherein the control device (9) is designed to adapt the control signals taking into account the result relating to a determined trailing edge (31, 32) in order to avoid the formation of a trailing edge (31, 32) in sheet metal parts (10) to be subsequently formed. Drawing press (1) for deep drawing sheet metal parts (10) characterized by a deep drawing tool (2) according to Claim 11 or Claim 12 .

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