Abstract:
An apparatus for introducing weakening cuts in a film or skin includes a cutting knife and a support arranged in opposition to the cutting knife, wherein the film or skin can be placed between the support and the cutting knife and borne by the support, and the cutting knife is movable relative to the film or skin. In order to ensure a defined residual wall thickness, the invention proposes that the distance between the cutting knife and the support in the direction of the cutting axis can be designed to be kept constant by a device.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of prior filed copending U.S. application Ser. No. 12/374,014, filed Jan. 15, 2009, the content of which is the National Phase of International application No. PCT/EP2007/052412, filed Mar. 14, 2007, which designated the United States and on which priority is claimed under 35 U.S.C. §120, and which claims the priority of German Patent Application No. 10 2006 034 287.9, filed Jul. 21, 2006, pursuant to 35 U.S.C. 119(a)-(d). 
         [0002]    The contents of U.S. application Ser. No. 12/374,014, PCT International application no. PCT/EP2007/052412 and German Patent Application No. 10 2006 034 287.9 are incorporated herein by reference in their entireties as if fully set forth therein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to an apparatus for introducing weakening cuts in a film or skin. 
         [0004]    Certain applications require the introduction of weakening cuts in flat elements to define, for example, a desired breaking point. One application involves the manufacture of instrument panels for motor vehicles with an integrated airbag, whereby the instrument panel breaks up at the designated area, in particular the weakening points, when the airbag is released, so that the airbag is able to emerge. The tem “film” or “skin” as used in the present application relates to plastic skins, films or respective flat workpieces, in which the presence of a cut in the material is targeted so as to establish a defined residual wall thickness regardless of the possibly locally fluctuating wall thickness as well as tolerances of a robot-guided movement of tool in relation to workpiece. Especially, when introducing a weakening in the skins of instrument panels of automobiles in the area of the airbag, which are considered safety structures, a high precision of the cut, a high process reliability, and a good process documentation is of great importance. 
         [0005]    A known apparatus for introducing such weakening cuts is shown schematically in  FIG. 7 . A cutting knife  102  of a respective cutting apparatus  100  is hereby guided across a support table  108  which supports a workpiece  110  to be weakened along a certain line. As a result of the distance of the tip of the cutting knife  102  to the support table  108 , a cut is made in the workpiece as the cutting knife  102  is moved, whereby a residual wall thickness remains which is designated by reference sign  112 . In the present case, the cutting knife  102  can be moved by a servomotor  104  and a spindle drive  106  in a direction of the cutting axis towards the support table and away from the support table. A sensor  118  is used to monitor and control the cutting depth of the cutting tool by measuring the distance (reference sign  116 ) to the metallic supporting table  108 . The geometric relation (reference sign  114 ) between sensor signal and residual wall thickness is referenced in the preliminary stage of the process, for example by a one-time calibration process. The received signal may in turn be used for controlling the executed cut or for controlling the cutting depth during the cutting operation. 
         [0006]    The main shortcoming of this arrangement is the offset disposition of the sensor in relation to the cutting axis. Especially when three-dimensional cutting contours are involved, this spacing causes determination of faulty distance values between cutting knife tip and support table, which do not reflect the actual situation at the cutting tool. Such possibly faulty measuring values do not allow execution of a respective compensation movement so that the cuts cannot be implemented with sufficient process reliability. 
         [0007]    It is an object of the present invention to provide an apparatus of this generic type which is able to precisely provide the wanted weakening with a predefined residual wall thickness. 
         [0008]    This object is solved by an apparatus for introducing weakening cuts in a film or skin, including a cutting knife, a support arranged in opposition to the cutting knife, wherein the film or skin can be arranged between the support and the cutting knife and borne by the support, and the cutting knife is configured for movement in relation to the film or skin, wherein a device is provided to maintain the distance between the cutting knife and the support constant in the direction of the cutting axis. 
         [0009]    A core idea of the present invention resides accordingly in the provision of a device which allows implementation of a constant distance between the cutting knife and a support along the cutting axis. When the distance between the support and the cutting knife is constant and the film rests continuously against the support, the presence of a constant residual wall thickness is inevitably realized. 
         [0010]    The device may hereby basically be constructed in two ways. 
         [0011]    One approach is characterized by providing a mechanical coupling, in particular a rigid mechanical coupling, between the cutting knife and the support. As a consequence, support and cutting knife are quasi connected to one another directly or indirectly via a mechanical construction, wherein—except for elastic effects—no change in distance can occur between both relevant parts. Such a coupling may be realized by means of a bracket for example which directly or indirectly connects the cutting knife and the support. It is also appropriate to provide a device which ensures a continuous support of the skin or film upon the support. Such a device may be realized by means of an elastic element, for example a spring, which maintains for example the combination of support and cutting knife under tension against the skin or film in one direction so that the skin or film rests upon the support at all times. The skin itself may hereby also be used as elastic element. 
         [0012]    According to an advantageous embodiment of the invention, the aforementioned bracket may be made of at least two parts, with a movable coupling unit being provided between both bracket parts in order to enable a relative movement (travel, pivoting) of both bracket parts relative to one another. Such a relative movement may be implemented in the form of a pivoting or a displacement. Both bracket parts should, of course, be fixed to one another, when the weakening operation—i.e. the cutting—is executed. 
         [0013]    To establish a cutting effect, cutting knife and film or skin must be moved in relation to one another. Either the cutting knife alone, or the film or the skin alone, or both elements simultaneously may hereby be moved relative to one another. 
         [0014]    The second basic embodiment, unlike the rigid connection of cutting knife and support, may involve an adjustment of both elements, which optionally may move relative to one another, such that the above-stated predefined residual wall thickness is guaranteed at all times. For this purpose, the position of the support and/or the position of the cutting knife in the cutting axis is to be ascertained. Both positions may be fed to a controller which determines here from the distance between the tip of the cutting knife and the support. In response to this signal, a drive can be activated either for the cutting knife or the support or possibly also for both devices so that the distance to realize a desired residual wall thickness can be reliably adjusted by a control process. Again, the fixed distance in prolongation of the cutting axis is a characterizing feature here. 
         [0015]    In order to introduce randomly configured weakening lines in a material, it may be advantageous to construct the cutting knife for rotation about its cutting axis. In this case, the cutting knife can always be adjusted in the desired manner to implement an optimum cut, when a change in direction between cutting knife and workpiece occurs. The bracket can in this case held in a fixed rotative position independent from the rotation of the tool or the axis  6  through intervention of a freewheel and a torque support on the robot&#39;s wrist joint. In this case, the abutment is to be configured as movable sphere to render possible a movement in cutting direction of the knife. The bracket can be held by this measure in a position which permits an optimum access to the workpiece. 
         [0016]    Of course, an additional rotary drive (e.g. as external robot axis) may hereby be provided for rotating the cutting knife and controlled in correspondence to the change in direction. It is also possible, to substitute the torque support and the sphere by a moving roll which is mounted onto an additional and synchronized rotary drive. 
         [0017]    A further embodiment is characterized by providing the cutting knife, disposed in opposition to the support, with an integral tracer which moves back against a stop, when a skin or film is deposited, and moves into contact against the cutting knife in the absence of a skin or film. The range of movement between the two just described positions is ascertained by a sensor. This embodiment is especially of interest, when damage to the forward tip of the cutting knife is intended to be determined. In the event, the tip of the cutting knife has for example broken off, the range of movement would exceed the desired residual wall thickness so that the deviation allows inference of either a faulty control or damaged cutting knife. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0018]    Several exemplary embodiments of the invention will now be described in greater detail with reference to the attached drawings. The drawings show in: 
           [0019]      FIG. 1  a schematic side view of a first embodiment of a cutting tool according to the invention, 
           [0020]      FIG. 2  a schematic side view of a second embodiment of a cutting tool according to the invention, 
           [0021]      FIG. 3  a schematic illustration of a third embodiment of a cutting tool according to the invention, 
           [0022]      FIG. 4  a schematic illustration of a fourth embodiment of a cutting tool according to the invention, 
           [0023]      FIGS. 5   a  to  5   c  various schematic illustrations, depicting respectively a multipart openable and closeable bracket of a cutting tool according to the invention, 
           [0024]      FIG. 6  a schematic illustration of a cutting tool according to the invention with a tracer, and 
           [0025]      FIG. 7  a schematic illustration of a prior art cutting tool. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]      FIG. 1  shows a schematic illustration of a cutting tool  10  for weakening of plastic skins and similar workpieces. A plastic skin  24  (hereinafter also called workpiece) is securely held between two clamping devices  26 . The cutting tool  10  includes a cutting knife  12  and—in prolongation of the cutting axis  14 —an abutment  18  which assumes in this embodiment also the function of a support. The abutment  18  is held in the housing of a load cell  22  which can determine the pressure upon the abutment  18 . Hereinafter, the term support relates in general to any device which directly supports the workpiece. 
         [0027]    The cutting tool  12  and the load cell  22  are rigidly connected to one another via a U-shaped bracket  16 . In this context, it is to be noted that the abutment  18  does not shift in relation to the bracket  16  and along the cutting axis  14  so that the distance between the abutment  18  and the tip of the cutting tool  12  is always the same. This distance  28  corresponds to the later residual wall thickness. A sphere  20  is provided at the end of the abutment  18  on a side proximal to the cutting tool and held rotatably. 
         [0028]    Not shown in  FIG. 1  is the mounting of the cutting tool. In order to be able to move the cutting tool for example in the direction of arrow  30 , it is held for example on a robotic apparatus which is able to move the cutting tool at least in one plane. However, basically all movement directions are suitable here in order to be able to move the bracket into the required positions, i.e. in x axis, y axis and z axis. 
         [0029]    The mode of operation of this first embodiment of the invention is thus clear and indeed simple. After inserting the workpiece  24  to be weakened in the intermediate space between the cutting tool  12  and the abutment  18  or moving the tool into the work area on the workpiece, the cutting tool  10  is moved in such a way that the workpiece  24  is elastically deformed and supported against the abutment  18 —here the sphere  20 —with a respective force. The load cell  2  measures the forces and ensures that the workpiece to be weakened continuously bears upon the abutment  18  acting as support. As the rigid unit comprised of cutting tool  12 , bracket  16 , and abutment  18  moves, the workpiece  24  is cut and weakened to ensure that a residual wall thickness remains in correspondence to a distance defined by the distance between the tip of the cutting knife  12  and the uppermost end of the sphere  20 . 
         [0030]    A structurally slightly modified embodiment of the invention is shown in  FIG. 2  and will be described hereinafter. Same reference signs designate hereby same elements as in  FIG. 1 . The difference between the embodiments in  FIG. 1  and  FIG. 2  resides in the disposition of the workpiece  24  to be weakened upon a plate  40 —also called support table. This support table assumes the function of the support. The sphere  20  of the abutment  18  now rolls on the lower surface of the support table. A spring  38  extends between an externally secured support block  36  and a support tab  34  of the bracket  16  and maintains the unit of cutting tool  12 , bracket  16 , and abutment  18  under tension to ensure a contact of the workpiece  24  on the support table  40 . This spring  38  urges the sphere  20  against the bottom side of the support table  40 , with this force being measured by the load cell  22 . The residual wall thickness is governed by the difference of the distance of the cutting knife from the abutment  18  minus the thickness of the support table  40 . 
         [0031]    A further embodiment of the present invention is also shown schematically in  FIG. 3 . Shown here are only the relevant elements with respect to adjustment or control. Other elements have been omitted for ease of illustration. 
         [0032]      FIG. 3  shows a cutting knife  12 ′ which is adjustable within certain ranges in the direction of its cutting axis by means of an actuator (e.g. servomotor  50  and spindle drive  52 ). The position of the cutting knife  12 ′ is ascertained by means of a distance sensor. The sensor  54  is connected via a signal line  60  with a controller  56 . The controller  56  has further a control line  58  to the servomotor  50  to receive the respective control signals. 
         [0033]    In addition, a further distance sensor  52  is arranged underneath the support table  40  and determines the distance to a measuring point  66  by means of a tracer. The measuring point is hereby located at the intersection of the cutting axis with the bottom side of the support table  40 ′. This distance information is also fed to the controller  56  via a signal line  64 . 
         [0034]    The controller  56  is able to determine from both signals of the distance sensors  54  and  62 , when suitably calibrated, the distance between the tip of the cutting knife  12 ′ and the measuring point  66 , which is arranged in the direction of the cutting axis on the bottom side of the support table  40 ′, and to readjust the knife position via the servomotor in dependence on the desired distance. The residual wall thickness is again governed by the distance between the cutting knife  12 ′ and the measuring point  66  minus the thickness of the support table  32 ′. 
         [0035]    The advantage of this surely more complex apparatus is the adjustability of the residual wall thickness. It is further possible to use support tables with variable or unknown thickness. The cutting knife is hereby first replaced by a distance sensor and then the thickness profile is determined by a one-time reference run in response to the movement. These reference data are stored and used together with an optionally variable desired residual wall thickness during the later cutting operation as desired value. 
         [0036]    In the embodiment of  FIG. 3 , the support table  40 ′ is moved with the skin positioned thereon along the arrow  27 . The controller  56  then determines continuously and in dependence on the signals of the distance sensors  54  and  62  the control signal for the servomotor  50 . Also in this way, it is possible to ensure a defined residual wall thickness through respective supervision upon the cutting axis, even though there is no rigid connection between the cutting knife  12 ′ and a support (here the support table  40 ′). 
         [0037]    The embodiment illustrated in  FIG. 4  corresponds substantially to the embodiment illustrated in  FIG. 1 . However, the bracket is now supported for rotation about the knife pivot axis  76 . The bracket is supported on the robot wrist joint  72  via a torque support  70  so as to maintain its position independent from a knife rotation. As a result of the rotatability of the cutting knife  12 ′, a reliable cut can be executed along any cutting line. The cutting knife  12 ′ is respectively rotated in response to any change in direction so as to attain an optimal cutting result. 
         [0038]    Of particular interest is also the easy accessibility of the workpiece by the cutting tool.  FIGS. 5   a  to  5   c  show three different embodiments which permit a respective insertion of the tool without any problem in the presence of a rigid coupling—at least during the cutting operation—between cutting knife and support. 
         [0039]    In the first embodiment according to  FIG. 5   a , the U-shaped bracket is made of two parts, that is with a first upper angular region  80  and a leg  84  which are connected to one another via a hinge. An abutment is arranged on the hinge-distal end of the leg  84 . As the leg  84  is pivoted in relation to the angular part  80  of the bracket, the receiving space is opened so that a skin element can easily be deposited. After placement of the skin element or insertion into a workpiece, the U-shaped bracket can be closed as the leg  84  is moved upwards. Of course, the two different elements of the U-shaped bracket must be fixed relative to one another during the actual treatment operation. 
         [0040]      FIG. 5   b  shows a further embodiment for placement of a skin part, whereby the angular part  80 ′ of the bracket is no longer connected anymore with the leg  84 ′ by a joint. Rather, the leg  84 ′ is now held by a respective guide on the other bracket part  80 ′ for linear movement. The linear movement and the securement of both elements is implemented by a hydraulic cylinder  86  which is supported on one side on the bracket part  80 ′ and on the other side on a support  88  for the bracket part  84 ′. As an alternative, the actuator may, of course, also be configured as pneumatic or electric drive. 
         [0041]    A further exemplary embodiment to ensure a reliable insertion or placement is shown in  FIG. 5   c . Compared to the embodiment in  5   b , the lower leg  84 ′ is now lowered not in its entirety but only the abutment  84 ″. The linear movement and the securement of the abutment  84 ″ is implemented by a hydraulic cylinder  86 ′ which is supported on a support  88 ′. The U-shaped bracket  80 ″ remains thus substantially rigid. 
         [0042]    A final embodiment of the invention is shown in  FIG. 6 . Reference is made only to the abutment  92  serving as support. This abutment is constructed at the same time with a measuring head and has a connection with a measuring tracer  94 . When the skin (not shown in  FIG. 6 ) is inserted, the measuring head  92  is shifted downwards against a stop and thus spaced at a distance from the tip of the cutting knife in correspondence with the residual wall thickness. When the skin is removed however, the measuring head  92  can advance in the direction of the cutting knife, with the movement being ascertained by the measuring tracer  94 . In this way, the distance of the support to the cutting tool can be determined. When this distance does not correspond to the desired residual wall thickness, then either the adjustment is faulty or the cutting knife has been damaged in the area of the tip. 
         [0043]    In summary, the cutting tool or the cutting knife and the abutment or the support are coupled to one another (passively or actively) such that the distance between both elements is precisely defined. The abutment thus establishes the movement of the cutting tool and thus the cutting depth directly on the cutting axis, when the workpiece rests upon the abutment. The use of a “virtual abutment” which determines the position of the residual wall thickness by means of a contactless operating sensor is also covered by the scope of the invention. This is true even when the residual wall thickness of the workpiece is not used directly as counterpiece, whereby instead the distal apparatus side and residual wall side of the workpiece are either known by the manufacturing specifications of the apparatus or determined with a reference run. 
         [0044]    Tolerances of a robot movement are compensated with the assistance of an active or passive compensating element which maintains the relative position between tool and abutment in the direction of the cutting axis at a defined parameter through movement of the workpiece, the workpiece-abutment unit and/or a synchronized individual movement of tool and abutment. The contact between abutment and residual wall thickness of the workpiece can be monitored by an integrated sensor assembly (force sensors, precision sensors, distance sensors). 
         [0045]    In summary, the process can be carried out by guiding the tool or the workpiece. In other words, the tool can be guided or held stationary. Any mechanical cutting tool can be used as tools, such as a blade, a milling cutter with spindle, ultrasonic knife, hot knife, perforation tool (e.g. oscillating needle), etc. 
         [0046]    An active coupling between abutment and tool can be realized by the use of any electric, pneumatic, mechanic, or hydraulic actuators, or combinations thereof. 
         [0047]    A direct mechanic coupling between tool and abutment can use external robot axes in order to keep the bracket away from a collision range. Moreover, when a variant is involved in which the tool is guided, the bracket can be mechanically suited to the robot in such a way that its position is independent from the axis of the robot to enable optimum access to the work area. The use of rotation-symmetric tools allows in addition the use of a bracket in an optimal position. 
         [0048]    As described above, optimal accessibility should also be ensured to the tool. 
         [0049]    The present invention provides high process reliability for weakening of plastic skins or similar workpieces, such as films etc. through one-sided cutting because a defined distance between a support and the tip of a cutting tool is ensured in the direction of the cutting axis.