Abstract:
A process for perforating workpieces and a tool for carrying out this process are envisaged. In the process described it is envisaged that a cutting tool is used to make an opening in a workpiece and stamping is carried out in the region of a cut edge of the opening purely by interlocking and frictional engagement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of German Patent Application Serial No. 102006017939.0, filed on Apr. 11, 2006 and entitled “Verfahren zum Lochen von Werkstücken” (Method of Punching an Opening into a Work Piece), the disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a process for perforating workpieces and a tool for carrying out this process. 
       BACKGROUND OF THE INVENTION 
       [0003]    Flat mouldings made of plastics are common in motor cars. With flat parts, in particular, openings have to be formed in them to accommodate functional units, requiring the use of suitable tools. Depending on the particular application the necessary openings have to be provided before or after the surface of the component is painted or varnished. 
         [0004]    For forming the openings, cutting tools have been designed which can be included in automatic production processes. However, one disadvantage which has become apparent is that shaping defects occur on the cut surfaces formed by the cutting as a result of the withdrawal of the edge and the formation of burrs and cracks, which both detract from the desired uniform appearance and also affect the strength of the components. 
         [0005]    With thermoplastic plastics, e.g. injection moulded parts, it is known to carry out a thermoforming process by applying a heated moulding surface in the region of the cut edge. 
         [0006]    The publication DE 197 30 859 A1 discloses for example a cutting tool and a process for perforating workpieces made of thermoplastic plastics. The cutting tool described has a blade running around its end face, which can be brought into engagement with a cutting plate in one operation and penetrates the workpiece arranged between its blade and the cutting plate, with a perforating punch, thereby producing a hole in the wall in the direction of cutting. This perforating punch has a thermoforming surface arranged counter to the blade and projecting beyond it at right angles to the cutting direction and heatable by means of a heating device, said thermoforming surface being adapted to be applied against the cut edge of the wall opening at the end of the operation. 
         [0007]    However, owing to the heat treatment carried out, the process described is only suitable for workpieces that can be exposed to the action of heat. A workpiece made of a polyurethane, for example, cannot be subjected to thermal processing of this kind. 
       SUMMARY OF THE INVENTION 
       [0008]    The process according to the invention for perforating workpieces, particularly workpieces which cannot be subjected to temperature treatment, envisages that in one operation a cutting tool is used to make an opening in the workpiece and moreover stamping is carried out in the region of a cutting or cut edge of the opening purely by interlocking and frictional engagement with the cutting tool. 
         [0009]    The making of the hole and the stamping in the region of the cut edge and hence in the region of the rim of the hole formed are preferably carried out in the same operation, i.e. in a single operation. The punching process is typically carried out before the stamping process. It is also possible for the two processes of punching and stamping to at least partially overlap in time. 
         [0010]    The process according to the invention can be carried out before or after the surface is painted. When the process is carried out before painting, the circumscribed stamping process ensures that the surface of the workpiece can be pre-treated in a manner which is favourable for the subsequent painting. If the process according to the invention is carried out after painting, it is ensured that the paint is not damaged and also no so-called “paint tears” are formed on the stamped openings. 
         [0011]    The process according to the invention may be carried out for example on a workpiece made of a polyurethane or a polyurethane elastomer. The punching force used is selected according to the thickness of the material. Typical values are between 1.5 and 7 t (metric). It should be noted that the process according to the invention can be used to treat workpieces that would be damaged by heat treatment. 
         [0012]    According to one feature, the tool used is a perforating punch having an encircling blade and a shaping surface which projects over the blade, at right angles to a cutting direction, and can be pressed against the cutting edge. The configuration of the blade or cutting edge determines the contour of the opening produced. During punching, the cutting edge engages in the workpiece. Depending on the length of the working stroke, a complete opening or only a partial opening and hence a recess may be produced in the workpiece. 
         [0013]    The shaping surface which does the stamping by pressing against the workpiece in the region of the cut edge may for example be curved, forming a radius. Alternatively, the shaping surface may be shaped into a chamfer. However, other designs of the shaping surface are possible, the construction of the shaping surface determining the form and nature of the cut edge after the process has been carried out. This shape is regularly adapted to the construction of the functional unit which is to be inserted. 
         [0014]    The tool according to the invention for perforating workpieces is used in particular for carrying out the process described hereinbefore, by making an opening in the workpiece. Tool steel is an appropriate material for the perforating punch. 
         [0015]    The tool is constructed, for example, as a perforating punch with an encircling blade and a shaping surface that extends beyond the blade, at right angles to a cutting direction, which can be pressed against the cut edge, so that stamping or cold forming take place in the region of the cut edge of the opening purely as a result of interlocking and frictional engagement. There is no need for a temperature process to achieve the desired results. 
         [0016]    Depending on the application, the shaping surface may be smooth or may have a structure, e.g. a grooved surface. This structure affects the nature of the surface of the workpiece around the rim of the opening. Thus, the surface can be pre-treated in this way for subsequent machining operations. 
         [0017]    Further features and embodiments of the invention will become apparent from the description and the attached drawings. 
         [0018]    It will be understood that the features mentioned above and those which will be described below may be used not only in the combination specified but also in other combinations or on their own, without departing from the scope of the present invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention is schematically illustrated in the drawings by means of exemplary embodiments and is described in detail hereinafter with reference to the drawings, wherein: 
           [0020]      FIG. 1  shows a preferred embodiment of the tool according to the invention; 
           [0021]      FIG. 2  shows a workpiece made of a polyurethane in sectional view; 
           [0022]      FIG. 3  shows a workpiece punched according to the prior art in sectional view; and 
           [0023]      FIG. 4  shows a workpiece punched according to the invention in sectional view. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]      FIG. 1  shows a preferred embodiment of the tool according to the invention, in this case a perforating punch designated  10 , which is intended for use in the process according to the invention. When the perforating punch  10  is used, it is moved in the cutting direction indicated by an arrow  12  towards a workpiece (not shown) which is to be machined. A preferably sharp-edged typically rotating blade or cutting edge  14  penetrates into the workpiece and produces the desired opening in one punching operation. The cross-section of the opening is determined by the cross-section of a front part  16  of the stamping punch  10  which is surrounded by the cutting edge  14 . 
         [0025]    Also, a curved shaping surface  18  is provided which, spaced from the blade  14 , projects over it at right angles to the cutting direction  12 . This shaping surface  18  is usually pressed against the workpiece in the region of the cutting edge, i.e. in the region of the rim of the punched-out opening or hole, in the same operation in which the punching is carried out, and in a stamping process brings about cold deformation of the workpiece in this region by interlocking and frictional engagement. As a result of the curved or bent formation of the shaping surface a radius is formed in the region of the cutting edge, as well as a smooth crack-free cutting edge. 
         [0026]      FIG. 2  shows in sectional view a flat workpiece made of a polyurethane material, generally designated  20 . This workpiece  20  has a relatively porous base part  22  and an integral skin  24 . This integral skin  24  is sensitive to external influences and is destroyed by a heating process, for example. Therefore, it is important that this workpiece is not subjected to any temperature treatment and hence not to any warm or hot stamping either. The workpiece  20  may be painted by applying a coat of paint to the integral skin  24  before or after the perforation of the workpiece  20 . 
         [0027]    The thickness of the workpiece  20  determines the force needed for the punching. In a controlled process, the force with which the tool is pressed against the workpiece  20  can be set or controlled or even regulated. 
         [0028]      FIG. 3  shows a workpiece  30  in which an opening  32  has been made using a process according to the prior art. Also shown is a coat of paint  34  which was applied before the punching process. 
         [0029]    In the region of a cut edge  36 , i.e. in the region of the rim zone of the opening  32 , a sharp edge has been formed, particularly in the coat of paint  34 , leading to an unsatisfactory appearance and hence to higher rejection rates. 
         [0030]      FIG. 4  shows another workpiece  40  in which an opening  42  has been formed according to the invention. This workpiece  40  is also covered with a coat of paint  44 . 
         [0031]    It is clear that both the workpiece  40  and the coat of paint  44  have been stamped in the region of a cut edge  46  and have been formed with a radius. The cut edge  46  is therefore smooth and free from cracks. 
         [0032]    This stamping is carried out according to the invention solely by cold forming using a contour on the punching die, so that the workpiece  40 , the coat of paint  44  and any integral skin provided are not damaged. 
         [0033]    The process according to the invention achieves a better consistency from one part to another, including the external appearance, and thus reduces the rejection rate. It can be carried out before or after painting. If the process according to the invention and hence the punching are carried out after painting, the production of variants before painting is no longer needed, which is a logistical advantage. 
         [0034]    It should be appreciated that the foregoing description is illustrative in nature and that the present invention includes modifications, changes, and equivalents thereof, without departure from the scope of the invention.