Patent Abstract:
Sheet metal forming is a manufacturing process in which flat sheet metal is drawn into a die cavity to form a product shape. Draw-in amount is the single most important stamping index that controls all forming characteristics (strains and stresses), formability failures (splits, wrinkles) and surface quality (distortions) on a panel. Adaptation of a new die set for repetitively stamping sheet metal parts to a part design specification is simplified by using a math-based simulation of the stamping operation under specified engineering stamping conditions for the specified part. The stamping simulations are used to create an engineered draw-in map comparing selected locations on the peripheral edge of the stamped part with corresponding locations on the peripheral edge of its original sheet metal blank. The resulting map of sheet metal draw-in dimensions reflect suitable displacements of the metal sheet between the binder ring and binder surface of the female die member at all such locations as the punch member of the die set executes its stamping operation. The engineered draw-in dimensions for a simulated part identify specific locations for adjustment of the binder ring/binder surface system in adapting the die set for production of parts.

Full Description:
TECHNICAL FIELD 
     This invention pertains to sheet metal formability and die making, and to methods for tryout of dies for sheet metal forming. More specifically, this invention relates to the use of a map of sheet metal draw-in or displacement distances around the periphery of a stamped sheet metal part in math-guided tryout of a three or four component die set comprising a punch, binder ring(s) and female die designed for making the part. 
     BACKGROUND OF THE INVENTION 
     A conventional three-component die set for stamping sheet metal parts consists of a punch, a binder ring and a female die. Such die sets are used to make many strong and light weight articles of manufacture. These articles include, for example, automotive body panels and other structure parts; aircraft and appliance sheet components; and beverage cans. Families of formable ferrous and aluminum sheet metal alloys have been developed for these manufacturing processes. 
     In a typical sheet metal forming operation, a flat blank of the metal is held at its periphery over the shaped cavity surface of a female die and the sheet is pulled into the cavity and against the shaped surface using a punch with a forming surface complementary to that of the female die. The sheet is drawn and stretched between the tools and assumes a desired shape. In making stamped parts such as automotive body panels, often characterized by deep pockets and small radii bends, the panel may be formed in stages using two or more sets of stamping dies each operated in a suitable hydraulic press. 
     In sheet metal stamping operations the blank is clamped at its periphery against the marginal surface of the female die cavity with a blank holder called the binder ring. The interaction between the binder ring, the interposed sheet and the binder portion at the margin of the female die is critical to making a part that is formed free of wrinkles or tears. The binder ring has a bead that presses adjacent sheet metal into a complementary depression, a trough, in the binder portion of the female die to create proper restraining forces that prevent the sheet metal from being drawn to the die cavity too freely (and cause wrinkles on the sheet metal) or too restrictively (thus causing panel splits). Thus, a peripheral ring of the blank material is crimped and grasped by the binder ring system. When operation of the stamping press brings the punch into contact with the blank, the metal sheet is drawn and/or stretched into the forming cavity. The binder bead/trough interactions at each point around the entire periphery of the blank control the local amount of sheet metal drawn into the forming die. The draw-in of too little metal over the binder ring system can lead to tears or cracks in the stamped part, and too much drawn metal can lead to wrinkles and surface distortions. The draw-in amount is controlled by the die design and die construction, the forming process parameters (binder force, lubrication, die set-up), and the sheet metal properties. 
     In a traditional die development and making process, a die is designed based on previous experience, and the die is validated through a series of physical tryouts. These tryouts are time consuming and costly and cannot guarantee the success of the die developments. For a typical automotive body panel, say a fender, the tryout alone could last twelve months and cost more than one million U.S. dollars. Today in math-based die development practice, the design of a die can be evaluated and reshaped through electronic tryout via advanced stamping simulation technology that consists of stamping CAE (computer aided engineering) programs, sheet metal forming simulation software (e.g., Pamstamp™ and Dyna3D™) and high performance computers. 
     After the die sets for a vehicle body panel, for example, are designed or developed successfully in the digital world, the dies are constructed and tried out in a tooling shop. However, there are differences between the engineering of a die set and its everyday use in a manufacturing operation. And there are differences in results obtained between digital simulation of die operation and physical parts produced in a stamping plant. One aspect of the problem is that die makers are not so familiar with math-based die engineering principles that they can make good use of the math-based work in tuning actual dies for everyday stamping operations and ordinary sheet metal material. Therefore, physical tryout periods for each set of manufacturing dies can still take weeks or months because there has been no robust procedure by which manufacturing people can detect and correct differences between an actual die set and the math-based simulation from which it was built. 
     It is an object of this invention to provide a process for conforming actual sheet metal stamping experience using a specific die set with state-of-the-art math-based simulation of the die set and stamping operation. It is a more specific object of this invention to provide a process for using the exact amount of sheet metal drawn in over the binder ring at selected locations around the periphery of the blank as a practical and effective basis to conform the operation of the physical die set to the simulated performance of the virtual die set. 
     SUMMARY OF THE INVENTION 
     This invention makes use of the capabilities of current mathematically based, computer executed sheet metal forming technology to produce a data set that can be used by stamping die tryout workers to rapidly produce defect free stamped parts using the stamping die set. This invention is a process that is based on the premise that the amount of sheet metal blank that is drawn into the die cavity is a critical manufacturing index that suitably reflects actual strains, stresses and thinning experienced by the stamped metal. It is perceived that the amount of draw-in can be used to anticipate stamping failures and that controlling the amount of draw-in at selected locations around the periphery of the blank is a simplest and robust way for die makers on the shop floors to tune the die set to make good stampings with a maximum efficiency and minimum efforts and costs. 
     In accordance with the practice of the invention, the math-based process is used to simulate the stamping of the part using the stamping die set made in accordance with the die design specified by the computer process. The math-based process is used to predict the sheet metal draw-in amount at sheet metal strain and stress levels throughout the stamping stroke of the punch that produce a defect free part. The mathematical simulation takes into account specified engineering die tryout conditions. These conditions include the material specification of the blank and its mechanical properties such as yield strength, ultimate tensile strength and coefficient of friction. The engineered die tryout conditions also include the thickness and shape (geometry) of the blank, the amount of binder travel, binder force (tonnage), forming tonnage, male bead and female bead (trough) configuration, lubrication, and the like. An engineered draw-in map is prepared comparing the periphery of the original blank with the shrunken periphery of the part as formed in the computer simulation. The draw-in map shows the linear dimensions in, e.g., millimeters, at locations around the plan view of the part of the draw-in of the sheet from the flat blank to the formed panel. In addition, like draw-in maps can be prepared at intermediate part forming stages based, e.g., on the apparent travel of the punch from initial contact with the blank to the completion of the die set forming stroke. 
     Die tryout workers can use the engineered draw-in map as a basis for correcting actual draw-in of the blank on the real die set. Where the sheet metal draw-in distance on the trial part is larger or smaller than the map dimensions suitable compensation adjustments are made to the bead shapes to reduce or increase sheet metal flow. Invariably, as the actual draw-in values around the periphery of formed parts are brought into conformity with the simulated stamping draw-in values, good parts are produced. 
     The advantage of this invention is that die tryout workers can focus on draw-in of the sheet metal as the part is formed, one of the occurrences that they regularly observe in the making of each stamping. Now, with the use of the engineered draw-in map, die tryout workers can more efficiently approach the die tryout process by conforming actual sheet metal draw-in dimensions with the math based simulation draw-in map and, where necessary, making adjustments to the beads at specific locations identified from the draw-in map. 
     Other objects and advantages of the invention will become apparent from a detailed description of a preferred embodiment of the draw-in map process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of the forming surfaces of a three-piece die for forming a vehicle fender panel. The figure comprises, from top to bottom, the forming and peripheral surface of the female die, the sheet metal blank, the binder ring surface, and the punch forming surface. 
         FIG. 2  is an enlarged sectional view of a broken out portion of  FIG. 1  showing the male bead on the binder ring and the bead trough on the peripheral surface of the female die with the interposed blank and underlying punch surface. 
         FIG. 3  is a plan view of the precut blank for forming the fender panel. 
         FIG. 4  is a plan view of the formed fender panel including a draw-in map. Drawn around the perimeter of the illustrated, formed sheet meal part is the outline of the original blank. Also marked around the perimeter of the original blank are directional arrows of the sheet metal draw-in and dimensions in millimeters of the amount of metal draw-in for a suitably formed fender panel. 
         FIGS. 5A–5D  are a series of four cross-sectional views (section  5 A— 5 A of  FIG. 4 ) of the position of the punch, binder ring, sheet metal blank and female die surface at progressive stages of the forming operation. In this example the press moves the female die surface downwardly pushing the blank against the binder ring and punch forming surface. 
         FIG. 5A  schematically shows the position of the die components as the binder ring has wrapped the blank against the female die surface, and the punch is first touching the surface of the secured blank. In this example, the punch is 137 millimeters from the completion of the die set forming stroke. 
         FIG. 5B  shows the position of the die components and sheet metal when the punch has advanced to a position 73 millimeters from the completion of the die set forming stroke. 
         FIG. 5C  shows the position of the die components and sheet metal when the punch is 19 mm from the completion of the die set forming stroke. 
         FIG. 5D  shows the completion of the forming stroke of the die set. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The practice of the invention is illustrated in connection with the stamping of an automotive vehicle fender outer panel. The fender panel may be stamped using a suitable low carbon steel metal blank or aluminum alloy blank, or the like. A shape for the fender is conceived by automotive designers. The design is transcribed into mathematically based dimensional and spatially locating data for use in computer assisted engineering design of a die set for the stamping of the fender panel. Commercially available computer based programs such as those identified above are used to then design a female die surface and a male punch surface and a binder ring shape suitable for controlling the stamping of the selected sheet metal material. Data concerning the thickness of the sheet metal, its physical properties and its deformation forming characteristics are used in the computer program in the design of the die set. Some panel shapes may require more than one stamping operation and, thus, more than one set of stamping dies to make the part. 
     The die components of a set are then made by known practices and as soon as they have been finish machined or otherwise shaped to the specification of the math based program, the components are ready for try-out with an actual blank of the specified sheet metal material.  FIG. 1  illustrates, in exploded view, the arrangement of the movable female forming die surface  110  (sometimes called the upper die surface in this specification), a sheet metal blank  112 , binder ring  114 , and a stationary male punch surface  116 . The full die members are not shown to simplify the illustration. 
     The female die member is secured in the upper platen of a suitable stamping press. The press is of known design and is also not shown to simplify the illustration. In this example, the female die surface  110  is movable. Female die surface  110  has a shaped cavity portion  118 , the surface of which is designed to shape the top surface  120  of blank  112  as the outer surface of a fender panel in a stamping operation. Female die surface  110  also has a peripheral surface  122  completely surrounding cavity portion  118 . During a stamping action of the press and die components, the peripheral portions of blank  112  are clamped against peripheral surface  122  of the upper die  110  by binder ring  114 . 
     Binder ring  114  is also secured in the press in a manner so that it can be separately moved to clamp an inserted blank  112  against peripheral surface  122  of the upper die when the upper die  110  is moving downwardly with the press rams. Binder ring  114  is shaped to define an opening  124  through which the punch die member with its punch surface  116  can enter to push against the lower surface  126  (see  FIG. 2 ) of blank  112  as it is moved downwardly by upper die surface  110 . Binder ring  114  also has a bead  128  which is illustrated in  FIG. 1  as a single continuous linear raised surface like a ring around the perimeter of opening  124 . Bead  128  will typically have different cross-sectional shapes in different regions around its ring configuration, and the bead ring may be formed in discrete linear segments. 
     The stationary punch die member, with its punch surface  116 , is secured to the lower platen of the press. 
     As may be apparent from the brief description of the die components, a fender panel stamping cycle is commenced with the die component members and surfaces in their open position as shown in  FIG. 1 . The lower platen of the press supports punch with punch surface  116  in the open position of the die set. Binder ring  114  has been lowered to permit the insertion and precise location of blank  112  between the upper die surface  110  with peripheral surface  122  and binder ring bead  128 . In a first closing step of the press, the upper die surface  110  is moved down to press the blank  112  against binder ring  114 . Binder ring  114  engages the bottom surface  126  of blank  112  to clamp the perimeter of the blank  112  against the peripheral margin surface  122  of the upper die surface  110 . Bead  128  on the binder ring  114  presses the overlying blank material against upper die surface  122  and crimps peripheral blank material into trough  130  (see Figure  2 ) formed in peripheral surface  122 . Trough  130  is shown only in a cross-section in  FIG. 2 . But trough  130  is formed in a ring-like linear path in surface  122  completely around the cavity surface  118  so that it overlies bead  128 . The opposing portions of the bead  128  and trough  130  cooperate in each portion of their respective rings to control the draw-in of sheet metal during stamping. As will be described further, the specific shapes and dimensions of bead  128  and trough  130  control the draw-in of the blank material when the punch surface  116  forces the blank  112  against cavity surface  118 . 
     When upper die surface  110  is moved to press blank  112  and binder ring  114  downwardly, the stationary punch with punch forming surface  116  engages the lower side  126  of blank  112  and continually pushes it into the cavity of the upper die into conformance with female die surface  118  to thus form the fender panel sheet. In the closing operation of the press, punch surface  116  passes through opening  124  in binder ring  114  to draw a portion of the blank into the upper die cavity  118 . Of course, metal at the edge of the blank  112  is pulled over the bead  128  of binder ring  114  and through the trough  130  of upper die  110  around the entire, usually enclosed path, of these linear, ring-like die elements. The shapes of the bead  128  and trough  130  in combination with action of the punch and female die surfaces  116 ,  118  controls whether an acceptable stamping is formed. It is the control provided by the bead and trough over the flow of blank material that largely determines whether the proper amount of metal is drawn and stretched into the female cavity. 
     In the example illustrated in  FIG. 1 , a panel is fully formed from an initially flat blank  114  of sheet metal into the fender member. In a subsequent operation the peripheral portion of the blank that is not used in the forming of the outer fender panel is trimmed away. 
       FIG. 2  is a somewhat enlarged view of a broken out section (at  2 — 2 ) of the die components and blank illustrated in  FIG. 1 .  FIG. 2  illustrates one cross-section of a bead  128  and trough  130 . As is known to die engineers, the height and width of the bead, or bead segments, and the radii of its corners are specified when the bead is formed on the surface of the binder ring (or on the surface of the upper die if the bead is formed there). Likewise, the corresponding dimensions of a trough, whether formed in the upper die or on the binder ring, are initially specified when the die set is made. However, if the initially specified shapes at mating regions of a bead or trough segment are not suitable they are changed, e.g., reduced or enlarged or provided with sharper or softer radii, during tryout of a die set. 
       FIG. 3  is a plan view of the flat, two dimensional outline, of a sheet metal blank  112  for stamping a fender outer panel on the die set surfaces shown in  FIGS. 1 and 2 . The blank will usually be of a suitable low carbon steel or aluminum alloy composition and have a thickness of, e.g., one to three millimeters. As stated, the mechanical and formability properties of the sheet metal are known and used in the computer aided design of the members of the die set. In this example, the blank is not of simple rectangular shape. The blank is provided with a two-dimensional outline generally resembling the outline of the fender to minimize scrap. But the shape and dimensions of the sides of the blank are important in assuring that the proper amount of metal is available on all sides for draw-in to the forming cavity, e.g., cavity surface  118 . And provision must be made in the blank shape for suitable engagement with the binder bead  128  and trough  130  combination. Referring to  FIG. 3 , edge  140  lies at the front end of what will become the fender panel;  132  at the top of the intended fender;  134  at the rear of the fender next to a door and edges  136  and  138  at the wheel well and bottom, respectively. But the locations of these edges on the blank  112  are located within a millimeter of specified design for use in the draw-in map used in the practice of this invention. Furthermore, the blank  112  is precisely located when it is places in the press between the upper die surface  110  and binder ring  114 . For simplicity of illustration, the total perimeter of edges  130 – 138  is identified as  144  in the draw-in map of  FIG. 4 . 
     After the upper die surface  110  has completed its full stroke and punch  116  has pushed the blank material into the upper cavity against female cavity surface  118 , the press then reverses its motion. The upper die  110  is raised and the stamped part  142  can be removed from the punch surface  116 .  FIG. 4  is a plan view of the stamped fender panel sheet  142 . Fender panel sheet  142  comprises the fender panel shape, corresponding to cavity surface  118 , plus peripheral edge material used in the stamping process. 
       FIG. 4  is an example of an engineered draw-in map essential to the practice of this invention. The outline of the formed sheet metal  142  is shown in plan view  FIG. 4 . Also superimposed around the perimeter of the deformed sheet metal part is the perimeter  144  of original blank  112 . The perimetrical outline of formed part  142  is smaller than the perimetrical outline  144  of blank  112 . The sheet material of the formed part  142  has experienced draw-in or displacement from the outline  144  of the original blank  112 . The precise amount of draw-in varies around the respective perimeter segments depending upon local flow of the sheet material over the binder ring bead system of bead  128  and trough  130 . 
     Also shown on  FIG. 4 , are several one and two digit numbers associated with directional arrows. The arrows indicate the direction of draw-in of the stamped sheet metal from the perimeter  144  of the original blank  112 . The numbers represent the dimensions, in millimeters, of the draw-in at the location of the associated arrow. The dimensions shown in  FIG. 4  are dimensions determined by a math-based simulation of the fender panel sheet metal  142  on a die set corresponding to the try-out die set and under specified engineered tryout conditions for the stamping operation of the type listed in the Summary of Invention section of this specification. Thus,  FIG. 4  represents a map of the idealized draw-in of the formed part from the original blank  112  shape in accordance with a math based simulation of the drawing process. Such a map may contain, for example, a draw-in dimension at points every 100 mm or so around the perimeter of the blank and formed piece. While several such draw-in dimensions are shown in  FIG. 4  as many as forty or more dimensions might be mapped in a fender panel like that illustrated. 
     In accordance with this invention, the data in the map of  FIG. 4  is used to compare with the actual draw-in dimensions at corresponding locations of a trial stamped part formed during tryout of newly made tooling. The trial stamping is made under the same engineered tryout parameters or conditions as imposed on the math-based simulation. Comparison of a trial part with the engineered draw-in map identifies locations, if any, where there is a significant difference between the experienced draw-in values and math-based simulation draw-in values. It is found that the die tryout process is very effectively shortened by focusing on reducing such differences in draw-in values to match the engineered draw-in. Where the trial part has experienced a significant difference in draw-in compared to the math-based map, the male beads and/or female trough elements are altered to correct the metal flow and the altered die set given a new trial with a new blank. The general relationship between bead and trough size and radii and metal flow is known. The die tryout process is greatly shortened by using a draw-in map as the sole basis of determining alterations to the bead and trough components of a die set. 
     The use of the math-based draw-in map can be used at intermediate stages of the stroke of the punch member of the die set. In other words, draw-in occurs progressively as the punch progressively forces the blank metal into the cavity of the female die member. 
       FIGS. 5A–5D  illustrate in schematic views how sheet metal draw-in occurs progressively in the forming of the part. These Figures also illustrate a cross-sectional view (at line  5 A— 5 A of  FIG. 4 ) of a bead  128  and trough  130  combination at two locations on the die set diametrically opposed to each other in the sectional view. The bead is illustrated on the binder ring and the trough on the female die surface but these locations can be reversed. Also the press operation is illustrated with the female die being lowered toward the punch but other die set closing modes may be employed. 
       FIG. 5A  shows binder ring  114  clamping sheet metal blank  112  against the peripheral surface  122  of the female die surface  118  when the upper die moves. Punch surface  116  just touches the bottom surface  126  of blank  112 , but the upper die must still travel a distance of, for example, 137 mm before the die set is fully closed and the stamped metal panel  142  is made. Sheet metal blank  112  is gripped by bead  128  on binder ring  114  and trough  130  on die surface  122  around the perimeter of the sheet. The bead  128  and trough  130  are seen at both sides of the blank  112  in these sectional views of  FIGS. 5A–5D . At the position of the punch surface  116  shown in  FIG. 5A  there has been no draw-in of the blank sheet metal and there is ample blank material (some broken off in  5 A) outside of each bead/trough location in this section. 
       FIG. 5B  shows punch surface  116  after the upper die has been moved downwardly over almost half of its forming stroke. For example, it may now be considered to be a distance of 73 mm from the completion of its stroke. Blank  112  is being deformed upwardly toward female die surface  118 . Although some metal stretching occurs, this deformation is largely accommodated by blank material being drawn inwardly over bead ring  128  and through trough  130  in the female die peripheral surface  122 . The cross-section of the blank  112  illustrated in these  FIGS. 5A–5D  experiences a significant amount of deformation and provision for adequate draw-in material must be provided. A useful math-based simulation draw-in map could be prepared for this portion of the stroke of the punch surface  116 . And a trial part could be prepared by programming the press to stop at this point on its stamping stroke. The draw-in dimensions of the trial part are compared with the math-based draw-in map at this stage of part-making to evaluate die performance at intermediate part formation. 
       FIG. 5C  shows the upper die surface  118  at a further stage of closure. For example, the upper die is now 19 mm from the completion of its forming stroke. More blank material will have been drawn between bead  128  and trough  130 .  FIG. 5C  further illustrates the progressive draw-in of blank metal in the stamping process. As observed with respect to  FIG. 5B  a math-based draw-in map can be made at any intermediate portion of the punch stroke for assessing suitable metal draw-in at that forming stage during the tryout of a die set. 
     Finally,  FIG. 5D  shows the punch and punch surface  116  at completion of die set closure with the fender panel now formed. The operation of the die set should be such that peripheral blank material is still within the grip of the binder ring system. 
     The practice of this invention utilizes math-based simulations to facilitate die tryout. The simulations, made under specified engineered stamping conditions, are used to predict sheet metal blank draw-in during the stamping of a specified sheet metal part. Trial parts are made under the same engineered stamping conditions and the trial part draw-in compared with a map of the math determined draw-in. Comparisons may be made with the fully formed part and at intermediate part forming stages. If the measured draw-in does not match the engineered draw-in, initial trial parts often are unsatisfactory because of defects such as wrinkles or splits or tears in the stamped metal. It is found that a most efficient way to correct such defects is to make use of the draw-in map as a basis for modifications to the binder system. Use of such a map can pin-point locations on the binder ring  128  where metal draw-in does not contribute to a defect free part. Comparison with actual draw-in at a trial part location indicates whether metal flow should be encouraged or restricted. After indicated changes have been made to the binder system a new trial part is made. This try-out approach is repeated to correct metal flow in the die set. 
     The practice of the invention has been illustrated by illustrative, but not scope limiting examples.

Technology Classification (CPC): 1