Page 448 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)
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428      Chapter 16  Sheet-Metal Forming Processes and Equipment

                                   l6.l3    Design Considerations in Sheet-metal Formmg

                                   As with most other processes described throughout this book, certain design guide-
                                   lines and practices have evolved with time. Careful design using the best established
                                   design practices, computational tools, and manufacturing techniques is the best ap-
                                   proach to achieving high-quality designs and realizing cost savings. The following
                                   guidelines apply to sheet-metal-forming operations, with the most significant design
                                   issues identified.
                                   Blank Design.  Material scrap is the primary concern in blanking operations. (See
                                   also Table 40.6.) Poorly designed parts will not nest properly, and there can be con-
                                                             siderable scrap between successive blanking opera-
                                                             tions (Fig. 16.55). Some restrictions on blank shapes
                                                             are made by the design application, but whenever pos-
                                                             sible, blanks should be designed to reduce scrap to a
                                                             minimum.
                                                             Bending.  In bending operations, the main concerns
                                                             are material fracture, wrinkling, and the inability to
                                                             form the bend. As shown in Fig. 16.56, a sheet-metal
                        `73`
                                                             part with a flange that is to be bent will force the
                                                             flange to undergo compression, which can cause buck-
                                                             ling (see also Hanging, Section 16.6). This problem
                                                             can be controlled with a relief notch cut to limit the
                                                             stresses from bending, or else a design modification
                                                             as shown in the figure can be made to eliminate the
                                                             problem. Right-angle bends have similar difficulties,
                                                             and relief notches also can be used to avoid tearing
                                                             (Fig. 16.57>.
                                                                  Because the bend radius is a highly stressed area,
          FIGURE l6.55  Efficient nesting of parts for optimum
          material utilization in blanking. Source: Courtesy of Society  all stress concentrations should be removed from the
          of Manufacturing Engineers.                        bend-radius location. An example is parts with holes
                                                             near bends. It is advantageous to move the hole away
                                                                     from the bend area, but when this is not pos-
                                     Better
                 Poor                `.i.-              Best         sible, a crescent slot or ear can be used
                      '                                              tabs and notches should be avoided, since
                    -`”i                                E; titr  to  (Fig. 16.58a). Similarly, in bending flanges,
                ._ `i~`                   |  g  eef           ,      these stress concentrations will greatly re-


                                                                     large radii should be used to reduce stress
            Closed    E'    3  X_ melellel           Flellel         duce formability. When tabs are necessary,
            corner            thickness (al          notch           concentration (Fig. 16.58b).
                                                                          When the bending and production of
                                                         ...FY
                                                                     ent the notches properly with respect to the
                                                                     notches is to be used, it is important to ori-

                                                                     grain direction. As shown in Fig. 16.17,
                                                                     bends ideally should be perpendicular to the
                                                                     rolling direction (or oblique if this is not
                                                           C  if
              Closed corner   3 >< material           Relief notch   possible) in order to avoid cracking. Bending
                               thickness                             sharp radii can be accomplished through
                                     (b)                             scoring or embossing (Fig. 16.59), but it
                                                                     should be recognized that this can result
          FIGURE l6.56  Control of tearing and buckling of a flange in a right»  in fracturing. Burrs should not be present
          angle bend. Source: Courtesy of Society of Manufacturing Engineers.  in a bend allowance, since they are brittle
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