Press Tools

7.Bending, Forming And Drawing Dies

7.1    BENDING DIES

          Bending is the uniform straining of material, usually flat sheet or strip metal, around a straight axis, which lies in the neutral plane and normal to the lengthwise direction of the sheet or strip. Metal flow takes place within the plastic range of the metal, so that the bend retains a permanent set after removal of the applied stress. The inner surface of a bend is in compression; the outer surface is in tension. A pure bending action does not reproduce the exact shape of the punch and die in the metal; such a reproduction is one of forming.

        Terms used in bending are defined and illustrated in Fig. 7-1. The neutral axis is the plane area in bent metal where all strains are zero.

7.1.1          Bend Radii: Minimum bend radii vary for the various metals; generally most annealed metals can be bent to a radius equal to the thickness of the metal without cracking or weakening.

7.1.2          Bend Allowances: Since bent metal is longer after bending, its increased length, generally of concern to the product designer, may also have to be considered by the die designer if the length tolerance of the bent part is critical. The length of bent metal may be calculated from the equation

                           A

                B  =                     2p  ( IR + Kt )                                                 (7-1)                           360

 

Where  B  = bend allowance in mm.  (along neutral axis)

                A  = bend angle in deg.

                        IR = inside radius of bend in mm.

                 t  =  metal thickness in mm.

 K = 0.33 when IR is less than 2t and is 0.50 when IR is more than 2t.

7.1.3          Bending Methods: Two bending methods are commonly made use of in press tools. Metal sheet or strip, supported by a V block (Fig. 7-2A), is forced by a wedge-shaped punch into the block. This method, termed V bending, produces a bend having an included angle which may be acute, obtuse, or of 90°. Friction between a spring-loaded knurled pin in the vee of a die and the part will prevent or reduce side creep of the part during its bending.

Edge bending (Fig. 7-2B) is cantilever loading of a beam. The bending punch, step1, forces the metal against the supporting die, step 2 - The bend axis is parallel to the edge of the die. The work piece is clamped to the die block by a spring-loaded pad, step3, before the punch contacts the work piece to prevent its movement during downward travel of the punch.

7.1.4         Bending Pressures: The pressure required for V bending is

                                K L S t²

                        P =           W                                                                             (7-2)

Where P = bending force, tons

            K = die opening factor: 1.20 for a die opening of 16 times metal thickness,

                   1.33 for an opening of 8 times metal thickness

            L = length of part, cm.

            S = ultimate tensile strength, tons per sq cm.

           W = width of V or U die, cm.

For U bending (channel bending) pressures will be approximately twice those required for V bending; edge bending requires about one-half those needed for V bending.

7.1.5    Spring back:

After bending pressure on metal is released, the elastic stresses are also released, which causes metal movement resulting in a decrease in the bend angle (as well as an increase in the included angle between the bent portions). Such a metal movement, termed spring-back, varies in steel from ½ to 5°, depending upon its hardness; phosphor bronze may spring back from 10 to 15°.

V-bending dies customarily compensate for spring-back with V blocks and wedge-shaped punches having included angles somewhat less than that required in the part. The part is bent through a greater angle than that required but it springs back to the desired angle.

        Parts produced in other types of bending dies are also over bent through an angle equal to the spring-back angle with an undercut or relieved punch.

7.1.6 Evolution of a Bending Die

        The production of a work piece of Fig. 7-4 in the die of Fig. 7-5 required blank development before die design began.

The straight length of the vertical leg is 25.0 – 1.5 or 23.4 mm. the straight length of the horizontal leg is 150.0 – 1.6 or 148.4 mm.

        The bend length (since IR is less than twice metal thickness) is, from Fig. (7-1)

                        90                        1.5

                B  =                  2 p  ( 1.6  +                  )

360                        3

 

                            =  3.3 mm.

 

The developed length is 23.4 + 148.4 + 3.3  = 175.1 mm.

        To hold the tolerance of ± ½ deg. allowed for the 90-deg bend, the designer decided that an edge-bending die, with a slight ironing action on the stock, be used.

        Based upon Fig. (7-2), the bending pressure needed without ironing is

                1.33 X 25 X 4.65 X  (0.15) ²

P   =                                                            = 0.483 ton

2 X 3.6

The total spring pressure required of six springs in the pressure pad is 480 kg.; each spring will supply a pressure of 80 kg. Commercial 25 mm. dia die springs, 50 mm. long, will easily supply this pressure. Almost any small OBI press will supply these pressures with an ample allowance for slight ironing of the blank and has a bed area large enough to accommodate a die set. There are no formulae for determining ironing pressure; it can be approximated by multiplying the yield strength of the metal by the thickness of the metal after reduction times its length.

Since the size of the blank to be sent is 250 x 175 mm. the area of a die set 350 mm. (right-to-left) by 250 mm. (front-to-back) allows for mounting of the punch and pressure pad on the upper shoe and the die block and heel to the lower shoe.

The blank is located on the die block against an end-stop pin and two rear-stop pins.  On the down stroke of the press, the pressure pad clamps the blank in this location.

 The descent of the punch forces the end of the blank against the end of the die block. Its wiping action results in some ironing of the blank, the amount of which is determined by the clearance between the heel block and the punch. To establish optimum clearance and to allow for wear on punch and heel block, shims can be inserted between the backup and heel blocks. The surface of the heel block against which the punch rubs can be hardened or can have a bronze wear strip as shown.

 

 

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