图6 弯曲方式
弯曲力的大小是可以通过对一根矩形横梁的简单弯曲的工艺过程的确定来估算。在此情况下的弯曲力是材料强度的函数，此弯曲力的计算式为：
P=KLST2/W 式中：P——弯曲力，吨（对于米制使用单位，吨乘以8.896数值以得到千牛顿单位）；
K——模具开启系数：16倍材料厚度（16T）时的开启系数为1.20，8倍材料厚度（8T）时的开启系数为1.33;
L——零件长度，英寸;
S——极限张力强度，吨/平方英寸； W——V或U形模具的宽度，英寸； T——材料厚度，英寸。
对于U形弯曲（槽形弯曲），弯曲力大约是V形弯曲所需要的弯曲压力的两倍，棱边弯曲则大约是V形弯曲所需要的弯曲压力的1/2。
回弹。所有金属材料均有一个固定的弹性模量，随之而来的是塑性变形，当施加在材料上的弯曲力消除时就会有一些弹性恢复（见图7）。在弯曲过程中这种恢复称为回弹。一般而言，这样的回弹在0.5°～5°之间变化，取决于固定的弹性模量、弯曲方式、模具间隙等。磷青铜的回弹则在10°～15°之间。
图7 弯曲中的回弹
减少或消除在弯曲工序中回弹方法可以根据下列工艺方法进行，如图8所示，在弯曲模具中产生的零件也可以通过等同回弹角度弯曲模上挖凹模或弹性缓冲式弯曲模而被过度弯曲来减少或消除回弹。
图8 减少或消除回弹的方法
从应用角度来说，有许多类型的压力机，诸如：闭式双点偏心轴单动机械压力机，冲压成形机，液压成形压力机，液压机，弯板机，三动式压力机，冲模回转压力机，双点压力机，双边齿轮驱动压力机，双点单动压力机，台式压力机，切边压力机，闭式单动（曲柄）压力机，肘杆式压力机，单点单动压力机，开式双柱可倾压力机，开式压力机，四点式压力机，四曲柄压力机，飞轮式螺旋压力机，摩擦传动螺旋压力机，闭式双点单动双曲柄压力机，摇臂式压力机螺旋式压力机和上传动板料冲压自动压力机等。
附件2：外文原文
Stamping Die Design
The wide variety of sheet metal parts for both the automobile and electronic industries is produced by numerous forming processes that fall into the generic category of \forming\Sheet-metal forming ( also called stamping or pressing )is often carried out in large facilities hundreds of yards long.
It is hard to imagine the scope and cost of these facilities without visiting an automobile factory, standing next to the gigantic machines, feeling the floor vibrate, and watching heavy duty robotic manipulators move the parts from one machine to another. Certainly, a videotape or television special cannot convey the scale of today's automobile stamping lines. Another factor that one sees standing next to such lines is the number of different sheet-forming operations that automobile panels go through. Blanks are created by simple shearing, but from then on a wide variety of bending, drawing, stretching, cropping , and trimming takes place, each requiring a special, custom-made die.
Despite this wide variety of sub-processes, in each case the desired shapes are achieved by the modes of deformation known as drawing, stretching, and bending. The three modes can be illustrated by considering the deformation of small sheet elements subjected to various states of stress in the plane of the sheet. Figure 1 considers a simple forming process in which a cylindrical cup is produced from a circular blank.
Figure 1 Sheet forming a simple cup
Drawing is observed in the blank flange as it is being drawn horizontally through the die by the downward action of the punch. A sheet element in the flange is made to elongate in the radial direction and contract in the circumferential direction, the sheet thickness remaining approximately constant Modes of sheet forming are shown in Figure 2.
Figure2 Modes of sheet forming
Stretching is the term usually used to describe the deformation in which an element of sheet material is made to elongate in two perpendicular directions in the sheet plane. A special form of stretching, which is encountered in most forming operations, is plane strain stretching. In this case, a sheet element is made to stretch in one direction only, with no change in dimension in the direction normal to the direction of elongation but a definite change in thickness, that is, thinning.
Bending is the mode of deformation observed when the sheet material is made to go over a die or punch radius, thus suffering a change in orientation. The deformation is an example of plane strain elongation and contraction
A complete press tool for cutting a hole or multi-holes in sheet material at one stroke of the press as classified and standardized by a large manufacturer as a single-station piercing die is shown in Figure3.
Any complete press tool, consisting of a pair( or a combination of pars ) of mating member for producing pressworked (stmped）parts, including all supporting and actuating elements of the tool, is a die. Pressworking terminology commonly defines the female part of any complete press tool as a die.
The guide pins, or posts, are mounted in the lower shoe. The upper shoe contains bushings which slide on the guide pins. The assembly of the lower and upper shoes with guide pins and bushings is a die set. Die sets in many sizes and designs are commercially available. The guide pins are shown in Figure 3.
Figure3 Typical single-station die for piercing hole
1—Lower shoe 2,5—Guide bushings 3—Cavity plate 4—Guid pin 6—Spring-loaded stripper 7—Punch 8—Support plate 9—Punch bushing 10—Fan-shaped block 11—Fixed
plate 12—Punch-holder plate 13—Backing plate 14—Spring 15—Stepping bolts
16—Upper shoe 17—Shank
A punch holder mounted to the upper shoe holds two round punches (male members of the die) which are guided by bushings inserted in the stripper. A sleeve, or quill, encloses one punch to prevent its buckling under pressure from the ram of the press.