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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 "sheet-metal 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. After penetration of the work material, the two punches enter the die bushings for a slight distance.
The female member, or die, consists of two die bushings inserted in the die block. Since this press tool punches holes to the diameters required, the diameters of the die bushings are larger than those of the punches by the amount of clearance.
Since the work material stock or workpiece can cling to a punch on the upstroke, it may be necessary to strip the material from the punch. Spring-loaded strippers hold the work material against the die block until the punches are withdrawn from the punched holes. A workpiece to be pierced is commonly held and located in a nest (Figure 2-3) composed of flat plates shaped to encircle the outside part contours. Stock is positioned in dies by pins, blocks, or other types of stops for locating before the downstroke of the ram.
Bending is one of the most common forming operations. We merely have to look at the components in an automobile or an appliance-or at a paper clip or a file cabinet-to appreciate how many parts are shaped by bending. Bending is used not only to form flanges, seams, and corrugations but also to impart stiffness to the part ( by increasing its moment of inertia ).
The terminology used in bending is shown in Figure 4. Note that, in bending, the outer fibers of the material are in tension, while the inner fibers are in compression. Because of the Poisson's ratio, the width of the part (bend length, L) in the outer region is smaller, and in the inner region is larger than the original width. This phenomenon may easily be observed by bending a rectangular rubber eraser.
Minimum bend radii vary for different metals, generally, different annealed metals can be bent to a radius equal to the thickness of the metal without cracking or weakening. As R/T decreases (the ratio of the bend radius to the thickness becomes smaller), the tensile strain at the outer fiber increases, and the material eventually cracks (Figure 5).
Figure 4 Bending terminology
Figure5 Poisson effect
The minimum bend radius for various materials is given in Table 1 and it is usually expressed in terms of the thickness. such as 2 T, 3 T, 4T.
Table 1 Minimum bend radius for various materials at room temperature
Material Condition
Soft Hard
Aluminum alloys 0 6T
Beryllium copper 0 4T
Brass,low-leaded 0 2T
Magnesium 5T 13T
Steels
Austenitic stanless 0.5T 6T
Low-carbon,lowalloy,HSLA 0.5T 4T
Titanium 0.7T 3T
Titanium alloys 2.6T 4T
Note :T——thickness of material
Bend allowance as shown in Figure 4 is the length of the neutral axis in the bend and is used to determine the blank length for a bent part. However, the position of the neutral axis depends on the radius and angle of bend (as described in texts on mechanics of materials).An approximate formula for the bend allowance, Lb is given by
Lb= α(R十kT)
Where Lb——bend allowance, in (mm).
α——bend angle, (radians) (deg).
T——sheet thickness, in (mm).
R——inside radius of bend, in (mm).
k——0.33 when R is less than 2T and 0.50 when JR is more than 2T.
Bend methods arc commonly used in press tool. Metal sheet or strip, supported by-V bock[Figure 6(a)],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 90°.Friction between a spring-loaded knurled pin in the vee die and the part will prevent or reduce side creep of the part during its bending.
Edge bending [Figure 6(b)] is cantilever loading of a beam. The bending punch forces the metal against the supporting die. The bend axis is parallel to the edge of the die. The workpiece is clamped to the die block by a spring-loaded pad before the punch contacts the workpiece to prevent its movement during downward travel of the punch.
Figure 6 Bending methods
Bending Force can be estimated by assuming the process of simple bending of a rectangular beam. The bending force in that case is a function of the strength of the material. The calculation of bending force is as follows:
P=KLST2/W
Where P-bending force, tons (for metric usage, multiply number of tons by 8.896 to obtain kilonewtons).
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, in.
S——ultimate tensile strength, tons per square in.
W——width of V or U die, in.
T——metal thickness, in.
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.
Springback in that all materials have a finite modulus of elasticity, plastic deformation is followed, when bending pressure on metal is removed, by some elastic recovery (see Figure 7). In bending, this recovery is called springback. Generally speaking, such a springback varies in sheet from 0.5°to 5°, depending upon finite modulus of elasticity, modes of bending, clearance of die and so on, but phosphor bronze may spring back from 10°to15°.
Figure 7 Springback during bending
Methods of reducing or eliminating springback in bending operations can be made according to the following operations, shown in Figure 8, and parts produced in bending die are also overbent through an angle equal to the springback angle with an undercut or relieved punch.
Figure8 Methods of reducing or eliminating springback
For the applications, there are many types of the presses, such as the single-action straight-slide eccentric mechanical press, punch press, hydro-former press, hydraulic press, press brake, triple-action press, turret press, two-point press, twin-drive press, two point single-action press, watch press, trimming press, closed-type single-action crank press, knuckle-lever press, one-point single-action press, open-back inclinable press, open-side press, four-point press, four-crank press, flywheel-type screw press, friction screw press, straight-side single-action double-crank press, rocker-arm press, screw press and top-drive sheet-metal stamping automatic press and so on.
冲压模具设计
对于汽车行业与电子行业，各种各样的板料零件都是有各种不同的成型工艺所生产出来的，这些均可以列入一般种类“板料成形”的范畴。板料成形（也称为冲压或压力成形）经常在厂区面积非常大的公司中进行。
如果自己没有去这些大公司访问，没有站在巨大的机器旁，没有感受到地面的震颤，没有看巨大型的机器人的手臂吧零件从一个机器移动到另一个机器，那么厂区的范围与价值真是难以想象的。当然，一盘录像带或一部电视专题片不能反映出汽车冲压流水线的宏大规模。站在这样的流水线旁观看的另一个因素是观看大量的汽车板类零件被进行不同类型的板料成形加工。落料是简单的剪切完成的，然后进行不同类型的加工，诸如：弯曲、拉深、拉延、切断、剪切等，每一种情况均要求特殊的、专门的模具。
而且还有大量后续的加工工艺，在每一种情况下，均可以通过诸如拉深、拉延与弯曲等工艺不同的成形方法得到所希望的得到的形状。根据板料平面的各种各样的受应力状态的小板单元体所可以考虑到的变形情形描述三种成形，原理图1描述的是一个简单的从圆坯料拉深成一个圆柱水杯的成形过程。
图1 板料成形一个简单的水杯
拉深是从凸缘型坯料考虑的，即通过模具上冲头的向下作用使材料被水平拉深。一个凸缘板料上的单元体在半径方向上被限定，而板厚保持几乎不变。板料成形的原理如图2所示。
拉延通常是用来描述在板料平面上的两个互相垂直的方向被拉长的板料的单元体的变形原理的术语。拉延的一种特殊形式，可以在大多数成形加工中遇到，即平面张力拉延。在这种情况下，一个板料的单元体仅在一个方向上进行拉延，在拉长的方向上宽度没有发生变化，但是在厚度上有明确的变化，即变薄。
图2 板料成形原理
弯曲时当板料经过冲模，即冲头半径加工成形时所观察到的变形原理，因此在定向的方向上受到改变，这种变形式一个平面张力拉长与收缩的典型实例。
在一个压力机冲程中用于在一块板料上冲出一个或多个孔的一个完整的冲压模具可以归类即制造商标准化为一个单工序冲孔模具，如图3所示。
图3 典型的单工序冲孔模具
1—下模座 2、5—导套 3—凹模 4—导杆6—弹压卸料板 7—凸模 8—托板 9—凸模护套 10—扇形块 11—固定板 12—凸模固定板 13—垫块 15—阶梯螺钉 16—上模座 17—模柄
任何一个完整的冲压模具都是有一副（或多副的组合）用于冲制工作的（冲压）零件组成，包括：所有的支撑件部分与模具的工作部分零件，即构成一副冲模。冲压（术语）通常将完整压制工具的凹模（母模）部分定义为模具。
导杆，或导柱，是安装在下模座上的。上模座则安装有用于导杆滑动的导套，分别装有导套与导杆的上模座与下模座组合成为木架。模架有许多规格与结构设计用于商业销售。
安装在上模座上的凸模固定装置固定两个凸模（模具中的突出部分），这两个圆形凸模则通过插入在卸料板上的导套进行导向。套筒，或凸模护套，是用来保护冲头，以免在冲压过程中被卡住。在冲穿工件材料后，两个冲头便进入到凹模一定距离。
凹模（母模）部分，即凹模，通常是由插入模具体内的两个模具导套组成的。因为冲头的直径是被冲孔的直径所要求的，所以有一定间隙的凹模直径是大于冲头直径的。
由于工件材料坯料或工件在冲制回程时与冲头附连在一起，所以把材料从冲头上剥离是必需的。弹压卸料板则保持冲头在冲制工件回程时缩回，使工件与工件剥离。一个冲制的工件通常是留在漏料槽内的，漏料槽是由包含整个零件外轮廓的平板组成。模座是由销钉支撑板以及其他的滑块下行程时定位的挡料块等定位的。
弯曲时一种最常见的成形工序。当我们仅将目光移至汽车或电器上的部件，或一个剪纸机或档案柜上时，就会发现许多零件都是由弯曲成形的。弯曲不仅可以用来成形法兰、接头、波纹，也可以提高零件的强度（通过增加零件的惯性矩）。
图4 弯曲术语
弯曲中所用的术语，如图4所示，应该注意的是，在弯曲中材料的外纤维是处于拉应力状态，而材料的内纤维则处于压应力状态。由于泊松比原因，在外部区域的零件（弯曲长度L）是小于原始宽度，处于内部区域的则比原始宽度大。这种现象可在弯曲一个矩形的橡胶板擦时容易观察到的。
最小弯曲半径对于不同的金属是变化的。一般而言，各种退火的金属板在没有断裂或变弱的前提下，可以弯曲成一个等同金属板厚的半径。随着R/T比值的减少（弯曲半径对厚度的比值变小），外纤维的拉应力增加，材料最终断裂（参见图5）。
图5 泊松效应
不同材料的最小弯曲半径参考表1，他通常是按照不同板厚来表示的，诸如：2T，3T，4T等。
表1 在室温状态下各种材料的最小弯曲半径
材料 状态
软 硬
铝合金 0 6T
钕青铜合金，钕合金 0 4T
黄铜，低铅 0 2T
镁 5T 13T
钢
奥氏体不锈钢 0.5T 6T
低碳钢，低合金钢，高强度铅合金 0.5T 4T
钛 0.7T 3T
钛合金 2.6T 4T
注：T——材料厚度。
弯曲容许范围，是指弯曲中的中性线（层）的长度，用来确定弯曲零件的坯料长度。然而，中性线（层）的位置是哟弯曲角度（正如在材料力学课本中所描述）来决定的。弯曲容许范围（Lb）的近似的公式为：
Lb=α(R+kT)
式中：Lb——弯曲容许范围，毫米；
α——弯曲角度（弧度），度；
T——金属板厚，毫米；
R——弯曲内层半径，毫米；
k——当半径R＜2T时为0.33，当半径R＞2T时为0.50。
弯曲方式通常用于冲压模具。金属钢板或带料，由V形支撑，参见图6（a）在楔形冲头的冲压力作用下进入V形模具内弹簧加载压花销和零件之间的摩擦将会防止或减少零件在弯曲期间的边缘滑移。
棱边弯曲，参见图6（b）是悬臂横梁式加载方式，弯曲冲头对相对支撑的凹模上的金属施加弯曲力。弯曲轴线是与弯曲模具的棱边相平行的。在冲头接触工件之前，为了防止冲头向下行程的位移，工件则被一个弹性加载垫片加紧模具体上。
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。
