外文翻译--薄板与板材的弯曲

Sheet and Plate BendingBending is a method of producing shapes by stressing metal beyond its yield strength, but not past its ultimate tensile strength. The forces applied during bending are in opposite directions, just as in the cutting of sheet metal. Bending forces , however, are spread farther apart, resulting in plastic distortion of metal without failure.The bending process appears to be simple; yet, in reality, it is a rather complex process involving a number of technical factor. Included are characteristics of the work piece material flow and required to from the bend, and the type if equipment used.In the large, varied field of sheet metal and plate fabricating, several types of bending machines are used. Press brakes predominate in shops that process heavy-gage materials, because they are well suited to such applications and also because they are adaptable to other metalworking operations, such as punching, piercing, blanking, notching, perforating, embossing, shearing, and drawing.Light-gage metal typically is formed with specialized bending machines, which are also described as leaf, pan, or box brakes; as wing folders; and as swivel bender. Equipment of this type is often manually operated.The principal kinds of equipment used to bend sheet metal and plate can be grouped into the following categories: 1. Mechanical press brakes-elongated presses with numerous tooling options. Work is performed by means of energy released from a motor-driven flywheel. These machines normally have a 3” or 4” stroke length.2. Hydraulic press brakesstretched C-frame presses that are likewise compatible with a wide range and diversity of tooling. High-pressure oil in hydraulic cylinders supplies the force, which is directed downward in most models. The stroking length usually exceeds 6”.3. Hydraulic-mechanical press brakespresses with drives that combine hydraulic and mechanical principles. In operation, oil forces a piston to move arms that push the ram toward the bed.4. Pneumatic press brakeslowtonnage bending machines that are available with suitable tooling options.5. Bending brakespowered or manual brakes commonly used for bending ligh-gage sheet metal.6. Special equipmentcustom-built bender and panel formers designed for spwcific firming applications.Bend allowanceBend allowance is the dimensional amount added to a part through elongation during the bending process. It is used as a key factor in determining the initial blank size.The length of the neutral axis or bend allowance is the length of the blank. Since the length of the neutral axis depends upon its position within the bend area, and this position is dictated by the material type and thickness and the radius and degree of bend, it is impossible to use one formula for all conditions. However, for simplicity, a reasonable approximation with sufficient accuracy for practical usage when air bending is given by the following equation:or where:L=bend allowance (arc length of the neutral axis) in. or mmA=bend angle, degR=inside radius of part, in. or mmt=metal thichness, in. or mmk=constant, neutral-axis locationTheoretically, the neutral axis follows a parabolic arc in the bend region; therefore, the k factor is an average value that is sufficiently accurate for practical applications. A value of 0.5 for k places the neutral axis exactly in the center of the metal. This figure is often used for some thicknesses. One manufacturer specifies k according to sheet thichness and inside radius of the bend; when R is less than 2t, k=0.33; when R is 2t or more, k=0.50.Types of bending The basic types of bending applicable to sheet metal forming are straight bending, flange bending and contour bending. Straight bending During the forming of a straight bend the inner grains are compressed and the outer grains are elongated in the bend zone. Tensile strain builds up in the outer grains and increases with the decreasing bend radius. Therefore, the minimum bend radius is an important quantity in straight bending since it determines the limit of bending beyond which splitting occurs. Flange Bending Flange bend forming consists of forming shrink and stretch flange as illustrated. This type of bending is normally produced on a hydrostatic or rubber-par press at room temperature for materials such as aluminum and light-gage steel. Parts requiring very little handwork are produced if the flange height and free-form-radius requirements are not severe. However, forming metals with low modulus of elasticity to yield strength ratios, such as magnesium and titanium, may result in undesirable buckling and springback. Also, splitting may result during stretch-flange forming as a function of material elongation. Elevated temperatures utilized during the bending operation enhance part formability and definition by increasing the material ductility and lowering the yield strength, providing less spring back and buckling. Contour Bending Single-contour bending is performed on a three-roll bender or by using special feeding devices with a conventional press brake. Higher production rates are attained using a three-roll bending machine. Contour radii are generally quite large; forming limits are not a factor. However, springback is a factor because of the residual-stress buildup in the part; therefore, overforming is necessary to produce a part within tolerance. Stretch Bending Stretch bending is probably the most sophisticated bending method and requires expensive tooling and machines. Furthermore, stretch bending requires lengths of material beyond the desired shape to permit gripping and pulling. The material is stretched longitudinally, past its elastic limit by pulling both ends and then wrapping around the bending form. This method is used primarily for bending irregular shapes; it is generally not used for high production. From Modern Manufacturing Process by D. L. Goetsch 薄板与板材的弯曲 弯曲是一种通过给金属施加超出其屈服强度但不超过其极限抗拉强度的压力来引起变形的方法。在弯曲过程中施加的力与金属薄板的切割一样，方向相反。但是，弯曲方向远处展开，引起在谨慎古的塑性扭曲而不会破坏。 弯曲过程似乎简单，但事实上，它是一种包含很多技术因素的相当复杂的过程。包含的因素有工件材料的特性、各变形阶段材料的流动和反应、工具设计对于成形弯曲所需要力的影响以及使用设备的类型。 金属薄板与板材的加工领域范围大、变化大，使用了几类弯板机。压弯机在加工大厚度板材的车间占优势，不仅因为它们和适合这样用，还业务它们适合于其他金属加工工序，如冲孔、落料、开缺口、穿孔、压花、剪边和拉延。 小厚度板材典型的成型方式是事业专用弯板机，也被称为薄板机、盘子或盒子压弯机；称为弯边机以及转盘弯折机。这种类型的设备常常由手工操作。 用于薄板与板材弯曲的机器主要类型可分为以下几类：1. 机械压弯机能选择多种工艺装置的延长了的压力机。由马达驱动的飞轮释放的能量来作功。这些机器通常具有3至4的行程长度。2. 液压式压弯机拉伸的C形架弯折机，也可兼容广泛的、多样的工艺装置。液压油缸里的高压油提供力，在大多数模型中力是向下的。行程长度通常超过6。3. 液压-机械式弯板机将液压与机械原理字和起来驱动的压力机。运行时，油液迫使活塞移动工作臂。工作臂推动推杆移向床身。4. 气动压弯机小吨位的弯板机，有适合的工艺装置选项。5. 压弯机动力或人力压弯机，通常用于弯曲小厚度金属薄板。6. 专用设备定制的折弯机以及为特殊成型用所设计的面板成形机。弯曲公差 弯曲公差是在弯曲过程中通过延长使部件尺寸增加的量。在确定毛坯的初始尺寸时，它被作为一个关键因素。 中心轴的长度或者弯曲公差的长度即为毛坯的长度。既然中心轴的长度取决于其所在弯曲区域内的位置，这一位置由材料的类型和厚度以及弯曲的半径和程度来确定，就不可能把一个公式用于所有情况。但是，为了简化，在气动弯曲时实际使用的具有足够精度的合理近似值由下面的方程给出： L=A/3602（R+kt）或 L=0.017453A（R+kt）其中：L=弯曲公差（中性轴的弧长）英寸或毫米A=弯曲角，度数R=部件内径，英寸或毫米t=金属厚度，英寸或毫米k=常数，中心轴位置理论上讲，中心轴在弯曲区呈抛物线状的弧形；因此，k因子是对于实际应用来讲足够精确的一个平均值。K值为0.5时，中性轴精确地位于金属的中心。该数常用于一定厚度的金属。一个制造厂按照薄板的厚度和弯曲内径来规定k值；当R小于2t时，k=0.33；当R等于或大于2t时，k=0.50。弯曲的类型 使用于金属薄板成形的基本的弯曲类型有直线弯曲、凸缘弯曲和成形弯曲。 直线弯曲 在直线弯曲件的成形过程中，在弯曲区的内侧晶粒受到压缩而外侧晶粒受到拉伸。拉伸应变在外侧经理产生并随弯曲半径的减小而增大。因此，最小弯曲半径是直线弯曲中很重要的量，因为它确定了弯曲极限，超过就会发生撕裂。 凸缘弯曲 凸缘弯曲成形由收缩凸缘成形和拉伸凸缘成形组成。这种类型的弯曲通常在室温下在液压或胶垫压力机上加工，如铝和小厚度钢等材料。 如果凸缘的高度和自由成形半径要求不高，用它来制造部件需要很少的手工工作。但是，对于具有较低弹性模量去强度比的成形金属，如镁和钛，可能产生不良的翘曲和回弹。而且，由于材料的延长作用，在拉身凸缘成形过程中可能引起撕裂。在弯曲工序中，利用提高温度，通过增加材料的延展性及降低屈服强度来增强部件的可成形性和边界成形，减少回弹和翘。 成形弯曲 单向成形弯曲是在一个三锟式压力机或使用专用进给设备与传统的压弯机。使用三锟式压力机可获得较高的生产效率。弯曲半径一般较大；成形限制不是一个要素。然而，回弹是一个要素，因为在部件内积聚了残余应力；因此，有必要过量成形以制造一个在公差反内的部件。 拉伸弯曲 拉伸弯曲可能是最复杂的弯曲方法，而且需要最昂贵的工艺装置和机器。而且，拉社弯曲需要材料的长度超过所许形状，好用来夹紧和拉拽。通过拉两端以及缠绕弯曲成形模，材料被纵向拉伸超过其弹性极限。这种方法主要用于不规则形状的弯曲；一般不用于大量生产。