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Sheet and Plate Bending
Bending 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 brakes—stretched 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 brakes—presses 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 brakes—low—tonnage bending machines that are available with suitable tooling options.
5. Bending brakes—powered or manual brakes commonly used for bending ligh-gage sheet metal.
6. Special equipment—custom-built bender and panel formers designed for spwcific firming applications.
Bend allowance
Bend 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 mm
A=bend angle, deg
R=inside radius of part, in. or mm
t=metal thichness, in. or mm
k=constant, neutral-axis location
Theoretically, 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
薄板與板材的彎曲
彎曲是一種通過給金屬施加超出其屈服強度但不超過其極限抗拉強度的壓力來引起變形的方法。在彎曲過程中施加的力與金屬薄板的切割一樣，方向相反。但是，彎曲方向遠處展開，引起在謹慎古的塑性扭曲而不會破壞。
彎曲過程似乎簡單，但事實上，它是一種包含很多技術(shù)因素的相當復(fù)雜的過程。包含的因素有工件材料的特性、各變形階段材料的流動和反應(yīng)、工具設(shè)計對于成形彎曲所需要力的影響以及使用設(shè)備的類型。
金屬薄板與板材的加工領(lǐng)域范圍大、變化大，使用了幾類彎板機。壓彎機在加工大厚度板材的車間占優(yōu)勢，不僅因為它們和適合這樣用，還業(yè)務(wù)它們適合于其他金屬加工工序，如沖孔、落料、開缺口、穿孔、壓花、剪邊和拉延。
小厚度板材典型的成型方式是事業(yè)專用彎板機，也被稱為薄板機、盤子或盒子壓彎機；稱為彎邊機以及轉(zhuǎn)盤彎折機。這種類型的設(shè)備常常由手工操作。
用于薄板與板材彎曲的機器主要類型可分為以下幾類：
1. 機械壓彎機——能選擇多種工藝裝置的延長了的壓力機。由馬達驅(qū)動的飛輪釋放的能量來作功。這些機器通常具有3＂至4＂的行程長度。
2. 液壓式壓彎機——拉伸的C形架彎折機，也可兼容廣泛的、多樣的工藝裝置。液壓油缸里的高壓油提供力，在大多數(shù)模型中力是向下的。行程長度通常超過6＂。
3. 液壓-機械式彎板機——將液壓與機械原理字和起來驅(qū)動的壓力機。運行時，油液迫使活塞移動工作臂。工作臂推動推桿移向床身。
4. 氣動壓彎機——小噸位的彎板機，有適合的工藝裝置選項。
5. 壓彎機——動力或人力壓彎機，通常用于彎曲小厚度金屬薄板。
6. 專用設(shè)備——定制的折彎機以及為特殊成型用所設(shè)計的面板成形機。
彎曲公差
彎曲公差是在彎曲過程中通過延長使部件尺寸增加的量。在確定毛坯的初始尺寸時，它被作為一個關(guān)鍵因素。
中心軸的長度或者彎曲公差的長度即為毛坯的長度。既然中心軸的長度取決于其所在彎曲區(qū)域內(nèi)的位置，這一位置由材料的類型和厚度以及彎曲的半徑和程度來確定，就不可能把一個公式用于所有情況。但是，為了簡化，在氣動彎曲時實際使用的具有足夠精度的合理近似值由下面的方程給出：
L=A/3602π（R+kt）
或
L=0.017453A（R+kt）
其中：
L=彎曲公差（中性軸的弧長）英寸或毫米
A=彎曲角，度數(shù)
R=部件內(nèi)徑，英寸或毫米
t=金屬厚度，英寸或毫米
k=常數(shù)，中心軸位置
理論上講，中心軸在彎曲區(qū)呈拋物線狀的弧形；因此，k因子是對于實際應(yīng)用來講足夠精確的一個平均值。K值為0.5時，中性軸精確地位于金屬的中心。該數(shù)常用于一定厚度的金屬。一個制造廠按照薄板的厚度和彎曲內(nèi)徑來規(guī)定k值；當R小于2t時，k=0.33；當R等于或大于2t時，k=0.50。
彎曲的類型
使用于金屬薄板成形的基本的彎曲類型有直線彎曲、凸緣彎曲和成形彎曲。
直線彎曲 在直線彎曲件的成形過程中，在彎曲區(qū)的內(nèi)側(cè)晶粒受到壓縮而外側(cè)晶粒受到拉伸。拉伸應(yīng)變在外側(cè)經(jīng)理產(chǎn)生并隨彎曲半徑的減小而增大。因此，最小彎曲半徑是直線彎曲中很重要的量，因為它確定了彎曲極限，超過就會發(fā)生撕裂。
凸緣彎曲 凸緣彎曲成形由收縮凸緣成形和拉伸凸緣成形組成。這種類型的彎曲通常在室溫下在液壓或膠墊壓力機上加工，如鋁和小厚度鋼等材料。
如果凸緣的高度和自由成形半徑要求不高，用它來制造部件需要很少的手工工作。但是，對于具有較低彈性模量去強度比的成形金屬，如鎂和鈦，可能產(chǎn)生不良的翹曲和回彈。而且，由于材料的延長作用，在拉身凸緣成形過程中可能引起撕裂。在彎曲工序中，利用提高溫度，通過增加材料的延展性及降低屈服強度來增強部件的可成形性和邊界成形，減少回彈和翹。
成形彎曲 單向成形彎曲是在一個三錕式壓力機或使用專用進給設(shè)備與傳統(tǒng)的壓彎機。使用三錕式壓力機可獲得較高的生產(chǎn)效率。彎曲半徑一般較大；成形限制不是一個要素。然而，回彈是一個要素，因為在部件內(nèi)積聚了殘余應(yīng)力；因此，有必要過量成形以制造一個在公差反內(nèi)的部件。
拉伸彎曲 拉伸彎曲可能是最復(fù)雜的彎曲方法，而且需要最昂貴的工藝裝置和機器。而且，拉社彎曲需要材料的長度超過所許形狀，好用來夾緊和拉拽。通過拉兩端以及纏繞彎曲成形模，材料被縱向拉伸超過其彈性極限。這種方法主要用于不規(guī)則形狀的彎曲；一般不用于大量生產(chǎn)。