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薄板 板材 弯曲 曲折

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薄板与板材的弯曲,薄板,板材,弯曲,曲折

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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:)(2360ktRALor)(1745.twhere: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 bendingThe 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 an