Mechanics Of Sheet Metal Bending

To understand the mechanics of sheet metal bending, an understanding of the material properties, characteristics and behaviors of metal, is necessary. Particularly important is the topic of elastic and plastic deformation of metal. Information on the properties of metals, with relation to manufacturing, can be found in an earlier section, (metal forming). It should be understood also that sheet metal bending produces localized plastic deformation and essentially no change in sheet thickness, for most operations. It does not create metal flow that affects regions away from the bend.

The force required to perform a bend is largely dependent upon the bend and the specific metal bending process, because the mechanics of each process can vary considerably. Proper lubrication is essential to controlling forces and has an effect on the process. In punch and die operations, the size of the die opening is a major factor in the force necessary to perform the bending. Increasing the size of the die opening will decrease the necessary bending force. As the sheet metal is bent, the force needed will change. Usually it is important to determine the maximum necessary bending force, to access machine capacity requirements.

The important factors influencing the mechanics of bending are material, sheet thickness, width over which bend occurs, radius of bend, bend angle, machinery, tooling and specific metal bending process. Bending a sheet will create forces that act in the bend region and through the thickness of the sheet. The material towards the outside of the bend is in tension and the material towards the inside is in compression. Tension and compression are opposite, therefore when moving from one to the other a zero region must exist. At this zero region no forces are exerted on the material. When sheet metal bending, this zero region occurs along a continuous plane within the part’s thickness, called the neutral axis. The location of this axis will depend on the different bending and sheet metal factors. However, a generic approximation for the location of the axis could be 40 percent of sheet thickness, measured from the inside of the bend. Another characteristic of the neutral axis is that because of the lack of forces, the length of the neutral axis remains the same. Fundamentally, to one side of the neutral axis the material is in tension, to the other side the material is in compression. The magnitude of the tension or compression increases with increasing distance from the axis.

If a relatively small amount of force is exerted on a metal part, it will deform elastically and recover its shape, when the force is removed. In order for plastic deformation of metal to occur, a minimum threshold of force must be reached. The force acting on the neutral axis is zero and increases with distance from this region. The minimum threshold of force required for plastic deformation is not reached until a certain distance from the neutral axis in either direction. The material between these regions is only plastically deformed, due to the low magnitude of forces. These regions run parallel to, and form an elastic core around, the neutral axis.

When the force used to create the bend is removed, the recovery of the elastic region results in the occurrence ofspringback. Springback is the partial recovery of the work from the bend to its geometry before the bending force was applied. The magnitude of springback depends largely on the modulus of elasticity and the yield strength of the material. Typically the results of springback will only act to increase the bend angle by a few degrees, however, all sheet metal bending processes must consider the factor of springback.

Tip 1: Methods Of Eliminating Springback

Techniques have been developed, in manufacturing industry, that can eliminate the effects of springback. One common technique is over bending. The amount of springback is calculated and the sheet metal is over bent to a smaller bend angle than needed. Recovery of the material from springback results in a calculated increase in bend angle. This increase makes the recovered bend angle exactly what was originally planned.

Another method for eliminating springback is by plastically deforming the material in the bend region. Localized compressive forces between the punch and die in that area will plastically deform the elastic core, preventing springback. This can be done by applying additional force through the tip of the punch after completion of bending. A technique known as bottoming, or bottoming the punch.

Stretch forming is a metal bending technique that eliminates most of the springback in a bend. Subjecting the work to tensile stress while bending will force the elastic region to be plastically deformed. Stretch forming can not be performed for some complex bends and for very sharp angles. The amount of tension must be controlled to avoid cracking of the sheet metal. Stretch forming is a process often used in the aircraft building industry.

STip 2: heet Metal Bendability

Bendability of sheet metal is the characteristic degree to which a particular sheet metal part can be bent without failure. Bendability is related to the more general term of formability, discussed in the sheet metal forming section. The bendability will change for different materials and sheet thicknesses. Also, the mechanics of the manufacturing process will affect bendability, since different tooling and sheet geometries will cause different force distributions.

Metal bending tends to be a less complicated process than deep drawing in the analysis of forces acting during the operation. One simple method to quantify bendability is to bend a rectangular sheet metal specimen until it cracks on the outer surface. The radius of bend at which cracking first occurs is called the minimum bend radius. Minimum bend radius is often expressed in terms of sheet thickness, (ie. 2T, 4T). The higher the minimum bend radius, the lower the bendability. A minimum bend radius of 0 indicates that the sheet can be folded over on itself. Anisotropy of the sheet metal is an important factor in bending. If the sheet is anisotropic the bending should be performed in the preferred direction. A test to determine anisotropy is discussed in the sheet metal forming section.

The condition of a sheet metal’s edges will influence bendability. Often cracks may propagate from the edges. Rough edges can decrease the bendability of a sheet metal part. Cold working at the edges, or within a part, can also reduce bendability. Vacancies within sheet metal can be another source of material failure while bending. The presence of vacancies will reduce metal bendability. Impurities in the material, particularly in the form of inclusions, can also propagate cracks and will decrease bendability. Pointed or sharply shaped inclusions are more detrimental to bendability than round inclusions. Surface quality of the sheet metal can make a difference in bending manufacture. Rough surfaces can increase the likelihood of the sheet cracking under force.

To mitigate these problems, and optimize the bendability of sheet metal, care should be taken all the way through the manufacturing process. High quality sheet metal comes from high quality metal. Effective refining techniques, along with a sound sheet metal rolling process should close up vacancies, break up or eliminate inclusions and provide a sheet metal product with a smooth surface. Edge treatment such as trimming, or fine blanking, can improve edge quality. Sometimes cold worked areas can be machined out. Annealing the part to eliminate regions of cold working and increase ductility also improves metal bendability. Bending operations are sometimes performed on heated parts, because heating will cause the metal’s bendability to go up. Sheet metal may also, on occasion, be formed in a high pressure environment, which is another way to make it more bendable.

Tip 3: Cutting And Bending Processes

Some manufacturing processes involve both cutting and bending of the sheet metal. Lancing is a process that cuts and bends the sheet to create a raised geometry. Lancing may be used to increase the heat dissipation capacity of sheet metal parts, for example. Another common process that employs both cutting and bending is piercing. Not to be confused with the forging process of piercing. Piercing is used to create a hole in a sheet metal part. Unlike blanking, which creates a slug, piercing does not remove material. The punch is pointed and can pierce the sheet. As the punch widens the hole the material is bent into an internal flange for the hole. This flange may be useful for some applications.

Tip 4: Metal Tube Bulging

Tube bulging is a sheet metal manufacturing process in which some part of the internal geometry of a hollow metal tube is subjected to pressure, causing the tube to bulge outward. The area being bulged is usually constrained within a die that can control its geometry. Total length of the tube will be decreased because of the widening of the bulging area. There are different metal bulging techniques employed in manufacturing industry.

One main group of processes uses an elastomer plug, usually polyurethane. This plug is placed within the tube. Pressure is applied to the elastomer causing it to bulge. Expanding outward, the plug bends the sheet metal tube. Upon removal of the force, the elastomer plug returns to its original shape and can be easily removed. Polyurethane plugs are durable and will create a good pressure distribution over the surface during bending. Hydraulic pressure may also be used to produce the same bulging effect. However elastomer plugs are cleaner, easy to remove and require less complicated tooling. Split dies are used to facilitate the removal of the part.