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Note: This is Part I of a two-part series looking at metal bending basics in a stamping press. Read Part II here.
As a professional consultant, I am often asked to address die problems in metal bending operations, like not being able to achieve the proper bend angle, inconsistent bend angle from part to part, and cracking in the radius of a bent part. As simple as it may seem, bending can be very challenging.
One of the most common forming methods performed in metal stamping dies, bending involves deforming metal along a straight axis. (This makes it different from flanging, which uses a curved axis.) It can be used to obtain any desired bend angle, although 90-degree bends are most common.
Items such as tabs and channels are created using the bending process. When bending is used to create U-shaped parts, it’s called U forming or channel forming.
One of the biggest challenges in metal bending is springback. Also known as elastic recovery, it is the tendency for material to return to its original flat shape when subjected to deformation.
Metals such as copper and mild steel are softer and have lower springback values than their higher-strength counterparts, such as high-strength steel or spring steel. Regardless of the springback value of the metal, to achieve the proper final bend angle, you have to bend the metal past the desired bend angle and allow it to return to the correct angle. Tool- and diemakers and engineers commonly refer to this process as overbending.
Despite the efforts of steel manufacturers, it’s nearly impossible to keep the metal’s mechanical properties consistent throughout an entire coil, and those differing properties affect the amount of springback that occurs.
The higher the yield and tensile strength of the material, the more likely that springback values will increase, requiring greater overbending to compensate. Thickness also makes a difference: Thicker metals exhibit lower springback values than thinner metals of the same type, mainly because a larger volume of material has been deformed and work-hardened in the radial area. In addition, thicker material is inherently stiffer than thinner material, so it retains its original shape more effectively.
The size of the inside bend radius also has a large impact on the amount of springback that will occur during the bending process. Larger radii will result in greater springback values, while smaller radii reduce the amount of springback. If the radius is too small, however, it may result in the metal splitting on the outside radius where it is in the greatest amount of tension.
All coil material is rolled and has a grain direction. Bending or forming with respect to the grain direction will affect the amount of overbending required, as well as the likelihood of splitting in the radial area. When splitting is a concern, bending transverse (across the grain) to the rolling direction is more desirable than bending parallel to the rolling direction. Be sure to pay close attention to both the size of the inside bend radius and the bending direction with respect to the rolling direction, especially if the material is high-strength or has poor stretchability.
Metal deformation speed also affects the amount of springback that occurs. Remember that metals are sensitive to strain rate, meaning that different forming velocities result in different amounts of stretch and stretch distribution.
The type, amount, and severity of strain being used to create the bend are other variables. When metal is strained or work-hardened, springback decreases. Tensile strain and compressive stresses are generated naturally during bending as the metal is stretched and compressed, respectively. Strain also can be created, such as by coining, which is squeezing the metal between a punch and a die to reduce its thickness and cause it to work-harden.
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