Mechanics Of Metal Sheet 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. 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 sheet metal bend is largely dependent upon the bending 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 processing of metal bending. In pressbrakes 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 metal material bending are material, sheet thickness, width over which bend occurs, radius of bend, bend angle, machinery, pressbrake tooling and specific metal bending process. Bending a sheet metal 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.
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