How does the plate bending machine bend the plate?
It is not easy to bend a thick steel plate to a narrow radius. There are many variables to consider, such as thickness, surface and edge conditions, and the chemical composition of the material. Plate bending is a process in which a force is applied to the plate, causing deformation along the bending axis, thereby forcing the plate to bend at the desired angle. At the moment of bending, the material is under tension, where the material expands on the curved outer surface and compresses or shrinks inside.
After the bending force is released, the residual stress in the material will cause the plate to rebound. To compensate for spring back, the plate needs to be bent to a greater angle than required. Experienced operators can calculate the expected amount of rebound based on a variety of factors, which can help speed up material construction with fewer attempts.
Specific operation points
Normally, the plate bending is performed on the pressure brake. The bending machine contains two tools: an upper tool (called a “punch”) and a lower tool (called a “die”). The board to be bent must be placed on the die and held in place, while the punch is lowered onto the board and the force applied causes it to bend. The bending angle is controlled by the depth at which the punch presses the plate into the die. The most common plate bending is called V-bending, in which the punch and die tools used are V-shaped.
When the punch does not push the material all the way to the bottom of the mold cavity, and there is space below it, we will perform “air bending”. When the punch pushes the material all the way to the bottom of the mold cavity, we have the “bottom line “. The bottoming process can better control the bending angle to reduce spring back. It should also be remembered that when the bending machine bends, the thicker and harder the plate, the greater the minimum bending radius.
While sheet metal gauges run from 0.005 to 0.249 inches thick, aluminum and steel plate thicknesses start at 0.250 in. and go all the way up to 13 in. or even more. Likewise, plate steel varies in strength from mild varieties to some very high-strength materials such as Hardox®. When it comes to very thick or high-tensile-strength material, traditional rules for determining minimum bend radii, minimum punch nose radii, die openings, bending force calculations, and tooling requirements may no longer apply—at least not in the same way that they do when working with thinner gauges.
Because the workpiece can be extremely thick and strong, you need to understand the variables and learn how to work with them. First, consider the material’s chemical composition, its surface, and edge condition, as well as its thickness, and determine whether the bend is with or across the material’s grain direction.
All forming, regardless of scale, involves some kind of plastic deformation. Material expansion occurs on the outside surface of the bend, and compression on the inside, and you need to know how to deal with both. The limits of material ductility will be the controlling factor for the minimum bend radius.
The strains associated with the plastic deformation when cold forming can cause the material to strain-harden. This can change the material’s mechanical properties in the area of the bend, where plastic deformation is occurring. At this point, ductility and resistance to fracture will need to be considered.
Minimum bending radius
No matter the material, its gauge or thickness, mild steel, and soft aluminum are much more ductile than high-strength materials and, therefore, can be bent to a sharper radius. That’s why when bending thick or high-tensile metals, you need to abide by a minimum inside bend radius. This will minimize the effects of strain hardening and cracking at the bend.
The material supplier’s product data sheets normally outline the extent to which the plate can be formed without failures, recommending minimum bend radii by material type and properties. Generally, low-carbon-content steel or soft aluminum is necessary for good formability and a tight inside radius; but as the level of carbon in the steel or the hardness of the aluminum increases, its ductility and formability are limited, increasing the minimum radius that can be produced.