20 questions about bending

Bending steel is the process of permanently transforming a flat steel member into a curve or angle. In steel processing, bending creates a permanent deformation of the material.

Three differently designed machines are used to obtain the required shape of an originally flat plate. Roll forming results in an open or closed section. Roll bending gives a circular section, truncated cone section or a shape with variable radius. Press braking creates a linear deformation. 

Roll forming

Roll bending

Press braking

Materials with high plasticity are better for forming. High plasticity means that the material more easily withstands sustainable deformation. The material’s ductility is also an important property. Good ductility means that a material can undergo a change of physical form without losing its mechanical properties.

What is most important when understanding the maximum hardness or maximum thickness, is the ability to be plastically (permanently) deformed without cracking. This property often goes by the name of ductility.

The basic principle for bending is that you need a force for influencing a permanent deformation in the material, and a support for the steel. For press braking, terms commonly used are punch and die. 

The key to obtaining a high quality bend in steel is to avoid or eliminate all notches in the machine and in the plate to be bent. Notches are crack initiation sites that might trigger cracks. The machine must be well maintained and lubricated. The plate edges should be ground to remove notches, in particular in the case of a large bending deformation. Plate surface notches close to the intended bending line should be removed.

Carefully fill in the plate and machine parameters as requested by the app SSAB BendCalc, run it and it will provide a set of recommendations to apply. To get the best result, adjust the process or parameters if needed. 

The SSAB BendCalc mobile app can be used to calculate all parameters needed for a successful bending operation. 

The most common dies are V-dies and U-dies, see pictures.

Bending will somewhat increase the original length of the workpiece, perpendicular to the bending line. Bend allowance is the extra material required to accommodate the bend in the material. 

During the bending operation, the workpiece will be deformed elastically and plastically. When the bending force is released, the elastic deformation will be reversed to zero, thus decreasing the bending angle. The press brake operator must overbend to the bending angle, which is angularly past the required bent angle, thereby compensating for the springback angle. 

During bending, the material will be stretched on one side and be compressed on the other. This means at a point between the sides, the deformation will be zero. This is the neutral axis.

The purpose of bending a plate is to obtain an angle between the parts on either side of the bending line. This is the bend angle.

Provided the available bending force is sufficient and the machine dimensions are compatible with the size of the workpiece, it is up to the operator to run the punch such that the desired bend angle is obtained after release of the load. 

The final bend angle depends on:

Machine geometry and settings as well as workpiece properties do have an influence. The punch radius is the dominating parameter. “Trial and error” is the way to get the required bending result. 

The bending radius depends on parameters such as:

It depends on how ductile the material is. A very ductile steel workpiece can be folded like a piece of paper, whereas a brittle one may crack at minor bend angles.

The minimum punch radii recommended for SSAB steel grades are based on a large number of tests and can be found in our Bending brochure and SSAB BendCalc. Using a smaller radius than recommended will increase the risk for cracking.

Tensile and compressive residual stresses are present in the vicinity of the bending line and balance each other. 

Use the SSAB BendCalc to calculate the recommended minimum distance. Too short a distance can cause the workpiece to slip and result in serious physical harm. Safety must always come first.