Sheet material itself, its properties and especially the variations in these properties, can influence the press brake bending process.
Sheet-metal, produced on large rolling mills, undergoes hot or cold rolling to reach final thickness: hot rolling typically for thicker sheet and cold rolling for thinner sheets due to the high loss of heat and difficulty in maintaining constant temperature in thin material.
Also, cold rolling better controls thickness tolerances and causes hardening of the surface layer.
Rolling stretches the crystal structure, causing material to acquire different mechanical properties across its length than across its width.
In other words, the material becomes anisotropic, and this affects the subsequent processing.
During bending, this can lead to variations in the bend angle.
Apart from this anisotropic nature, unavoidable variations occur in material properties as a result of minute differences in material composition and rolling conditions.
This also results in variations in stress/strain curves, not only between different batches of sheet materials, but even within a single batch.
Springback is the phenomenon by which sheet rebounds on either side of the bend after the bending tool has been removed. Why?
In the center of the sheet-not exactly the geometrical center, but close to it-resides a zone with low stress in which, even under large bend forces, only elastic deformation occurs.
This part of the sheet’s cross-section, therefore, wants to return to its original shape after bend force is lifted.
The extent to which springback occurs depends on the nature of the sheet material: The stiffer the material, the greater the spring-back.
Soft materials exhibit springback limited to no more than 0.5 deg., and steel to 1 deg., but springback in stain-less steel can amount to as much as 3 deg.
Bend angle also is a determining factor. The smaller the relative effect on the elastic area in the neutral zone, the smaller the springback.
This is the case with small bend angles and small bend radii (meaning a sharp tool).
For example, a steel sheet 0.8 mm thick bent with a bend radius of 1S exhibits spring-back of 0.5 to 1 deg.
The same sheet bent with a bending radius of 77S results in springback of as much as 30 deg.
With a leg length of 100 mm, each degree of deviation will mean that the end of the sheet will have a spatial deviation of 1.7 mm.
For post-processing, such as robotic welding, a deviation of this size will soon exceed acceptable tolerance limits.
In practice, it is relatively easy to correct for springback when bending a sheet, providing that influential parameters are known.
For calculating springback for cold-rolled steel, a formula offered by Benson is D = R / (2.1 x S).
where R is the radius of the angle in mm and S is the sheet thickness in mm.
Using this formula, a steel sheet 0.8 mm thick, and given a bend radius of 20 mm and a bend angle of 90 deg., has a springback value of 11.9 deg.
To calculate springback for other materials, Benson uses a correction factor (0.5 for copper, 0.75 for hot-rolled steel and 2.0 for stainless steel).
Keep in mind that under certain air bending conditions, negative spring-back can occur, particularly when employing dull tools in combination with a large punch angle as deformations then can occur in the sheet between the punch and die surface.
When coining, given high pressing pres-sure and a sharp top tool, this tool can press into the sheet past the neutral zone.
In that case, the plastic phase is achieved everywhere and springback is reduced to virtually zero.
Galling of the bend tool-particles of material or part flakes cling to spots on the tool during bending-is especially a concern with the bend radii in the bottom tool.
Galling can result in damage to tools and to the sheet surface.
This problem can be minimized by selecting an optimum bend radius for the V-die (Fig. 7) and by hardening the relevant bend radius.
Hardened surfaces are much less sensitive to galling.
When high tonnage is exerted, deflection unavoidably occurs lengthwise in the top and bottom tools.
As a result, the top and bottom tools no longer remain parallel during the bend process, bringing variations in the bend angle over the length of the product (Fig. 8).
This adversely affects post-bend processes such as robotic welding.
In the past, this problem often was remedied by shimming the bottom tool to acquire a crowning that compensated for the deflection.
Today, computer-controlled or centrally adjustable crowning systems quickly and accurately compensate for deflection over the entire machine length.