Sheet metal springback is a natural phenomenon that occurs when the pressure from the punch is released. The material partially returns to its original shape; therefore, the actual angle of the finished part may differ from the angle set on the machine. For example, a part bent to 88° may open up to the required 90° once it is removed from the die.
Springback is a natural phenomenon that occurs once the pressure from the punch is released. The material partially returns to its original shape, which is why the actual angle of the finished part may differ from the angle set on the machine. For example, a part bent to 88° may open out to the required 90° once removed from the die.
In a single-off production run, a slight deviation is often not a problem. In mass production, however, it affects assembly, the tightness of joints, the geometry of housings and the appearance of the finished product. An incorrect angle may make it difficult to screw components together, fit fittings or assemble parts following subsequent manufacturing operations.
Springback cannot be completely eliminated, but its impact on the bending result can be minimised. The key factors are: the type of material, its thickness, the bending radius, the choice of tools, the working method and the correct settings of the CNC press brake.
What is springback in sheet metal?
During bending, the outer layer of the sheet metal is stretched, whilst the inner layer is compressed. When the pressure is released, some of the residual stresses subside. The material then tends to return partially to its original shape, which causes a change in the angle.
In practice, this means that the workpiece often needs to be bent slightly more than the final dimension specifies. This adjustment is known as over-bending. Its value is not constant, as it depends on the material, thickness, V-groove width, punch radius and the way the process is carried out.
The same press brake, the same punch and the same die may produce different results for carbon steel, stainless steel and aluminium. Therefore, for precision parts, one should not rely solely on the default settings. A trial run on an actual batch of material is essential.
Material and its properties
The type of material is one of the most important factors affecting the springback of sheet metal. Standard structural steel usually behaves predictably and allows a consistent angle to be achieved easily. Stainless steel is more springy, so it often requires a greater bend angle for the same thickness.
Aluminium can also spring back significantly, particularly in harder alloys. At the same time, it is more susceptible to surface damage. Attempting to minimise springback by reducing the radius too aggressively can lead to cracks, scratches or surface deformation.
The yield strength of the material is also significant. The higher the strength of the sheet, the greater the proportion of energy that can be recovered after unloading. High-strength sheets, stainless steel and certain aluminium alloys therefore require greater process control than ordinary carbon steel.
Sheet thickness and actual material dimensions
Sheet thickness affects the inner radius, the required pressure and the final angle after stress relief. In mass production, it is important to bear in mind that the nominal thickness of the material does not always correspond to the actual value. Sheet metal described as 2 mm may have a slight deviation, which will alter its behaviour during bending.
If the next batch of material comes from a different supplier or has different properties, previously saved adjustments may need to be checked. This is precisely why inspecting the first component after a change in sheet metal batch is very important. It allows settings to be corrected quickly before a larger number of non-conforming components are produced.
Rolling direction and part preparation
The bending result can be influenced by the rolling direction of the sheet metal. The material has a different structure along and across the rolling direction. For demanding parts, it is advisable to maintain a consistent sheet orientation so that subsequent parts are bent under comparable conditions.
Bending alternately along and across the rolling direction can result in slight differences in the bend angle, even with the same press settings. In series production, clearly marking the direction of the material and following a consistent workpiece preparation procedure are helpful.
Before bending, remove any coarse contaminants, chips and cutting irregularities from the surface. An edge prepared following incorrect cutting may not lie correctly on the die, which increases process variability.
Bending radius and die selection
The bending radius has a direct impact on springback. With a larger radius, the material is deformed more gently and therefore returns more easily to its original shape. A smaller radius usually reduces springback, but requires greater force and may increase the risk of cracking on the outer side of the bend.
The punch radius should be matched to the type of sheet metal, its thickness and the requirements of the finished part. A radius that is too small may damage the material, whilst one that is too large may increase the angular deviation after stress relief.
The width of the V-groove in the die is also important. A wider die opening usually increases the bending radius and may cause greater springback. A narrower die allows the material to be deformed more deeply, but increases the required pressure. A V-groove that is too small may overload the machine, the tools and the workpiece.
Tool selection should be based on the manufacturer’s tooling table, material data and trials on actual workpieces. Particularly with long workpieces, the correct punch–die combination helps to maintain the angle along the entire length of the bend.
Pre-bending as the primary angle correction
The most commonly used method of compensating for springback is pre-bending. If the final part is to have a 90° bend, the operator sets a smaller angle, for example 87° or 88°. Once the pressure is released, the material opens up slightly and reaches a value close to the required one.
The exact value of the over-bend should be determined through trial runs. It is not advisable to apply a single correction for all materials. Stainless steel, aluminium and high-strength sheets may require different settings to structural steel of the same thickness.
A CNC press brake makes the work easier, as it allows proven adjustments to be saved in the programme. This means that subsequent batches can be produced more quickly and with greater repeatability.
Bending method versus springback
In free-air bending, the sheet metal comes into contact mainly with the edges of the die and the tip of the punch. This is a fast, flexible method commonly used in production. It allows different angles to be achieved with the same set of tools, but is more susceptible to variations arising from the material’s properties.
Full-depth bending provides greater control, as the material is pressed more firmly into the die’s shape. It can reduce springback compared to free-air bending, but requires more precisely matched tooling.
Coining produces the least springback. In this method, the material is heavily deformed in the bend area. However, this requires significantly greater pressure, which is why it is not always economically or technologically viable.
CNC press brake settings
A modern CNC press brake allows programmes to be saved for specific materials and workpieces. The operator can set the angle correction, the position of the stops, the punch depth and the compensation parameters. This makes it easier to reproduce tried-and-tested settings for subsequent batches.
Even a well-prepared programme is no substitute for inspecting the first workpiece. After the first bend, the angle should be measured, compared with the specification and, if necessary, corrected. In mass production, it is advisable to check samples during the process as well, particularly after a change in material batch or tools.
For long components, beam deflection compensation is of great importance. Without it, the workpiece may have a different angle at the ends and a different one in the middle. A mechanical or CNC compensation system helps to distribute the pressure evenly along the entire length of the bend.
Condition of tools and workstation preparation
The punch and die must be clean, undamaged and correctly aligned. Swarf, dirt, marks from previous machining operations or damage to the working surfaces can affect the bending result. In the case of precision parts, even the slightest unevenness can alter the final angle.
You should also check that the sheet metal is correctly supported against the stops. Incorrect positioning causes the bend line to shift. This can lead to assembly errors even when the angle itself is correct.
Regular inspection of tools, keeping the worktable clean and ensuring consistent part positioning help to minimise process variability. In mass production, it is advisable to treat these activities as an integral part of the production process, rather than as an additional check carried out only after a problem has arisen.
How can springback be minimised in practice?
The best results are achieved by combining several measures. The bending radius and die width must be selected to suit the type of sheet metal, a test bend must be carried out on the actual material, and the verified correction must be recorded in the press programme.
- select tools appropriate for the thickness and grade of the sheet metal,
- check the actual thickness of the material,
- take the rolling direction into account,
- carry out a test bend and measure the first part,
- apply the appropriate overbend,
- for long parts, use deflection compensation,
- check the condition of the punch, die and stops regularly.
In the case of difficult-to-bend materials, it is worth testing different bending methods and assessing which one ensures the smallest deviation whilst maintaining the safety of the workpiece and the tools.
Summary
Springback in sheet metal is a natural consequence of the bending process. Its extent depends on the material properties, thickness, bend radius, die width, working method and machine settings. It cannot be completely eliminated, but its effects can be effectively compensated for.
The most important factors are technical trials, the correct selection of tools and inspection of the first workpiece. A well-adjusted CNC press brake, tried-and-tested tools and saved adjustments help to maintain repeatability, minimise waste and speed up the production of bent components.
