Achieving accurate first-piece bending results
November 15, 2001
Adaptive bending allows press brake operators to measure a bend angle during the forming process and feed the information to the numerical control. The article discusses springback and how to determine it and the fact that when air bending, 90 percent of problems result during initial setup, and only 10 percent result from springback. It also discusses using an angle control system, methods of measuring angles, and requirements for angle measurement systems.
In conventional press brake bending, the bend angle obtained often differs from the programmed angle even though it is produced on a CNC machine.
To overcome this problem, the bend angle can be measured during the forming process and this information fed to the numerical control. This process is adaptive bending.
Operators often use air bending because of its flexibility; a single punch and die set allows them to form a range of bend angles without changing tools. Once they have chosen the tooling, operators must determine what position to program the punch in the Y axis (ram position) to obtain the correct bend angle. The material thickness (S) and material characteristics also are important considerations.
(A-Left) This bending radius is small because the material is bending around the punch. (B-Right) The higher the material's tensile strength is, the more springback there will be.
Press brake ram repeatability always has been a key element in producing an accurate bend angle. The focus now, however, is on the effects of material variations on maintaining critical angular tolerances.
Material variation — the nonuniformity and varying dimensional tolerances inherent in all materials — is a concern in achieving both accurate first pieces and consistent parts throughout a production run.
Many think that a bend angle's inconsistency is due to the amount of springback attributed to the variations in material type and thickness. Research shows that while springback does influence the accuracy of a bend angle, its role in this process is limited to approximately 10 percent of the overall problem.
When the punch contacts the sheet, the sheet begins to bend elastically. It exceeds the yield point quickly, and the flow curve as represented by the material's strain hardening determines the bending process's outcome.
In materials with low strain hardening, the yield stress remains at the same level until the punch's force overcomes it. The material flows first where the bending moment is the largest, which typically is in the center, leaving the adjacent material unaffected. The material bends around the punch, so the bending radius will be small, as shown in Figure 1A.
In materials with a high strain-hardening condition, the yield stress increases as material strain increases. The material flows first where the bending moment is the largest, but because of strain-hardening effects, the adjacent material offers more resistance, resulting in a larger bending radius, as depicted in Figure 1A.
Variations in strain hardening influence the bend angle. A 10 percent increase in strain hardening results in a bend angle that is 1.5 degrees smaller. The relationship between plate thickness and the bend angle obtained also is shown in Figure 1A. A plate thickness that is 10 percent greater results in a bend angle that is 3 degrees smaller.
Up to this point we have considered only the bend angle under strain. As soon as the strain is removed, the material springs back elastically and evolves toward a new state of equilibrium, resulting in the actual bend angle. The higher the material's tensile strength is, the more springback there will be.
Figure 1Billustrates this fact. If the tensile strength increases by 10 percent, the springback will be 0.2 degree higher. The thickness also influences the springback. If the thickness is 10 percent more, the springback will be 0.3 degree less.
Comparing Figures 1A and 1B illustrates that 90 percent of the problems in air bending result during the initial forming, and only 10 percent result from springback.
This angle measurement method uses a laser device with two receivers placed symmetrically toward it.
Figures 1A and 1B imply that for material with a large variation in thickness and tensile strength, a precise bend angle can't be guaranteed simply by moving the punch to the same Y-axis position. Only by controlling the punch position based on the measured bend angle is it possible to obtain the same bend angle on materials with varying properties.
An adaptive angle control system controls the angle in real time during the bending process. The system allows the machine to adapt to variations in material and compensates for them.
As the operator initiates the press brake's bending sequence, the sensing device transmits digital information in real time to the CNC unit, which processes it and subsequently recalculates the correct depth adjustment to obtain the correct angle. It does not interrupt the bending process or result in lost production time.
One way to measure the bend angle is to fit the die with half disks and measure their inclination. Another means of measuring the bend angle is with a laser device with two receivers placed symmetrically toward it (seeFigure 2). The device is set so that each receiver obtains the same intensity of reflected radiation. The device's inclination determines the bend angle.
This system of angle measurement determines the bend angle by measuring distance.
A number of systems have been developed to determine the bend angle by measuring distance. Knowing the geometry then permits users to calculate the bend angle (see Figure 3).
A third group of angle measurement systems is based on projection and image processing (see Figure 4). With longitudinal projection, the shadow of the bent sheet is recorded. A different technique uses a laser pattern projected onto the bent sheet and a camera to view the projected image.
This angle measurement system is based on projection and image processing.
If a slant line is projected, the projection will turn to the extent that the bend angle changes. If a cross, two points, or two lines are projected, the position or the distances between them will change. With knowledge of the sheet position, the laser source, and the camera, users then can determine the bend angle.
A new method of angle measurement is projecting a laser line on both the workpiece and the tooling (seeFigure 5). Two measuring systems, positioned in the front and back of the tooling, each measure the angle between the workpiece and the die. Each measuring system sees two rows of points within its measuring range.
One line is the projection on the sheet; the other line is the projection on the die. The gradient for both lines is determined within the axis coordinates of the camera. The difference between both gradients is the angle B, which has no relation to the position of the measuring system. Bend angle A = 360 - B1 - B2.
To be effective, an industrial angle measurement system should satisfy several requirements:
This angle measurement method involves projecting a laser line on the workpiece and the tooling.
The system should be able to transfer the measured values quickly to the numerical control so that it does not slow down the bending process. A data flow in the range of 100 measurements per second typically is required.
The angle measurement system should be set up so that it is not an obstacle for certain bends. It also should be able to measure the bend angle for short, upright edges and Z profiles.
Off-center bending does not affect this method of projection.
Setup time should be quick. Having to adjust the measuring system when the tooling or the bend angle changes is a disadvantage. A number of angle measurement systems require calibration after a tool change because the geometric condition of the setup has changed. This can be a time-consuming process. The newer laser angle control measurement devices do not require calibration after each setup.
An efficient angle measuring system should be able to function in a harsh working environment and should not be influenced by the tool's orientation or the material being formed.
Off-center bending requires correct angle measurement. Figure 6shows that a method of projection on the workpiece and the tooling is not influenced by off-center bending.
The springback data for stainless steel is stored in the machine's CNC.
In Figure 1, variations in material properties have a limited effect on the variation in springback. That's why it is common practice to use adaptive angle control only under strain and to apply an empirical value for the springback. Figure 7shows the springback data for stainless steel, stored in the CNC's database.
If the operator knows the characteristics of the material to be bent in advance, and the material properties are ±10 percent, the maximum error on the bend angle will be ±0.3 degree. It's important to know what type of material will be used for bending, as both the bend angle and bend allowance data depend on the material.
If the material tolerance and bend allowance data are too high, the developed part length will be wrong, causing faulty dimensions to the bent product that cannot be modified adaptively.
As demand increases to produce complex components to a higher degree of accuracy and repeatability, adaptive bending is an important means of producing accurate formed parts from the first blank and with minimal setup time.