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Better bend accuracy on the press brake through crowning

Crowning keeps things straight on the press brake

To guarantee accurate, repeatable forming results, it’s essential to compensate for the deflection that inherently occurs in the beam (ram) and table of the press brake when load is applied. Without deflection compensation, it’s likely that a workpiece will have some form of deformation at its center when it’s bent along the full length of the press brake. This is especially so for press brakes 8 feet or longer, 80 tons or more, and when bending long or large parts, but it also can be the case when forming shorter workpieces. To keep the bend angle consistent over the full length of the part, a press brake needs a crowning system, either in the beam, in the table, or in both.

In this age of short batches, complex parts, and quick turnaround, crowning ensures efficiency, part accuracy, and repeatability. In high-tonnage applications, crowning helps produce straight bends in challenging, high-tensile-strength materials.

Crowning Basics

Most modern press brakes use a servo-hydraulic system in which a closed-loop system controls the position of the ram. At each end of the beam, under the left and right pistons, a position-measuring system monitors the movement and positioning accuracy of the beam. The machine control processes this information and uses it to position the beam to an accuracy of 0.01 mm. It does this by activating the left and right servo valves at the top of each hydraulic piston.

During a bending cycle, the servo-hydraulic system ensures that the piston at each end of the beam reaches the programmed position established in the machine control, resulting in the correct bend angle. Because press brakes tend to deflect under load, particularly in the center of the machine between the pistons, the angle in the center of the press brake will be more obtuse than the angle underneath the ram. Without intervention, this deflection will cause the bend angle to vary over the length of the press brake.

To compensate, press brakes can be equipped with a crowning system that generates an opposite force, or “curve,” to offset the deflection of the beam and table. The crowning system ensures that the ram and table are parallel during bending (see Figure 1).

Crowning has been used since the late 1970s and early 1980s. The demand for higher-tonnage press brakes, high-tensile-strength steels, and the need to bend longer lengths have fueled the need for crowning, as machine frame deflection is greater in these applications.

Before crowning existed, press brake manufacturers had to pregrind a curve into the table of the press brake to offset about 75 percent of machine deflection. Today systems are more advanced. Using programmable crowning systems, operators enter the sheet thickness, length, die opening, and material tensile strength data into the control, which automatically determines the real deflection of the table and ram, and preloading is obtained for each bend. Together, the press brake’s hydraulic system and crowning achieve an accurate bend angle with minimal operator intervention.

Choices in Crowning

Crowning is generally used on press brakes 80-ton, 8 ft. or larger in size. Crowning can be applied to lower-tonnage press brakes, but this depends on the length of the press brake table and the application. The longer the distance between uprights, the greater the machine deflection. Also, the higher the machine tonnage, the greater the need for deflection compensation. For bending both thick and thin materials, a crowning system is recommended.

Hydraulic crowning and CNC mechanical wedge-style crowning are the two most common types of crowning systems. A wedge-style crowning system can be purchased as an add-on to a press brake through a press brake manufacturer or tooling supplier. Hydraulic crowning is built into the machine. The cost of these systems is comparable, so selection is more a question of accuracy, efficiency, and application.

Hydraulic crowning applies pressure to the cylinders during the bend stroke to compensate for machine deflection. Most of these systems are not able to apply crowning across the entire bed of the machine. Instead, the amount of deflection that the crowning system adjusts for depends on the cylinder location. Some hydraulic crowning systems use only a center cylinder or a three-cylinder system, and some provide crowning over 60 percent of the bed, but not the full bed length.

Figure 1
If the upper and lower beam are not parallel during bending, the bend angle will differ along the length of the workpiece.

The hydraulic cylinders all use the same hydraulic line. As a result, no single cylinder can apply the full crowning force; this force is applied over all the hydraulic cylinders, whether there are two, three, or 10.

A wedge-style crowning system consists of two rows of wedges across the length of the bed; one row is fixed and the other is movable (see Figures 2 and 3). The wedge in the middle of the bed has a higher slope than the wedges under the pistons, and the angle of the slope decreases toward the ram from the middle of the bed.

When activated, the movable wedge creates a curve with the high point at the center of the bed and the low points at either end of the bed below the pistons to create a spline curve in the table. The crowning system, automatically controlled, compensates for the bed deflection and the beam under different bending forces. In a wedge-style system, the table is designed to bear a certain load depending on the tonnage required. Certain crowning systems provide a programming accuracy of ±0.01 mm and positioning repeatability of ±0.005 mm.

The wedge system accounts for crowning for the full length of the bed because it is machined specifically for that bed. It also can handle deflection compensation of higher tonnages and shorter bed lengths. Crowning can be adjusted to bend at any position on the bed. This adjustment is made in real time via the control, so no production time is lost in the process.

In contrast, a universal wedge system is not particular to a press brake and is built from a standard wedge design. It can be controlled by the CNC and it will account for deflection, but it is not specific to the application or its materials. It also does not use a central database to factor the amount of deflection correction required, so its crowning ability is limited and more complex.

A universal crowning system can be retrofitted to an existing press brake to upgrade its capabilities. It also can be fitted to a smaller-tonnage, entry-level press brake not offered with crowning to give the operator more control over the bending process.

A crowning system retrofit can be beneficial and is sometimes necessary, particularly when the application changes or evolves. For instance, if an HVAC manufacturer moves from producing residential components that can be formed efficiently without crowning to making much larger, heavier components for industrial units, the new part profile dictates the need for crowning. The crowning system will increase efficiency by eliminating the number of adjustments and setups to produce the longer bend lengths.

Deflecting Forming Challenges

By ensuring a consistent angle across the length of the bend, crowning also reduces setup operations, minimizes scrap, and eliminates the need for operator intervention or for a highly skilled press brake operator.

Without a crowning system, overcoming machine deflection is a manual operation using shims or through adjustment of the dies. Calculating the amount of shimming required can be a time-consuming process, often one of trial and error, producing costly scrap. Even experienced setup operators will generate some amount of scrap when making parts on a machine with a manually adjusted crowning bed.

A CNC crowning system automatically produces these calculations. It also reduces the level of skill needed to operate the machine, thus improving efficiency and productivity.

Figure 2
This wedge-style crowning system is custom-made for each press brake. The associated components are ma

However, no crowning system can compensate for problems with tooling, such as poor punch and die alignment and asymmetric sliding of the workpiece. Nonprecision or unevenly worn tooling will form parts with inconsistent angles. Tooling problems are in part responsible for problems associated with nonuniform angles along a bend line.

Precision-ground tooling helps ensure accurate bend angles along the entire bending length. The dies have a progressive radius on both sides of the V opening, allowing the material to flow into the die more evenly and with less drag (see Figure 4).

High-tonnage Applications

A CNC crowning system compensates for the symmetric deflection of the press brake. When material properties are the same over the length of the part, standard crowning is sufficient. But when material properties are variable and more challenging—particularly with heavier or longer workpieces and in high-tensile-strength materials such as S1100, S1300, Hardox®, and Armox®—a more advanced programmable crowning system, controlled by the CNC, provides more compensation for the machine deflection and, ultimately, more flexibility.

High-tensile-strength materials require more force to bend, have more springback, and, therefore, require more crowning (see Figure 5). The use of high-tensile materials is increasing, especially in large bending applications (12 ft. or longer), as manufacturers look to reduce the weight of the end product without jeopardizing structural integrity.

Crowning is even more critical for tandem, tridem, or quadem press brakes, which combine two, three, or four press brakes, respectively, of similar or dissimilar tonnage in a synchronized configuration. These systems synchronize the machine cylinders every 5 milliseconds and continually check to ensure that there is constant straight-line movement in the machine. Some of these machines use yet another type of crowning system that incorporates a dual crowning table. It’s essentially two CNC crowning systems on top of each other with an asymmetric but complementary crowning profile.

Critical for Accuracy

Crowning systems have become more precise and capable, able to compensate for high-tensile-strength materials. They also can place more crowning in certain areas, be it left, right, or in the center of the press brake bed.

As the manufacturing landscape changes, the crowning system has become a critical element for achieving bending accuracy and a productive and profitable bending operation.

Steven Lucas is press brake product manager for LVD Co. nv. Images courtesy of LVD Strippit, 12975 Clarence Center Road, Akron, NY 14001, 800-828-1527, www.lvdgroup.com.