The working principle and comparation of hydraulic compensation and mechanism compensation in CNC press brake have been introduced in the text. The hydraulic compensation has been used on the CNC Press Brake for more than ten years which makes it a maturity technology, while the mechanism compensation is a new technology which has been used on the CNC Press Brake in recent years. It is concluded that the mechanism compensation device has a good effect in the manufacturing process of CNC Press Brake.

Why we need a crowning compensation system for press brake

CNC press brake is an important equipment in sheet metal processing, its work precision directly affects the bending precision of the workpieceIn the bending process of the workpiece, due to the maximum loading pressed on both ends of the slider, the reaction force during the plate bending causes concave deformation on the lower surface of the slider, especially the shape variable of the middle part of the slider is the largest.

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 2400mm 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.

Hydraulic Crowning and Mechanical Crowning can generate a upward elastic deformation in the middle of the worktable to offset the deformation of the machine tool slider,Nowadays,most of press brake manufacturer 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.In the market,there are many NC control system of press brake,in this article,we will take DELEM DA6x system for example.First of all,we need to understand how the two crowing system to work,

Hydraulic Crowning

The worktable is made up of a thick main board and two relatively thin side boards, which totally the worktable is made up of three iron plates.

The press brake is installed two hydraulic cylinders at two sides, Install the auxiliary hydraulic cylinder in the lower part of the worktable. During bending process, it generates an upward force on the worktable, which forms the automatic crowning system.

When the workpiece is bending, the hydraulic valve according to the compensation value calculated by the numerical control system automatically to open the proportional valve volumes, fill the hydraulic oil into the compensation cylinder, then the piston in the compensation cylinder will be pushed out thin side boards a little,this is the key process,so that cause to the middle of the main board upward elastic raised up deformation,this  compensation is the result of improving the precision of the workpiece is achieved.

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.

Hydraulic Crowning

Mechanical Crowning

One of the mechanical compensation crowing system is composed of upper and lower beds,there are two same slope in the upper and lower beds,Before compensation, they are fully fitted together,in the compensation,The bottom bed is driven by the servo motor in the direction shown in Figure 2 and 3 The upper and lower beds are still fitted together at A but  they are separated at B. the upper beds generate a convex elastic deformation by the action of the bottom bed.

Figure 1. Mechanical crowning main board

The other one is A wedge-style crowning system.which consists of two rows of wedges across the length of the bed; one row is fixed and the other is movable,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 cnc system calculate the compensation value according to the magnitude of the load force when the workpiece is bent(this force will cause the slider and the table to have a deflection).the servo motor move the 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 curve correctly match with deflection of table and slide which caused by bending force in the full length of the bed..then we can say the crowning compensation system, automatically controlled.

A wedge-style crowning system

Options of the crown systems

Hydraulic crowning and CNC mechanical wedge-style crowning are the two most common types of crowning systems, hydraulic crowning is an traditional method 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. So the hydraulic crowing system working precision is weaker.If there is debris in the hydraulic oil, the hydraulic proportional valve spool will be stuck, and the working accuracy will be more weaker. because the compensation pressure is activated from the entire hydraulic pressure, when the compensation valve is working, the entire system pressure will be partially lost..while, CNC mechanical wedge-style crowning is a universal modern compensation system,Some Crown mechanisms can ensure designing ± 0.01 mm precision and position± 0.005 mm position reproducbility,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,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.