In recent years, the application cases of industrial robots have increased rapidly, mainly in the fields of welding, spraying, and handling, and there are not many applications in sheet metal bending.
At present, 40% to 50% of the press brake bending machines in the sheet metal processing workshops in the European and American markets are equipped with robotic automatic bending systems, while China’s bending automation has just begun.
In the next 10 years, the demand for bending robots around the world will rise linearly.
Numerical controlled sheet metal flexible bending cell with the robot as the core execution part is a set of highly automated equipment combinations, which has the advantages of high efficiency, high quality and high flexibility.
In the flexible bending cell, selecting the appropriate component combination can provide better support for improving bending efficiency and flexibility.
The bending accuracy depends on the accuracy of the press brake machine, the positioning accuracy of the robot, and the coordinated control of the robot and the press brake machine;
The difficulty of collaborative control lies in the speed matching between the robot and the press brake machine, and the robot supporting the moving trajectory of the workpiece;
The poor following effect will seriously affect the bending angle and the flatness of the sheet surface, thereby affecting the quality of the finished product.
Composition of the robotic press brake bending cell
The standard press brake bending cell (Fig.1) uses robots and press brake machines as the core, and the gripper, loading table, unloading table, positioning table, turning frame, hand changing device and various detection sensors are auxiliary components.
The gripper is the “hand” of the robot that replaces the human and picks and places the workpiece.
Fig.1 Overall layout of the press brake bending cell
The gripper of a bending robot is generally constructed by mounting multiple suckers on a metal frame.
The loading and unloading platforms usually use stacking pallets, and some use conveyor belts or rollers to transport raw materials and transfer finished products.
Oily sheets are prone to adhesion, which can cause multiple sheets to be picked up at one time. A splitting device (such as a magnetic splitter) and detection sensors can be installed next to the loading table to ensure that the sheet is grasped as a single sheet.
The positioning table is an inclined platform with flanges, and micro convex balls are distributed on the table.
The robot transfers the steel sheet to the positioning table, and the plate slides freely to the retaining edge by gravity.
Because the position of the positioning table and the retaining edge are fixed, when the robot picks up the sheet again, the position of the plate and the gripper is relatively accurately fixed, which provides a reference for the next bending.
The turning frame is a fixed frame of the grasping device.
When the robot needs to change the position to pick up the workpiece, the workpiece can be placed on the turning frame to fix it, and the robot can hold the workpiece again at the new position.
On some special occasions, you can also use the press brake dies to clamp the workpiece and change the grip position.
Robotic bending cell work process
The work of the bending cell is divided into six processes as shown in Fig.2, including:
- Turn over
Fig.2 Workflow of the bending cell
The entire stack of sheets to be processed is manually placed on the feeding table. A sheet detection switch is installed on the feeding table to prevent the robot from grabbing the tray after all the sheets are processed.
The robot moves to the position of the loading table, and detects the height of the plate by an ultrasonic sensor installed on the gripper. According to the detection data, it will automatically run to the appropriate position to grab the sheet.
After the sheet is grasped, the thickness of the sheet is measured by the thickness measurement device, so as to avoid grasping multiple sheets at a time, which causes processing failure.
After passing the thickness measurement, then prepare for alignment.
The robot runs to the position of the positioning table and places the sheet on the positioning table for precise positioning (Fig.3).
Fig.3 Sheet positioning
After positioning, grab the sheet again and prepare for bending.
(4) Turn over
According to the needs of the process, determine whether a turning frame is needed.
If necessary, the robot will move to the turning frame position and place the sheet on the turning frame, the robot releases the sheet, and moves to the other side of the sheet to grab the sheet.
The robot moves to the position of the press brake machine, places the sheet flat on the lower die, and accurately positions it by means of the rear finger sensor of the press brake.
After the positioning is completed, the robot sends a bending signal to the press brake machine and cooperates with the press brake machine to complete the bending action.
Judge whether need to bend again to decide whether to perform consecutive bending, as shown in Figure 4.
Fig.4 Robotic sheet bending
The technical difficulty of bending lies in the cooperative action of the robot and the bending machine, that is, bending following.
When the robot grips or supports the sheet during bending, the sheet is deformed. The robot needs to follow the sheet to make a circular motion according to a specific trajectory algorithm, and always maintain a relatively fixed position with the sheet.
The robot runs to the position of the unloading table. Due to the difference in the forming of the workpiece, there are a variety of palletizing actions, such as conventional matrix palletizing, single and double-layer cross palletizing, positive and negative buckle palletizing, etc., as shown in Figure 5.
Fig.5 Sheet palletizing
Technical key points of the robotic press brake bending cell
At present, whether it is a universal standard six-axis robot or a bending robot optimized for the bending process on the robot arm span or shape, it needs the support of the bending following algorithm, and it is rare that it does not follow the bending.
If there is no good following effect, the gripper or sucker gripper will pull the workpiece due to the poor following trajectory, forming sheet wrinkles and affecting the forming quality.
Establishing an accurate robot bending and following motion model can help to establish a good following trajectory algorithm, thereby obtaining excellent following effects.
Fig.6 Schematic diagram of the bending process
Fig. 6 is a schematic diagram of a bending process, and a mathematical model of bending following is obtained based on it, as shown in Fig. 7.
Fig.7 Bending motion model
Each parameter in Figure 7 is expressed as:
- 1) Arc radius of top punch: R, unit: mm;
- 2) Arc radius of lower die: r, unit: mm;
- 3) Lower die opening: V, unit: mm;
- 4) Lower mold angle: ∠b, unit: ° ;
- 5) Workpiece thickness: T, unit: mm;
- 6) Thickness from the neutral layer to the upper surface of the workpiece: λ, unit: mm;
- 7) Workpiece bending angle: ∠a, unit: ° ;
- 8) The amount of the press brake ram descending from the clamping point: S, unit: mm.
The relationship between the bending angle and the amount of bending descending calculated according to the mathematical model is:
S = [r×TAN(45°-1/4×∠b)+V/2)×SIN(90-1/2×∠a)-(r+R+T)]/COS(90-1/2×∠a)+(r+R+T)
According to the mechanical parameters in Table 1, the formula of the relationship between the bending angle and the amount of descending can be used to obtain the trajectory curve of the displacement change of the bending angle from 180 ° to 10 ° in the X and Z directions, as shown in Fig. 8.
Table 1 Bending die information and necessary information of the workpiece
Fig.8 Relationship between bending angle and robot trajectory
With the continuous development of sheet metal manufacturing, robot bending has more and more broad application prospects.
Compared with the development of special bending robots, the development of a robot bending follow model algorithm suitable for general six-axis robots and applied to general robots, the development cost will be lower.
With the cooperation of most excellent brands of robots and other auxiliary hardware in the industry, it can quickly promote robot bending applications.