The CNC bending machine is an important equipment in sheet metal processing, and its working accuracy directly affects the bending precision of the workpiece. During the bending process of the workpiece, the maximum force at the two ends of the slider and the reactive force during the bending of the sheet metal cause concave deformation on the lower surface of the slider, with the maximum deformation occurring in the middle part of the slider, resulting in varying angles along the length of the final bent workpiece. To eliminate the adverse effects of slider deformation, it is necessary to compensate for the deflection of the slider. The usual compensation methods are hydraulic compensation and mechanical compensation, both of which create upward elastic deformation in the middle of the worktable to counteract the deformation of the machine tool slider, ensuring the accuracy of the machining joint surface and improving the workpiece precision. Currently, major machine tool manufacturers abroad use mechanical compensation devices, while domestic manufacturers choose the appropriate compensation method based on actual conditions.
Difference between compensation and no compensation
Introduction to two compensation methods
I. Hydraulic compensation method:
The hydraulic automatic deflection compensation mechanism of the worktable consists of a set of oil cylinders installed in the lower worktable. The position and size of each compensating cylinder are designed based on the deflection compensation curve obtained from finite element analysis of the slider and worktable. Hydraulic compensation is achieved by the relative displacement between the front, middle, and rear three plates, to realize the bulging compensation of the middle plate. The principle is to achieve bulging through the elastic deformation of the steel plate itself, so the compensation amount can be adjusted within the elastic range of the worktable.
II. Mechanical compensation method:
Mechanical compensation consists of a set of inclined convex wedge blocks. Each convex wedge block is designed based on the deflection curve obtained from finite element analysis of the slider and worktable. The CNC system calculates the required compensation amount based on the magnitude of the load force during workpiece bending (this force will cause deflection deformation of the slider and worktable upright plate), and automatically controls the relative movement of the convex wedge blocks to effectively compensate for the deflection deformation of the slider and worktable upright plate. Ideal mechanical deflection compensation is achieved by controlling the position to realize "pre-bulging." A group of wedge blocks forms a curve in the length direction of the worktable that matches the actual deflection, ensuring consistent clearances between the upper and lower molds during bending and ensuring consistent angularity of the bent workpiece in the length direction.
Principles of the two compensation methods
As there are many types of CNC bending machine control systems, here we will only use the DA-66T series CNC system from the Dutch company DELEM as an example to illustrate the working principles of hydraulic compensation and mechanical compensation.
I. Working principle of hydraulic compensation
Below the worktable is a thick main board and two relatively thin side boards, all three iron plates are joined together. In the position where the compensation cylinder is placed, the middle main board is noticeably more convex. When bending the workpiece, the hydraulic valve opens the proportional valve according to the compensation value automatically calculated by the CNC system, allowing hydraulic oil to enter the compensation cylinder. The piston in the compensation cylinder will push out a small part, causing the middle main board to undergo upward elastic deformation, thereby improving the precision of the workpiece. Of course, the bending precision of the workpiece is also related to the internal parameter settings of the CNC system.
II. Working principle of mechanical compensation
The mechanical compensation mechanism consists of upper and lower cushion plates and the worktable, all connected through disc springs and bolts. The upper and lower cushion plates are composed of multiple segments of wedges with different slopes. Without mechanical compensation, there is a noticeable difference in slope between the ends and the middle of the upper and lower cushion plates, as the middle part of the machine tool requires the largest amount of deformation compensation. With the addition of mechanical compensation, the slopes of the upper and lower cushion plates become the same. Before compensation, they are completely fitted together. When compensation is needed, the lower cushion plate moves to the left under the drive of the motor. The upper and lower cushion plates remain fitted together at the left end but separate at the right end. Due to the different slopes of each segment of the upper and lower cushion plates, the upper cushion plate undergoes upward convex elastic deformation under the action of the lower cushion plate. This type of mechanical compensation mechanism uses integral upper and lower cushion plates, but it can also use segmented wedge blocks with slopes. These blocks are driven left and right by a threaded screw, thereby compensating for the deformation of the machine tool slider and improving the bending precision of the workpiece. The electrical control principle of this segmented wedge block mechanical compensation mechanism is the same as that of the integral upper and lower cushion plate mechanical compensation mechanism. This type of segmented wedge block mechanical compensation mechanism is currently more common in high-tonnage bending machines. Through mechanical compensation, it counteracts the deformation of the machine tool slider, ensuring the accuracy of the machining joint surface and improving the precision of the workpiece.
Comparison of the two compensation methods
1. Advantages of hydraulic compensation:
1. Over time, hydraulic compensation does not suffer from wear issues, whereas the screw and wedge blocks in mechanical compensation will experience wear over time.
2. Hydraulic compensation occupies less space, while mechanical compensation will occupy more free space in both height and width.
3. With hydraulic compensation, the sheet metal cannot shift during use because the lower worktable maintains a flat state and is in contact with the surface of the bent sheet metal. The contact point between the upper beam and the sheet metal is stable when clamping, whereas mechanical compensation only takes effect after bending and may produce unpredictable errors.
4. Hydraulic compensation can be adjusted even when the workpiece is not removed, which is not possible with mechanical compensation.
2. Advantages of mechanical compensation:
1. Mechanical compensation offers long-term stability, reducing the maintenance difficulty and frequency of hydraulic compensation (e.g., due to seal damage causing oil leakage) over the lifespan of the machine tool.
2. Because there are more compensation points, mechanical compensation can achieve precise deflection compensation along the entire length of the worktable, allowing the bending machine to achieve linear compensation during operation and improve the bending effect of the workpiece.
3. Mechanical compensation uses a linear scale to measure the position of the return signal, serving as a numerical control axis for digital control, making the compensation value more precise.
