1. Introduction
Sheet metal bending machine is a kind of widely used sheet metal processing equipment.
The sheet metal bending machine uses a relatively simple universal bending die, which can fold a variety of complex parts, equipped with the corresponding process equipment, and can also complete the groove, shallow drawing, punching, voltage ripple and other processes.
In addition, it has the characteristics of simple die, good versatility, convenient adjustment and replacement, and can flexibly realize various sheet metal forming processes.
Modern manufacturing technology is one of the important means to develop the economy.
As an important branch of modern manufacturing technology, the development and research of precision machinery manufacturing technology are also highly valued by economically developed countries.
The development and promotion of precision and ultra-precision machining technology have improved the machining accuracy and technical level of the whole machinery manufacturing industry.
It also generally improved the quality, performance and competitiveness of mechanical products.
Related industries have higher and higher requirements on the accuracy of bending, so improving the accuracy of bending has become an urgent issue in the research and development of bending machine.
The bending accuracy of the press brake machine is the primary factor to determine the bending quality of the plate.
The existence of sheet metal processing error affects the subsequent assembly, increases the workload of trial die repair and shape correction after forming, prolongs the product development cycle, and restricts the further promotion and application of incremental bending forming, especially for the application of large workpieces formed by high strength and high springback steel plate.
Therefore, it is of great practical significance to analyze the mechanism of the processing error of the sheet metal bending machine, find out the main factors affecting the processing error, and put forward the effective methods and measures to improve the processing accuracy of the bending machine, so as to improve the market competitiveness and added value of the sheet metal bending machine.
In this article, PBH110- 3100 CNC sheet metal bending machine is used as the research object.
According to the theory of elastic mechanics, the analysis model of bending error is established, and the influence of machine tool manufacturing accuracy on metal sheet bending accuracy is analyzed.
1. Definition and national standard of plate bending accuracy
According to the national standard GB/T 14349-93, the working accuracy inspection of the bending machine mainly includes two parts: bending angle and straightness, as shown in Table 1.
Requirements for test pieces during working accuracy inspection:
1) Length of specimen:
- When the length of the worktable is less than 2000mm, it is the length of worktable;
- When the length of the worktable > 2000mm ~ 3200mm, it is 2000mm;
- When the length of the worktable is more than 3200mm, it is 3000mm.
2) The width of the specimen shall not be less than 100 mm.
3) Thickness of specimen:
- Nominal force ≤ 1000 kN, it is 2mm;
- Nominal force > 1000 ~ 2500 kN, it is 3mm;
- Nominal force > 2500 ~ 6300 kN, it is 4mm.
4) The material of the test piece is A3 steel plate, and its tensile strength σb ≤ 450 MPa.
Table 1 Bending accuracy inspection diagram of sheet metal bending machine
| Inspection items | Diagram | Tolerance | Inspection tools | Inspection methods | |
| Bending angle of specimen |  |
Accuracy classification | On the whole length | Universal angle meter | The universal angle ruler shall be placed on the outer surface of the bending test piece and measure it several times (at least three places per M). The error is calculated by the difference between the maximum and minimum angle of the angle ruler and the specified bending angle of 90°. |
| I | ± 30′ | ||||
| II | ± 1° | ||||
| III | ± 1°30′ | ||||
| Bending straightness of specimen |  |
Accuracy classification | On the whole length | Feeler gauge inspection level ruler | Place the inspection surface of a 1000 gauge against the edge of the bending specimen, and measure the gap between them with a feeler gauge. The error is calculated by the maximum reading value within any 1 m length. |
| I | 0.30 | ||||
| II | 0.75 | ||||
| III | 1.00 | ||||
2. Analysis of the influence of the flatness (longitudinal) of the worktable on the bending accuracy
2.1 Angle accuracy of workpiece bending
When there is flatness error in the longitudinal direction of the worktable, it will affect the depth of the upper die tool entering the notch of the lower die, and the depth deviation ΔH is equal to the longitudinal flatness error (a) of the worktable.
The influence of the flatness (longitudinal) of worktable on bending angle error is shown in Figure 1.
Fig. 1 Effect of flatness error of worktable on bending angle error.
The depth deviation △H of the upper die tool entering the lower die notch is:
The bending angle of the workpiece is:
Then the angle deviation of the bending workpiece is:
2.2 Straightness accuracy of workpiece bending
When the worktable has flatness error, the influence of flatness error on the straightness error of bending workpiece is shown in Figure 2.
Fig. 2 Effect of flatness error of worktable on bending straightness error.
The straightness error of the bending workpiece is equal to the longitudinal flatness error.
That is:
2.3 Workpiece bending edge accuracy
The plane error of worktable has no influence on the accuracy of sideline.
3. The influence of the flatness (transverse) of the worktable on the bending accuracy
The influence of horizontal flatness of worktable on bending accuracy is similar to that of vertical flatness.
3.1 Workpiece bending angle accuracy
The bending angle error of the workpiece is:
3.2 Straightness accuracy of workpiece bending
The bending straightness error of the workpiece is:
3.3 Workpiece bending edge accuracy
The horizontal flatness of worktable has no influence on the accuracy of bending sideline length.
4. The influence of slider stroke on working face perpendicularity and bending accuracy
4.1 Workpiece bending angle accuracy
The influence of slider stroke on perpendicularity of the working face and bending accuracy is shown in Figure 3.
Fig. 3 Influence of slider stroke on perpendicularity of working face and bending accuracy.
When the perpendicularity deviation of the slider stroke to the worktable is t, the depth H ‘of the slider into the die is:
The bending angle of the bending workpiece is:
The bending angle error of the bending workpiece is:
4.2 Straightness accuracy of workpiece bending
When the perpendicularity deviation of the slider stroke to the worktable is t, the straightness error of the bending workpiece is:
4.3 Workpiece bending edge accuracy
When the perpendicularity deviation of the slider stroke to the worktable is t, the distance S1 from the bending center line of the upper die is:
It can be found that:
The actual bending edge length L ‘is:
The length error of bending sideline is:
5. Conclusion
The influence of various factors of manufacturing accuracy of PBH110-3100 CNC sheet metal bending machine on workpiece bending accuracy is shown in Table 2.
Table 2 Analysis of the influence of various factors on bending accuracy
| Parameters | Bending force | 1100 kN | Slide thickness | 60 mm | Slide stroke | 220 mm | |
| Worktable length | 3100 mm | Worktable height | 890 mm | Plate thickness | 5 mm | ||
| 2660 mm | Worktable thickness | 60 mm | Die width | 40 mm | |||
| Slide height | 1300 mm | Deflection calculation
Correction |
1.5 mm | Upper die height | 120 mm | ||
| Classification | Project | Deviation | Influence on bending accuracy | ||||
| Tolerance | Actual value | Straightness error △θ | Angle deviation
△l |
Side length error
△L |
|||
| Machine tool manufacturing accuracy | Flatness of worktable | Longitudinal (a) | 0.08 | 0.06 | 0.172 | 0.06 | × |
| Horizontal (b) | 0.03 | 0.02 | 0.057 | 0.02 | × | ||
| Perpendicularity of slider stroke to working face | 0.25 | 0.2 | -0.003 | 0.000 | 0.182 | ||
It can be concluded from the analysis table that the bending angle deviation is about 3 times of the flatness of the worktable.
In the national standard, the requirement for the flatness of the upper die mounting surface is low, and the bending angle error is large.
Bending machine manufacturers can improve the flatness of the worktable according to the actual production.
However, the bending angle has little effect on the perpendicularity of the working face due to the stroke of the slider.
The flatness of the worktable will directly reflect the straightness deviation of bending, and the straightness error of bending is less affected by the slider stroke on the perpendicularity of the working face.
