Common defects of bending parts and methods of treatment and prevention

With the wide application of bending parts in construction machinery products, in order to meet the product quality and production needs, their quality standards have gradually increased. In the cold state, the CNC Press Brake can use the equipped universal mold (or special mold) to bend the metal sheet into the workpieces of various geometric cross-sections. The rationality of the bending process directly affects the final forming size and appearance. Through the control and prevention of the bending defect process, adopt corresponding effective technological methods to improve the quality of the workpiece.

1. Common defects and technological measures of bending workpieces

(1) Fracture: After the material is sheared or punched, burrs or small cracks often appear on the edge, and it is easy to form stress concentration and be fractured when bending. It can be seen from Figure 1 that the shear surface when the workpiece is fractured (burr surface) ) Outwards, the cracking position occurs in the shearing zone and kneading zone of the workpiece. The technological measures taken for this are:

① After shearing or punching the workpiece, ensure that the shearing face is inward when bending, that is, it is under compression, as shown in Figure 2, the forming effect is better.

Figure 1                                        Figure 2                

Bending workpiece with shear face outward         Bending workpiece with shear face inward

② Deburr the workpiece before bending, and use a manual grinding machine to grind the broken part after bending.

(2) Springback: During the bending process of metal materials, plastic deformation and elastic deformation coexist. At the end of bending, springback occurs due to the recovery of elastic deformation, as shown in Figure 3. The rebound phenomenon directly affects the dimensional accuracy of the workpiece and must be controlled. The technological measures adopted for this are: ① Angle compensation method. If the bending angle of the workpiece is 90 °, the opening angle of the lower groove (V-shaped) of the Press Brake can be selected as 78 °.

Figure 3

Rebound of bending workpiece

② Correction method through increasing the compression time. Performing pressure correction at the end of bending to extend the contact time of the upper die, workpiece and lower groove of the Press Brake to increase the degree of plastic deformation at the corner of the lower groove, and the rebound trends of the fibers in the tension and compression zone can resist  and reduce.

(3) Protrusion: For the bending of thick plates (thickness t≥6mm), the situation is special. The area near the bending line will have a long fiber structure during the bending process, resulting in the formation of protrusions at the corners.Through the observation of the workpiece formed in the workshop, the size of the rounded protrusion increases with the increase of the plate thickness. For parts with high quality requirements, the rounded protrusions will affect the dimensional accuracy of welding or assembly, and need to be removed by process measures. The technological measures adopted in this regard are: ① If the material is cut by thermal cutting equipment, the craftsman makes a concave arc in the center of the blanking diagram with the bending line as the center of the blanking chart to remove the excess sheet material As shown in Figure 4, R is the radius of the arc.

②For workpieces with concave arcs that are not cut by sheet metal, manually grind to the required dimensional accuracy is required after the bending is completed. If the assembly accuracy is high, the workpiece needs to be milled.

Figure 4

Process drawing with concave arc

(4) Sliding material:

①Case 1: The bending line and the edge line of the workpiece are not parallel,which is one end has a fulcrum on the bottom groove of the Press Brake, and the other end has no fulcrum. The actual workpiece is shown in Figure 5, where L is the cut Seam length.

②Case 2: The bending line of the workpiece to be bent is parallel to the edge line of the workpiece, but there is no fulcrum at both ends on the lower groove of the Press Brake. The technological measures adopted for this are: ① Increase the cutting treatment.

For case 1, the craftsperson adds a slit along the direction of the workpiece bending line to the blanking diagram, the length of which is the outward deviation of the workpiece bending line, and the offset width is half of the width of the lower groove of the selected Press Brake.

② Increase the margin processing.

For case 2, the craftsman increased the size of the blanking chart so that the edge has a fulcrum on the lower groove of the Press Brake to meet the bending. After the bending process is completed, it will be processed according to the product drawing, and the cut will be sharpened to meet the product quality requirements.

Figure 5

Process drawing with slit

(4) Indentation or slip: When the workpiece is pressed and bent, it is affected by the extrusion of the upper die and the lower groove of the Press Brake, and there are different degrees of indentation or slip. Generally, the surface roughness of carbon steel parts is not high, and slight scratches have no effect on it. However, for some special materials such as 430-2B, 12Cr17Mn6Ni5, and the appearance of the mirror surface, certain technical measures need to be taken to protect it. The technological measures taken for this are: ① For carbon steel, increasing the width of the lower groove can increase the radius of the forming fillet and reduce the extrusion of the lower groove on the workpiece.

②For special stainless steel materials,we should make preparations before pressing and bending, and lay paper on the bottom groove of the Press Brake to isolate. If both sides of the workpiece need to be protected, the workpiece can also be laid along the direction of the bending line. Those with better production conditions can use anti-indentation pads.

2. Prevention of technical problems in bending workpieces

1) Bending workpiece with holes:

When the bending workpiece has a round hole or an oblong hole, as shown in Figures 6 and 7, where L is the distance from the edge of the hole to the edge of the bending area, and t is the thickness of the plate. With the method of deformation before bending, the material will be pulled after bending, which will cause the external dimensions of the bending piece to fail to meet the design requirements, and the hole will also be deformed. Technological measures must be taken to prevent it.

Figure 6

Workpiece with round holes

Figure 7

Bending a workpiece with a long round hole

The technological measures adopted for this :

1.For round holes, if L ≥ 2t, the hole can be formed before the workpiece is bent, if there is a slight hole deformation, the drilling machine needs to be re-perforated; if L<2t, the workpiece needs to be shaped after completion drilling.

2.For long round holes, deal them with the above round hole conditions. Under normal circumstances, ensure that the length of the round hole is not greater than 20% of the width of the plate (in the direction of the bending line), that is, La≤0.2Lb, but in special cases, the actual work should be analyzed concretely.

(2) Fracture prevention of non-shear parts:

The fibrous structure formed during the rolling process of the steel plate is anisotropic due to its directionality. In the actual operation of the workshop, when the direction of the fiber is parallel to the direction of the bending line, the tensile strength of the material is poor, and it is easy to cause cracks at the rounded corners. The technological measures taken for this are: ① If the overall shape of the material is limited, the direction of the tissue fiber is parallel to the direction of the bending line, and the radius of the bending corner needs to be increased at least twice the minimum bending radius of the plate. ②When the tissue fiber direction is perpendicular to the bending line, the material has a greater tensile strength, and the bending radius can be the minimum bending radius. ③ When pressing and bending two-way workpieces, the direction of the tissue fiber and the direction of the bending line must be at a certain angle.

(3) Adding process holes to the bending the workpiece :

The bending line of the workpiece is along one of its edge lines, as shown in Figure 8, where the two-dot chain line is the bending line. If the bending process is performed directly on the Press Brake, the material will be torn. Process holes must be added for prevention, as shown in Figure 9, where φ is the diameter, and the two-dot chain line is the bending line, and the diameter of the process hole is also regulated. The technological measures taken for this are: adding process holes along the direction of the bending line at the corners of the bent workpiece, and the diameter of the process holes is about 1.5 to 2.0 times the thickness of the plate.

Figure 8

Process chart without adding process holes

Figure 9

Process diagram with adding process holes

(4) Interference and anti-knife:

As shown in Figure 10, H is the height of the bending workpiece and B is its width. When H≤B, the bending workpiece may interfere with the panel of the CNC Press Brake due to the workpiece height being too high; when H≥B In addition to interference, if the workpiece is small, the straight edge of the bend may also contact the upper knife of the Press Brake, and the knife resistance phenomenon may occur.

Figure 10

Bending interference

When interference or anti-knife phenomenon occurs in the bending of the workpiece, the forced application of external force will cause the bending angle of the workpiece to fail to meet the requirements. The process measures adopted for this are: ① For the interference phenomenon, the bending and reverse deformation method is adopted. The first knife process first bends the obtuse angle in the middle of the workpiece, the second and third knife processes are bent on both sides, and the fourth knife process reverses the middle ( The reverse side of the first knife) is flat. ② For the phenomenon of anti-knife, you can replace the straight knife and use a curved knife to bend, as shown in Figure 11, where H is the height of the bending workpiece and B is its width.

Figure 11

Bending resistance correction

(5) Bending line positioning of special-shaped parts:

Due to the complex shape and size of some bending parts, non-straight edges and intersection with the bending line will cause the edge of the workpiece to be difficult to touch the CNC ruler's ruler and difficult to draw the bending line by hand. The technological measures adopted for this are: ① Prepare when cutting the medium and thin plates. For medium and thin plate parts, laser cutting machine or fine plasma cutting machine can be selected for cutting. When cutting, the bending mark cutting slit is cut directly on the workpiece. This cutting slit has little effect on the thin plate and can be repaired later. ② Prepare after cutting the thick plate. Cut a work piece that is the same as the original shaped piece as a scribing tooling template. The thickness is generally 2 to 4 mm. Cut a V-shaped opening at the bending line to facilitate the positioning of the scribing line.

3. Conclusion

According to the workshop production process, combined with work experience, reasonable solutions are provided for the prevention of common defects and technical problems in bending workpieces. These technical measures have been verified by production to meet the quality requirements and improve the quality of the workpiece.