How to Solve Common Defection on Bent Parts?

Estimated reading time: 10 minutes

As bent parts widely used in construction machinery products, their quality standards have been gradually improved to meet product quality and production needs. In the cold state, the CNC bending machine can use the equipped general (or special) mold to fold the metal sheet into various required cross-sectional shapes.

Whether the bending process is reasonable or not will directly affect the final shape, size and appearance of the product. Aiming at bending defects, through process control and prevention, corresponding effective process measures are taken to improve the quality of the workpiece.

Fracture

After shearing or punching, burrs or fine cracks often appear on the edge of the material, and stress concentration is easily formed when bending and it is broken. When shearing (surface with burrs), it is sheared outward, and the cracking position occurs on the shearing and kneading belts of the workpiece.

The technical measures adopted for this are:

After shearing or punching, the cutting surface should be inward when the workpiece is bent, that is, under pressure, and the forming effect is better.

Before bending, the workpiece is deburred, and after bending, the bent part can be ground with a manual grinding wheel.

Rebound

In the bending process, the plastic deformation and elastic deformation of the metal material are synchronized. When the bending ends, spring-back occurs due to the recovery of elastic deformation. The spring-back phenomenon directly affects the dimensional accuracy of the part, so it must be controlled.

The technical measures adopted for this are:

Angle compensation method. If the bending angle of the workpiece is 90°, the opening angle of the lower slot (V-shaped) of the bending machine can be 78°. The pressure duration increases the correction rate.

When the bending is over, the contact time between the upper die, the part and the lower groove of the bending machine is extended by pressure correction to increase the plastic deformation of the corners of the lower groove, so that the spring-back of the fibers in the tension and compression zone cancel each other out, thereby reducing the spring-back.

Bulge

The bending situation of medium and thick plates (thickness t≥6mm) is special. The fiber structure in the area near the bending line becomes longer, which leads to bulges on the bending corners. Through the observation of the formed parts in the workshop, the size of the raised fillets increases with the rise of plate thickness. For some workpieces with high-quality requirements, the rounded protrusions will affect the dimensional accuracy of welding or assembly, and process measures need to be taken to eliminate them.

The technical measures adopted for this are:

When hot cutting equipment is used for cutting when the processing staff unfolds the plate and cuts the blank, the blanking drawing is made as an inner concave arc with the bending line as the center, and the excess plate is removed, where R is the radius of the arc.

For the workpiece that has not been cut by concave arc cutting, after the bending of the cutting plate is completed, it is manually grinded to achieve the required dimensional accuracy. If the assembly accuracy is high, the workpiece needs to be milled.

Slippery material

1. The bending line of the part to be bent is not parallel to the edge line of the part, that is, one end has a fulcrum at the lower groove of the bending machine, and the other end has no fulcrum. In the blanking diagram, a slit is added in the direction of the bending line of the workpiece. The length of the slit is the length of the outward offset of the bending line of the workpiece, and the offset width is half of the width of the lower groove of the selected bending machine.
2. The bending line of the workpiece to be bent is parallel to the edge of the workpiece, but there is no fulcrum at the lower groove of the bending machine at both ends. The technicians increase the size according to the blanking drawing, so that the edge of the lower groove of the pipe bender has a fulcrum to meet the pipe bending requirements. After the bending process is completed, cut allowances and trim cuts are carried out according to the product drawings to meet the product quality requirements.

For situation 1, the craftsman adds a slit to the blanking diagram along the direction of the bending line of the workpiece, the length of which is the outward offset of the bending line of the workpiece, and the offset width is half of the width of the lower groove of the selected bending machine, or increase the margin deal with.

For situation 2, the craftsman increases the size of the blanking chart so that the edge has a fulcrum on the lower groove of the bending machine to satisfy the bending. After the bending process is completed, the cutting allowance is processed according to the product drawing, and the cut is trimmed to meet the product quality requirements.

Bent parts indentation

In the press bending process, the workpiece is squeezed by the upper die and the lower groove of the press brake to produce different degrees of indentation or slip marks. The surface roughness of ordinary carbon steel parts is not required to be high, and it will not cause slight damage. However, for some special materials, such as 430-2B, 12Cr17Mn6Ni5, etc., the appearance of the mirror surface needs to be protected by certain technological measures.

The technical measures adopted for this are:

For carbon steel materials, the bending width of the lower groove should be increased to increase the forming fillet radius and reduce the extrusion of the lower groove on the workpiece;

For special stainless steel materials, the preparatory work before bending is to isolate paper at the lower groove of the bending machine. If both sides of the workpiece need to be protected, the workpiece can also be separated from the paper in the direction of the bending line. With good production conditions, anti-indentation pads can be used.

Prevention of process problems in bending workpieces

Bent parts with holes

When the bent parts have a round hole or an oblong hole, as shown in the figure below, 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. If the hole is formed before bending and is within the bending deformation, then After bending, the material will be pulled, which will cause the shape of the bent part to fail to meet the requirements of the design drawing. At the same time, the hole will also be deformed, and process measures must be taken to prevent it.

The technological measures taken in this regard are:

• For round holes, if L≥2t, the hole can be formed before the workpiece is bent. If there is slight hole deformation, the drilling machine needs to re-pierce; if L<2t, the workpiece needs to be drilled after the workpiece is formed.
• For long round holes, follow the above-mentioned round hole situation. Under normal circumstances, ensure that the length of the round hole is not more than 20% of the width of the plate (along the direction of the bending line), that is, La≤0.2Lb, but in special cases, it should be based on the actual workpiece The shape is analyzed in detail.

Prevention of breakage of non-sheared parts

The fibrous structure formed in the rolling process of the steel plate has anisotropy in the mechanical properties of the material due to its directionality. In the actual operation process of the workshop, when the fiber direction is parallel to the direction of the bending line, the tensile strength of the material is poor, which is easy to cause cracks at the rounded corners.

The technological measures taken in this regard are:

1. If limited by the overall shape of the material, the direction of the tissue fibers is parallel to the direction of the bending line, and the bending fillet radius needs to be increased to at least 2 times the minimum bending radius of the sheet.
2. When the direction of the tissue fibers is perpendicular to the bending line, the material has greater tensile strength, and the bending fillet radius can be the minimum bending radius.
3. When bending a bidirectional workpiece, the direction of the tissue fibers must be at a certain angle with the direction of the bending line.

Increase process holes for bent parts

The bending line of the workpiece is along with one of its edge lines, as shown in the figure. The double-dotted line is the bending line. If the bending process is directly performed on the bending machine, the strip will tear. It is necessary to increase the process hole for prevention, as shown in the figure, where φ is the diameter, and the two-dot chain line is the bending line. At the same time, the process hole diameter is standardized.

The technological measures adopted for this are: adding craft holes along the direction of the bending line at the corners of the bent workpiece, and the craft hole diameter is about 1.5 to 2.0 times the thickness of the plate.

Interference and anti-knife

As shown in the figure, H is the height of the bending part, and B is the width. When H≤B, the bending part may contact the panel of the CNC bending machine due to the high height of the workpiece, and interference will occur; when H≥B, In addition to interference, if the workpiece is small, the straight edge of the bending machine may also contact the upper knife of the bending machine, resulting in an anti-knife phenomenon.

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 technological measures taken in this regard are:

• For the interference phenomenon, the bending anti-deformation method is adopted. The first knife process first bends an obtuse angle in the middle of the workpiece, the second and third knife processes are bent on both sides, and the fourth knife process counter-presses the middle (the backside of the first knife) to be flat.
• For the anti-knife phenomenon, you can replace the straight blade and use a scimitar to bend, as shown in the figure, where H is the height of the bending workpiece and B is the width.