Common defects and process measures of bending workpiece
1. Fracture
After cutting or punching, burrs or small cracks often appear on the edges of materials, which are easy to form stress concentration and be broken when bending. When the workpiece is broken, the shear surface (with burrs) is outward, and the cracking position occurs in the shear crack zone and kneading zone of the workpiece. The technological measures taken are:
(1) After cutting or punching, the workpiece shall be ensured to be inward when bending, that is, it is under pressure, and the forming effect is good.
(2) Before bending, the workpiece shall be deburred. After bending, the broken part can be polished with a manual grinder.
2. Rebound
In the bending process of metal materials, plastic deformation and elastic deformation exist at the same time. At the end of bending, springback occurs due to the recovery of elastic deformation. Springback directly affects the dimensional accuracy of the workpiece and must be controlled. The technological measures taken are:
(1) Angle compensation method. If the bending angle of the workpiece is 90 °, the opening angle of the lower groove (V shape) of the bending machine can be selected as 78 °.
(2) The pressurization time increases the correction method. At the end of bending, pressure correction is carried out to extend the contact time of the upper die, workpiece and lower groove of the bending machine, so as to increase the plastic deformation degree at the fillet of the lower groove, so that the rebound trend of the fibers in the tension and compression area resists each other, so as to reduce the rebound.
3. Bulge
For the bending of medium and heavy plates (thickness t ≥ 6mm), the situation is relatively special. The fiber tissue near the bending line will grow during the bending process, resulting in the formation of bulges at the bending fillet. Through the observation of the workshop formed workpiece, the size of the fillet bulge increases with the increase of the plate thickness. For some workpieces with high quality requirements, the fillet bulge will affect the dimensional accuracy of welding or assembly, and technological measures need to be taken to remove it. The technological measures taken are:
(1) If the thermal cutting equipment is used for blanking, the process personnel shall make an inward concave arc with the bending line as the center of the blanking drawing when the blanking is carried out, and remove the excess sheet metal, where R is the arc radius.
(2) For the workpiece without concave arc of sheet metal cutting, manually grind it to the required dimensional accuracy after bending. If the assembly accuracy is required to be high, the workpiece needs to be milled.
4. Sliding material
(1) The bending line of the workpiece to be bent is not parallel to the edge line of the workpiece, that is, one end has a fulcrum on the lower groove of the bending machine, and the other end has no fulcrum. The technologist adds a slit along the bending line of the workpiece to the blanking drawing. Its length is the outward offset of the bending line of the workpiece, and the offset width is half of the lower groove width of the selected bending machine.
(2) The bending line of the workpiece to be bent is parallel to the edge line of the workpiece, but both ends have no fulcrum on the lower groove of the bending machine. The process personnel increase the size of the blanking drawing so that the edge has a fulcrum on the lower groove of the bending machine to meet the bending. After the bending process is completed, the cutting allowance shall be processed according to the product drawing, and the cutting opening shall be polished to meet the product quality requirements.
5. Indentation or slip
When the workpiece is pressed and bent, it is squeezed by the upper die and the lower groove of the bending machine, resulting in indentation or sliding injury to varying degrees. Generally, the surface roughness requirements of carbon steel parts are not high, and slight scars have no effect on them. However, for 430-2b, 12cr17mn6ni5 and other special materials, the appearance of the mirror needs to be protected by certain technological measures. The technological measures taken are:
(1) For carbon steel materials, increase the width of the bending lower groove, increase its forming fillet radius, and reduce the extrusion of the lower groove on the workpiece.
(2) For special stainless steel materials, make preparations before bending, and lay paper on the lower groove of the bending machine for isolation. If both sides of the workpiece need protection, the workpiece can also be laid paper along the direction of the bending line for isolation. Those with better production conditions can use anti indentation pads.