Discussion and solution of chip jumping of the hot

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Discussion and solution of chip jumping problem of high-speed progressive die

Abstract: This paper expounds several main factors affecting the "chip jumping" in the production process of high-speed progressive die, and puts forward solutions and measures accordingly, which has a certain reference for the design and manufacture of progressive die

in the metal cold stamping industry, high-speed progressive dies have high production efficiency, high product accuracy, easy to realize automation and mechanization, and are suitable for the needs of mass production. Therefore, they are widely used in the stamping manufacturing industry of automotive, electronics, household appliances and other products. At present, the mainstream speed SPM (strike per minute) of domestic molds is concentrated in 40 – 2000, and more than 4000spm punches and molds have been developed internationally. However, whether in the past or now, the "chip jumping" of high-speed progressive die has always been a problem that annoys designers and producers. As long as "chip jumping" occurs occasionally, it will leave bruises on the product surface and produce batches of defective products. At least, the production capacity of the manufacturing department will be affected, and it is necessary to stop the machine to repair the mold and re frame the mold for debugging; The key is to break the knife edge, break the punch, and damage the expensive mold and high-speed punch spindle. And "chip jumping" is a high frequency problem in the high-speed progressive die industry. The so-called "chip jumping" refers to the phenomenon that the chip or product that should have fallen from the die edge jumps out of the die surface with the punch due to various reasons in the process of mold production, as shown in Figure 1. From the point of view of mechanics, the chip jumping is because the bite force F between the chip and the side wall of the die edge is less than the upward adsorption force f it receives, so it jumps out of the die and enters the die surface, as shown in Figure 2. This paper specifically analyzes the causes of chip jumping and its solutions for high-speed progressive dies

1 analysis of the causes of chip jumping in high-speed progressive die

the two mechanical factors of chip jumping have been mentioned in the previous article, which are described below

(1) the upward adsorption force of chips is too large, resulting in chips jumping out, including the following parts

1) the chips are adsorbed by the vacuum of the punch. During punching processing, the material cut by the punch is bent in the center due to the action of bending moment, but it is tightly pressed with the punch around, as shown in Figure 3: there is an upward force of atmospheric pressure under the chip, and there is a vacuum negative pressure between the top and the punch, resulting in a pressure difference adsorbed on the punch. With the opening of the mold, it jumps out of the mold surface. In addition, in the production of high-speed punching and cutting, in order to heat the die and lubricate the convex and concave die, we often coat the material with cutting oil before it is sent into the die, which will produce a similar effect as adsorbent. If the volatility of cutting oil is poor, the viscosity is high, and the amount of cutting oil is too large, the vacuum adsorption between chip and punch will be more obvious

2) effect of electromagnetic force. Many parts on the mold are processed by grinding. The existing grinding machines use the magnetic force of the electromagnetic platform to clamp parts. If the residual magnetism of the part is not degaussed after processing, the chip of iron-based material will jump due to the magnetic force rising with the adsorption of the punch

3) punch piston effect and acceleration. As shown in Figure 4, when the die is closed to the bottom dead center, the discharge plate and materials inside the die are tightly wrapped around the punch and tightly pressed on the knife edge of the die, forming a relative vacuum negative pressure. At this time, the upper die picks up and opens, and the punch is first extracted from the die. Because the chip is subject to the pressure difference between the lower atmospheric pressure and the upper vacuum, it rises with the punch, just like the piston moving in the cylinder, which is called the piston effect. Due to the influence of acceleration and inertia, the faster the punch rises, the easier the piston effect will occur. In the production site, it is often encountered that the chips jump frequently when the mold is in high-speed normal production. At this time, the chip jump will not occur when the mold running speed SPM (speed per minute) is reduced, which is caused by the piston effect

4) the influence of punch wear. After long-term use of the die, the effective cutting edge of the punch will be worn. After the chip is cut off, the burr will become larger, and the burr will form a large burr with thick root according to the worn punch edge shape. Due to the extrusion effect of the die, it will be tightly adhered and wrapped on the punch cutting edge, as shown in Figure 5. As the punch rises together, it will adsorb and jump out of the die surface

(a) the normal shape of the punch (b) the shape of the punch after wear (c) the chip adheres to the punch

(2) the bite force between the chip and the side wall of the die is too small, which is mainly affected by the following aspects

2.1 impact of blanking clearance. Figure 6 shows the cross-sectional shape of the chip after punching. From a theoretical point of view, the chip after punching is connected with the die: the part touched by Wang Zhe is the bright belt. When the blanking gap is appropriate, the bright belt usually accounts for 1//3 of the full section, and the proportion of the bright belt of high-precision blanking will be higher. For example, Japan can achieve nearly 100% by using the reverse cutting method. When the proportion of the bright belt of the chip in the section is larger, the contact area with the knife edge is larger, and the biting force between the two is also larger. When the blanking gap is too large, the tensile effect of the material increases, which is close to bulging fracture, and the proportion of the bright band decreases. Due to the elastic recovery of the material, the chip size shrinks towards the solid direction, and the size of the punched chip is smaller than the size of the die. In this way, the bite force of the chip on the knife edge will become weak, and the chip is easy to jump out of the knife edge with the rise of the punch. However, large blanking clearance is conducive to reducing the blanking force and improving the service life of the knife edge

2.2 the peripheral shape of chips is simple. When the shape of the chip is simple, the cutting line of the whole periphery is relatively simple and short, and its internal stress change and material strain are also simple. They all point to the same center of the entity. The peripheral center of the chip shrinks uniformly, and there is a uniform gap with the die edge, which reduces the contact area between the chip and the side wall of the die and reduces the biting force. That is why the simpler the shape of the chip is, the easier it is to jump out of the die. The most common problem on the production site is that the round hole chips jump and crush the stamping products. Due to the length of the cutting line, there are multiple solid centers for the chip with complex shape, its internal stress and strain are complex, and the shrinkage around the periphery is inconsistent, which leads to its close engagement with the die edge, increases the friction force, and effectively reduces the probability of chip jumping up

2.3 surface roughness of die blade. Friction force between chip and die edge f=u × N. F: friction, u: friction coefficient, N: positive pressure. To increase the friction force F, only by increasing the friction coefficient and positive pressure can it be achieved. The positive pressure n can be controlled by designing a reasonable blanking clearance. The friction coefficient u depends on the roughness of the friction surface. In order to ensure the sharpness of the knife edge and easy blanking, current mold manufacturers usually use high-precision machine tools such as slow wire cutting and optical curve grinder to process the concave die knife edge. The size can be controlled within ± 0.002mm, and the surface finish can also reach below Ra0.2. Therefore, the side wall of the die edge is very smooth, the friction coefficient u is very small, and the friction force between the chip and the side wall of the knife edge will be reduced, resulting in easy chip jumping

2.4 influence of mechanical and physical properties of die punching materials. If the hardness of the material is high, it will be brittle, and the effective depth of being sheared will be small. The material is basically pulled apart soon after being sheared. Most of the whole shear surface is fracture zone, and the proportion of bright zone is very small. The radial shrinkage of the material is large, so the biting force is weak, and it is easy to chip jumping. The material with good plasticity is easy to be sheared, the proportion of bright bands is large, the radial shrinkage of the material is small, and the engagement with the die is good. Relatively speaking, it is not easy to jump chips

2.5 influence of excessive mold maintenance and blade wear. In order to make it easy for chips to fall out of the die, there is a slope or flare under the knife edge of the female die we designed, as shown in Figure 7. After regular maintenance and grinding of the upper surface of the knife edge, if the effective section of the blade edge has been completely ground off, the blanking gap will become larger, causing chip jumping. For the punch, the total length becomes shorter after maintenance, and the depth of cutting into the die is too shallow. It often needs an experimental machine to tighten the experiment. The chip material is close to the die surface in the die, and it is easy to be adsorbed and brought out by the punch. After the side of the die is worn, the blanking gap is too large, and the bite force between the chip and the side wall of the die is small, causing chip jumping

(3) deformation and ejection of chips, as shown in Figure 8, for some chips without closed cutting line, due to the lack of mutual engagement of one or several die side walls, downward bending will occur during punching. Generally, select different parts to test at least 3 hardness values. Due to the influence of table vibration and punch rise, bending sometimes produces upward reversal, thus jumping out of the die surface. The precision products produced by the high-speed progressive die have thin material and small volume. Because the self gravity of the chip is very small compared with other forces it receives, the analysis of the impact of chip jumping can be ignored

2 countermeasures for chip jumping of high-speed progressive die

theoretically, whether the chip jumps out of the die and enters the die surface depends on the difference between the upward adsorption force it receives and the downward biting force of the side wall of the die on the chip. As long as the biting force is increased and the adsorption force is reduced, the chip jumping can be improved and prevented

2.1 improvement and prevention of design section

the key to comprehensively evaluate the advantages and disadvantages of a mold lies in the good and bad design. Therefore, we must consider comprehensively when designing, not only to prevent chip jumping, but also not to lose other properties of the mold. Otherwise, the mold is congenitally deficient, and subsequent debugging is difficult to improve

2.1.1 design reasonable blanking clearance

many textbooks have studied the reasonable blanking clearance of different materials. Generally speaking, when the single-sided blanking gap is greater than 5%t, the chip material cut by most of the materials will be smaller than the size of the die edge, so the biting force will be too small and it is easy to jump chip. When the single-sided blanking gap is less than 3%, the bite force between chip and die edge will be strong. From the perspective of preventing chip jumping, the smaller the blanking gap is, the better. However, the smaller the gap is, the contact pressure of the punch and die will easily cause the compression fatigue damage of the die blade, and the blade will collapse, which will affect the life of the die and produce large burrs at the same time. In the design of high-speed progressive die, it is recommended to use the blanking clearance in Table 1:

2.1.2 the shape of the cutting edge

when designing the cutting edge, try to avoid too simple shape and complicate the shape, including adding some clamping grooves. As shown in Figure 9, ide-cut (edge cutting) shape, a shape is simple, and chip jumping is easy to occur. However, if the design is changed to B shape, the shape cut-off line will be increased, and at the same time, the chip material will increase the clamping groove. The shape is complex, and it is easy to be bitten by the knife edge of the die, so it is difficult to chip jumping. The best design should be C-shaped, and the clamping groove is swallow tail star, which effectively improves the biting force with the knife edge. The relationship between chip shape and chip jumping difficulty is shown in Figure 10 according to the experience accumulated in our daily design: from left to right, chip jumping is from easy to difficult. When designing the cutting tool, we will inevitably encounter some cutting with simple shape, and we must punch out this shape. In addition, the progressive die is a continuous production, the material belt needs to be positioned, and it is designed with circular holes. In order to prevent this kind of chip jumping, the knife edge can be made into a component type in the design, as shown in Figure 11, and a gap of 0.002 -0.005mm can be deliberately staggered, so that the chip during punching will deform, increase burr, and remain in the die

2.1.3 in order to effectively cut off the chips and prevent the chips from jumping out, the punch must be completely cut into the concave die according to theoretical experience

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