Desvendando o efeito da folga e da velocidade na qualidade das peças estampadas

Unraveling the effect of backlash and speed on the quality of stamped parts

1. Die from punching

As shown in Figure 1, the die used in the punching experiment has an upper mold stroke of 45 mm.

Figure 1 Experimental mold

When the punching process is completed, the distance between the bottom of the upper mold and the top surface of the lower mold is 0.3 mm, as shown in Figure 2.

Figure 2 Experimental schematic diagram

This arrangement is made to prevent the blade from penetrating too deeply into the lower mold, thus preventing excessive wear on the upper mold.

The material used in this experiment is a copper alloy (see Figure 3), with a thickness of 0.23mm.

Figure 3 Copper alloy

2. Experimental Conditions

This experiment used four different punch gaps of 0 mm, 0.01 mm, 0.02 mm and 0.03 mm, at two punch speeds of 30 mm/s and 80 mm/s, making a total of eight experimental parameters.

The experiment was conducted on a press designed specifically for punching and used a Keyence digital microscope to observe the shape of the part after punching.

3. Experimental Results and Discussion

Figure 4 compares the burrs on the punched parts when the punching speed is 30 mm/s and the punching gaps are 0 mm and 0.03 mm.

Figure 4 Burrs (30 mm/s) at cutting gaps of 0 mm and 0.03 mm
a-0mm, 30mm/m, b-0.03mm, 30mm/s

It is clear from Figure 4 that for a gap of 0mm, the surface of the punched part is very smooth and burr-free, whereas with a gap of 0.03mm, the burrs remain at the break, measuring approximately 75μm in width.

This may be due to the rounding of the blade edge due to wear when the gap is too large, thereby reducing the stress concentration at the blade edge and causing the crack to appear further away from the blade edge.

As the punching speed increases to 80 mm/s, the effects of different punching clearances on the punched parts are noticeably different.

For a clearance of 0 mm, the cross-section of the drilled part remains burr-free, but the edge of the part is much fuller than when the speed is 30 mm/s (see Figure 5a), almost devoid of material loss due to cutting. Therefore, the surface quality is better than when the speed is 30 mm/s.

Figure 5 Burrs (80 mm/s) at cutting gaps of 0 mm and 0.03 mm

In contrast, with a gap of 0.03 mm, the burrs on the drilled part are noticeably larger and wider than when the speed is 30 mm/s, as shown in Figure 5b. Similar to the case of 0mm gap, the edge of the part produced with a gap of 0.03mm is also quite complete.

Figures 6 and 7 show cross-sectional images of punched parts formed under two different punching speeds and different punching gaps, magnified 500 times.

Figure 6 Cross section of punching parts with punching gaps of 0mm and 0.03mm (30mm/s)
a-0mm, 30mm/m, b-0.03mm, 30mm/s

Figure 6 illustrates that when the punch gap is 0 mm, the shiny and fractured zones of the punched part are very noticeable.

Furthermore, the bright zone is wider, while the fractured zone is flatter. This suggests that when the punching speed is 30 mm/s and the punching gap is 0 mm, the punched material undergoes a complete process of plastic shear deformation and fracture.

Figure 7 Cross section of punching parts with punching clearances of 0mm and 0.03mm (30mm/s)

In the case of a gap of 0.03 mm, distinct bright and fractured zones do not appear, but quite noticeable collapsed corners and a gradual process of plastic flow are observed.

This reveals that an increase in clearance is unfavorable for rapid fracture of the material. Too large a punch gap weakens the shear deformation of the material and increases plastic flow, significantly impacting the cross-sectional accuracy of the punched part.

On the other hand, when the punching speed increases, as can be seen in Figure 7a, the bright zone decreases and the fractured zone increases with a gap of 0 mm. This indicates that an increase in punching speed accelerates the shear deformation and fracture process of the punched material.

Figure 7b also demonstrates that even with an increase in gap under high-speed punching conditions, the shear deformation and fracture process of the punched part remains clear and complete. This shows that an increase in punching speed is beneficial to ensure the quality and accuracy of the punched part cross section.

4. Conclusion

The impact of clearance and punching speed on the cross-sectional quality of the punched part is evident.

For thinner copper alloy punched parts, choosing a smaller gap has a direct impact on suppressing burr formation: smaller gaps lead to rapid shear deformation and fracture processes, resulting in smooth surfaces and a burr-free result .

Furthermore, an increase in punching speed can accelerate the elastic and plastic deformation of the material, controlling the plastic flow and improving the surface quality.

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