Page 23 - MetalForming Magazine September 2022

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In addition to using the correct press, efforts to mitigate snapthrough and reverse tonnage should focus on the tool and on press speed (inertia).
While a low press speed during blanking helps reduce inertia, doing so may bring an undesirable result: a lower production rate. Using a servo- driven press or an alternative-motion mechanism, such as a link drive or knuckle joint, allows for low forming speeds while maintaining optimum production rates.
Adjustments to the tool set also can reduce the tonnage and snapthrough generated by the process. The first approach: Use shear angles whenever possible. A roof-top punch is recom- mended to limit loads. However, con- sider its effects on hole/blank geom- etry.
Staggering punches also can help, if performed properly. The objective: Have the second punch in contact with the material just in time to absorb the snapthrough from the first punch. In other words, energy released by the first tool fracture should be used during the second tool contact and so on. For this purpose, stagger punches at a dis- tance shorter than the shear zone of the material, typically 10 to 30 percent of the material thickness. This method- ology also helps to improve the timing of the blanking/piercing loads, thus reducing the peak load of the process.
Finally, using properly timed cush- ions helps to reduce the slide acceler- ation during snapthrough, avoiding reverse tonnage.
In summary, snapthrough and reverse tonnage can limit the produc- tion rate and even the process feasi- bility itself. Therefore, spend time eval- uating them and keeping them to a minimum. To minimize snapthrough and reverse tonnage, control the cut- ting speed, and use sharp and properly timed tools mounted into a press with tight stack-up clearances and robust components. And, good practice includes working with the press man- ufacturer to understand machine capa- bilities, and measuring reverse tonnage and snapthrough values. MF
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MetalForming/September 2022
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