Page 21 - MetalForming Magazine September 2022
P. 21

 Reverse tonnage and snapthrough—they’re not the same—could destroy your press. Find out why, and how to minimize these loads.
BY DAVID DIAZ-INFANTE
 Tension Compression Tension
  DON’T LET TONNAGE Put You in Reverse
While most stampers are aware of reverse tonnage, not many can differentiate it from snapthrough. Consequences of expe- riencing one or the other differ, and, while one typically is monitored in press operations, the other is not. To understand the difference between reverse tonnage and snapthrough, let’s review how stamping forces interact with the press.
Generating Snapthrough and Reverse Tonnage
In operations such as blanking or pierc- ing, the stress config- uration leading to for- ward tonnage (Fig. 1) may be reversed. In these operations, when the material reaches its plastic limit and fractures, all of the energy stored
David Diaz-Infante, Ph.D., is a servo press scientist at Nidec Press & Automation, Min- ster, OH; www.nidec- pa.com.
during deformation suddenly is released. This leads to a quick accel- eration of the slide components, gen- erating snapthrough. Fig. 2 shows a load vs. time graph, with snapthrough represented by a sudden drop in load. A large-enough drop takes the con- necting rods from compression into tension, creating reverse tonnage.
Despite their close relation, snapthrough and reverse tonnage can
exist independently, with any of these situations possible:
• Snapthrough with reverse tonnage (in most cases)
• Snapthrough without reverse ton- nage (snapthrough not large enough) • Reverse tonnage without
snapthrough (in some high-speed applications).
Snapthrough
Snapthrough, or quick acceleration of the slide compo- nents, can lead to excessive noise, vibration, loose components, leaks, premature wear, broken tools, part- quality defects and press-component fatigue. Due to its nature, the effects of snapthrough should be measured using accelerometers, as it cannot be measured with strain gauges attached to the uprights.
The greater the stack-up clearances on a
 18 MetalForming/September 2022
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Fig. 1—Stresses on a press during stamping (forward tonnage).















































































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