Managing Reverse Tonnage—What Every Press Technician Should Know
July 26, 2024Comments
Cutting, punching and blanking stresses produced by stamping operations generate unloading forces that can damage dies and presses. These forces commonly are referred to as snapthrough, reverse unloading or reverse tonnage.
Stampers must anticipate the magnitude of these forces during the die-design process so that engineers may assign presses properly and incorporate countermeasures that safeguard the die and the press. Press technicians should understand the cause and effects of these unloading forces and know the implemented countermeasures to prevent unintentionally negating their benefits.
Understanding Reverse Unloading
Thicker and higher-strength materials require increased punch-to-die cutting clearance and greater force to shear the material as compared to working with mild steel. Both conditions produce increased unloading forces—some great enough to damage presses.
For comparison, a person using a three-hole paper punch (Fig. 1) experiences similar unloading forces. Punching holes in a single sheet of paper (Fig. 1-a) requires relatively little effort. Punching the maximum number of sheets that fit into the hole punch (Fig. 1-b)—think of this as thicker material—requires substantially greater force. When the punches penetrate the pages, the built-up forces suddenly release, sometimes resulting in smashed fingers and knuckles on the tabletop.
Changing the work material from paper to tempered shim stock (Fig. 1-c) also results in large unloading forces even though the shim-stock thickness is equivalent to a single sheet of paper. This is due to the shim stock’s higher tensile strength and the corresponding increase in force required to perforate it.
Other causes of reverse unloading: punch velocity at the time of material contact, the amount of deflection in the press structure and bearing clearances in the press drivetrain.
Reverse-unloading forces introduce large downward accelerations to the upper-die half when suddenly released. These forces essentially work to separate the upper die from the bottom of the ram on every stroke. If the die-clamping system has insufficient clamping force, the upper-die half could separate from the bottom of the ram on each stroke, causing fatigue to the upper-die mounting fasteners. 
