Peter Ulintz Peter Ulintz
Technical Director

Estimating Reverse Tonnage During Die Design

April 22, 2022

Cutting, blanking and punching stresses produce unloading forces in stamping presses, referred to as snapthrough or reverse tonnage. Because thicker and higher-strength materials require increased cutting clearance and greater force to shear as compared to mild steel, both will generate proportionally increased unloading forces. Understanding the magnitude of these forces during the die-design process allows engineers to properly assign presses and incorporate countermeasures into the die design.

Typical Blanking Operations

To better visualize what occurs during a typical blanking operation, refer to Fig. 1. 

tooling-by-design-force-time-curve-blankingThe Y axis depicts press force in terms of forward (positive) tonnage and reverse (negative) tonnage. The dashed horizontal line represents zero tons. The X axis represents cycle time in milliseconds (ms). The curve shows the rapid buildup and release of press forces with respect to time during the working portion of the press cycle.

As the ram travels downward, the die contacts the work material at point A. Resistance builds quickly as the slide is counter-pressured upward—eliminating all clearances in the mechanical connections between the slide and the press crown. As the ram continues to descend, it builds force until reaching the shear strength of the material (point B). The punch begins to penetrate the material until it overcomes the strength of the material (point C) and the blank fractures (breaks) free.

The steep incline downward illustrates the release of energy after fracture. At point D, the curve passes below the zero-tonnage line where a negative force (reverse tonnage) is generated in the press frame (point E). 

For most blanking applications, the complete energy release portion of the cycle occurs within 20 ms. From load-monitoring systems and knowledge of press deflection, it has been shown that the energy release can generate downward slide accelerations approaching 10 g, or 10 times the acceleration of gravity (Wonsetler and White, The Use of Hydraulic Shock Dampers to Arrest the Reverse Load of Blanking Presses, W-Technologies, Inc., 2002).

Finally, the elastic deflection stored in the press frame dissipates through high-frequency oscillations occurring between points E and F.

The Impact of Negative Tonnage on Die Design

High-tensile unloading forces introduce large downward accelerations to the upper die half. These forces essentially work to separate the upper die from the bottom of the ram on every stroke. Insufficient force of the hydraulic die-clamping system or mechanical clamping could cause the upper-die half to separate from the bottom of the ram on each stroke, causing fatigue to the upper-die mounting fasteners.

Uncontrolled, high reverse loads can fatigue the press structure. Fatigue-related cracks can propagate in the slide, drive linkage, crown, uprights and bed. Eventually, this results in a catastrophic (sudden) fatigue failure. Reverse loads also have been associated with quality issues due to premature wear of punches and dies.

tooling-by-design-reverse-loading-yield-strength-tensile-strengthMost presses are designed for approximately 10 percent of their maximum-rated forward load in reverse tonnage (check with your specific machine builder). For example, an 800-ton press with 10-percent reverse-tonnage rating should not exceed 80 tons of reverse load.

A press equipped with a suitable tonnage monitor can measure reverse load. Unfortunately, during the die-design process, tonnage-monitor data are not available. Instead, the die designer must estimate the reverse tonnage to ensure no overloading of the selected press, sufficient holding force of the upper-die clamping system and no damage to the die itself.

Estimating Reverse Tonnage

The ratio of material yield strength (YS) to ultimate tensile strength (UTS) was reported to be a critical factor in determining the amount of forward tonnage converted to reverse tonnage when blanking in a press (Wonsetler and White). Empirical tests were conducted to predict the unloading force (Fu) as a function of the YS/UTS ratio:

Fu = [0.893 (YS/UTS)2 ] + 0.107

When plotted as a graph, the strong relationship that the ratio of yield strength to tensile strength has on reverse loading can be observed (Fig. 2). Use this graph to estimate the reverse tonnage produced by a blanking process without dampers. 

Assume a 24-in.-dia. blank produced from 0.182-in.-thick steel with 30,000 psi YS and 60,000 psi UTS. The YS/UTS ratio here: 0.5.

Find the reverse-load factor from the curve for 0.5 YS/UTS ratio. This example yields a reverse-load factor of 0.33.

Next, estimate the force (tonnage) required to produce the blank:

P x t x Ss/2000  = 308.7 tons


P = part perimeter (24 in.)(π) = 75.40 in.

t  = material thickness = 0.182 in.

Ss = material shear strength = 45,000 psi

Obtaining accurate shear stress data (Ss) is quite difficult; thus, we use approximations. The approximate shear strength of mild steel measures between 70 and 80 percent of its tensile strength. The above example used 75 percent.

Now multiply the estimated blanking force by the reverse load factor:

308.7 x 0.33 = 101.87 tons

The result must be less than the rated capacity of the press in reverse load.

An 800-ton press with a 10-percent reverse-tonnage rating would be overloaded in reverse tonnage (101.87 tons) even though the blanking tonnage (308.7 tons) falls well within the forward-tonnage rating of the press. This is critical information for the die designer as the proposed process must change. 

Here, the designer has several choices.

Shear can be added to the die cavity to reduce the blanking tonnage. The force would need to be reduced to at least 200 tons. A reverse load factor of 0.33 would result in a reverse load of 66 tons. Within the rated capacity of the 800-ton press, this amount also leaves a margin of safety for die wear, which will increase the blanking force.

Or, add approximately 80 tons of counterpressure inside of the die using nitrogen or hydraulic cylinders. Shock dampers mounted to the press bed—outside of the die—provide another option. Any counterpressure—inside or outside of the die—proportionally increases the blanking tonnage. Be sure that the increased force does not exceed the rated capacity of the press. Adding 80 tons of counterpressure in our previous example increases the blanking tonnage to approximately 389 tons and the reverse tonnage to 128 tons. But the 80 tons of counterpressure counteracts this 128 tons, resulting in 48 tons of reverse load.
The final option: Select a different press with greater reverse-tonnage capacity. MF

Industry-Related Terms: Bed, Blank, Blanking, Die, Point, Ram, Stroke, Tensile Strength, Thickness
View Glossary of Metalforming Terms

Technologies: Tooling


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