Page 19 - MetalForming Magazine May 2022

Tooling by Design
By Peter Ulintz
Estimating Reverse Tonnage During Die Design
A BCD
Unloading
Reverse Tonnage
20 ms
Peak Forward Tonnage
F
Peak Reverse Tonnage
E
TIME (ms)
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 dur- ing a typical blanking operation, refer to Fig. 1.
The 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—elimi-
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His back- ground includes tool and die making, tool engi- neering, process design, engineering manage- ment and advanced product development. As an educator and technical
presenter, Peter speaks at PMA national seminars, regional roundtables, international conferences, and college and university programs. He also pro- vides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
Fig. 1—This curve plots force vs. time for typical blanking operations.
nating 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 illus- trates the release of energy after frac- ture. 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 accel- eration 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 intro- duce large downward accelerations to the upper die half. These forces essen- tially 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, caus- ing 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
16 MetalForming/May 2022
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REVERSE FORWARD TONNAGE TONNAGE