Specifying sheet metal with little technical information also can affect die performance. For example, specifying Type 1008 steel allows for the supply of a wide range of properties. A low-carbon, vacuum-degassed (VD) steel could fall within this specification range as well as a Type 1006 drawing steel (DS) or a Type 1008 commercial steel (CS). All have less than a 0.10-percent weight of carbon and are aluminum-killed. And, all perform differently in the die.

In many shops, internal struggles persist around who decides on the material to be used for production. Is it the purchasing department (“we got the best price”), engineering department (“we want the most formable material”), estimating (“this is what we quoted”) or sales (“our competitor quoted a cheaper grade”)? The real answer: The die decides. 

If a stamping die could talk to its designer, it might say, “If you want me to produce this part to those specifications, using that lubricant and this die design, then I am going to need sheet metal with the following attributes…”

The problem that arises: The die speaks a language that most designers don’t understand. The language of the die is mechanical properties: strain, strain-rate, n-value, r-value, yield stress, elongation, coefficient of friction, etc. The die doesn’t understand tradenames, composition, Rockwell hardness or standards. To be successful, designers must learn the language of the die and how it applies to their designs.

Influence of the Press

Die designers also must understand that a centered ‘load’ and a centered ‘die’ are not the same. In a stamping die, forces distributed unevenly across the press slide cause the slide to tip and move laterally in the direction of the greatest force. A load not centered under the press ram may alter critical die clearances. If this movement occurs while punches are engaged with other die components, excessive wear or damage can occur. Rapid die wear increases die-maintenance costs and compromises the dimensional consistency of the stamped parts. 

Also important: the press tonnage rating. The available tonnage decreases in mechanical presses as the working distance above bottom dead center (BDC) increases. This ‘de-rating’ of tonnage occurs because the relationship between the crankshaft angle and the pitman has reduced mechanical advantage higher up in the stroke. This holds true for flywheel-drive mechanical presses and their servo-drive counterparts. Tonnage de-rating proves particularly important with deep drawing, where forming can begin several inches above BDC.

Cutting and blanking stresses all produce unloading forces in stamping presses—known as snapthrough or reverse tonnage. High-tensile snap-through forces introduce large downward accelerations that can damage the die and the press. 

A press factor not as well-understood as tonnage is working energy—a function of the press load and the distance through which the load must be applied. Energy expends with each stroke of the press, and must be replaced prior to the next stroke. In combination, the flywheel stores and delivers the required work energy while the electric motor restores depleted energy by maintaining flywheel speed and avoiding excessive slowdown. 

Forming and drawing processes can consume large amounts of press energy due to their long working distances. For example, pushing 50 tons through 1 in. of forming requires 50 in.-tons of energy. On the other hand, drawing 3 in. deep will require 150 in.-tons of energy. Changing to a high-strength material with twice the strength would require a press with at least 300 in.-tons of available energy. 

Want to learn more about progressive dies and deep drawing processes? Consider attending PMA’s Progressive Die Technology seminar (October 6-7, 2021) and our all-new Deep Draw Boot Camp (November 9-11, 2021). Both events will be held live in Cleveland, OH. Visit www.pma.org for more information or contact Marianne Sichi at msichi@pma.org. MF

Technologies: Tooling

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