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Ask the Stamping Expert: Which tool steels are best for high-speed progressive dies?

How to find the right mechanical properties for the application

Ask the Stamping Expert: Which tool steels are best for high-speed progressive dies?

Many metal can lids would typically be stamped on a high-speed press. But tool steel selection depends on the application and environment. Stampers need to know the steel’s mechanical properties to make the right choice. Getty Images

Q: What tool steels do you recommend using in high-speed progressive dies?

A: I have been asked this exact question many times—and there is no one best response. It depends not just on the application, but also the environment. You need to know the tool steel’s mechanical properties, heat-treated or not, describing how it will react or flex when outside mechanical forces are applied:

  • Tensile Strength. Most metal fabricators put a lot of weight on tensile strength when choosing a tool steel to be used in the construction of a progressive stamping die, but shear strength and compression strength are related and valuable to know as well. I typically do not consider tensile strength when choosing a tool steel, because I value other properties much more. But in part/product design, it is one of my top considerations, since it measures how much tension force can be applied until deformation.
  • Fatigue Strength. This is another property I generally do not relate to my tooling, unless it’s an extreme forming application. But if I manufactured springs, it would be at the top of my list, since it measures how many times the metal can be cycled through loading and unloading before failure.
  • Elasticity. While rarely considered, elasticity is very important when tooling needs to flex and return to its original shape without any permanent deformation.
  • Hardness. Hardness and toughness are the two most critical properties that I look at when choosing tool steel for cutting and coining. Hardness is the ability of the material in its heat-treated state to withstand indentation. If you are stamping light-gauge material (less than 0.01 inch), opt for the hardest possible cutting tooling—carbide—unless the geometry is so fine it does not hold up. If so, back down on hardness and increase toughness, such as a Crucible CPM® grade of tool steel.
  • Toughness. This is the ability of the material to absorb energy and shock to withstand degradation. I once worked on a job that was required by design to have a forming arbor with a nonsymmetrical load. We tried several materials based on hardness that didn’t work, but when we focused more on toughness, sacrificing hardness and some tooling life, we got the results we wanted. We ended up choosing Crucible CPM 3V, a high-toughness material designed to provide maximum breakage resistance. Heat treating, stress relieving, and triple tempering were essential in this case. We also added cryogenic freezing between the last two tempers.
  • Coefficient of Expansion. Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature. Steel grows when it is warmed. We once worked with a 2.5-foot-long die chase manufactured from A6—the standard material for chases. The progression on the final die strip produced was +/-0.002 in. on a 2-ft. length of strip. We did not notice that when the chase was heated up 20 degrees F, it would grow 0.002 in. We corrected this problem by adding environmental controls to maintain the temperature in stamping at exactly 75 degrees F.

Several other mechanical properties—malleability, ductility, abrasion resistance, and brittleness—are important in very specific applications, but fatigue, hardness, and toughness are the main players.

But in the end, in the high-speed environment, I estimate 20 percent of problems are related to materials and 80 percent are related to design and build. Eliminating vibration with a robust tool design, staggering punches to minimize snap-through shock, and minimizing the press stroke can do more than any punch material can do. In one application punching thin metals, we were able to increase tooling life fivefold by eliminating vibration. Carbide does not wear like conventional steel—the edges break down and disintegrate—so no vibration yields virtually no wear.

About the Author
Micro Co.

Thomas Vacca

Micro Co.

Has a shop floor stamping or tool and die question stumped you? If so, send your questions to kateb@thefabricator.com to be answered by Thomas Vacca, director of engineering at Micro Co.