Tooling Tips for High-Speed Stamping

Saturday, November 1, 2008
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Punch-to-matrix clearance
Tooling Tips
directly affects wear and slug control. High-speed stamping applications magnify these effects.

How fast is fast? We define high-speed stamping as an operation that generates special needs due to fast operating speeds. These special requirements generally relate to stock and slug control and excessive wear problems.

Speed-related problems typically start when press speeds exceed 100 strokes/min.

“Tool chipping and wear cause the most headaches for high-speed stampers,” says Jim Angelo, regional sales manager for Dayton Progress, citing an increase in the amount of high-speed stamping of hard materials such as full-hard stainless steel as well as tempered steels. “In these applications, alignment is critical, as is proper tool-steel selection and clearance. Typically, high-speed stampers experiencing excessive punch chipping and wear need to upgrade to a high-performance high-speed tool steel, such as a particle-metallurgy (PM) steel. Also, many experience slug-pulling problems due to using too much clearance. Punching out regular mild steel, a per-side clearance of 10 to 12 percent of material thickness. But at high speeds, you need a tighter clearance to avoid slug pulling—we recommend eight percent or less per side depending on the material and its hardness.”

Punch-to-die clearance can be described one of two s: total, and per side—the distance between the cutting edges of the punch and matrix.

The matrix offers some of the best methods of slug control for high-speed stamping applications. Minimal land lengths allow slugs to enter the taper sooner and prevent slug jamming.

Methods of holding a few slugs in the matrix can prevent slug jamming and pulling.

A 1⁄8-in. land is the industry standard. This works well in most applications. Because high-speed applications tend to deal with thin material, you can usually reduce the land. Land length should not exceed three to four times the material thickness.

Also in high-speed applications, use of minimum land lengths will effectively minimize slug pulling. A reverse taper also will increase tool life, as well as reduce slug pulling by tapering down to a dimension less than the slug diameter for a short distance before tapering back out at a quarter degree per side or more. Increased taper is designed to prevent soft part material from sticking in the relief area, which minimizes slug stacking.

Finally, basic die principle number one is to ensure precise alignment to prevent off-center loading and uneven tool wear, says Angelo, and in high-speed applications this rule rings particularly true. “I see die shops fabricating high-speed tooling by stacking their die sections in a wire-EDM machine, to ensure precise alignment,” he explains, “cutting the holes in mating sections at the same time.”

Patented Slug-Control Grooves

The practice of grinding precision grooves in the matrix land was designed specifically for high-speed stamping applications. The opposing grooves create small ears on the slug. The grooves spiral in opposite directions, holding the slugs to one side of the matrix to prevent them from pulling up when the punch withdraws.

The grooves vary in size based on material thickness and punch-to-matrix clearance, and can be applied to tapered and counter-bored relief matrixes as well as to shapes. They are ideal for use in applications where punch-to-matrix clearance does not exceed 10 percent per side.

Also during high-speed stamping, metalformers should keep punch entry to a minimum (0.015 to 0.030 in. is ideal) to prevent removal of the small tabs created by the grooves, but as much as 0.060 in. is acceptable. These tabs are essential for retaining the slugs in the matrix.

Tool-Steel Considerations

Tooling for high-speed applications requires a high degree of wear and temper resistance, typically indicated by a tool steel’s alloy content. While chrome offers some temper and wear resistance, molybdenum, tungsten and vanadium prove more effective when used in sufficient amounts.

The chart (Fig. 1) lists some of the commonly used tool

Typical Tool-Steel Compositions
Fig. 1
steels and their alloy content. Each alloy element listed in the table contributes to a specific characteristic in the finished steel. But these alloys also can create undesirable side effects, particularly when used in excess. Alloy elements also can react with each other, which may enhance or degrade the final result.

If toughness was the only factor in selecting a tool steel, S7 would be the obvious choice. Unfortunately, this toughness is achieved at the expense of other characteristics necessary in most stamping applications. Also, toughness tends to drop as alloy content increases. Higher alloy content also demands a higher price.

The manufacturing process also affects the toughness of a tool steel. The PM process can greatly enhance toughness of a given tool-steel grade over its conventionally made counterpart. For example, note the difference in toughness between M4 and CPM M4 (an alloy from Crucible Materials Corp.) in the chart below (Fig. 2).

Hardness also affects toughness. Any given grade of tool steel has greater toughness at lower hardness. Be aware that the lower hardness may have a negative effect on other characteristics necessary in achieving sufficient tool life.

Tool-Steel Characteristics
Fig. 2
Compressive Strength

At first glance, high-speed stamping applications do not appear to require a great deal of force to perforate or blank parts, as parts generally are small and manufactured from thin, relatively soft material. Because the part material is thin and typically soft, it tends to stack up and jam in the matrix or die plate. If the punch lacks the necessary compressive strength, it fails.

Two factors affect compressive strength: alloy content and punch-material hardness.

Alloy elements such as molybdenum and tungsten contribute a great deal to compressive strength. Also, higher hardness of a given steel grade increases that steel’s compressive strength.

Alloys such as molybdenum, tungsten and cobalt that contribute to compressive strength also tend to improve temper resistance and red hardness. This is important in high-speed applications where heat buildup in the tool is a concern.

Temper Resistance

Temper resistance is the tool steel’s ability to maintain hardness after exposure to heat. Compromising hardness of a given tool steel reduces wear resistance and strength.

Several factors influence temper resistance. Alloy content is the primary contributor. The level of refractory-alloy elements—molybdenum, tungsten and cobalt—significantly affects a steel’s resistance to being tempered to a lower hardness. Other alloy elements, such as vanadium and chrome, also provide some temper resistance.

The temperature where a tool steel’s temper resistance is affected is directly affected by the tempering temperature used during initial heattreatment. A service temper within 50 F of the tempering temperature directly affects a tool steel’s temper. The bar graph (Fig. 3) illustrates the temper-resistant advantage of high-speed and high-alloy steels such as M2, CPM-M4 and CPM-10V over common cold-work tool steels like A2 and D2.

High load applications such as high-speed stamping of stainless, spring and HSLA steels call for tool steels with a combination of shock resistance and high compressive strength—M2 or CPM-M4 perform best in these applications.



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See also: DAYTON Lamina Corporation, Crucible Materials Corporation

Related Enterprise Zones: Tool & Die

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