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When a component or a part is marked with a steel stamp, the process is basically the same as cold forging - material is being moved around or compressed. As a result, for a stamp to be durable and effective in producing a clear mark, it must be hard, have sharp edges and a smooth surface along the entire relief of the character.
In the 1980s, the EDM process was applied to the manufacture of steel marking stamps by a number of manufacturers in an effort to produce stamps faster and at a lower cost. Those objectives were never met, and the real result has been a loss in stamp tool life and consistent mark readability (see Figure 1). Production-proven tests have shown engraved stamps will typically last twice as long, and in some instances up to eight times as long.
For those reasons, Smith & Wesson (Springfield, MA) recently switched to engraved roll marking dies instead of those manufactured by the EDM process. Tom Phipps, the CEO of Columbia Marking Tools (Chesterfield Township, MI) points out, "If you look at the EDM process, it involves intense local heating, which metallurgically changes the surface of the stamp characters, but also the subsurface to a depth of 0.005 in (0.13 mm). Typically, you apply the EDM process after heat treat. And during the process, carbon in the steel material disintegrates, and decarburization occurs. Remember, it is the carbon in the steel that makes it tool steel. So stamping die life is automatically reduced as much as 400 percent by applying the EDM process."
WATCH YOUR Ps and Qs
An EDM'd surface works like sandpaper, where build-up naturally occurs on the rough surface during stamping action. This commonly occurs on closed characters like Ps, Os, Ds and As and causes readability problems because these characters appear washed out.
Because the thin EDM surface layer is untempered, it can cause premature failure where high impact or tensile stresses are encountered, or under conditions of stress cycling. Three layers are actually created:
1) The spattered EDM surface layer, which is created when the expelled molten metal and small amounts of electrode material form spheres and spatter the surface of the characters forging edge. This spattered material is now soft and is easily removed during typical forging action of a stamping operation.
2) A white recast layer is also formed by the expelled molten metal, solidifying in the crater. This molten metal is rapidly quenched by the dielectric fluid. Typically, micro cracks are formed in this very hard, brittle layer. Also, if this layer is too thick or is not reduced or removed with polishing, the effects can cause premature failure of the stamp characters in most, if not all, applications.
3) The last layer is the heat-affected zone (HAZ), or annealed layer, which has only been heated, not melted. The heat sinking ability of the material and the power used for the cut determines the depth of the recast layer and the heat-affected zone. This altered metal zone directly influences the quality of the stamp character's surface integrity.