When a component or part is marked with a steel stamp, the process is basically 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 have a smooth surface along the entire relief angle of the character.
Over the past 20 years, the EDM process has been applied to the manufacture of steel marking stamps in an effort to produce stamps faster and at a lower cost. Those objectives were really never met, and the real result has been a loss in stamp tool life and mark readability. Production-proven tests have shown that ultra high-speed milled (up to 50,000 rpm) engraved stamps that are double-pass milled will typically last twice as long, and in some instances up to 8 times as long as EDM stamps. Columbia Marking Tools uses a proprietary ultra high-speed engraving and milling system to provide the highest quality marking stamps, which in many cases is a lower cost process than the now aging EDM technology.
"Manufacturers of hand tools, for example, produce millions and millions of the same kind of components. They are very sensitive to quality and readability of their marks, which are not only functionally important, but also present a look that exudes quality and precision," explains Mike Francisco, regional sales manager for Columbia Marking Tools. "This is what we recently found at Snap-On Tools, a leading global developer, manufacturer, and marketer of tool and equipment solutions for professional tool users. Our ultra high-speed milled stamps were tested on a series of wrench sockets and proved to be better than the EDM stamps in every respect." (Shown here, one of the Snap-On sockets that has been marked with the Columbia engraved roll marker. The roll marking stamp is being held in the background.)
Getting It Right the First Time
Columbia's stamp design engineers, using 3D Solid Works® engineering software, worked closely with manufacturing engineers at Snap-On to develop tooling drawings from their CAD data so the stamps were right the first time. Snap-On has developed its own quick-change tooling for its roll marking machines, which Columbia incorporated into the engraved stamps so they could be easily interchanged with the current EDM roll marking stamps on the existing equipment. The engraved stamps were also coated with Columbia's proprietary ACC™ Auto Clean electroless nickel coating for rust prevention and dirt rejection.
"We find that a lot of manufacturers do not fully understand the quality issues related to stamps produced with the EDM process," says Francisco. "When we can visually demonstrate the difference, the benefits become obvious and improved quality is a given. Because steel marking stamps are considered secondary operation tools, it's not uncommon to find stamps operating for up to 6 months without being replaced. Combine this situation with the increased use of more environmentally friendly water-soluble lubricants and release agents and you'll find compacted dirt in the character grooves, especially on EDM stamps." (Photo shows two brand new stamps: the engraved stamp is at the lower left, the EDMed stamp at the upper right.)
Effects of Heat
"What manufacturers need to understand about the EDM process," Francisco explains further, "is that it involves intense local heating, which metallurgically changes the surface of the stamp characters as well as the subsurface to a depth of 0.005 in. Typically the EDM process is performed after heat treatment, and during the process, carbon in the steel material disintegrates and decarburization occurs. It's important to remember -- it's the carbon in the steel that makes it 'tool steel.' As a result, EDMing the surface automatically reduces stamping die life as much as 20%."
In addition, because the EDM surface layer is untempered, premature failure may occur where high impact to tensile stresses are encountered. In simpler terms, the EDMed surface works like sandpaper, with buildup naturally occurring on the rough surface during stamping and grabbing onto surface dirt. This happens on closed characters like P, O, D, and A, ultimately causing character blowout or severe premature wear so the characters appear missing or washed out. And when characters are small, the problem is compounded.
As the issue of overall part quality has evolved, manufacturers continue to search for improvements in every operation in the manufacturing process, including part marking. For some of those manufacturers, one area of improvement that has added value, but not cost, has been replacing EDM stamps with stamps that have been engraved with a precision ultra high-speed milling processes.
