December 11, 2007
Modernization has brought us many advancements, and two big advancements that fabricators must deal with are in technology and commerce. On the technology side, advancements have led to modern alloys; on the commerce side, we have increased globalization. Many modern ferrous alloys are harder to cut than carbon steel, and while the cutting is more difficult, globalization is increasing the competitive pressures and reducing the profit margins. Other advancements have modernized band saw blades, giving fabricators a fighting chance to stay profitable.
Steel is all around us. It's in the cars we drive, the bridges we cross, and the buildings we live and work in. The unique strength properties and overall versatility of steel have made it the material of choice since it was adopted by the construction and fabrication industries nearly 150 years ago. As a key component of our transportation infrastructure, steel has been, and continues to be, an important player in increasing globalization for many industries. This globalization, although helping to sustain the steel industry, has changed it as well. Manufacturers now are required to make advancements every day to keep up with this changing marketplace.
Advancements in the steel industry are vital to both economic competitiveness and growth. According to the American Iron and Steel Institute (AISI), many of the steels made today did not exist as recently as 20 years ago. Most grades of steel in use today are lighter, stronger, thinner, and easier to shape. Advancements in galvanizing and coating steel—particularly high-strength steels—have led to a greater ability to resist decay and corrosion more than any other material.
But these steel advancements have brought with them new challenges for band saw blade manufacturers. The special treatments, added elements, and different melt source processing techniques tend to alter the properties of steel and make it more challenging to machine. For example, the elements added to make steel corrosion-resistant can work-harden when they are sawn and cause premature blade wear and band breakage due to fatigue.
In addition, globalization means that standard items ordered from traditional local steel service centers can come from a variety of sources, both domestic and foreign. A variety of sources typically leads to variability in the steel's machinability, which directly affects tool life and, therefore, profits for manufacturers.
Increasing costs are forcing manufacturers and machine operators to seek more economical ways to cut steel. Modern, advanced metals have pushed equipment-makers to generate new saw machine designs and improve saw blades to help keep manufacturing costs under control. Over the years a number of band saw blade advancements have helped fabricators sustain profitability and remain competitive in the global economy.
Improvements include more durable cutting edge and backing materials for bimetal blades, improved carbide-tipped products, coatings, heat-treating techniques, and new tooth forms designed with advanced computer modeling techniques. Many manufacturers are working to blend these technologies together.
Increasingly sophisticated techniques for modeling tooth forms, in combination with improved band saw design, have led to improvements in carbide-tipped blades for a wider range of sawing applications. A hollow section such as a tube or pipe is a unique cutting challenge because it is an interrupted cut—the blade cuts the two walls, but the cut is interrupted in the empty space between the walls—which can create substantial vibration. For such applications, blades are typically bimetal, have zero rake angle (or a negative rake angle), and have a low back clearance angle. New tooth form designs are now also allowing carbide-tipped blades to be used for this application, one which was previously not an option. These elements provide a durable tooth tip to withstand both the stresses of making interrupted cuts and vibration.
In addition, as carbide blade technology has become more widespread, manufacturers have developed new carbide grades to optimize its effectiveness. Producing blades with the toughness and durability necessary for band sawing used to mean using grades of carbide that could not withstand abrasive cutting. New chemical content and grain size distributions allow manufacturers to produce band saw blades without such compromises.
As tooth forms and edge materials improve, the limitation in blade life is often now the durability of the backing steel that supports the tooth structure.
Blade surface preparation techniques can reduce the occurrence of band breakage caused by metal fatigue. Such breakage can be the result of many factors, or combination of factors, including improper band tension; band guides in poor condition or improperly adjusted; improper tracking of the band on the wheels; and conditions in which bands are required to twist severely between the wheel and guides. In all these cases, the stresses on a blade usually begin at points that metallurgically are susceptible to cracking.
Coatings. To help minimize the effects of these new materials on overall blade life, many manufacturers suggest using a physical vapor deposition (PVD) coating to stand up better to new material challenges. PVD coatings commonly used for cutting tools are titanium nitride (TiN) and aluminum titanium nitride (AlTiN). TiN can provide a Vickers hardness of approximately 2,500 Hv; AlTiN provides approximately 4,100 Hv.
Surface Compression. Blade surface preparation, which takes place after heat treatment and tempering, can significantly reduce the likelihood of cracking under stress. The blade preparation process places a very thin external layer of the blade material under extreme compression, which makes it less likely to crack. This altered backing steel increases fatigue resistance and fracture toughness and decreases notch sensitivity. Surface preparation also ensures that each tooth tip is rounded into a stable, maintainable surface that can endure the rigors of cutting forces and vibrations, resulting in a more consistent and longer- lasting cutting tool.
Peening. Peening is one blade surface preparation technique increasingly used to combat stress fractures. It is a relatively new process to the band saw industry, but has been used successfully in the aerospace industry where fatigue life is a constant concern. Unlike applications in other industries, the relatively thin cross section of a band saw blade creates a major challenge for manufacturers in determining the optimum stress configuration for the band material.
Peening or shot peening is a process of stressing the surface layer of a material by bombarding it with a selected medium (usually round steel shot) under controlled conditions. In addition to steel shot, several other media can be used, such as glass and ceramic bead and cut wire. Each media is selected based on the material being peened and desired outcome. Once this is determined, the ability to achieve the desired end result is through experimentation with several key parameters, including the material and size of the shot media and the intensity and duration of the process. Peening is a complex process, but the finished product is simple: a blade with improved resistance to breakage and a much more consistent edge.
The industrial and global forces that have created newer, harder, and tougher materials and made cutting more competitive aren't going away any time soon. Today's difficult-to-cut materials, such as high-strength steel (HSS); high-strength, low-alloy (HSLA) steel; and ultrahigh-strength steel (UHSS) might be commonplace tomorrow, and other, more challenging alloys are likely to appear. Where such alloys lead, blade, sawing machine, and coating manufacturers are sure to follow.
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