If you call your product unique, it had better be one of a kind. The L. S. Starrett Co. is so certain that its patent pending process for manufacturing bi-metal saw blades is sui generis that the Athol, Mass., firm has trade-named the product Bi-Metal Unique. Originally developed for manual and portable power saws, the company is now at work on band saw blades for stationary production machines.

For 35 years, the high-speed steel tooth surface of a bi-metal blade has been bonded to the high-strength steel backer or carrier strip by fusion processes, first by electron-beam welding and later by laser welding. What both processes have in common, and what Starrett claims is its Achilles heel, is melting the HSS strip to fuse it to the carrier.

"Melting produces a heat-affected zone," says John Caron, global director of research and development for saws at Starrett. "That zone is unavoidable with electron beam or laser welding."

The heat-affected zone is a stressed area that can cause teeth to shear from the carrier. Teeth are especially vulnerable during interrupted cuts, such as sawing a cross section of tube or pipe. As one tooth breaks contact with the cut and the next tooth enters the cut, the shear force is so great that tooth breakage occurs frequently, according to Caron.

In typical situations it's the entire tooth that separates at the heat-affected zone, and the broken tooth can become imbedded in the workpiece. Contact with the broken tooth, Caron states, can result in catastrophic blade failure that shears off multiple teeth.

Diffusion Bonding Trumps Welding
Starrett's R&D department determined that eliminating the heat-affected zone would reduce the tooth-stripping problem, and it found a promising alternative to welding in diffusion bonding. "It's a solid-phase process that uses only 10 percent to 20 percent the amount of heat rise found in welding," says Caron. "Over a five-year period we developed the process for manufacturing bi-metal saw blades."

Caron describes solid-state diffusion bonding as a combination of low heat and high pressure that applies HSS wires to both sides of the carrier with pressure rolls in an inert atmosphere.

In the electron-beam and laser processes, one HSS wire is butt-welded to the edge of the carrier. The joining line is only the thickness of the carrier and the fusion zone is perpendicular to the teeth. In diffusion bonding, with HSS wires bonded to the sides of the strip, the contact area is increased by 170 percent, and the fusion zone is parallel to the teeth. This configuration produces what Starrett engineers call grooved-tooth geometry.

"If you look at the front of the tooth you'll see two cutting surfaces separated by the carrier," Caron explains. "The blade structure is like a sandwich."

The carrier strip wears when cutting begins, and the wear forms a small U-shaped groove between the two HSS edges. The groove is 0.001-inch to 0.002-inch deep and remains at a constant depth, wearing at the same rate as the teeth.

The ‘Split-Chip Advantage'
Starrett registered the name Split-Chip Advantage to describe the twin-tooth design, which produces two chips. "The two teeth break each chip into two pieces, and that is a significant benefit in reducing chips bonding to the saw teeth," Caron says.

Chips bonded to the teeth produce friction that increases as tooth temperature rises. Split chips tend to curl and fall away as strings instead of chunks, and they're less prone to cling to the teeth.

"Producing twice as many chips roughly half the size allows the blade to run cooler," Caron says. "Coolant flow carries the chips away faster, and the center groove forms a coolant passage that directs coolant into the tooth gullets."

Starrett has brought the technology along slowly. Bi-Metal Unique made a modest debut 18 months ago in manual hacksaw blades manufactured at Starrett's Itu, Brazil, plant, and the blades were first distributed in the South American market. The initial offering was made there because the relatively low cost of labor creates a huge market for hand tools. The line has since expanded to include portable power tools: hand-held band saws, jigsaws and reciprocating saws. Production has expanded to Starrett's Mount Airy, N.C., factory.

New to North America
The North American debut for Bi-Metal Unique was last September at the International Manufacturing Technology Show in Chicago, with a display of the hand saw blades only. Within the next six months Starrett will begin beta testing of band saw blades for stationary production saws. The first out will be 1-inch wide by 0.035-inch thick, one of Starrett's highest-volume products. Larger sizes will be brought out as the technology progresses.

Initially, Starrett intended to use Bi-Metal Unique only for hacksaw blades. But as R&D progressed, company engineers were "amazed as to where we should go with the technology," says Caron. "Through all the testing we've done on those items, we've discovered that we need to go further than we intended in the beginning because the benefits are greater than we imagined. I've been in the saw blade business for 30 years, and this is the first process change in tooth joining I've seen."

Nevertheless, Caron sees a finite boundary to the technology. "Every time we step up to a larger blade, we encounter a much tougher application," he says. "There may be some upper limitations on where we go with diffusion bonding."

Increased durability and higher cutting speeds were the goals of the project, and Starrett claims up to 22 percent longer blade life and 20 percent faster cutting. But these tests were performed only on the hacksaw blades. No working data are yet available for large band saw blades.

The cost of diffusion bonding for band saw blades is expected to be competitive with fusion welding. Although diffusion bonding is more costly, the process of making the highspeed steel for it is not as critical. The metallurgy of the M-42 steel used for fusion-welded blades must have vacuum melting properties. This isn't a factor with diffusion bonding, which eliminates molten metal.

Even if the cost is higher, Caron is confident that diffusion-bonded blades will prove cost-effective by their improved performance. Even at a slight premium, he says "if you can buy a blade that lasts longer and cuts faster, there will be a saving over the life of the blade."

As diffusion-bonded band saw blade production ramps up, for a time Starrett will continue to offer fusion-welded blades as well. But Caron expects to convert exclusively to diffusion bonding for blades 1-inch wide and smaller. In the offing: diffusion-bonded blades larger than 1-inch wide. How much larger remains to be determined.

Don't expect diffusion bonding for carbide blades. It isn't the next technology target, according to Caron, because further development is needed for larger bi-metal bands. Experiments have, however, been run with diffusion bonding for powder metal blades. Stay tuned.

Investing in R&D
L.S. Starrett's commitment to R&D is an investment that Caron believes is essential to meet customer demands. "Our customers are the agents thatdirect us to pursue R&D," he says. "We are reaching out further than we ever had to in the past because of the competitive nature of the U.S. and
world marketplaces.

"The evolution of the sawing industry is speed and quality of cut," he continues. "Sawing must be done at a very fast rate, and separation quality needs to far exceed what it used to."

Tight tolerances are a given, and scrap reduction is also a goal. The saw kerf must be minimized, and the cut surface is expected to be a finished edge.

Caron points out that for many manufacturers, sawing is becoming a first machining operation. Companies want to eliminate edge grinding or milling before they do additional processing. "The goal," Caron concludes, "is to minimize both the amount of time and the amount of waste in using new sawing technologies."

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