Bi-Metal Blades COBALT-STEEL ALLOY-TIPPED BLADES. EQUIPMENT REVIEW by Dave Boyt. 4 Sawmill & Woodlot

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EQUIPMENT REVIEW by Dave Boyt The author used a bi-metal band saw blade to slice cleanly through the metal wire in a large old black walnut log, leaving (debatably) interesting points of metal detail

Bi-Metal Blades COBALT-STEEL ALLOY-TIPPED BLADES At first glance, the 26-inch-diameter, black walnut log looked like a good candidate for slabbing into tabletops, even though it was only 6 feet long.

To see if I could salvage it, I ordered a couple of bi-metal blades which were advertised to cut through nails.

At $60 each, the cost was still only about half the value of a single slab, so it seemed a good investment.

1 EQUIPMENT REVIEW by Dave Boyt The author used a bi-metal band saw blade to slice cleanly through the metal wire in a large old black walnut log, leaving (debatably) interesting points of metal detail in the lumber. The character of this one slab gave it enough value to more than pay for the blade. Bi-Metal Blades COBALT-STEEL ALLOY-TIPPED BLADES At first glance, the 26-inch-diameter, black walnut log looked like a good candidate for slabbing into tabletops, even though it was only 6 feet long. On closer inspection, the wire hanging out of its side and the blue discoloration were warning signs of the metal inside. I cut a lot of urban lumber, and hated to throw this log on the firewood pile. To see if I could salvage it, I ordered a couple of bi-metal blades which were advertised to cut through nails. The tips of these blades are a cobaltsteel alloy the same material used in hack saw blades so it seemed like a reasonable idea. At $60 each, the cost was still only about half the value of a single slab, so it seemed a good investment. The blades, purchased from Norwood, had a 3/4- inch tooth spacing a little closer than the 7/8-inch spacing I normally use, and felt a bit stiffer, but otherwise appeared comparable to conventional blades. After the second slab, I found out just how tough these blades are. Dozens of bright dots of metal appeared where the blade sliced through them, with no apparent change in the quality of the cut. It seems that when the tree was a sapling, it had been used as a living fence post (a common practice here in the Missouri Ozarks). The blade had no problem at all cutting through the mild steel fence wire. I was (fortunately) on my last cut when the blade finally met its match a 3/8-inch anchor bolt at a height that suggest- 4 Sawmill & Woodlot Top left: The damage to the tooth of the blade depends on where it strikes the nail. If it catches in the gullet, it can break off a tooth. Bi-metal blades are as prone to this type of damage as a conventional carbon steel blade, but advancing the feed very slowly in areas suspected of having metal minimizes tooth damage. (Photo based on a drawing provided by Lenox) Bottom photo: Typical damage to a band saw blade that hit a nail. The strike tore the tip off of one tooth and damaged the one behind it. While it is possible to grind down the two affected teeth and continue to use the blade, this one is destined for the scrap pile.

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3 Cobalt Alloy Tip in. Bi-metal blades have a narrow cobalt alloy strip welded to a carbon steel backing before grinding or cutting the tooth profile. The result is a tip that is much more wearresistant than hardened steel tips on conventional blades. (Photo based on a drawing provided by Lenox) ed that it had once supported a clothesline. As much as I hated to lose a blade that could cut through so much metal, it went to the scrap pile. There is no question that it had paid for itself many times over in the value of the slabs, which had the character of bluestained grain swirling around the wire like eddies in a river. What Are Bi-metal Blades? Bi-metal (a.k.a. cobaltized) blades are a composite of a carbon steel backing with a thin strip of cobalt steel alloy wire welded to the edge. The teeth are then ground out to form the profile. Steve Rearick, product manager at the M. K. Morse Company in Canton, Ohio, describes the process: We use electron beam and laser welders to bond the alloy wire to the backing. Both processes produce excellent welds about inch wide. It is pretty impressive that a weld about three times the thickness of a sheet of paper is all that is holding a inch tip to the blade, and it can take the impact. Steve noted that the process requires very tight quality control. Every inch of blade coming out of our welders is inspected, so we are very confident in our highspeed edge remaining intact. We have never had a problem with tip separation. The result is a sawmill blade that uses the properties 6 Sawmill & Woodlot The wire hanging out of the side of this walnut log was a dead give-away that there would be metal inside (confirmed by the blue discoloration on the end of the log), making it a good candidate for trying out a bi-metal blade s wire-cutting ability. Cutting slowly to avoid damage to the tooth, a bi-metal blade was able to cut through dozens of strands of fence wire and staples with little loss in performance. The blade, however, met its match when I tried to cut lengthwise through this bolt on the last cut. of both types of steel to their best advantage. The more flexible carbon steel backing can run at high speeds for a longer period of time without fatigue failure, while the harder, stiffer cobalt alloy tip maintains its edge much longer, explained Steve. This means longer run times between blade changes especially useful for production mills or for cutting extremely hard woods such as Osage orange and black locust that quickly dull conventional carbon steel blades. But how are the blades when it comes to cutting through wire? My first experience was very good (until I hit the bolt). To find out more, I called Lenox Corporation in East Longmeadow, Massachusetts. Metal-cutting blades are a specialty of Lenox, so it was only natural for them to apply their expertise to their band saw sawmill blades. Joe Mailo, research and development engineer with Lenox, had a few thoughts on the matter. Joe noted that cutting through metal was not exactly what Lenox had in mind when they came out with the bi-metal Woodmaster B sawmill blade. I can t say it is a great blade for cutting metal in wood, he explained. When a wood blade with 7/8-inch tooth spacing runs into a piece of metal, it can strip off a tooth it doesn t matter what kind of blade it is. Close tooth spacing is great for metal, but does not allow the tooth to effectively cut wood (if in doubt, try cutting wood with a hacksaw). Compared to cutting metal in wood, cutting a 10-inch-diameter bar of stainless steel is not difficult at all, he concluded. That said, Joe did say that a cobalt alloy blade tip will certainly hold up better to metal than a conventional carbon steel blade. The best way to cut metal in wood is with a very slow feed rate so that the tip chips away at the metal without catching it in the gullet. The blade may come out with some damage, but no blade with standard tooth spacing will come out any better. According to Joe, bi-metal blades are gaining acceptance for production sawing, such as breaking down logs into cants for the pallet industry where the cost per cut is more important than the cost of the blade. It is hard to put a number on how much longer they run, but I would say that the cost per cut far outweighs the cost in blades, says Joe. For example, if you could run 2.5 hours cutting white oak with a

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5 $23 carbon steel blade, you should get at least three times the life with a $60 bi-metal blade, and only have to change blades between shifts. When you figure time for blade change plus sharpening cost, it more than pays for itself. He noted that the band sawmills in the U.S. lean more toward using the cheaper carbon steel blades one time, then disposing of them, while European mills lean more toward using bi-metal blades and resharpening them. At a cost of $60 or more per blade, bi-metal blades are worth the time and expense of re-sharpening. Randy Panko, sales manager for the blades division of Wood-Mizer, says that, as long as it is done properly, bi-metal blades can be resharpened several times, and that grinding out the micro-cracks that form in the gullet extends the blade life. While the blades have standard profiles and can be resharpened with conventional equipment, Randy says that a full profile CBN (cubic boron nitride) grinding wheel gives the best results. We have no problem sharpening bi-metal blades with them, he explained. You do need a different wheel for each profile, but one wheel should be good to sharpen 800 to 1,000 bands. By running coolant, Randy says the body of the blade remains resilient, while the tip maintains its hardness. He also noted that the Wood-Mizer sharpening service centers use this system in their resharpening service. So When Do You Use the Different Types of Blades? Randy sums it up this way: Under normal sawing conditions, a straight carbon steel blade works fine. They have a heat-treated tip that is harder than the body of the blade, so they get good life. Use bi-metal blades for larger-diameter logs, dry logs, reclaiming beams, and hardwood species. He noted that demand for bi-metal blades is especially strong in tropical regions for cutting tough, exotic woods. Randy cautions, however, that although bi-metal blades can cut three times as much wood as carbon steel blades, it is tempting to push the bi-metal blades harder, which can cause stress cracks. When asked how he would advise someone on which blade works best, Randy s answer was simple and to the point. Buy a few blades of different compositions and tooth geometry, and see what works best for your application. With the number of variables involved blade size, band wheel diameter, horsepower, log size, species, frozen wood, and experience of the sawyer this is good advice. Keep track of your blades and when you find what works well, stay with it. In the final analysis: Bi-metal blades really pay off in a production setting where changing blades once a shift saves on downtime and on hard woods that quickly dull carbon steel blades, but are a less-thanperfect solution to milling wood with metal. As Joe Maiolo with Lenox put it, As far as a blade for cutting metal in wood on a band sawmill, there is no such animal. That said, with a slow feed speed through the metal, bi-metal is a much better option than a standard carbon steel, and may allow you to mill valuable wood that would otherwise go to waste. Of course, it would be a good idea to warn your customers (or make a note to yourself) that the wood has metal before you put it through a jointer, planer, or other woodworking equipment! Dave Boyt has a BS degree in Forest Management and an MS in Wood Technology. He manages a tree farm (2006 Missouri Tree Farm of the Year), and operates a band saw sawmill. Laser welding a cobalt alloy strip onto a carbon steel band saw blade. When the tooth profile is cut into the blade, the strip forms a hard, durable tip. 8 Sawmill & Woodlot M.K. MORSE MFG. PHOTO

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