Ground saw blades provide the long life and accuracy that CNC production bandsawing demands.

Production metalsawing, particularly of bars, tubing, pipes, and structural parts, is changing. Increasingly, manufacturers are using metalcutting service centers or dedicated sawing companies for their cut-to-length components. This change in production also is changing the market for bandsaws.

While demand for manually controlled bandsaws holds steady, demand for CNC production bandsaws among such service centers is on the rise. With CNC bandsaws, operators can enter the number and length of parts into the CNC and then go on to other work as the saw cuts the material unattended.

The increase in contract sawing and in the use of CNC production bandsaws also is causing a shift in saw-blade requirements. Operators need blades that last longer to make unattended sawing more productive. Even though a blade on a modern saw may only require 2 to 3 minutes to change, the cost of the blade and the time involved in making the change add up. Any lengthening of the chipmaking time between blade changes lowers the cost per cut.

The need for longer lasting blades is now being met by bimetal bandsaw blades with teeth formed by a precision grinding process rather than by milling, the traditional method (Figure 1). Precision-ground teeth have been available on some blades used by butchers and woodworkers for years, but these ground teeth are just now becoming available on blades for the metalworking industry. The blades are so new that the variety of pitches is limited. The good news is that different pitches are being added regularly. Presently, ground bandsaw blades are available with 2-3, 3-4, or 1.3 pitch, M-42 cobalt-HSS cutting edges in precision or claw-tooth form.

Bimetal Blades

Blades made entirely of HSS were introduced in the mid-1950s. At that time, they were a major breakthrough compared to carbon-steel blades, because HSS retains its hardness at higher temperatures. This allowed HSS blades to be used at higher speeds and feeds. While HSS blades had a greater tooth hardness, they had limited flex life, a critical problem for bandsaw blades. The flex problem was solved in 1965 with the introduction of bimetal bandsaw blades.

For most bandsawing operations, HSS bimetal blades are the most efficient, cost-effective product available. Although they are more expensive than carbon-steel blades and blades made entirely of HSS, they provide the lowest cost per cut. Because blades represent about 5% of the total cost of a sawing operation, the incremental expense of higher blade cost is likely to be recouped through higher machine output, better finish, lower reject rate, and less downtime. With ground-tooth blades available at prices comparable to blades sporting milled teeth, the utility of an HSS bimetal blade is extended even further.

The first step in the manufacture of HSS bimetal bandsaw blades is to electron-beam weld a ribbon of HSS to a spring-steel backing, which provides the flexibility for high band speeds and long life. The second step is to subject the weld to a nondestructive test to identify and remove weld defects. The third step is to form the teeth on the composite strip.

For this step, the teeth originally were formed by punching and filing, a slow and somewhat inaccurate process.

To overcome these problems, various methods of shaping teeth, such as broaching, have been tried. Until recently, blade manufacturers have found milling to be the most successful method. As a result, most manufacturers mill their blades' teeth. But even milling has its limitations. A high-speed milling cutter is accurate when first sharpened, but subsequent use decreases the accuracy of the tooth form until the cutter is resharpened. Worn milling cutters tend to produce rougher surface finishes and irregular tooth heights.

On these new blades, saw teeth are ground with an abrasive wheel that is dressed on each pass using a diamond dressing wheel. It's like using a resharpened milling cutter for each tooth formed. The consequence is that a tooth produced by grinding has a smoother surface finish with fewer stress risers in the gullet and has a more uniform tooth height and tooth form.

Figure 1: Close-up of the ground Penetrator 3 bandsaw blade from DoALL Co. (right) compared to a milled blade.

Beat the Heat

As a bandsaw blade moves across a workpiece, chips form at the tip and follow the curvature around the tooth's gullet. The smoother the gullet, the less resistance the chip gliding along it encounters. As a result, when the gullet is smooth, chip formation generates less friction and heat.

Heat is the chief enemy of saw-blade life. Anything that can be done to reduce the heat created during chip formation improves blade life by preventing the teeth from softening and by reducing metal fatigue.

About 75% of the heat generated during sawing is created by the chip being formed, another 20% by the friction of the tool against the workpiece, and about 5% by the resistance of the workpiece surface to the saw tooth. Because 80% of the heat generated is dissipated by the chip, a cooler chip can absorb and carry away more heat. Chips made by ground bimetal blades meet less resistance than chips made by milled bimetal blades do, so they are cooler. The role the chip plays in creating and carrying away heat is also why cutting fluids that flush chips from the work area are so important to production sawing.

Longer Blade Life

Reduced heat from smoother gullets is just one advantage of using a ground bimetal bandsaw blade. The blade's uniform tooth height evenly distributes the load at the tips of the teeth as the blade moves across the surface of the workpiece. This helps lengthen blade life.

If a tooth is slightly higher than the surrounding teeth, it will encounter greater pressure, because it will take a larger chip than the teeth in front of it or behind it. The taller tooth will wear more quickly and may break more readily than the other teeth do. Conversely, a tooth that is too short will not be subjected to the same cutting load as the teeth on either side of it. This creates added stress on the surrounding teeth.

Uniform tooth height also increases cutting life by evenly distributing the wear on the saw teeth. Uniform tooth height also plays a role in tooth set, which affects the surface finish of the cut. Tooth set is the amount the teeth are offset to each side of the blade to provide clearance for the blade body in the cut. Teeth are set by passing the blade through a forming machine that bends the teeth to either side. When height is consistent, the set also will be consistent, because each tooth tip will be uniformly displaced from the same centerline.

Accurately set teeth on a rigid, well-maintained production power bandsaw can produce cuts with a 125µin. to 250µin. finish and an accuracy (in terms of flatness and parallelism) in the 0.002" range per 1" of workpiece height. After the teeth are set, the blades are hardened and multiple-tempered by heating and cooling in controlled-atmosphere furnaces to improve both hardness and toughness. Typically, HSS tooth tips on bimetal blades are hardened to R_C 66 to 69.

Testing

Factory tests have verified that a precision-ground bandsaw blade delivers longer blade life than a milled blade can. In a typical comparison test, cuts were made on 4" round AISI 4150 steel, using 1 1/4"-dia. bimetal blades with a 2-3 pitch on a production power bandsaw cutting at 12 sq. in./min. (Table 1). As indicated in Table 1, the ground blade cut 3580 sq. in. more steel than the milled blade - an increase of 42%. The time elapsed before there was a need for a blade change went from about 1 1/2 shifts to slightly more than 2 shifts.

Table 1: Cutting performance of a ground blade and a milled blade

While optimal blade life depends partly on the unique procedures and equipment of each user, the goal in any sawing operation is to obtain the lowest possible cost per cut within quality parameters.

Shops can go a long way toward meeting this goal by using a sturdy, well-maintained bandsawing machine, the proper cutting fluid, and blades matched to the job. Blades and cutting fluids are a small portion of the overall cost per cut, so trying different products is an inexpensive way of investigating cost savings. The precision-ground bimetal blade is the type of new sawing product that merits testing and can help shops achieve additional reductions in cost per cut, particularly in production applications.

About the Author

John Whalen is vice president, metallurgy, with DoALL Co.'s saw-blade division, Des Plaines, IL.