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 RC 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.
Related Glossary Terms
- abrasive
abrasive
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
- backing
backing
1. Flexible portion of a bandsaw blade. 2. Support material behind the cutting edge of a tool. 3. Base material for coated abrasives.
- bandsaw
bandsaw
Machine that utilizes an endless band, normally with serrated teeth, for cutoff or contour sawing. See saw, sawing machine.
- bandsaw blade ( band)
bandsaw blade ( band)
Endless band, normally with serrated teeth, that serves as the cutting tool for cutoff or contour band machines.
- bandsaw blade ( band)2
bandsaw blade ( band)
Endless band, normally with serrated teeth, that serves as the cutting tool for cutoff or contour band machines.
- bandsawing
bandsawing
Long, endless band with many small teeth traveling over two or more wheels (one is a driven wheel, and the others are idlers) in one direction. The band, with only a portion exposed, produces a continuous and uniform cutting action with evenly distributed low, individual tooth loads. Often called band machining.
- broaching
broaching
Operation in which a cutter progressively enlarges a slot or hole or shapes a workpiece exterior. Low teeth start the cut, intermediate teeth remove the majority of the material and high teeth finish the task. Broaching can be a one-step operation, as opposed to milling and slotting, which require repeated passes. Typically, however, broaching also involves multiple passes.
- centers
centers
Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.
- clearance
clearance
Space provided behind a tool’s land or relief to prevent rubbing and subsequent premature deterioration of the tool. See land; relief.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
- cutting fluid
cutting fluid
Liquid used to improve workpiece machinability, enhance tool life, flush out chips and machining debris, and cool the workpiece and tool. Three basic types are: straight oils; soluble oils, which emulsify in water; and synthetic fluids, which are water-based chemical solutions having no oil. See coolant; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- dressing
dressing
Removal of undesirable materials from “loaded” grinding wheels using a single- or multi-point diamond or other tool. The process also exposes unused, sharp abrasive points. See loading; truing.
- fatigue
fatigue
Phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.
- filing
filing
Operation in which a tool with numerous small teeth is applied manually to round off sharp corners and shoulders and remove burrs and nicks. Although often a manual operation, filing on a power filer or contour band machine with a special filing attachment can be an intermediate step in machining low-volume or one-of-a-kind parts.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- grinding
grinding
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
- hardness
hardness
Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.
- high-speed steels ( HSS)
high-speed steels ( HSS)
Available in two major types: tungsten high-speed steels (designated by letter T having tungsten as the principal alloying element) and molybdenum high-speed steels (designated by letter M having molybdenum as the principal alloying element). The type T high-speed steels containing cobalt have higher wear resistance and greater red (hot) hardness, withstanding cutting temperature up to 1,100º F (590º C). The type T steels are used to fabricate metalcutting tools (milling cutters, drills, reamers and taps), woodworking tools, various types of punches and dies, ball and roller bearings. The type M steels are used for cutting tools and various types of dies.
- metalcutting ( material cutting)
metalcutting ( material cutting)
Any machining process used to part metal or other material or give a workpiece a new configuration. Conventionally applies to machining operations in which a cutting tool mechanically removes material in the form of chips; applies to any process in which metal or material is removed to create new shapes. See metalforming.
- metalworking
metalworking
Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.
- milling
milling
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling cutter
milling cutter
Loosely, any milling tool. Horizontal cutters take the form of plain milling cutters, plain spiral-tooth cutters, helical cutters, side-milling cutters, staggered-tooth side-milling cutters, facemilling cutters, angular cutters, double-angle cutters, convex and concave form-milling cutters, straddle-sprocket cutters, spur-gear cutters, corner-rounding cutters and slitting saws. Vertical cutters use shank-mounted cutting tools, including endmills, T-slot cutters, Woodruff keyseat cutters and dovetail cutters; these may also be used on horizontal mills. See milling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- pitch
pitch
1. On a saw blade, the number of teeth per inch. 2. In threading, the number of threads per inch.
- sawing
sawing
Machining operation in which a powered machine, usually equipped with a blade having milled or ground teeth, is used to part material (cutoff) or give it a new shape (contour bandsawing, band machining). Four basic types of sawing operations are: hacksawing (power or manual operation in which the blade moves back and forth through the work, cutting on one of the strokes); cold or circular sawing (a rotating, circular, toothed blade parts the material much as a workshop table saw or radial-arm saw cuts wood); bandsawing (a flexible, toothed blade rides on wheels under tension and is guided through the work); and abrasive sawing (abrasive points attached to a fiber or metal backing part stock, could be considered a grinding operation).
- sawing machine ( saw)
sawing machine ( saw)
Machine designed to use a serrated-tooth blade to cut metal or other material. Comes in a wide variety of styles but takes one of four basic forms: hacksaw (a simple, rugged machine that uses a reciprocating motion to part metal or other material); cold or circular saw (powers a circular blade that cuts structural materials); bandsaw (runs an endless band; the two basic types are cutoff and contour band machines, which cut intricate contours and shapes); and abrasive cutoff saw (similar in appearance to the cold saw, but uses an abrasive disc that rotates at high speeds rather than a blade with serrated teeth).
- shaping
shaping
Using a shaper primarily to produce flat surfaces in horizontal, vertical or angular planes. It can also include the machining of curved surfaces, helixes, serrations and special work involving odd and irregular shapes. Often used for prototype or short-run manufacturing to eliminate the need for expensive special tooling or processes.
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