Prime cuts

April 1, 2001
The right bandsaw blade can mean the difference between success and scrap.

The right bandsaw blade can mean the difference between success and scrap.

It's common knowledge that choosing the right bandsaw blade results in smoother, faster, and more accurate cuts to improve workpiece quality and minimize blade changes. But it goes beyond that — keep in mind one crooked cut in a 30-in. block of Inconel, for instance, can ruin a $10,000 workpiece. This scenario may not be exactly the same for every shop, but it effectively illustrates the most important reason for using the proper bandsaw blade: to avoid scrapping parts.

But how do shops select the right sawblade? The answer depends on several key factors including the hardness, size, and shape of a shop's feedstock and the condition of its saws. By closely examining these factors, shops can determine the specific blade construction, tooth geometry, pitch, and set for their particular application.

Identifying the material to be cut is a critical first step since every material type has its own special sawing requirements. Even relatively soft, nonmetallic fiber-glass and graphite composites can be abrasive enough to wear out a saw-blade. Obviously, metals are harder, and each metal alloy creates a different cutting condition.

Depending on the material being cut, a blade's tooth gullets may be large or small. For example, soft, ductile, non-ferrous metals such as aluminum, brass, and bronze produce large chips that can quickly clog a sawblade's tooth gullets, resulting in jagged cuts, tooth wear, and possible breakage. For such materials, large tooth gullets minimize clogging.

On the other hand, ferrous metals are generally harder, and their machineability is often determined by the alloys added during the steelmaking process. For instance, alloyed steels will often include vanadium, chromium, and other additives. Although these enhance certain properties in the steel, they also increase hardness, making it tougher for a sawblade to cut.

For such hard-to-cut metal alloys, stronger teeth with smaller tooth gullets are often more appropriate. The reason is that stainless steels, for example, have added nickel content, which makes the material workharden as it is being cut. Like stainless steel, D2 also workhardens as it is cut, which puts tremendous stresses on saw-blade teeth. In addition, this material smears, which increases friction as the blade passes through the bar-stock. The mate-rial's rough surface finish can also compound blade wear.

To further prevent blade wear or tooth breakage, shops should also examine feedstock geometry and surface condition before choosing a bandsaw blade. Thin-walled materials such as tube, pipe, and sheet are best cut with fine-pitch blades (more teeth per inch). In contrast, coarse-pitch blades (fewer teeth per inch) work well for large solid shapes (typically 2 in. in diameter or greater). The large tooth gullets of coarse-pitch blades minimize clogging and distribute feed forces more evenly among fewer teeth to improve penetration into larger workpieces.

In addition to examining the material to be cut, evaluating the condition of the bandsaw doing the cutting is a critical step in choosing a blade. Older, poorly maintained machines can cause more vibration than newer, more stable ones. For such old machines, a bimetal blade with greater shock resistance might be the answer. On the other hand, newer saws can take better advantage of the higher cutting speeds possible with a carbide-tipped blade. In either case, shops should perform routine maintenance to their bandsaws to avert costly damage and maximize blade life.

After examining material and machines, shops need to specify a blade construction that matches their particular application. Deciding between a bimetal or carbide-tipped blade and picking the correct tooth geometry, pitch, and set all come into play.

Bimetal blades are constructed with hardened tool-steel teeth welded to a fatigue-resistant spring-steel backing. Carbide-tipped blades, like bimetal ones, have a spring-steel backing, but their teeth are hard abrasion-resistant carbide.

Carbide-tipped blades cost more, but under the right conditions, cut faster and last longer. They also require more powerful saw machines.

Both bimetal and carbide blades come with different tooth geometries for ferrous and nonferrous feedstock of different sizes. Tooth geometry, including the rake angle, clearance angle, and gullet depth, is tailored to different applications.

For example, softer steels are easy to penetrate and require a strong design that maximizes resistance so that teeth don't break off. On the other hand, harder steels require a tooth design that penetrates easily, thereby reducing the cutting forces required.

Bimetal and carbide blades can also have constant or variable-pitch teeth. The preferred choice of most sawblade users is the variable pitch. The ability of this tooth form to cut a variety of shapes and sizes gives the user the widest range of cutting and the least amount of blade changes. This tooth form comes in both fine and coarse.

Variable-pitch blades change the number of teeth per inch periodically to counter vibration on workpieces with surface defects. They can also reduce blade changes and minimize machine downtime.

Coarse-pitch blades are better suited for cutting high-strength or hard-surface alloys such as stainless steel and titanium. The reason is that they have fewer teeth per inch, which lets them penetrate workpieces more easily and cleanly. However, if a shop's feedstock changes frequently, variable pitch offered in small pitch form may be more appropriate.

Along with pitch, the right tooth-set combination is also important when specifying a saw-blade. Tooth set is the extent a tooth is bent away from the blade width to make the cut a little wider than the blade itself. This provides clearance for the blade to travel through the cut unrestricted and prevents friction and binding. It also increases the amount of material removed.

When specifying tooth set, shops have many choices. Such choices include blades with a raker set that alternates right-set, left-set, and unset teeth or one with a combination set in which each unset tooth is followed with set teeth in a left-right, left-right pattern. Both of these tooth-set combinations often work fine when cutting softer steels.

For harder more difficult-to-cut stainless steels and metal alloys, a special tooth set may be required, like Bahco's Milford Triple-Set tooth. These blades, which are available in carbide only, have a high, unset raker tooth with chamfered edges followed by two low unchamfered teeth — set left and right. This tooth combination produces three distinct chips with much lower cutting forces than conventional bimetal blades.

Triple-Set blades prevent workpieces from closing in and seizing the blade during cuts, resulting in better kerf and swarf clearance and longer tool life. The blade's large gullets also minimize clogging and distribute feed forces evenly among fewer teeth.

Bandsaw fit for automation

The KBS620DG-NC structural bandsawing machine fitted with the M150NC pusher feed carriage automates a traditionally manual job. Working in unison with the machine via NC control, the feed carriage runs on a substantial linear bearing mounted on a 62-in. solid steel rail. It grips and pushes material through from the back end, so there is no need for a powered roller conveyor on the infeed side. The system, from Kaltenbach in Columbus, Ind., makes it possible to straight and miter cut structural beam sections up to 36 in.

The bandsaw features a frequency-controlled main drive motor for precise cutting speed adjustments and an automatic electrohydraulic back-off. Because the KBS620 rotates about the band/fence intersection, there is no length offset on miters. In addition, the saw's vertical top clamp securely holds bundles and extremely short pieces of material.

Quick sawing setup

Featuring automatic controls and random cut positioning with a digital stock-index-length readout, DoAll's C-430A cutoff saw sets up fast to maximize production. This double-column saw has a full 17-in. capacity — even with nested stock. An independent 5-hp band drive motor with an AC inverter control provides variable band speeds between 60 and 400 fpm using 1 1 /4-in. blades.

Both an index vise mounted on an under-slung carriage and an idler conveyor that supports stock further promote smooth automatic material feed along with excellent cutoff repeatability. And, for fast indexing, the saw has a rapid-travel index vise paired with a saw-head rapid-approach control.

The C-430A is well suited for cutting structurals, tubing, and other mild steel products. It is also equipped with an automatic work-height sensor, out-of-stock sensor, and a preset cut counter.

Other standard features include an independent pump for flood-coolant application through the saw guides, spring-loaded zero-clearance faced saw-guide inserts, and a powered band brush.

Sawing the hard stuff

For exotic and hard materials, the CT2000 saw from HE&M Saws, Pryor, Ok., sports band wheels weighing 800 lb each. These hefty wheels dampen the blade vibration often associated with cutting hard materials.

The machine has a 20-in.-high, 20-in.-wide part capacity and a blade size of 2 in. 266 in. 0.050 in. For power, the hydraulic CT2000 comes equipped with a 20-hp motor.

Running nonconventional blades

Instead of conventional blades, the CM75AN-CNC saw from Amada Cutting Technologies uses disposable circular carbide ones. According to the manufacturer, the saw cuts all kinds of materials 5 to 10 faster than regular blades and leaves a surface finish that seldom requires a secondary operation.

The machine accommodates round material from 0.5 to 3.00 in. and square stock from 0.55 to 2.56 in. Its indexing system positions parts to ±0.004 in., and a 5.5-Kw motor powers the saw's blade to speeds of 40 to 150 rpm.