A carbide-tipped blade with triple-set tooth design provides good swarf clearance for long tool life.

by Paul Duffy, Babco Milford Bandsaws
Edited by James J. Benes

Stainless steel is, arguably, the metal most sensitive to bandsawing and blade condition. Variations in throughput and blade life are common even when sawing with the same blade type. One shop may saw through only 1 in.2/min and wear out a blade every day, while another cuts at 7 in.2/min with the blade lasting a week, or more.

Stainless chips do not readily break up and clear. Rather, they are stringy and adhere to saw blades. These clingy chips also clog the blade's gullets and increase cutting heat and friction. In a bimetal blade, heat generated by cutting softens both the blade and workpiece, dulls the teeth and welds chip particles to the teeth. Also, at high cutting temperatures, softened stainless moves aside rather than being cleanly cut off. Then, as the blade passes, the material moves back and closes the kerf, pinching the blade.

Blade selection is key to productivity and blade life when cutting stainless. In addition, the work must be properly clamped, blade speed must be correctly set, blades must be properly installed and run-in, and machines and blades have to be kept in top working order. Above all, the teeth must be protected and carefully handled throughout the blade's life. Damage to a single tooth starts a chain reaction that inevitably leads to blade failure.

Carbide or bimetal blades?
With good practice, a carbidetipped blade cuts stainless steel two to three times faster than a bimetal blade. New bonding processes create stronger bonds between tooth and backing than brazing, and the bonds retain their strength at temperatures three to four times higher than those recommended with brazements. Although carbide-tipped blades sometimes are more expensive, time and cost savings resulting from higher throughput quickly offset the higher costs. Additionally, blade cost represents only 1 percent to 2 percent of the total cost of stainless steel bandsawing.

Some metalcutting bandsaw blades feature ground, rather than milled, teeth which improve dimensional uniformity by a factor of five. The consistent tooth height of a ground blade uniformly distributes cutting loads from tooth to tooth. This results in more even tooth wear, which extends blade life and reduces the risk of overloading a tooth. More important for stainless-steel cutting, grinding creates a smooth surface, so teeth resist metal adhesion.

Carbide-tipped blades are vulnerable to misuse and selection is still somewhat limited, although this is changing. Bimetal blades may not cut as fast, but they are more forgiving of lessexpert handling than carbide-tipped ones.

Operator skill level is an important factor in choosing between bimetal and carbide-tipped bandsaw blades. With either type of blade, shops should specify the widest possible set for cutting stainless steel. Although kerf losses increase, a wide kerf protects against stainless steel's tendency to pinch, bind and close-up behind the blade.

A triple-set blade creates the largest clearance between workpiece and blade. Rather than having teeth set right-left, a triple-set blade is set centerleft/right-center, and so on. The center tooth makes it possible to set the others wider.

Blade pitch is more a function of material thickness than composition. A basic rule-of-thumb is specifying pitch that engages at least three or four teeth in the cut. A variable-pitch blade works best for material prone to chatter or harmonic vibrations.

Handling and operating
Blade life also depends on how the blade is unpacked and installed, especially carbide blades. Dropping can damage teeth, which leads to immediate blade failure. Operators should keep the plastic tooth guard, shipped with most blades, in place until the blade is fully installed. If the blade must lie on the floor, do it on a wood or other soft surface and keep the teeth facing up. And, if the blade must be stepped on to invert it, wrap the affected area with a wiping cloth or piece of cardboard.

Stainless steel sawing benefits from recent developments in bandsawing equipment. Today's bandsaws sport the rigidity and power for shops to take full advantage of carbide-tipped blades. However, a bandsaw table that is not level over-twists blades and causes failure. To reduce the risk of over-twisting, shops should bring the blade guides in close and rigidly clamp the workpiece.

Proper running-in of a new blade, particularly a ground-tooth bimetal blade, can double its life. Running-in, or "going-light," hones the burrs and sharp edges, and eliminates stress risers. The resulting smooth surface reduces the tendency of stainless steel chips to cling to the blade.

To run-in a new bimetal blade, run it at full bandspeed and half feed for the first ten minutes, afterward increase feed in 25-percent increments. For example, run at half feed for the first piece if it is a heavy solid and for the first three pieces if hollow. Shops should not slow blade speed during run-in because it increases cutting forces on each tooth.

A general rule for setting speeds and feeds is the heavier the section, the slower the speed. The blade should not run at a low feedrate until it is engaged fully in the cut and cutting forces are distributed over at least ten teeth to prevent overloading any single tooth. Speeds and feeds always should be set before the blade contacts the work and the blade should not be touching the workpiece when starting the machine. Also, blade brushes must be in good condition and properly aligned, and tooth gullets thoroughly cleared of chips before re-entering the work.

Typical cutting speeds for bandsawing stainless steel
Metal grade	Bimetal blade	Cutting speed (fpm) Carbide-tipped blade
Austenitic 201, 202, 301-304 316, 317, 330	95-140 60-110	215 200
Ferritic 405, 409	95-145	215
Martensitic 410, 420	110-175	215

 

The L.S. Starrett Company has developed a preventative maintenance guide as part of its standard saw service program. The guide consists of a 20-point checklist designed to cover practical machine care. The parameters on the checklist are summarized as follows: Condition and adjustment of band guides — If the guides are worn or loose, misaligned cuts and premature blade failure could result. Band wheel and bearing conditions — Worn flanges and out-of-round wheels can cause blade stress that, in turn, can cause blade failure and/or poor cuts. Blade tension — Low tension will speed wear on the blade. Excessive tension can cause blade failure, and also can damage the machine. Always set tension within the manufacturer's specifications. Blade alignment — Improper alignment to the vise or bed will lead to out-of-spec and crooked cuts, and blade fatigue. Feed and speed calibration — Once the blade is properly guided, it is imperative that the feed and speed of blade and saw are calculated to the machine manufacturer's recommendations. This will optimize blade and, ultimately, machine performance. Hydraulic and transmission oil level — Check the level and the condition of both the hydraulic and transmission oils, and add or replace them as needed. Coolant quantity, condition and flow — Check the level and the concentration, as well as the presence of tramp oils in the machine's coolant, and verify sufficient flow. Belts and drives — Check belts for wear and cracking. Check pulleys and gearbox for wear or backlash. Guide arms in proper position and secured — Loose guide arms can fatigue a blade, reduce its life and result in out-of-square cuts. Blade tracking and proper rotation — If a blade is not riding on the band wheels correctly, the result can be fatigue on the blade, excessive wear to the wheels, and possible machine frame damage if a blade rides off the wheels and cuts into the saw. Blade and/or wheel brushes — Check to ensure correct positioning and performance. These play a vital role in cleaning either the blade gullets or the surface of the bandwheels. They are often neglected or overlooked. Lubrication — Ensure that moving parts are checked and/or lubricated as recommended by the manufacturer's specifications. Calibrate work height and slow approach controls — Saws equipped with these options should be checked and adjusted according to the manufacturer's recommendations. Safety switches — Check safety switches and guards for proper placement and operation to ensure operator safety. Adequate vise holding — Check mechanical and hydraulic parts to ensure that the workpiece is held securely. Freeness and/or looseness in ways and slides — On both bandsaws and power hacksaws, check that vise parts and the material indexing mechanism move freely, and that the slide on a power hacksaw moves freely. Feed clutches and feed controls, springs and hydraulics — On power hacksaws check and adjust feed clutches, controls, springs and hydraulics according to manufacturer's recommendations. Condition of blade holder pins — On power hacksaws, check pins for excessive wear and replace them as needed. Secureness of blade holders in frame — On power hacksaws, ensure that blade holders are firmly in place in the frame. Loose holders can cause excessive fatigue on the blade and out-of-tolerance cuts.

Paul Duffy is a member of the bandsawing support team at Bahco Milford Bandsaws, Throop, Pa.