Efficient sawing of aluminum: though it's easy to saw, this material can be difficult to saw productively and cost-effectively. Here are some tips

Modern Machine Shop, Nov, 2002 by Dallas Stuck, Frank Tielli

Depending on the alloy, soft aluminum can be about four times easier to cut with a bandsaw than free-machining 12L-12 or 12L-15 steel. However, with car makers, pump-and-valve companies and other manufacturers using more aluminum today, shops accustomed to sawing steel have to learn how to saw aluminum effectively in order to maximize productivity and minimize cost-per-cut. Bimetal and carbide-tipped blades cut soft alloys quickly, but abrasive aluminum still takes a toll in terms of reduced blade life and lost shop time. Bandsawing aluminum and other non-ferrous metals productively requires an informed approach to blades, bandsaw machines and the sawing processes. The right saw blade used properly improves throughput on aluminum workpieces and enhances the competitiveness of the entire bandsawing operation.

Even with easy-to-cut aluminum, the least expensive blade is not always the most cost-effective. One aluminum foundry switched from inexpensive carbon steel bandsaw blades to a strong-set carbide blade and realized impressive time and cost savings. On Type 356 or 319 aluminum castings, a single carbide blade (a Bahco 3869 Carbide Triple Set blade) outlasted 40 carbon steel blades that together cost six times as much.

Dramatically fewer shutdowns for blade changes saved more than enough shop time to offset the higher price of the carbide blade. Uninterrupted cutting also relieved a persistent production bottleneck to enhance overall productivity.

For any bandsawing application, an accurate measure of productivity includes more than the price of the blade spread over the number of finished parts. Blade manufacturers figure overall cost-per-cut by including labor and sawing machine burdens as well. The calculation amortizes the total cost of the job over the number of good-quality finished pieces. (See table.) When cutting aluminum, a lower calculated cost-per-cut can readily justify a higher cost per blade.

Soft, Sticky And Abrasive

Despite its relative softness, aluminum still poses sawing challenges. The high cutting speeds possible when sawing aluminum generate intense heat that weakens ordinary saw blades. "Gummy" aluminum chips weld themselves to blades, increasing tooth stresses and smearing workpieces. Above all, the abrasive microstructure of aluminum accelerates saw blade wear and shuts down sawing operations for blade changes.

Aluminum is fundamentally abrasive. Aluminum that is 99 percent pure is softer but more abrasive than the Type 6051 alloy used in aircraft structures. Compared to wrought shapes, aluminum castings are even worse. Silicon content in common casting alloys ranges from 6 to 20 percent. More complex castings, such as pump cases with thin cooling fins, typically use alloys with more silicon to improve casting mold flow. The higher silicon content creates more abrasion and more hard spots that reduce saw blade life. Oxide inclusions also provide additional hard spots that take a toll on saw teeth.

Compared with the carbon steel blades commonly used to cut aluminum, advanced bimetal and carbide-tipped blades provide clear productivity advantages. However, the best choice of a bimetal or carbide blade is determined by factors including the bandsawing machine, the workpiece and the economics of the job.

A single machine in the aluminum foundry described earlier, for instance, used 10 carbon steel blades per month cutting abrasive castings. While bimetal blades lasted longer, the modest improvement in blade life was hard to justify at four times the cost of carbon steel. However, in the same application, carbide blades almost twice the cost of bimetal paid their way in radically greater productivity. While carbon steel blades produced anywhere from 200 to 5,000 parts before they needed replacement, the carbide blade showed no wear after 70,000 castings.

The Need For Speed

Soft aluminum alloys are so easily penetrated by bandsaw teeth, it is easy to overfeed and overload the blades. High blade speeds increase throughput to decrease chip load per tooth. They help evacuate chips better to prevent crooked cuts and broken teeth. While bandsaws might cut free-machining steel productively at around 300 sfm, the recommended speed cutting wrought aluminum shapes is 500 to 1,000 sfm. To cut risers and gates on aluminum castings with carbide blades, the surface speed should be 3,000 to 4,000 sfm.

Higher speeds support higher feed rates. To optimize chip load in aluminum, blade feed rates should be set around 20 to 25 square inches per minute. Feed pressure is generally much lighter than that used on steel, and it is fine-tuned for the best chip formation. Within the usual 28,000 to 32,000 psi range, blade tension should be reduced slightly for the best blade life cutting aluminum.

Pick A Blade

Though carbide-tipped blades provide the best abrasion resistance for high speed aluminum cutting, not all bandsawing machines and operations can use them effectively or economically. At modest speeds from 500 to 1,000 sfm in aluminum, abrasion resistant carbide teeth can sometimes outlast their spring steel backing. To get the best return on a carbide blade investment, blade speed, feed and tension should be adjusted so backing and teeth wear at about the same rate.