by William Goodwin

In the past, machining aluminum with high-speed steel and carbide tools seldom exceeded 4,000 sfm (surface feet per minute). Today, cutting speeds of 15,000 sfm and higher are becoming commonplace, which means faster spindle speeds and feed rates and in turn more pieces completed per shift. But to achieve these speeds, you need advanced cutting tools and techniques.

Machining soft materials not so easy
A hydraulic press can work some forms of aluminum as easily as the guy in the neighborhood Italian restaurant works raw pizza dough. That’s not true when you want to machine it.

Because aluminum and other non-ferrous metals and synthetics are relatively soft, you need special tooling and knowledge to machine these materials at high speed.

Increased thermal expansion can distort the workpiece. Poor chip removal can cause extra work to dig galled metal fragments from tools and untwist chips from tool holders.

While it’s true that it doesn’t take much to simply cut aluminum, doing it with reliability and economy at high speed and high production rates requires diamond or carbide tools, rigid mounting and specialized techniques.

Toolmakers have learned that the critical issues for high-speed milling of aluminum and other "soft" materials are horsepower, cutter construction (geometry and material) and chip evacuation.

Horsepower equals speed
Though it requires relatively less horsepower to machine aluminum than steel, the horsepower required for high-speed machining soft materials is much greater than the power of conventional machines. As a rule of thumb, it takes about 1 HP for every 1,000 RPM of spindle speed just to rotate the tool. A typical machine with a spindle speed of 15,000 RPM might require as much as 50 HP for machining aluminum.

What does "high speed" mean when it comes to cutting aluminum and other relatively soft materials? Most manufacturers agree that high speed generally refers to spindle speeds over 10,000 RPM and as high as 50,000 RPM. High speed also indicates high surface speeds, between 3,000 to 30,000 surface feet per minute (sfm). Speeds above 30,000 sfm are considered to be ultra-high and today are used only in experimental machining.

Spindle speed needed to generate a certain surface speed is directly related to the cutting tool diameter, leading to the development of ever larger cutter diameters in high-speed applications.

High-speed cutters for soft materials are different
Tools for cutting soft metals and plastics differ significantly from tools for cutting steel. Cutters for soft materials require higher rake angles to create a shearing action in the relatively "gummy" material and to aid in chip removal. The cutting surfaces must be highly polished to reduce chip galling. The gash angle (axial rake) on the teeth must be greater to help pull the chip from the pocket and prevent clogging.

The helix angle of aluminum-cutting end mills is as much as 50 percent greater than conventional steel-cutting end mills. This helps throw chips clear of the workpiece at high cutting speeds.

High-speed machining end mills for aluminum and other soft materials have fewer flutes, which allows greater room for chip removal. End mills up to one inch in diameter usually require only two flutes.

The cutting surfaces are usually carbide or diamond, with the diamond-tipped or diamond-coated cutting edges being the choice for cutting alloys that have large, hard particles of silicon (sand). Many have high-velocity, high-pressure coolant manifolds for through-the-spindle coolant delivery in cutter bodies from 3 to 12 inches.

Chip removal
With stock removal rates commonly exceeding 100 cubic inches per minute, chip evacuation can be a limiting factor in effective high-speed milling.

In milling tools and drills, flute volume and the degree of spiral (helix) determine chip removal rate. The 30 to 35 degree helix of steel-working end mills, for example, must increase to 37 to 45 degrees to handle chip removal in high-speed aluminum milling. Wide parabolic flutes with a large gash radius (also referred to as axial rake) are ideal for machining soft metals and polymeric solids. For heavier feed-rates, three flutes provide greater flute volume than two. Although more flutes restrict chip space, they remove more chips per revolution.

Three methods of chip control are available for high-speed cutting:

Cutting fluids. Washing chips away with cutting fluids often does not work well at high rotational speeds because these fluids get atomized before they flush chips away. Systems that deliver cutting fluid through the spindle coolant mounting screws can work.

Compressed air. High-speed soft material machining usually does not produce enough heat to require a coolant, so compressed air can be used to blow chips away. However, flying chips and a high noise level may present problems.

Vacuum. Although as noisy as the compressed air, a vacuum system can efficiently collect chips.

Chip thickness is also important in milling aluminum. When high cutting speeds are used, a slow feed rate will cause excessive rubbing instead of cutting, creating thick chips which lead to over-heating, discoloration and poor tool life.

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