Materials Force Equipment Development

Manufacturing Engineering, Oct 2007 by Morey, Bruce

Machines and tooling must adapt to deal with the new generation of materials, such as titanium and composites

Driven by the need to reduce weight, industries from energy exploration to aerospace are adopting composites and titanium alloys in ever-greater volumes. New titanium alloys such as titanium 5553 are providing higher strength, up to 180 ksi (1241 MN/m^sup 2^) yield, and good corrosion resistance and fatigue properties. Traditional epoxy-based composites are the material of choice in many new aircraft. Metal-matrix composites, if rare, are no longer laboratory specimens.

These new materials present different and unique machining challenges. Composites are abrasive on typical tools, and produce potentially damaging dust. Titanium, on the other hand, while not particularly harder than many other metals, retains its hardness at high temperatures. Cutting temperatures for titanium alloys can reach 1100-1200°C if not properly controlled. Material removal rates can be as little as one-fourth the rate achieved when machining more common metals. Titanium also conducts less heat than other materials-the chip does not transport heat away from the cutting tool. Machining this material has its own rules.

Coupled with titanium's tendency to chatter, these issues mean the ideal cutting machine is dynamically stiff with higher horsepower and torque at lower rpm, according to Dan Cooper, senior application specialist for Cincinnati Machine (MAG Cincinnati, Hebron, KY). "The typical spindle speed when machining titanium may be 200-300 rpm, compared to 10,000-15,000 rpm for aluminum," says Cooper.

To meet these special requirements, MAG Cincinnati last year introduced a new machine, the Ti-Profiler, that produces 0.33 hp (0.25 kW) per rpm, compared to typical aluminum cutting machines that might produce 0.03 hp (0.022 kW) per rpm, according to Cooper. He describes the Ti-Profiler as a vertical, five-axis multispindle (three, four, or five-spindle) profiler capable of delivering up to 340 hp (254 kW) in total. A variation on the design of Cincinnati Machine's Wide Range profiler, the Ti-Profiler runs at a maximum of 3500 rpm compared to the 7000 rpm of the Wide Range. This profiler can produce large parts. "Right now, we are quoting a machine whose rails are 490' [149-m] long," says Cooper, "and can produce up to five parts that are about 45' [1.1-m] wide."

Greg Hyatt, vice president and chief technical officer for the Machining Technology Laboratory, Mori Seiki USA (Rolling Meadow, IL), agrees that machining the harder titanium alloys, like Ti 5553, requires a machine that delivers high torque at low rpm. The typical parts are complex as well. "Almost all of the machines we provide for these applications are five-axis machines. To improve the damping for these applications, our NMV and NT machines use an octagonal ram. It provides the damping of the box-way design, but eliminates the thermal deformation of box ways. The spindle centerline is not displaced, even with heavy utilization of the high-speed positioning."

Hyatt says that the larger horizontal machining centers used on the tougher beta-alloys, like Ti 5553, deliver 1000 N*m of torque, in applications where cutting speeds are below 500 rpm. He reports cutting speeds for alloys like Ti 5553 at 150-250 fpm (45.7-76.2-m/min) with a carbide tool.

"We have observed a dramatic increase in the use of composites," notes Hyatt, led by aerospace applications. "Machining composites is as different from titanium as possible." Composites require much higher spindle speeds coupled with higher acceleration rates. Spindle speeds are approximately 10,000-20,000 rpm, and accelerations exceed 1 g.

Composites produce dust that may be a health hazard for both humans and machines. To control dust, Mori Seiki is developing a "zero chip" spindle and tool combination, in collaboration with Kennametal (Latrobe, PA). "As we cut composites, we suction the dust and chips through the tool and spindle, and capture it behind the machine," explains Hyatt. They are targeting a mid-2008 introduction of the tool and spindle.

Tooling engineering for hard alloys or composites presents its own problems.

Tools designed for machining titanium and other hard alloys must have specific features, according to Michael Standridge of Sandvik Coromant (Fair Lawn, NJ). Features include:

*Positive cutting geometries with sharp edge lines,

*Insert shapes that provide the proper lead angle to allow chip-thinning and resist notch wear,

*Tools with coatings and grades of carbide that have the proper balance between heat resistance and toughness, and

*High-pressure coolant.

"Titanium and heat-resistant superalloys combine relatively low density (compared to steel) with high strength, and need to be sheared. A positive cutting geometry does that," says Bruce Carter, product manager for rotating tools for Sandvik Coromant. Optimal cutting angles vary depending on the application, but are typically around 11°. "We also recommend using round inserts and inserts with radius corners on them, which spread the chip out over a greater distance on the cutting edge," he explains. This geometry contributes to chip thinning and better Cutting action.