High-Speed Machining for AEROSPACE

Manufacturing Engineering, Mar 2008 by Morey, Bruce

Successful high-speed machining requires attention to the cutting tool, spindle, and machine dynamics

Enabled by advances in tooling and control capabilities, high-speed machining is finding wider acceptance in aerospace applications. Although especially good for aluminum, the technique is finding its place in composites and hard-metal machining as well.

Competitive pressures are constantly forcing manufacturers to machine parts more efficiently. At the same time, builders of aerospace structures are requiring stronger, lighter structures with closer tolerances. High-speed machining (HSM) can cut cycle time while also allowing manufacturers to cut finer, thinner parts than ever before.

HSM is a term used loosely by many people, according to Wayne Reilly, applications manager, Haas Automation Inc. (Oxnard, CA). While some say it is any spindle speed over 10,000 rpm, others have a little more complex definition, according to Reilly. "It really depends on the context of the term," he says. "A tool manufacturer might define it by rpm, and a machine-tool manufacturer might define it as some large-block lookahead in the CNC controller. The technique tends to use fast speeds and feeds and lighter cuts, while conventional machining is usually slower with heavier, deeper cuts." For example, VMCs from Haas offer HSM options with spindle ratings up to 30,000 rpm and drive systems rated up to 30 hp (22.4 kW).

"Rather than discuss spindle speed alone, I like the term high-performance machining," says Randy Von Moll, platform manager-aluminum, MAG Cincinnati (Hebron, KY.) His definition includes the dynamic response of the machine as well as spindle speed. He describes five parameters that define high-performance machining: i) spindle rpm, ii) spindle power, iii) high feed and toolpath rates, iv) high acceleration and decelerations and v) high accuracy. The last three terms uniquely define the dynamic response of the machine, rather than spindle properties. "You really need the combination of the high-performance spindle with the high dynamic response of the machine tool to efficiently cut alloys like aluminum," says Von Moll.

Categorizing aircraft parts into two broad categories of 'thin-plate' and 'thick-plate', he believes what defines HSM for parts up to 50mm thick are spindle rotations of 30,000 rpm and spindle ratings to 80 hp (60 kW). High speed for thick plates-thicknesses greater than 50 mm-is 18,000 rpm and spindle ratings to 135 hp (100 kW).

"The high dynamic response parameters are not really different for machines cutting thin or thick," explains Von Moll. "For either, acceleration/deceleration should be about 0.5 g, and you should offer as rapid a [noncutting] traverse as possible, at least 1500 ipm [38 m/min]."

The acceleration/deceleration (ace/dec) significantly influences the cutting time in complicated pocketed parts, where the tool has to make many direction changes.

The traverse time affects cutting time, especially 'parasitic' time, which can be as high as 20% of total cycle time in cutting aluminum. Parasitic time includes positioning the tool for a new cut or moving to a tool changer. "In lean terms parasitic time is an easy waste to eliminate. Achieving rapid-traverse rates in combination with high ace/dec rates is what led MAG Cincinnati to introduce our HyperMach Vertical series of profilers several years ago. These machines have a rapid-traverse rate of 4000 ipm [101 m/min]. Nobody can machine that fast-that speed is for reducing parasitic loss," explains Von Moll. Overall, the HyperMach Vertical profilers boast X-axis travel up to 33 m, Y-axis up to 3500 mm, and Z axis up to 1250 mm with additional A and B or C axes, and a spindle speed as high as 30,000 rpm. Most HyperMach Vertical Series installations feature twin independent spindles carried on a common X-axis gantry structure. "In response to the market demand for increased efficiency for monolithic parts up to 2000 × 4000 mm, the HyperMach Horizontal Series is being introduced and demonstrated at IMTS 2008," says Von Moll.

"Take a small chip and go as fast as you can," is the way Alan Hollatz, proposal engineer for Makino Corp (Mason, OH) defines HSM. "A smaller depth of cut at high speed leads to less cutting heat transferring either to the part or to the tool," explains Hollatz. "It also transfers less cutting force to the part and the machine." He describes conventional machining methods larger cuts with lower spindle rpm-as prone to distorting parts that are as thin as 0.030'' (0.76 mm) in modern designs. Less force transferred also means reduced fixturing requirements.

When machining aluminum, Hollatz advises going as fast as possible in finish cuts. "If we have a 30,000-pm spindle, we are going to try and run at 30,000-rpm," explains Hollatz. "We also try to limit the diameter of the tools we use. The faster we go, the smaller the tool diameter because of centrifugal force causing imbalances." As an example, one of the larger Makino machines has a top spindle speed of 33,000 rpm with a 107-hp (80-kW) motor where he does not recommend using any tool bigger than a 50-mm-diam cutter. For most cuts, a 25 mm or smaller diam cutter is most efficient.