High-Speed Machining for AEROSPACE

Manufacturing Engineering, Mar 2008 by Morey, Bruce

HSM for composites requires less horsepower and torque than for aluminum, according to Crick. The machine itself does not need to be massive, as it must be to cut titanium, but still needs to be rigid to overcome vibration and harmonics. He reports that most spindle speeds are 10-13,000 rpm, although they can go much higher. He cites one example of composite HSM at a large domestic aerospace OEM where cuts in composite material of 0.012-0.016'' (0.3-0.4 mm) are made on a 24,000-rpm machine.

Units originally designed for cutting metals are used to machine most composite materials at present. Crick believes the ultimate goal is to build a composite machine tool that is lighter and purpose-built for composites. One trend such a machine has to address is the growing size of composite parts in aerospace.

"You can have very large composite parts, for instance wing skins that are up to 100' [30-m] long," says Crick, "even whole fuselage sections such as the new 787 with barrel sections that are more than 20' [6m] in diam and 30' [9-m] long. The interfaces have to be machined to tight tolerances between one fuselage section and the other on these large structures. Other parts can be long and 'stringy,' such as spars, stringers, struts, and floor beams."

To address machining these long, thin parts that have a lot flexion-Crick describes them as acting like wet noodles-MAG Cincinnati has developed a purpose-built extrusion mill. Adaptable to either aluminum or composites, it is a feed-through process with a 13 × 8' (4 ' 2.4-m) work zone, a spindle rated at 24,000 rpm that uses up to 12 tools no more than 25-mm in diam. Parts can be up to 40' (12-m) long.

"While nearly all applications and materials can benefit from HSM, free-cutting materials, like aluminum or composites, tend to benefit most," says Reilly, the applications manager for Haas Automation. "But even hard mold steels benefit from HSM because of the trend to hard milling, where high feeds and speeds are used with light cuts." Titanium, a material growing in importance in aerospace applications, is certainly one of those metals.

"Aluminum cutting machines are more like a Formula 1 race car and titanium-cutting machines are more like a bulldozer," says Dan Cooper of MAG Maintenance Technologies' Productivity Solutions (Hebron, KY). "There is a drastic difference in spindle rpm. Although the principles of high-speed machining-a shallow cut at high rpm-is still sometimes relevant for titanium." In particular, thin-walled parts are best machined using HSM principles, says Cooper, describing for example, a customer part with dimensions of 0.030" (0.76-mm) thick and up to 3" (76-mm) high. "Such a thin, high wall cannot be roughed with the old, conventional process. Deep cuts at low rpm and high torque distort the part and deflect the cutting tool. This is especially true for newer components machined from 5553 Titanium alloy," says Cooper.

Titanium's low thermal conductivity and a high modulus of elasticity, combined with its strength, makes it a difficult material to cut, according to Cooper, "While torque and dynamic stiffness may not be as important to composites and aluminum, they are very important for machining titanium," says Cooper, limiting how high a speed one can achieve when compared to aluminum.