Aerospace: Sky is the limit

By Eric Ostini, GF Machining Solutions

The aerospace industry always has tested its parts suppliers on their ability to machine difficult materials. Critical engine components and airframe structures require components that are light yet uncompromisingly strong and durable, and the exotic alloys and composites being developed are beginning to further challenge the limits of conventional machine technology and part processing methods.

Superalloys like Kovar and Inconel provide many benefits, but the high cutting forces required to machine them generate a tremendous amount of heat and workhardening. Moreover, Inconel and other superalloy chips are gummy and stick to virtually everything in a machining center. Composites, in addition to being extremely hard, are brittle and abrasive, requiring special cutting tools to avoid delamination and achieve desired surface finishes.

If those challenges aren’t enough, lower-tier manufacturers now face the aerospace OEMs’ proprietary black art of formulating aerospace materials that are unique creations and closely guarded business secrets. Operating within this highly secretive and competitive arena has a steep, expensive learning curve that limits the advance of generalized machining parameters for efficient production. In many cases, machining methods used on one formula of these cutting-edge substances do not apply to others.

Conventional milling that uses shearing and cutting forces to machine has been pushed hardest by the continual development of new component materials. Milling remains feasible for many components, such as landing gear assemblies, seats, interior fixtures and panels. However, critical high-stress parts like engine components demand near-perfect integrity devoid of microcracking caused by the high thermal forces of machining. This is why conventional machine shops often struggle to establish throughputs that are efficient and profitable while maintaining proper thermal conditions.

To keep pace, tooling suppliers have had to advance cutting tool technology. Little more than a decade ago, polycrystalline diamond tools were among the hardest tools available that also provided a high degree of abrasion resistance and tool life. The new generation of aerospace materials, unfortunately, quickly wore down PCD tooling to the extent that a single part required multiple tool changes to complete.

In addition to pushing machine tool cutting parameters to the limit, complex shapes, such as a blisk, present workholding challenges. Image courtesy of GF Machining Solutions

Today, conventional milling of complex aerospace parts calls for cutting tools sintered with manufactured diamonds in which the shape of the part being milled is engineered into tool geometry. While these advances solve some of the challenges of aerospace part machining, they are extremely expensive, and their cost effectiveness is not always clear. Further, the materials used in today’s aerospace sector are not only battering cutting tools but wearing out the machine tools. As a result, a growing number of aerospace part manufacturers have adopted other technologies like electrical discharge machining, laser machining and additive manufacturing.

Contactless Cutting

EDM has taken a leading role in many aspects of aerospace manufacturing. Because the EDM electrode never contacts the workpiece, precision seal slots and other high-tolerance features can be machined without imparting stress to the part. Overall, EDM’s ability to cut very hard materials and complex shapes without the thermal distortion associated with conventional machining has made it a chief technology in aerospace manufacturing. However, electrical discharge machine tool builders have had to work hard to adapt the technology to aerospace’s unique challenges.