As part of continuing efforts to offer lighter, stronger, more cost-efficient products, manufacturers develop and apply high-performance workpiece materials. Sandwiched composites are good examples of that trend.
Aerospace manufacturers, in particular, rely heavily on sandwiched composite structures in critical components such as aircraft wing skins, fuselage sections, cabin walls and floors. However, machining a stack of materials of differing strengths and physical properties presents several layers of challenges. The main goal is to avoid bending or fraying the core structure or delaminating the face sheets. Sandwiched composite material that is cut unevenly or deformed loses its strength, much as creasing corrugated cardboard destroys its rigidity.
Like corrugated cardboard, a sandwiched composite is comprised of a lightweight core structure, usually resembling the hexagonal cells of a honeycomb, backed by rigid facing sheets. Depending on strength requirements, the honeycomb cells may be formed from high-tech paper, cardboard, carbon-fiber-reinforced plastic or aluminum. The face sheets can be paper, plastic, aluminum or titanium and are bonded to the open ends of the honeycomb cells. A balance of bending, compression and shear forces among the elements of sandwiched composites results in materials that are lightweight, rigid and remarkably strong.
Sandwiched composite parts typically are flat or mildly curved panels that range in thickness from 0.250" up to 0.500". The panels are fabricated to near-net-shape and finish-machined to trim outer edges, mill out widows and other various shaped openings and holes. For finish machining, shops must use high-speed endmills specifically designed for such sandwiched composites.
Sharp endmill cutting edges and high cutting speeds are key factors in cleanly machining sandwiched composite materials. The situation is much like slicing bread - cut too slowly, and the bread compresses instead of shearing cleanly. While on the other hand, fast-moving, sharp cutting edges generate clean cuts. When machining sandwiched composites, slow cutting speeds can distort the face sheet and the honeycomb structure itself.
High cutting speeds, however, generate heat, and that poses problems because many of the constituents of sandwiched composites are heat sensitive. Accordingly, light radial engagement - on the order of five percent of cutter diameter - minimizes heat generation. For the same reason, feedrates are kept low. Despite the light engagement and low feedrates, high cutting speeds help maintain productivity.
For instance, a 0.500"- diameter Seco Jabro 860 solid-carbide endmill at a 10% radial engagement would run at a speed of 400 surface feet per minute (SFPM) with a feedrate of 0.005 inches per tooth (IPT). Such parameters would apply to a sandwiched composite material with an internal Ti-Al honeycomb structure.
Tool geometry can also enhance productive machining of sandwiched composites. For instance, Seco engineered its Jabro 860 endmills specifically to machine these types of composite materials. The tool's double-helix tool flute configuration eliminates fiber breakout, prevents delamination and improves part edge finishes. As the Jabro 860 rotates, the flute helix on the lower part of the tool forces material upward while the upper helix forces material down. The opposed and balanced cutting forces make for a clean cutting action.
The components of sandwiched composites can be abrasive, so the tools used to machine them usually are manufactured from micrograin carbide to maximize edge integrity and wear resistance. To further resist abrasive wear, some cutting edges receive diamond coatings. Seco's DURA thin diamond coating, for example, is applied via CVD and combines low surface roughness for lubricity with high adhesion characteristics that reinforce its wear resistance. The coating requires a balance in tool engineering in that it is thin enough to minimally affect sharpness but thick enough to provide resistance to abrasion.
Machining the open construction and lightweight materials characteristic of sandwiched composites involves very low cutting forces, so high-torque machine tool spindles are often unnecessary. However, most sandwiched composite parts are big, such as expansive aerospace wing skins, and the machines that cut them are very large in size as well as powerful.
Industry is examining different methods of machining sandwiched composites, including waterjet and other abrasive cutting methods. All the alternatives have advantages and disadvantages. The bottom line is that flawless machining of sandwiched composites, especially those intended for aerospace applications, is crucial. Any imperfection on the skin of the wing can be a crack initiation site, and vibration and other forces in the aircraft will cause a crack to grow. In the interest of reliability and safety, manufacturers will continue to employ the cutting tools and techniques that have been developed and proven over time when machining sandwiched composite materials.
Authored by Don Graham, Manager of Education and Technical Services at Seco Tools, LLC
For more information contact:
Seco Tools, LLC
2805 Bellingham Drive
Troy, MI 48083
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