*CFRP stands for Carbon Fiber Reinforced Plastic.
The unrelenting passion of the aerospace industry to enhance the performance of commercial and military aircraft is constantly driving the development of improved high-performance structural materials. Composite materials are one such class of materials that play a significant role in current and future aerospace components. Composite materials are particularly attractive to aviation and aerospace applications because of their exceptional strength and stiffness-to-density ratios and superior physical properties. A composite material typically consists of relatively strong, stiff fibers in a tough resin matrix. Man-made composite materials, used in the aerospace and other industries, are carbon- and glass-fibre-reinforced plastic (CFRP and GFRP respectively) which consist of carbon and glass fibres, both of which are stiff and strong (for their density), but brittle, in a polymer matrix, which is tough but neither particularly stiff nor strong. Very simplistically, by combining materials with complementary properties in this way, a composite material with most or all the benefits (high strength, stiffness, toughness and low density) is obtained with few or none of the weaknesses of the individual component materials.
Composite materials are important to the Aviation Industry because they provide structural strength comparable to metallic alloys, but at a lighter weight. This leads to improved fuel efficiency and performance from an aircraft.
In the figure below you can see that the body of the Boeing 787 Dreamliner consists of 50% Composite materials compared to the Boeing 777 that uses only 12% of composite materials.
Machining of Composites
Machining of composites may look like machining metal, but that appearance is deceiving.
Parts made of a composite material such as the carbon fiber reinforced plastic (CFRP) increasingly being used for aircraft components can be set up and run on the same machine tools as metal parts. The CFRP might even be machined with similar cutting tools as the metal parts, though this is less likely. The very mechanism of material removal is different.
In metal cutting, that mechanism is plastic deformation. The material is softer than the tool, and the chip flows over the cutting edge. But in machining of composites—the focus here will be CFRP—there is no chip to speak of. Instead, the material removal mechanism might be better described as shattering. Rather than shearing material away, the impact of the cutting edge fractures the hard carbon fibers. In the process, the cutting edge undergoes considerable abrasion that can lead to rapid wear.
In any cutting tool application, tool geometry drives cutting performance and tool material drives life. This is true of composites machining as well. However, in composites, tool material also becomes a driver of performance. Composites can cause the tool to wear so rapidly that the geometry can change rapidly as well—unless the edge material can withstand the abrasion well enough to hold its geometry and stay sharp. In a way, machining composites actually turns the machining process upside down, because the burden of the shop’s attention shifts to different parts of the process. An aircraft part machined out of metal might involve a high-power machine tool that relies on just commodity tooling and simple clamps to secure the work. By contrast, milling and drilling of composites can generally be done with a much lighter-duty machine. However, high-end cutting tools are likely to be required, as well as custom-built work holding that closely supports the part throughout the machining process to prevent its thin, rigid walls from vibrating and fraying.
One factor that influences the cutting behavior or “machinability” of composite materials is the Fibre Orientation. The fibre orientation refers to the placement of each layer onto one another to form a ply. Most common are those plies with 0-90 degrees, 0-45-90 degrees and 0-45-90-135 degrees stacking patterns. When machining the 45 and 135 degree layers a most difficult to cut, cutting tools suppliers have developed several geometries to successfully machine those types of CFRP.
In the picture below is an example of typical milling cutters for machining composite materials. Most often these types of cutters have a diamond coating.
As mentioned earlier the cutting tools for machining composites are different from standard milling cutting tools. This can be clearly seen in the following illustrations.
The standard style end mills generate cutting forces in only one direction. With a positive helix cutter, this will have the tendency to lift the workpiece while causing damage to the top edge.
The compression-style router generates cutting forces into the top and bottom surfaces of the workpiece. These forces stabilize the cut while eliminating damage to the workpiece edges.