Machining composites is a dusty, arduous and abrasive process that is difficult for cutting tools and requires a combination of the right strategy and expertise to properly navigate their dynamics.
Fiber-reinforced composite wing components are often stacked with aluminum and titanium and can present challenges to the machining process, including tool safety and final surface quality. Sandvik Coromant's aerospace industry is increasingly turning to composite materials to make modern aircraft lighter, stronger, and more efficient. Composites are 20% lighter than metal and stronger than aluminum, steel, and titanium, making them ideal for lightweight, large, structural and airframe components. They also provide engineers with greater design flexibility, as they can be molded into complex shapes that are not possible with traditional materials.
The downside for manufacturers is that these materials are difficult to machine because they are abrasive and prone to chipping. For example, the extremely high abrasive content of carbon fiber reinforced polymers (CFRP) presents a variety of challenges, from rapid wear of cutting tools and reduced cutting quality, to high temperatures, vibrations, and unstable cutting conditions. When cutting CFRP, standard carbide cutting tools may only have a 5% lifetime of their usual tool life when cutting CFRP.
There is no simple formula for overcoming these challenges, as composites can have such a wide range of properties. However, there are a few key best practices and cutting principles that every store can apply to navigate these dynamics and improve operations.
Know your material
The first thing to understand is that composites can contain a wide range of constituent materials through matrix or layered reinforcements, which means they can have a wide variety of physical and chemical properties. In addition to carbon fiber, composites may have other reinforcements such as glass fibers, aramid (Kevlar), or other fiber synthetics. Ceramic or metal powders and various additives such as silica or silica can also be added to improve material properties.
Understanding the composition of composites and how they behave during machining is essential to achieve an optimal machine setup. For example, cutting a unidirectional band (UD) material consisting of a single layer can present unique challenges. When the fibers are arranged in the same direction, they do not provide support for the material structure and are susceptible to pulling and abrasion during processing operations. Factories need to approach the cut from the proper angles and orientations, and in some cases, use compression tools that approach the cut from the top and bottom of the material.
Another challenge with composites is that they are often stacked in various combinations with other materials such as titanium, aluminum, or copper. When drilling through these combinations in a single hole, the shop needs to understand how the drill bit will interact with each layer of material. With stacked carbon fiber and aluminum, the operator can maintain the same aggressive cutting speed during the cutting process, but for harder materials such as titanium, the cutting speed and feed rate may need to be adjusted during each layer drilling operation.
Control the dust
Composites generate a large amount of harmful dust during processing. Not only is this harmful to respiratory health, but dust particles are also extremely damaging to machine electronics and equipment. In addition, most aircraft manufacturers cannot allow debris to remain within the aircraft structure, so effective dust removal is essential. To control these particles, install a suitable cartridge dust collector and, if possible, seal the machine housing. If the spindle is properly closed, a simple workshop vacuum attachment is also sufficient.
Get tough with tools
Due to the extremely high wear resistance of composite materials, HSS and even tungsten carbide tools can fail quickly. Frequent tool changes due to breakdowns can lead to reduced productivity, especially when machining large structural components such as wing boxes or drilling thousands of rivet holes. In these applications, it is worthwhile to invest in polycrystalline diamond (PCD) tools, as they have a service life of 10-15 times longer than standard cutting tools, improving process safety and surface quality. These tools stay sharp for longer and provide a cleaner cut in composites, reducing the risk of wear or failure that can lead to end-of-life components.
High-performance solid carbide drill bits, such as Sandvik Coromant's CoroDrill 863, are designed to easily drill through a wide range of carbon fiber reinforcements and metal stacks.
Diamond tools may have a diamond coating on their surface, applied using a chemical vapor deposition (CVD) process, or they may be made from diamond veins sintered into carbide. Vein cutting tools represent a significant advance in composite machining, enabling PCDs to be placed in spiral veins for better spiral geometry and chip flow. This reduces cutting forces, especially at the drill outlet.
A good cutting tool supplier will perform extensive testing of PCD tools using composites to provide clear data on performance and life expectancy, helping to achieve more predictable results. They can also extend the life of the tool through restoration, an area that has improved in recent years. Previously, PCD tools could be reground two or three times, now you can regrind some tools up to 10 times. The cost of regrinding is usually 20% of the cost of a new cutting tool, so this significantly increases the return on investment.
Eliminate delamination
Ineffective machining processes can lead to delamination of composite laminates, which can adversely affect the structural integrity and surface quality of the part. In order to avoid this layer separation, it is essential to use cutting tool geometries that minimize material lift. For example, choosing a milling tool with a low helix angle (up to 10° to 15°) for material removal can prevent the milling operation from pulling the material apart.
Also, as soon as the cutter starts to dull, it immediately starts pushing the material instead of making a smooth cut, which leads to chipping. To avoid this, choose a tool that retains a sharp edge longer, such as a drill with a PCD tip, and don't push the limits of life expectancy – when the cutting tool starts to lose edge, replace the cutting tool. To maintain sharpness for longer, tool wear can be managed through strategies such as positioning the tool in a side milling operation to cut further on the groove. This results in a more even distribution of tool wear and longer tool life.
Specially designed compression spiral tools, such as the CVD-coated CoroMill Plura from Sandvik Coromant, help to reduce delamination and suspension (uncut fibers) when trimming.
There is a wide variety of sharp tools to choose from that are designed to improve performance when used with composites. For example, compression spiral cutters, such as Sandvik Coromant's CVD-coated CoroMill Plura, help reduce delamination and suspension (uncut fibers) during trimming, while textured PCD end mills perform well in orthogonal cuts such as grooving, trimming and grooving. For drilling, textured diamond drill bits are often equipped with special edge chamfers, radii, and even a modified tip (similar to a woodworking drill bit) to prevent delamination or chipping upon penetration.
Typical machine operators listen during the cutting process, and when the sound is not right, they tend to reduce the cutting speed and feed rate. This is also a common practice to protect the life of cutting tools. But in composites, this careful processing method often leads to problems. The sound of composite machining is more difficult to read, and too low feed and chip load (the width of the chips produced by each rotation of the cutting edge) can lead to friction, chatter, delamination, and excessive tool wear.
The machinist must also learn to read the wear patterns on the cutting tool and adjust the toolpath accordingly. If the cutter is excessively worn, the operator may want to consider adjusting the cutter in the Z direction to use more tools to reduce wear and avoid delamination. For drilling, switching to a pointed drill bit may prevent delamination on the back of the hole. Due to the diversity of composites, it is often best to experiment with different types of tools and process parameters, or to work with a tool provider to find the right geometry.