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The aerospace industry is increasingly turning to composite materials to make modern aircraft lighter, tougher and more efficient. Composites can weigh 20% less than their metal counterparts and are stronger by weight than aluminum, steel and titanium, making them well-suited for lightweighting large structures and airframe components. They also offer engineers greater design flexibility because they can be molded into complex shapes that aren't possible with traditional materials.

The downside for manufacturers is that these materials are difficult to machine because they are abrasive and prone to splintering. The extremely abrasive content of carbon fiber-reinforced polymers (CFRP), for example, will cause challenges ranging from rapid wear on cutting tools and compromised cutting quality, to elevated temperatures, vibration and unstable cutting conditions. A standard carbide cutting tool may last only 5% of its usual tool life when cutting CFRP versus traditional metal machining.

There is no simple formula for overcoming these challenges because composite materials can come with such a wide range of characteristics. However, there are a few key best practices and cutting principles that every shop can apply to navigate these dynamics and improve operations.

Know your material

The first thing to understand is that composites can have a wide range of constituent materials that are incorporated through matrices or layered reinforcements, which means they can have a wide variety of physical and chemical properties. Composites may have other reinforcements besides carbon fiber, like strands of glass, aramid (Kevlar) or other fibrous synthetics. Ceramic or metal powders, along with various additives such as silica or silicon dioxide, may also be added to enhance material properties.

Knowing the makeup of your composite material and how it behaves during the machining process is essential for achieving an optimal machine setup. For example, cutting into a unidirectional (UD) material composed of a single layer can present unique challenges. When the fibers are aligned in the same direction, they do not lend support to the material structure and are susceptible to pulling and fraying during machining operations. Shops will need to approach the cut from the proper angle and direction, and in some cases will need to use a compression-style tool that approaches the cut from both the top and bottom of the material.

Another challenge with composites is that they are often stacked with other materials — such as titanium, aluminum or copper — in all sorts of combinations. When drilling through these combinations in a single pass, shops need to understand how the drill will interact with each layer of material. With stacked carbon fiber and aluminum, operators can maintain the same aggressive cutting speed for the duration of the cut, but with harder materials like titanium, the cutting speed and feed rate may need to be adjusted during the drilling operation for each layer.

Control the dust

Composite materials create plenty of harmful dust during the machining process. Not only is this a health hazard to breathe, but dust particles are also extremely damaging to machine electronics and equipment. Additionally, most aircraft manufacturers cannot allow debris to become trapped inside aircraft structures, making efficient dust removal a must. To control these particles, install a suitable cartridge-style dust collector and seal the machine tool enclosure if possible. A simple shop vacuum attachment can also suffice if the spindle is properly enclosed.

Get tough with tools

Because of the extremely abrasive nature of composites, high-speed steel and even cemented carbide cutting tools can quickly fail. Frequent tool changeouts because of failures add up to lost productivity, especially when machining large structural components like a wing box or drilling thousands of holes for rivets. In these applications, investing in polycrystalline diamond (PCD) tools are well worth the cost because they'll last 10-15 times longer than a standard cutting tool, improving process security and surface quality. These tools maintain their sharpness longer and provide a cleaner cut in composite materials, reducing the risk of fraying or failure that can lead to scrapped components.

Diamond tools may have a diamond coating on the surface, applied using a chemical vapor deposition (CVD) process, or they can be made with diamond veins that are sintered into the carbide. Veined cutting tools represent a significant advancement in composites machining, enabling the PCD to be placed in a helical vein for better helical geometries and chip flow. This reduces cutting forces, particularly on drill exits.

A good cutting tool provider will run extensive tests on PCD tools using composite materials to provide clear data on performance and life expectancy to help drive more predictable results. They'll also be able to extend the life of the tool through reconditioning, which is an area that has seen improvement in recent years. Where previously a PCD tool could be resharpened two or three times, you can now resharpen some tools up to 10 times. The cost of resharpening is typically 20% the cost of a new cutting tool, so this dramatically increases the return on investment.

Eliminate delamination

Ineffective machining processes can result in delamination of composite laminates, which can have detrimental consequences to the structural integrity and surface quality of the component. To avoid this layer separation, it's essential to use cutting tool geometries that minimize lifting of the material. For example, choosing a milling tool for material removal with a low helix angle — 10° to maybe 15° at most — will keep milling operations from pulling the material apart.

Additionally, once a tool begins to dull, it will immediately begin pushing the material instead of making a smooth cut, which then causes splintering. To avoid this, choose tools that maintain a sharp edge longer, like drills with PCD tips, and don't push the limits on life expectancy — change out cutting tools when they start to lose their edge. To maintain sharpness longer, manage tool wear with strategies like positioning the tool to cut further up on the flutes in side-milling operations. This disperses tool wear more evenly and extends the life of the tool.

There is a wide range of sharp tools available that are designed for improved performance when used with composites. For example, compressive spiral cutters, like Sandvik Coromant’s CVD-coated CoroMill Plura, help reduce delamination and hanging (uncut fibers) when trimming, and veined PCD end mills work well for orthogonal cutting such as slotting, trimming and pocketing. For hole-making, veined diamond drills are often supplied with special edge chamfers, radii or even a modified brad point (similar to a woodworking drill) to prevent delamination or splintering when breaking through.

A typical machine operator listens during the cutting process and tends to reduce the cutting speed and feed rate when something doesn’t sound right. This is also a common practice used in efforts to conserve cutting tool life. But in composites, this careful approach to machining will often lead to problems. The sounds of composite machining are much harder to read, and going too low on the feed and chip load (the width of chips produced by a cutting edge on each rotation) will lead to rubbing, chatter, delamination and excessive tool wear.

Machinists must also learn to read the wear patterns on cutting tools and adjust tool paths accordingly. If milling tools are showing excessive wear, operators may need to consider adjusting the cutter in the Z-direction to use more of the cutter to reduce wear and avoid delamination. And for drilling, switching to a brad point drill might prevent delamination on the backside of a hole. Because of the variety of composite materials, experimenting with different types of cutters and process parameters, or partnering with a tool provider to find the correct geometry, is often the best approach.

Be aggressive

Cutting into composites, especially in sandwiched honeycomb materials used in aircraft floor panels and cabin dividers, can be challenging — most of the time the tool just pushes the panel away. The key here is to stay aggressive, maintaining cutting parameters of 6,000-10,000 rpm and high chip loads in the range of 0.002-0.003 inch per tooth. This also helps minimize heat buildup in the material by transferring the heat through the chip and to the cutting tool. This is especially important when working with thermosets and thermoformable/thermoplastic materials, so the material doesn’t adhere to the tool. If tools are operated with less than a 0.001-inch chip load, they will be destroyed very quickly.

Using PCD or CVD tools here will enable operations to maintain the aggressive cuts needed. For example, Sandvik Coromant’s CoroDrill 863 with -O geometry is available with PCD or CVD options specially designed for drilling composite materials. The optimized drill features a distinctly formulated substrate for better adhesion with the coating and a high axial rake angle for reduced delamination. The PCD-veined CoroDrill 859V is optimized for UD materials that may use a prepreg system as well as epoxy and bismaleimide. The double-angle geometry helps reduce delamination on demanding materials.

Another consideration is the use of coolant. Managing heat with coolants is not typically an option because the coolant can enter into the composite laminate and contaminate the material. To keep heat down, shops can use a compressed air or a cold air gun in the cutting zone with an adequately sized dust collection system as a low-cost solution.

Secure your setup

Rigid and secure toolholding is essential, preventing chatter in the cut that can ruin an investment in good machining tools. Hydraulic chucks will minimize run-out and the chance of tool pullout — events that can risk scrapping a $40,000 CFRP airfoil. And at higher spindle speeds (anything above 20,000 rpm) it's best to have the toolholders balanced as a complete assembly, with the cutting tool and retention knob installed to keep the toolholder secure.

Gripping the composite workpiece is also important. Vacuum fixtures are useful if the workpiece has enough surface area to create sufficient gripping force. In most cases, jigs are the best option, especially for the complex, 3D shapes common with composite workpieces. These should be designed to avoid any unsupported cutting areas while still gripping securely enough to prevent movement and vibration in the part.

Team up with tool providers

With so many variables involved with machining composites, getting the right machine setup with the right processes can be daunting. Proving out a process on a new component or material through testing takes time and resources that many shops don't have. However, it’s important to remember you’re not on your own. Partnering with tool providers can lead to much more than tooling recommendations; they can be an invaluable resource for engineering and support. Beyond tool selection guidance, they can also help with toolpath programming, simulations and machining new components at test facilities using the same type of equipment as your shop. Providers like Sandvik Coromant can even tailor customer-specific tools to meet the demands of a component.

Composites are among the most challenging materials any machinist will face. Machining these materials is a dusty, arduous and abrasive process that is hard on cutting tools. But with the right combination of cutting tools, cutting strategies and know-how from in-house teams and/or tool provider partners, any shop can set itself up for more successful aerospace composites production.