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Overview of Composite Materials
Composites are indispensable machining materials with massive potential, which drives innovation in new, emerging industries. These manufactured materials often comprise two or more constituent materials with considerably varying chemical or physical properties, which are distinct on a macroscopic level within the final composite structure.
Hence, their combination of unique properties, including high mechanical strength, electrical insulation, chemical, impact, and fire resistance. Composite materials such as carbon fiber exhibit a remarkable strength-to-weight ratio than titanium, steel, and aluminium.
For this reason, they are widely embraced for diverse applications across industries such as automotive, marine, aerospace, and wind energy. Even though there is a wide range of composite materials available, and each material type with varying machinability, they are compatible with various machining techniques.
Common Composite Materials
Some of the common examples include the following:
- Polymer Matrix Composites (PMC): These are the most widely used composite materials and are otherwise known as Fiber-reinforced polymers (FRP). They possess a polymer-based resin as the matrix and reinforcing fibers, including carbon, glass, and aramid.
- Metal Matrix Composites (MMC): These composite materials are common in the automotive industry. The matrix is a metal (aluminium) and contains reinforcing fibers such as silicon carbide.
- Ceramic Matrix Composites (CMC): These types of composites are increasingly found in extremely high temperature environments. These composite materials use a ceramic as the matrix and strengthen it with short fibers or whiskers like boron nitride and silicon carbide.
Standard Techniques for Machining Composites
Manufacturers, product engineers, and machinists employ different techniques for composite machining to achieve desired results. These techniques are generally categorized into conventional and non-conventional machining techniques. More so, each manufacturing process delivers distinct capabilities that provide an edge over another.
Conventional Machining Methods
They include:
- Milling: It involves the controlled removal of material from a blank (composite workpiece) with rotating multipoint cutting tools. CNC milling usually occurs at controlled feed rates and cutting speeds to avoid machining defects. However, this composite machining process provides good results for surface profiling and edge trimming.
- Drilling: The drilling process makes precise cuts in composite materials to create accurate holes for specific purposes. Machinists often use diamond-coated cutting tools and stepped drills to make holes of a certain diameter and depth in machined composite parts. Machinists maintain controlled speeds and moderate feed rates when drilling composite materials to maintain structural integrity and ensure complete assembly.
- Turning: This conventional cutting technique involves rotating the composite workpiece while the cutter shears material from it to create specific cylindrical component shapes. The turning process offers tight tolerances and higher accuracy in making composite bearings and shafts.
Non-Conventional Machining Methods
They are:
- Electrical Discharge Machining: EDM uses controlled electrical sparks to make desired cuts on conductive composites to achieve precise parts. Electrical discharge machining is a non-contact method that provides quality surface finishes. As such, you don’t have to worry about mechanical stress or tool wear when you choose EDM for composite materials machining.
- Ultrasonic Machining: This non-conventional composite machining technique uses high-frequency vibrations to remove material from composite workpieces. Not only does this technique offer great accuracy with minimal heat, but it can also cut brittle materials like ceramics and glass.
- Laser Machining: In laser machining, fiber or CO2 lasers drill and cut composite materials to desired shapes. This technique delivers clean cuts with minimum heat-affected areas due to its non-physical contact working principle. While fiber lasers are ideal for metal matrix composites, CO2 lasers work best for organic compounds. Both types offer superior quality components with clean edges and no mechanical stress.
- Water Jet Cutting: Water jet cutting employs high-pressure water containing abrasive materials to shape composite materials to the desired geometry. Machinists often embrace water jet cutting technology because it preserves fiber integrity when cutting composites, especially thick materials. It also produces clean edges on machined composite parts without compromising the material structure. Moreover, this technique is ideal for creating parts for applications where tight tolerances and complex shapes are non-negotiable.
Expert Tips for Effective Composite Materials Machining
Most manufacturing experts often encounter various issues when machining composite materials due to the abrasive nature of composites, resulting in complications such as rapid tool wear, elevated temperatures, and compromised cutting quality.
In this section, we will explore some helpful practices that would help improve machining skills and confidence as you use composite materials for various purposes:
Enter the Matrix (Cut into the Resin-rich Binder Materials)
When machining composites, it is crucial to note that they are made with a wide range of constituent materials. These constituents are in two groups, namely matrices and reinforcements (reinforcing fibers).
For instance, the FRP (Fiber reinforced Plastic) composites are produced from strands of carbon, aramid (Kevlar), glass, or other fibrous synthetics. These reinforcement fibers are held together with a type of resin in a thermoset or thermoplastic system. Meanwhile, manufacturers often use added ceramic or metal powders as reinforcement, and additives like silica or silicon dioxide are commonly used to improve the properties of a composite material.
Therefore, you should understand that the cutting tool is penetrating materials with varying hardness, density, and thermal reactions.
Install a Suitable Dust Bowl
Just like sanding the hull on a sailboat, composite machining generates fine particle matter, which causes a health hazard if inhaled. For example, fiber glass and carbon fiber composites are types of fiber reinforcement that create plenty of harmful dust during machining processes, like their other phenolic-filled relatives.
These composite dust particles are dangerous to health if inhaled and can cause extreme damage to machine controllers and equipment on the machine floor. Most aircraft manufacturers, being the largest consumers of composite components, cannot afford to let debris become trapped inside aircraft structures for safety purposes.
For this reason, efficient carbon fiber dust removal is crucial when machining. Therefore, it would be a good practice to always install a compatible cartridge-style dust collector in the machine and seal the machine tool enclosure to effectively control these dust particles.
Choose the Right Tools for Superior Performance
The high abrasiveness of composites is an innate attribute that poses a variety of complications, especially when cutting at high spindle speeds with high-speed steel and cemented carbide tools.
However, turning tools and indexable milling cutters with polycrystalline diamond (PCD) tips exhibit longer tool life and produce desired surface quality when cutting composites, even though they cost more than carbide cutting tools.
AT Machining can provide these engineered machining tool solutions. “Veined” tools generally provide optimized cutting and long-lasting precision in machining composite materials.
The right tool geometries and material ensures machine uptime and desired results when cutting composites like CFRP (Carbon Fiber Reinforced Polymers).
Use Sharp Tools
The use of sharp cutting tools with the appropriate tool material and cutting edge geometry is a non-negotiable necessity when machining composite materials. Expert machinists recommend using specially designed compressive helix cutting tools to minimize delamination and uncut fibers when trimming.
Conversely, veined PCD is best suited for orthogonal cutting like pocketing, trimming and slotting. To avoid splintering or delamination when making holes in composites, ensure to use veined diamond drills with special edge chamfers, radii or a special Brad point (like in a woodworking drill).
More importantly, the ideal approach to machining demanding materials like composites is to employ cutting tools with incredibly sharp teeth and a positive cutting action in any situation.
Optimize Composite Machining Parameters
In most cases, composite cutting can be incredibly challenging, especially when machining honeycomb materials used in cabin dividers and aircraft floor panels. The workpiece just pushes away most of the time. However, you can tackle these machining challenges with plenty of RPM and a steady pace.
Typically, how fast the cutting edges should move over the composite workpiece ranges from 550 to 760 meters per minute or more. The cutting process begins with 0.076 mm per tooth using a 6.3 mm wide cutter with 2-flute tool/ cutting edges, spinning at about 30,000 spins per minute and 4.570 mm per minute feed.
However, reduce the spindle speed to 1/3 (one-third) of the mentioned values when cutting solid composites. You can also consult the cutting tool provider for the right machining parameters when in doubt.
Maintain a Firm Grip on the Cutting Tools
While high-quality diamond cutters and diamond-coated tools are essential to ensuring successful CFRP machining operations, it might be difficult to achieve without the right tool holder to hold the cutter in position.
Experts recommend using hydraulic chucks to reduce run-out and the possibility of tool pullout during material removal. More so, keep the tool holders balanced with the retention knob installed and the cutting tool when machining composites at higher spindle speeds, such as 20,000 RPM or more.
Stabilize the Workpiece
Holding the composite workpiece in place is just as important as using a top-quality tool holder on the cutting tools. Since composites have an arduous and abrasive nature that makes them hard on cutting tools, it is crucial to hold the workpiece in place for the cutting operations to prevent micro-delamination.
Experienced machinists recommend using vacuum fixtures to secure the workpiece, provided it possesses adequate surface area to create sufficient gripping force. Special jigs are also ideal options for securing such materials in place.
Ensure the design of the workholding device can avoid unsupported cutting areas as it holds onto the composite workpiece tightly to prevent vibration and sudden movement. More importantly, it should support the complex three-dimensional shapes typical of composite components.
Maintain Proper Attack Angle
Whether you rip the fibers out or you cut them cleanly hinges on whether you use the right attack angle. Besides, the weave-like internal structure of composites makes them relatively challenging to cut. While the use of dull tools is a key cause of delamination and hanging fibers, the abrasive nature of composites is a primary cause of poor tool life.
Therefore, it is important to learn to assess and identify wear patterns on cutters and adjust tool paths duly, even though you can alleviate some of these challenges by using the right cutting tools.
Utilizing a brad point might be effective in preventing delamination at exit points. Similarly, climb milling cutting offers better results than conventional cutting in most cases since the cutting action pulls reinforcement fibers into the cut. Furthermore, it is typically an ideal approach to experiment with composite cutting with a variety of cutters and parameters due to the variety of composite materials.
Manage Generated Heat
Keeping the cutting zone cool is key to ensuring efficient composites machining. Although, generous application of coolant helps lubricate cutting tools, minimize generated heat, and remove debris from the cutting zone. It creates an abrasive paste that can be incredibly challenging to deal with when cutting composites.
In worst cases, composite materials can absorb water and swell, damaging their integrity. A cold air gun is a low-cost and effective solution for cooling composite workpieces and removing dust particles. However, ensure to collect the resulting composite dust tornado in an adequately sized collection system.
Although researchers record that cryogenic cooling systems are effective in keeping composites cool during machining, such cutting-edge solutions may be too expensive or inaccessible to all but the most die-hard manufacturers committed to manufacturing composite components.
Conclusion
While composite materials are one of the most challenging materials any manufacturer will ever machine, they are not impossible to cut. These materials foster groundbreaking possibilities in modern manufacturing. As such, it is important to think outside the box and devise innovative solutions to ensure quality throughputs and optimized material performance. We have discussed some of the effective composite machining strategies you need to understand to elevate your project!
AT-Machining is the right CNC machine shop to contact when you need professional assistance with your composite machining project. We are a top machining services provider that caters to the needs of various industries. Our team of experienced experts leverage our bleeding-edge machine and unmatched machining capabilities to meet stringent industry standards and elevate your project to the next level. Don’t hesitate to contact us for a free and instant quotation!
