The story of carbon fibre dates to 1879. Inventor, Thomas Edison, used carbon fibres as filaments for early light bulbs. Although these fibres lacked the tensile strength of today’s carbon fibres, their considerable tolerance to heat made these fibres ideal for conducting electricity. It wasn’t until the late 1950s that high tensile strength carbon fibres were discovered. Rayon became the first precursor used to create these modern fibres.
The benefits of these high-strength fibres were clear. They weighed a fraction of the weight of steel yet contained much greater tensile strength. Another important benefit of carbon fibre was its high modulus or resistance to stretching.
In the recent decades, carbon fibres have found wide applications in commercial and military aircrafts, recreational, industrial and transportation markets. They are used in composites with a lightweight matrix and are ideally suited for applications where strength, stiffness, lower weight and outstanding fatigue characteristics are critical requirements.
Thus, composite materials or composites are materials made from two or more constituent materials with significantly different physical or chemical properties, that when combined, produce a material with characteristics different from the individual components.
CFRP is an acronym for ‘Carbon Fiber Reinforced Polymer’. Each carbon filament thread is a bundle of many thousand carbon filaments. A single such filament is a thin tube with a diameter of 5-8 micrometres and consists almost exclusively of carbon.
Properties of CFRP
CFRPs are non–homogenous and possess high tensile strength than alloy steels and super alloys. It has very high thermal, corrosion and chemical resistance. Since it has a high strength to weight ratio, it finds application in aircrafts where fuselage, wings, frames and engines are being made of CFRP for increased fuel efficiency.
Did you know that the Airbus A350 and Boeing Dreamliner 787 aircrafts have 50% composites by weight? Studies also reveal the overall weight of a passenger car can be brought down by 30% when CFRP is used as metallic substitute on the vehicle body and structures. As the momentum for EVs pick up, we expect CFRPs will find more prominence in the automotive industry soon.
Challenges in machining composites
Machines can sometimes be handheld units, less rigid, which can pose unstable cutting conditions. For machining bigger structures and assemblies, either handheld drilling units or Automatic Drilling Units (ADUs) will be mostly used. Customers also use robotic drilling machines for large parts where some flexibility is needed. 5-axes gantry machines are also popular but there will be limitations in terms of the size of CFRP panels that can be machined in these machines. Machining conditions will be either with compressed air suction or Minimum Quantity Lubrication (MQL) arrangement.
The CFRPs are extremely abrasive and hence, the tool develops flank wear rapidly. Minimum tool wear is sufficient to distort the surface quality in these materials.
Delamination (separation of layers) is caused between the CFRP sheets due to too much pressure at the cutting point. Splintering is another phenomenon which happens due to debonding and fibre pull out due to dull cutting edge and too much temperature of the resign. Metal exits in CFRPs can have ‘exit burrs’ because of high cutting pressure and the cutting edge not being sharp or sometimes worn out.
How can we overcome these challenges?
Possible remedies can be low thrust double angle point geometry tools for drilling. Engineered geometries that can minimise friction and PCD tipped tools offer a great value and enhance productivity. In this aspect, Kennametal has a strong portfolio supplemented with newly developed ‘DAL’ and ‘SPF’ point geometries. DAL is meant for CFRP + metallic exits which minimises the exit burr condition and the ‘SPF’ is meant for CFRP drilling without the metal exit. It can be offered with advanced diamond coating or be tipped with PCD versions for superior performance. These new geometries & grades in Kentip FS have also been developed, which is a proven modular drilling platform and can save cost for our customers.
In fact, Kennametal is the first and the only company to offer modular drilling solutions for CFRP drilling off-the-shelf. For edge trimming and side milling, the company also offers ‘burr style’ routers. This provides excellent temperature control while machining at higher cutting speeds.
In some cases, where we need great ramping capabilities, we offer down cut style routers which produce downward cutting forces and thus, eliminate surface delamination. Lastly, on the safety front, since CFRP is an extremely abrasive material and can spread thin in the air that we breathe, prolonged exposure can cause respiratory illness. Extreme care must be exercised and the powdery dust coming out of the machining process must be vacuum absorbed immediately.