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Enhancing performance and building lighter, stronger components has been a driving forces in the aerospace industry, which has motivated manufacturers to push the limits of innovation. This includes designing tooling solutions for drilling rivet holes in carbon fiber-reinforced polymer (CFRP) composites, including thermosets or thermoplastics, for use in fuselages, spoilers, wing skins and other vital components.

Global tooling company Kennametal (Pittsburgh, Pa., U.S.) says it understands the intricacies of complex rivet hole drilling — especially when machining layered stacked plates. With global demand for CFRP increasing thanks to its significant lightweighting benefits, some machining challenges can come into play including frequent tool changes and setups. Kennametal is continuously developing cutting tool systems that provide increased performance to overcome those challenges and meet industry requirements.

Challenges of drilling rivet holes in composites

Drilling rivet holes is often performed in CFRP or CFRP-hybrid material stacks (such as with aluminum or titanium). Machining CFRP can be challenging due to their abrasive and anisotropic nature; the strength and stiffness of the material changes based on fiber direction and how the composite is layered. Drilling through these materials often leads to delamination, fiber pullout or even hole misalignment if not managed correctly. Therefore, it becomes paramount that chip control and tight tolerances are maintained, fiber damage is avoided and burr formation is minimized to preserve workpiece integrity.

There are several ways to ensure users achieve the best outcome for drilling composites, including removing material in smaller increments, using the proper tools and feed rate, and optimizing the drilling cycle to ensure a clean hole.

“To tackle the challenges of machining composites, it’s crucial to employ strategies that enhance precision and maintain the quality of the workpiece,” says Steve Gray, Kennametal technical program manager – CFRP and aerospace assembly and future solutions engineering. “Consider using specialized tools, adjusting your machining parameters and refining your process to achieve optimal results.” 

One issue with stacked laminates is maintaining sharp cutting edges, which reduces the force required to machine, minimizing the risk of burr formation and delamination. It is also important to watch for wear in the tool, which can cause excessive friction and generate more heat, leading to higher risks of delamination and fiber pullout. Using diamond-coated or polycrystalline diamond (PCD) drills can increase cutting performance and tool life when machining multi-material stacks, making it much easier to maintain tight tolerances. 

Another possible aid is to use a pecking cycle (also known as peck drilling or peck milling). This technique uses multiple, shallow passes instead of one deep pass and is an effective way to remove chips and maintain low temperatures in a CFRP/metal stack. It prevents the hole from accumulating metal chips which could erode the CFRP as they exit. It also prevents tool overheating, which prevents the resin from reaching its glass transition temperature which could damage the composite workpiece.

Indeed, heat buildup is a major factor that can damage the part, as explained above, but also reduces the lifespan of cutting tools. Notably, thermoplastic composites are less prone to delamination but more susceptible to heat buildup and deformation during drilling. Meanwhile, thermoset composites have excellent thermal stability and will not melt under high temperatures, but the heat generated during drilling can still cause thermal degradation and affect the composite’s mechanical properties. CFRP-titanium hybrid composites pose further issues by requiring techniques that deal with both materials.

Employing coolant strategies is another option to manage temperature control. For example, providing minimum quantity lubrication (MQL) on the cutting edge of the tool can reduce friction and heat buildup.

Sustainable solutions for an eco-friendly environment

In addition to improved composites cutting efficiencies, cutting tool suppliers are also making sustainability a top priority — that is, finding solutions that minimize waste and reduce energy consumption. The MQL technique is ideal for minimizing waste — by applying a specific amount of lubrication directly to the cutting zone of a rivet hole while drilling, the amount of coolant fluid used is significantly reduced. Cryogenic cooling using liquid CO₂ is another effective method to reduce heat at the cutting edge of the tool. This technique keeps the tool extremely cold, which helps reduce tool wear and maintain the cutting edge for a longer period. 

“We are regularly approached by customers who ask for ways to reduce their carbon footprint,” says Georg Roth, global portfolio manager – drilling and threading tools at Kennametal. “This goes for everything from improving tool life and reducing carbide consumption to the use of recycled package material.”

Along with using edge inserts to prevent waste or using modular toolholder systems, reconditioning and regrinding services aim to restore existing cutting tools for longer service life. This contributes to sustainability by using less material and leads to the reduction of cost per hole.

Improving design and performance

As drilling tools and techniques continue to advance, the integration of materials like CFRP and thermoplastics has led to significant innovations in Kennametal’s technology for rivet hole drilling layered material stacks.

The modular drill, for example, offers high rigidity and can be used on a variety of materials. The ultra-high polished flutes achieve efficient chip evacuation, and the coupling is completely protected from chip flow and contact with the workpiece.

Split point fiber (SPF) solid carbide drills offer a material-specific design to machine composites and composites stacks. The multilayered chemical vapor deposition (CVD) diamond coating provides increased tool life with high wear resistance. A 90°-point angle design increases the centering capability of the cutting tool and minimizes delamination.

Double angle (DAL) drills tackle CFRP-metal stack drilling operations. The double-angle point design offers optimal centering capabilities and minimizes burrs when exiting the metal side of the stack. DAL drills can be applied in all stack combinations: CFRP-titanium-aluminum (CFRP-Ti-Al) as well as CFRP-Ti, CFRP-Al and straight Ti or Al. Highly polished chip flutes ensure optimal chip evacuation, even when MQL is applied.

The correct drill point geometry is also crucial in rivet hole drilling, as it ensures accurate hole diameters and alignment, effectively reduces heat and facilitates efficient chip removal — ultimately resulting in stronger structural integrity in the aircraft. 

The drilling and countersinking tool is another high-precision system, achieving 1° angular countersink tolerances in aerospace fastener hole applications. Designed to be clamped in a standard hydraulic chuck, HiPACS consists of three standard components: a reducer sleeve with a built-in high-precision pocket seat for a countersinking insert, a PCD countersinking insert, and solid carbide or PCD drills with SPF and DAL point geometry. This easy-to-assemble system can be used to drill and chamfer in one operation. Each component can be exchanged independently from one another, so only the worn piece can be replaced while the others can continue to be used.

HiPACS precision tooling system insert sits inside the flute to enable optimal formation of the transition radius/chamfer, preventing a step between the hole and countersink. Additionally, the system’s flexibility can reduce the inventory in traditional monoblock tools. The straight shank enables height adjustment within 10 millimeters. This precision system offers clearance which allows the drill to maintain runout of 3-5 microns.

In a practical application, a Tier 1 aerospace supplier was looking to reduce the costs and complexity of its monoblock assemblies. Kennametal stepped in and replaced the existing monoblock tooling assemblies with the company’s HiPACS system, which achieved lowest cost per hole and resulted in a significant line-item reduction.  

A pilot can help

Rivet heads must not protrude above the airplane’s skin, as this would create turbulence and drag. Instead, a flush surface is achieved by countersinking, which enables the rivet head to sit level with the surface. Due to accessibility challenges with machinery and equipment, countersinking operations often have to be done manually. To solve that challenge, S piloted PCD countersinks are designed for ease of handling in manual countersinking applications. Accuracy in depth control is provided by the microstop unit. This allows the countersink to provide consistent quality when machining.

The future of rivet hole drilling

Rivet hole drilling is a critical process in aerospace manufacturing and the shift toward composites is pushing the advancements of cutting tool technologies. Keeping that in mind, Kennametal sees new directions for composites holemaking. Sensors, for example, are being developed to guide, monitor and adjust as drilling operations are in-process to prevent wear on the tool and damage to the material. This would be done in real time and not only optimize the holemaking process but also improve efficiencies.

For example, Kennametal has recently been collaborating with a customer on a “one way assembly” project which involves developing sensor-based technology aimed at streamlining stack drilling in aircraft assembly. By using sensors to ensure holes meet specifications, the need for cleaning, disassembly and inspection could be eliminated, saving significant time and cost of aircraft components.

The aerospace industry’s drive for better performance and lighter components has led to cutting tool innovations and will continue to do so as the demand for CFRP grows.