In practical applications of CFRP products, extensive riveting and threaded connections are required to meet assembly specifications. Consequently, hole-making processes represent the most frequently employed machining technique in CFRP manufacturing. Due to CFRP’s strong anisotropy, its machining mechanism differs significantly from that of metallic materials. During hole-making, a series of defects readily occur, leading to a reduction in overall product strength and severely limiting the widespread adoption of CFRP products. Additionally, the high hardness of carbon fibers causes significant tool wear during hole-making, resulting in low machining efficiency.
CFRP Drilling Process
Currently, the primary structural components of aircraft are still manufactured using lightweight metals such as aluminum alloys and titanium alloys. To further reduce aircraft weight, the airframe skin is typically constructed from thinner materials. However, the weldability of aluminum and titanium alloys is inferior to that of steel, making them prone to welding defects such as porosity, gas bubbles, and microcracks. Consequently, achieving smooth, uniform welds comparable to those in steel is unattainable. Additionally, during flight, uneven thermal and mechanical stresses on the aircraft surface readily induce cracks at welded joints. These cracks lead to fatigue failure, posing a threat to flight safety. Consequently, riveting or bolted connections are typically employed in aircraft skin and structural assembly to circumvent welding-related issues. Riveting and bolting offer simple operation, high strength, and reliability, while also facilitating disassembly and maintenance. However, both methods require machining numerous connection holes for assembly. In the current aerospace field, CFRP is gradually replacing traditional metallic materials as the primary material for aircraft skins and structural components. This shift generates substantial welding and assembly operations, necessitating the machining of a large number of connection holes.
Drilling Force Characteristics
Rivet holes for assembly are typically small in size, and drilling remains the primary machining process for small-scale operations on CFRP components. Drilling technology is a fundamental method for hole machining, typically performed on drilling machines, boring machines, or milling machines. The twist drill is the primary cutting tool in diamond drilling. It rotates while simultaneously performing a precise axial feed motion along the tool’s axis to machine holes in the workpiece matching the tool’s diameter. Most CFRP components utilize CNC equipment for hole machining, though handheld tools are also employed for hole machining during assembly processes.
Influence of Cutting Parameters on Drilling Quality
In machining processes, cutting parameters are critical factors affecting processing quality. In drilling operations, controllable cutting parameters primarily include spindle speed and feed per tooth. Spindle speed directly affects the cutting speed at each point along the tool’s cutting edge, while feed per tooth directly determines the thickness of the removed material. The selection and adjustment of these parameters are crucial for optimizing machining results and enhancing product quality. During the cutting process, the proper selection and adjustment of cutting parameters can effectively improve machining efficiency, reduce processing costs, elevate product quality, and extend tool life.
Tool Wear
Rapid tool wear during CFRP hole machining is also a primary cause of high processing costs and poor machining quality. Increased axial forces resulting from tool wear are the main factors leading to delamination damage, microcracks, and other forms of damage. During CFRP drilling, cutting temperatures are significantly higher than those for metallic materials, indicating that cutting heat is not the primary cause of rapid tool wear. Carbon fiber materials possess hardness comparable to high-speed steel. Consequently, friction between the high-hardness carbon fiber particles and the tool during CFRP machining is the principal factor causing tool wear.
Specialized Drilling Process for CFRP
CFRP hole machining presents challenges due to the material’s high hardness and susceptibility to machining damage. Simply improving cutting tools remains insufficient to overcome the difficulties in CFRP processing. Lasers serve as a vital tool for machining high-strength materials, but the high temperatures generated during laser processing can exceed the glass transition temperature of the resin matrix. Different feed rates and laser types significantly influence the size of the thermal damage zone in CFRP, and the extent of this zone directly impacts the material’s mechanical properties. Consequently, lasers are typically employed for CFRP with alternative matrices—such as graphite, ceramic, or metal—which offer significantly superior mechanical properties to resin materials. Components fabricated from these matrices are better suited for high-temperature, corrosion-resistant, or high-radiation environments, yet they are often more challenging to machine and substantially more expensive. Electrical discharge machining (EDM) can also be applied to CFRP processing. However, issues such as substrate melting during machining and low processing efficiency severely limit the widespread adoption of this method. High-pressure water jets are utilized for cutting CFRP and GFRP, as well as for cavity machining. Nevertheless, due to constraints related to surface perpendicularity and nozzle diameter, high-pressure water jets are not suitable for machining small-diameter holes.
CFRP Helical Milling Process
Helical milling, also known as orbital drilling, involves a milling cutter rotating rapidly around its own axis while simultaneously moving along a helical trajectory in three-dimensional space. Thus, the tool’s motion during helical milling comprises three independent movements: self-rotation, orbital rotation around the hole axis, and linear feed along the hole axis. This unique motion pattern confers inherent technical advantages over conventional drilling, such as superior machining quality, lower tooling costs, broader applicability, and reduced processing temperatures.
Currently, no alternative machining process effectively replaces drilling for producing small-diameter holes (e.g., rivet holes) in CFRP. To address issues like poor machining quality and severe tool wear in CFRP hole processing, we applied the helical milling process to CFRP hole machining while optimizing aspects including machining performance, equipment, tools, and process parameters.