Milling CFRP with Single-Crystal Diamond: The Impact of Rake Angle

## The Challenge of Machining Advanced Composites Carbon fiber-reinforced polymers (CFRPs) are essential in aerospace, high-performance automotive, and renewable energy due to their exceptional strength-to-weight ratio, fatigue resistance, and durability. However, their complex, layered structure makes them notoriously difficult to machine. Achieving a clean cut without delamination, fiber pull-out, or excessive tool wear is a significant engineering challenge. While diamond-based tools are known to offer superior performance, the extremely high cost and manufacturing complexity of commercial single-crystal diamond (SCD) tools have limited their use in both industry and research. This gap hinders the development of optimized machining strategies for CFRPs. ## Custom-Fabricated SCD Tools for Precision Research In collaboration with Koç University, Appsilon Advanced Materials addressed this challenge by developing a novel fabrication process for custom SCD milling tools. This work, detailed in a recent scientific paper, focuses on isolating the effect of a key geometric parameter: the axial rake angle. Our process involved several advanced techniques: 1. **Diamond Synthesis:** Single-crystal diamonds were grown using a specialized Microwave Plasma-Assisted Chemical Vapor Deposition (MPCVD) reactor. 2. **Precision Cutting:** The lab-grown diamonds were sliced into 1 mm thick plates using a water jet-guided laser. 3. **Brazing:** The polished diamond segments were bonded to 316L stainless steel tool bodies using an active braze alloy in a custom-built, high-temperature vacuum furnace. This critical step ensures the diamond cutting edge can withstand extreme machining forces. Using this method, we produced a set of 12 mm diameter milling tools identical in every respect except for their axial rake angles, which were precisely ground to 0°, 5°, and 10°. This allowed for a direct, controlled comparison of their cutting performance. ## Putting the Tools to the Test The custom-fabricated SCD tools were tested in a 5-axes CNC machining center on pultruded CFRP profiles. To understand the physics of the cutting process, we mounted a Kistler rotary dynamometer on the machine spindle. This allowed us to measure the tangential, radial, and axial cutting forces—as well as the spindle torque—with high fidelity during each milling pass. A full factorial experimental design was implemented, testing all three rake angles at multiple feed rates (10, 20, and 30 μm/tooth) and cutting speeds (30 and 45 m/min). This systematic approach provided a comprehensive dataset on how tool geometry and cutting parameters interact. ## Key Findings: A Counter-Intuitive Result The experimental results were clear and, in some ways, surprising. While it's conventional in metal cutting for a positive rake angle to reduce forces, our findings show the opposite is true for milling CFRP with ultra-sharp SCD tools. * **Neutral Rake Angle is Optimal:** The 0° axial rake angle tool consistently produced the lowest tangential and radial cutting forces, as well as the lowest spindle torque, across all tested speeds and feeds. * **Forces Increase with Rake Angle:** As the axial rake angle was increased from 0° to 5° and 10°, both cutting forces and torque rose significantly. This suggests that for brittle, heterogeneous materials like CFRP, a neutral angle provides a more efficient shearing action at the fiber level, whereas a positive angle may induce more complex stress states and higher resistance. * **Exceptional Durability:** Post-machining inspection via SEM imaging showed no noticeable chipping or wear on the SCD cutting edges, confirming the high durability and structural integrity of the brazed tools. * **Superior Surface Quality:** The 0° rake angle tool produced a very clean machined surface with limited matrix smearing, highlighting its effectiveness in achieving precise fiber cutting. ## Conclusion: A New Paradigm for CFRP Milling This study demonstrates that the conventional wisdom for tool geometry in metal cutting does not directly apply to the high-performance milling of CFRPs with single-crystal diamond tools. The key takeaway is that a **neutral 0° axial rake angle is the most efficient configuration**, minimizing cutting forces and torque, which in turn can lead to reduced delamination, better surface quality, and longer tool life. By successfully fabricating and validating these custom tools, we have not only uncovered fundamental insights into CFRP cutting mechanics but also paved the way for more cost-effective, high-performance tooling solutions in advanced manufacturing.