Drilling of GFRP and CFRP Composite Materials With Process Monitoring

Jonathan William Koski

doi:10.7273/000005199

Fiber reinforced polymer (FRP) materials are continuing to see expanded use in industries that demand exceptional strength to weight ratios. Aerospace structures are becoming dominated by FRP materials for superior mechanical properties and resistance to fatigue relative to aluminum alloys. The production of FRP parts has traditionally come with high labor and material cost, but advances in technology for layup automation and production of source material are reducing cost on both sides of the equation and making these cutting-edge materials more accessible. However, machining of FRP parts is still often required after curing to net shape and challenges still exist due to rapid tool wear, inconsistent machining performance, and a high rate of defects. Some of these challenges can be overcome by an experienced machinist with a keen eye observing the process closely, but better advancements in machining automation will be critical to increase production rates and further reduce manufacturing costs of FRP parts. A critical part of improving this automation is to understand how to use sensor feedbacks generally to make interventions during the process, whether that means changing out a worn tool or modifying a machining parameter to prevent a defect. In this research, drilling is performed on a hybrid form of glass fiber reinforced polymer (GFRP) that is built with two different layup techniques. The experiment involved tool material, feed rate, and state of tool wear as variables. From the sensor data collected during drilling, relations were analyzed with the state of drilling through the hybrid composite layers and hole quality outputs. Additionally, a second experiment involved deep hole drilling in carbon fiber reinforced polymer (CFRP) with the intent to observe chip clogging. This study involved the use of three tool materials. Sensor data was reviewed for evidence of chip clogging and relation to hole quality outputs. From the hybrid GFRP drilling experiment, it was found that the hand layup side of the plate had more resistance to cutting than the vacuum bagged side. Both high speed steel (HSS) and carbide tools were used, with the carbide tool showing exceptional wear resistance, even when drilling numerous sacrificial holes in more abrasive CFRP. It was also found that progressive tool wear resulted in reduced magnitude of vibration, and worn tools produced more consistent hole diameter than fresh tools. The deep hole drilling of CFRP experiment found chip clogging to be confirmed with HSS tooling, and cobalt tooling to a lesser extent. The presence of chip clogging with carbide tooling was inconclusive and further research is needed. Spectrogram analysis showed changes in frequency response that correlated well with the onset of chip clogging, especially for HSS tooling.

Drilling of GFRP and CFRP Composite Materials With Process Monitoring

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