Process monitoring for reliable machining of CFRP structures

Basic quality requirements in machining of CFRP components, besides the geometric precision, are thermally undamaged and delamination-free cutting surfaces and edges. Machining of large CFRP structures implies a high risk of component recovery cost and time, since machining operations are late within the production process chain of those components. Moreover, machinery for processing large CFRP structures is very expensive. These reasons substantiate the need for online-monitored reliable high-productivity CFRP machining capable of preventing rework and waste. Existing process monitoring and control systems developed for metal machining are not suitable for CFRP because the material-specific machinability in terms of tool wear, forces and surface quality call for different detection mechanisms. Accordingly, this study is dedicated to experimental investigations of CFRP machining in order to determine suitable signals and sensors for online monitoring of the relevant tool wear and quality parameters. As the edge trimming of large CFRP components is most difficult to control up to now, the study focuses on this operation. Promising first results with regard to preventive monitoring of tool wear and surface temperature were obtained by force and acceleration evaluation. Experimental studies have been performed in CFRP machining in order to establish a basis for online monitoring with focus on edge trimming. The findings can be summarized as follows: - According to the anisotropy of CFRP the cutting force, the surface temperature and the main spindle power as well as the feed drive torque exhibit a periodic variation - Continuously progressing tool wear with feed travel and local cutting edge chipping are related to an increase of the feed or active force in milling - The amplitudes of the feed force and of the acceleration at the spindle head, which correspond to the spindle speed and its first two harmonic multiples, indicate a clear difference between new and worn cutters - The maximum surface temperature easily exceeds the temperature limit of epoxy resins and is related to the active force - Automated online detection of delamination remains a challenge.