Contents lists available at ScienceDirect Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro Quality monitoring and control for drilling of CFRP laminates L. Romoli ⁎ , A.H.A. Lutey Department of Engineering and Architecture, University of Parma, 43124, Parma, Italy ARTICLE INFO Keywords: CFRP Drill wear Delamination Tool condition monitoring Process quality ABSTRACT Widespread uptake of Carbon Fiber Reinforced Plastics (CFRP) in the aerospace and automotive industries over the past two decades has highlighted the importance of machining composites in large production environments. The abrasive nature of carbon fibers and inherently heterogeneous structure of CFRP laminates create tool wear conditions that are unlike those typical of metal machining. The present work quantifies thrust force, tool flank wear and push-out delamination (POD) for drilling of 8 mm thick CFRP laminates with 8 mm high-speed steel drill bits. Thrust force, directly responsible for delamination effects, is found to increase with feed rate and cutting speed, while flank wear increases at lower feed rate and higher cutting speed. POD is found to increase with feed rate and tool wear, with excessive values of the latter found to change the material removal mechanism from cutting to tearing of fibers resulting in increases in local workpiece temperature and matrix degradation. A quality monitoring and control strategy for drilling of CFRP laminates is then proposed based on these ob- servations by exploiting a fuzzy logic algorithm to predict POD based on thrust force and flank wear mea- surements. Feed rate, in turn, is controlled based on the POD to ensure acceptable machining quality over the entire lifespan of the tool. The developed approach offers an effective and simple way of maximizing throughput and tool duration while guaranteeing part quality. Introduction The massive uptake of Carbon Fiber Reinforced Plastics (CFRP) in the aerospace and automotive industries over the past two decades has been accompanied by stringent requirements for testing and quality control to ensure safety and performance. The use of CFRP in passenger aircraft such as the AIRBUS A350 now makes up approximately half the total mass, up from just 5% twenty years ago [1]. Drilling of CFRP, accounting for almost two-thirds of all non-metal removal, is a complex process employed prior to joining of parts with rivets or screws where complex geometry and/or different materials prohibit production as a single component [2]. Drilling is strongly affected by the tendency of fibers to detach from the matrix or break under the action of machining forces, which can potentially lead to delamination and failure. Amongst the various drilling techniques available today, mechanical chip re- moval remains the most frequent choice for CFRP, with four variations generally considered: conventional drilling, grinding drilling, vibration- assisted twist drilling and high-speed drilling. Of these alternatives, conventional drilling remains the most widely adopted due to its sim- plicity and low-cost setup, which are well-suited to the manual nature of most CFRP part production [3]. Other mechanical and unconven- tional drilling techniques such as pulsed laser drilling [4,5] generally provide better quality holes with higher efficiency but require far more complex and expensive facilities [6]. Conventional CFRP drilling in manufacturing is therefore likely to be of relevance for some time to come. Drilling of CFRP parts is an inherently heterogeneous process due to the coexistence of high-strength fibers and a polymer matrix with very different mechanical and thermal properties [7,8]. CFRP laminates comprising adjoining fabric layers are more sensitive to drilling forces than are metal alloys [9], as axial thrust can lead to delamination and interlayer cracking at the exit side of a hole where low laminate thickness at the end of machining provides lower resistance to tool penetration [10]. Drilling of CFRP without compromising laminate in- tegrity is therefore an important challenge for manufacturing engineers and has seen much attention over the past two decades. Despite the large quantity of experimental data available, generalized guidelines for drilling of CFRP are still somewhat limited. Uncertainties often exist relating to the properties of the laminate itself, which can be affected by residual porosity from the manual lay-up of pre-impregnated fabrics. This, in turn, can affect machining operations as well as the ideal choice of cutting tool geometry [11]. Within this context, manufacturers are often faced with mixed information regarding ideal process parameters. A general monitoring and control strategy would therefore provide a https://doi.org/10.1016/j.jmapro.2019.02.028 Received 22 May 2018; Received in revised form 15 November 2018; Accepted 21 February 2019 ⁎ Corresponding author. E-mail address: luca.romoli@unipr.it (L. Romoli). Journal of Manufacturing Processes 40 (2019) 16–26 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. T