ORIGINAL ARTICLE Analysis of thrust force and characteristics of uncut fibres at non-conventional oriented drilling of unidirectional carbon fibre-reinforced plastic (UD-CFRP) composite laminates Norbert Geier 1 & Tibor Szalay 1 & Márton Takács 1 Received: 8 June 2018 /Accepted: 15 October 2018 # Springer-Verlag London Ltd., part of Springer Nature 2018 Abstract Carbon fibre-reinforced plastic (CFRP) is an often-used structural material in the high-tech industries, like aerospace, wind turbine, sport, automobile, robotics and military. Due to both the growing application area of composites, and the advanced construction requirements, the used thickness of the CFRP plates increases, and the necessity of drilling holes on the sides of the plates (normal II direction) becomes even more important. Many researchers studied the machinability of UD-CFRP using numerous drilling exper- iments at the normal I direction. However, drilling experiments at normal II and axial directions were not published yet. The main objective of the present study is to analyse and discuss the influence of a non-conventional drilling direction on hole-quality parameters and on the thrust force. Drilling experiments were carried out in unidirectional CFRP at non-conventional drilling direction, based on central composite inscribed design. Influences of feed rate and cutting speed were analysed using response surface methodology (RSM) and analysis of variance (ANOVA) techniques. Characteristics of uncut fibres were analysed using digital image processing (DIP). The results have proved that the effect of the cutting speed is more significant when drilling UD- CFRP at the non-conventional drilling direction than at the conventional one. Furthermore, the specific feed force (k f ) in the case of the non-conventional drilling direction was more than three times higher than the k f in the case of the conventional one. Keywords CFRP . Machinability . Optimisation . Thrust force . Uncut fibres Nomenclature A c (%) Area factor d (mm) Tool diameter F t (N) Thrust force k f (N/mm 2 ) Specific feed force n (rev/min) Rotational speed s* (same as the mean) Standard deviation v c (m/min) Cutting speed v f (mm/rev) Feed rate x (mm) Lengths of the longest uncut fibre α (1) Significance level θ (°) Fibre cutting angle ϕ (°) Fibre orientation angle 1 Introduction Carbon fibre-reinforced plastic (CFRP) is an often-used con- struction composite material in the aerospace, marine, human- oid-robotics, sport, wind turbine and automobile industries due to its specific mechanical properties, damage tolerance and corrosion resistance [1–4]. In order to meet the micro and macro geometrical requirements of the CFRP parts, me- chanical machining of these materials is often necessary. However, the machining behaviour of CFRP is difficult be- cause of the non-homogeneity, anisotropy of the material, and the intensive abrasive wear-effect of the carbon fibre- reinforcements on the tool. Mechanical joining CFRP components is often preferred by rivets and bolts [5], and the most common machined fea- tures are therefore through and stage holes [2, 6]. By applying improper machining technologies, several geometrical errors could occur: like delamination, fibre pull-outs, uncut fibres and matrix burnings [7]. These errors often cause loss of strength of CFRP components and often result in additional machining operations and costs. Researchers and tool- * Norbert Geier geier@manuf.bme.hu 1 Department of Manufacturing Science and Engineering, Budapest University of Technology and Economics, Műegyetem rkp. 3., Budapest 1111, Hungary The International Journal of Advanced Manufacturing Technology https://doi.org/10.1007/s00170-018-2895-8