Influence of fibre orientation on cutting force in up and down milling of UD-CFRP composites

Carbon fibre–reinforced polymer (CFRP) composite materials are favoured due to their excellent specific mechanical properties in industries where low weight and high strength are required [1, 2]. For example, almost 50% of the structural elements of the Boeing 787 airliner consist of composite materials [3]. By using the novel composites, engineers were able to achieve 20% weight loss and 35% maintenance time reduction over previous models (Boeing 767 and 777). In the aerospace industry, as well as in the automotive, wind turbine, military, sports, and aerospace industries, manufacturers strive to laminate CFRP components in a single operation (moulding and hardening); however, they often require further processing before they can be used or assembled [4‐6]. These may include (i) removing material build-up in the dividing plane of the laminating tool, (ii) removing excess material from the flange of the laminating tools, (iii) smoothing the mating surfaces of the laminated composites, and (iv) making holes for assembly of components [7‐9]. Typically, these post-manufacturing needs are met by various machining techniques, like conventional drilling, helical milling, tilted helical milling, wobble milling, side milling, or edge trimming [6, 10‐13].

Nevertheless, the cutting of CFRP composite materials is complicated and expensive: (i) due to the inhomogeneity and anisotropy of the material, the characteristic geometrical errors caused by the machining and the chip formation mechanisms are significantly dependent on the machining directions; (ii) carbon fibres have a strong abrasive wear effect, which should be considered for the cutting tool and for the machine tool also; and (iii) heat dissipation is also problematic due to the low thermal conductivity of polymers and the dangers of using coolant lubricants (polymer wicking) [2, 4‐6, 14, 15]. Because of these cutting features and conditions, CFRP materials are referred to as difficult-to-cut materials, which can result in a variety of micro- and macro-geometric material defects like delamination, uncut fibres, matrix burning, fibre pull-outs, or micro-cracking [13, 16‐26]. Although centuries of experience in the field of metalworking have been accumulated, this theoretical and practical knowledge cannot be directly applied to the cutting science of fibre-reinforced technical polymer composite materials that have been researched for only a few decades.

Investigation of cutting forces are often unavoidable for modelling (i) tool wear, (ii) chip forming mechanisms, (iii) micro-, and (iv) macro-geometric errors caused by machining [27‐29]. The mechanical and thermodynamic properties of quasi-homogeneous materials are quasi-isotropic so that the machinability properties are less directional [30‐34]. However, this is not true for a fibre reinforced composite material [35], the machinability of fibre-reinforced materials is therefore strongly direction-dependent. In this paper, the effect of the fibre orientation angle (ϕ—the angle between the direction of the fibres and the vector of the feed rate, as illustrated in Fig. 1) is investigated on the cutting force (F), which describes the direction dependence of the composite. Cutting force dependence on the fibre orientation angle is discussed already in some key papers [36‐40]; however, there are still many lacks of knowledge in cutting force optimisation in UD-CFRP in the case of up and down milling.

Li et al. [36] carried out orthogonal machining experiments in unidirectional CFRP composite and found that the effect of the fibre cutting angle (θ—angle between the direction of the fibres and the vector of the cutting speed, as illustrated in Fig. 1) and the depth of cut are significant on the cutting force and their interaction effect is also significant. Voss et al. [37] analysed the influence of fibre orientation on the cutting force and on the quality of machined features in milling CFRP. They observed that fibre cutting angle has a significant effect on cutting force and on surface quality. They showed that cutting force can be decreased by increasing the rake and the clearance angles; furthermore, optimal cutting process parameters were defined to maximise machining quality and minimise cutting force. Li et al. [41] analysed the machinability of UD-CFRP by up milling experiments and concluded that there is a strong correlation between surface roughness and vibration of cutting force signals. Furthermore, they showed that the fibre orientation angle has a significant effect on the cutting force. Xu et al. [42] conducted machining experiments in CFRP and proved that the feed rate has the most significant influence on the cutting force that closely correlates with delamination and burr formation. They confirmed and validated these findings in Xu et al. [43] too.

Wang et al. [44] analysed the influence of fibre cutting angle on the cutting force components in milling UD-CFRP with a small radial depth of cut of (a_e = 0.1 mm). They observed that the radial cutting force is the most effected by the fibre cutting angle, followed by the tangential force, while the passive force component is less influenced by the θ. He et al. [38] investigated the cutting force in slot milling of CFRP using a two-straight-flute carbide mill. They resulted that fibre cutting angle of θ = 135° causes the largest tangential forces and specific energies, while the tangential cutting force at θ = 45° is the smallest. Wang et al. [45] conducted milling experiments in CFRP and analysed the influences of process parameters on the cutting force using response surface methodology. They proved that feed rate has the most significant effect on cutting force, followed by the cutting speed and the radial depth of cut. Sui and Wang [39] conducted slot milling experiments in UD-CFRP. They proved that the cutting speed has only just a little effect on the cutting force, while the effect of fibre orientation and chip thickness is significant on it. The total machining power should be kept in minimal in order to improve the flexural strength of CFRP and improve the quality of machined features; according to Ashworth et al. [8], the minimisation of cutting force is therefore relevant and necessary.

Modelling of cutting forces in orthogonal cutting of CFRP composites are often based on laws of physic (e.g., Kienzle model) [46, 47]. Nevertheless, in the case of more complex technologies like drilling or milling processes, cutting forces are often modelled by statistic, semi-mechanistic, or numerical techniques [48, 49]. Each modelling approaches have their advantages and limitations, which have to be addressed before selecting them. Mechanistic models have a physical aspect which enables to understand the analysed process; however, it is extremely difficult to use them to model complex systems or processes (advanced shaped tools, difficult tool path or non-homogeneous materials etc.). In contrast, the physical meaning of statistical models is strongly limited, but they are often preferred in recent cutting-edge-research areas like in smart manufacturing, self-organisation, intelligent machining, digital twin or real-time process monitoring, and diagnostics solutions [50]. Polynomial models are often preferred in these sectors because it is easy and fast to calculate with these formulas by computers, even in quasi-real-time.

Although experimental and simulation studies and modelling of cutting forces have been addressed in many of scientific studies, only a few papers have been published on the effect of up and down milling strategies on cutting forces. Moreover, it is not possible to clearly derive from these articles the knowledge required to create (i) a special (tilted) trochoidal toolpath for, e.g. efficient slotting, or (ii) an optimised zig-zag (up and down alternately) type toolpath for CFRP milling. Therefore, the main objective of the present experimental study was to analyse the influence of the fibre orientation on the cutting force in climb (down) and conventional (up) milling of unidirectional carbon fibre reinforced polymer (UD-CFRP) composites. The other main goal was to develop an adequate polynomial model that is capable of force optimisation.

The rest of the paper is organised as follows: First, the experimental setup is introduced; then, the experimental results are presented. Finally, the results are compared and discussed based on the analysis of chip removal mechanisms.

Edge milling experiments were performed in a hand lay-up laminated, epoxy resin-based unidirectional CFRP composite. The ratio of the applied FM20 resin to the MH3124 hardener was 100:35, respectively, while the reinforcement was a dry unidirectional carbon fibre fabric. The thickness of the laminate was t = 25 mm. The important mechanical properties (tensile strength, interlaminar shear, hardness, and impact strength) of the applied composite are measured by using a Zwick Z250 and a Zwick Z020 tensile testers, a Zwick H04.3150 hardness tester, and a Ceast Resil Impact Junior impact tester, respectively. The measurement setups were repeated five times, and the average and the deviations were calculated, as summarised in Table 1.

The machining experiments were conducted on a VF 22 vertical spindle milling machine. The chips were removed from the cutting zone with a NILFISK GB733 industrial vacuum cleaner. Coolant lubricant was not used for the experiments (dry machining). An uncoated HSS end mill with a diameter of D = Ø50 mm, with z = 5 cutting edges (clearance angle of α = 10°, rake angle of γ = 25°, and helix angle of λ = 40°) was used for the milling experiments. The machining experiments were designed using the full factorial experiment design method. The factors and their levels are shown in Table 2. The 4th level (ϕ = 90°) was repeated five times in order to calculate reproducibility deviation for regression analysis and graphical adequate analysis.

The values of the technological parameters not listed in the table were fixed in order to fix their influences on the cutting force, as follows: cutting speed of v_c = 230 m/min, feed rate of v_f = 397 mm/min (feed per tooth of f_z = 0.054 mm/tooth), axial depth of cut of a_p = 7 mm, and radial depth of cut of a_e = 3 mm. The schematic drawing of the experimental milling setup can be seen in Fig. 1.

Cutting force was measured with a KISTLER 9281B three-component dynamometer, and collected using a LabVIEW measurement program at a sampling frequency of f_m = 18,000 Hz for t = 10 s per experimental setting. The spindle of the milling machine was equipped by an eccentric switch (a metal element), which position was detected by an OMRON E3F-DS10B4 proximity sensor, in each spindle rotations. The force data and the data provided by the proximity sensor was collected simultaneously, the exact position of the cutting edges could be therefore determined.

The analysis of tool wear is not in the scope of this study, however, it had to be monitored in order to minimise its influence on the analysed response variables. The tool wear criterion was defined in a maximum of VB = 0.3 mm wear in length, measured from the tip of the tool edges on the clearance face of the tool edges. The tool wear was measured by digital image processing of pictures provided by a Dino-Lite AD7013MZT digital microscope. Images were collected after each experimental setup. The tool wear on the clearance surface did not reach the defined tool wear criterion, the tool wear therefore not influences significantly the experimental results of this study. Representative images of clearance faces of the cutting tool is shown in Fig. 2.

The collected force signal is noisy mostly due to the tool vibration, the filtering of data is therefore often necessary. Frequency filtering was applied at the measured force values using the fast Fourier transformation (FFT) and a Butterworth low pass filter with a cut-off frequency of f_c = 400 Hz (cutting edges enter the workpiece at the frequency of f = 1000v_cz(Dπ)⁻¹ ≈ 122 Hz). The principle of applied frequency filtering is illustrated in Fig. 3. The original force signal is first transformed by using the FFT, then the Butterworth filters the high-frequency signals, then the spectrum is inverse transformed by inverse FFT.

The main steps of optimisation parameter calculations are, as follows: (i) selecting an evaluation period t_k = 1 s in the filtered force diagram (Fig. 3), during which the tool continuously machines, then (ii) dividing the evaluation phase into m = 10 equal subsections, and then (iii) calculating the maximal parameter in each subsection (F_f1 in Fig. 3), finally (iv) averaging the features calculated in the subsections for the evaluation phase (based on Fig. 3: F_f = 10⁻¹(∑F_fi)). The resulting F(ϕ) force (cutting force) was calculated by Eq. (1).where F denotes the cutting force, ϕ is the fibre orientation angle, while f indicates the direction of the feed rate (y in Fig. 1), r indicates the radial direction (x in Fig. 1) and p indicates the passive (z in Fig. 1) direction.

During the evaluation, polynomial models were fitted to the force parameters, and they were adequately examined by (i) regression analysis and (ii) graphical adequate analysis. If the R squared (R²) of the deg = 1 regression model was smaller than R² = 0.9, then the degree (deg) of the polynomial model was increased by deg = deg + 1, until R² ≥ 0.9. A general deg = 5 polynomial model used here is expressed by Eq. (2).where F denotes the cutting force, ϕ is the fibre orientation angle, while a, b, c, d, e and f are regression coefficients. The results of the regression analysis are summarised in Table 3.

The reproducibility was determined by averaging the empirical standard deviation values obtained five times by repeating the experimental set-up of ϕ = 90 °, using Eq. (3).where n = 5 the number of repeated experimental settings, m = 10 is the number of evaluation subsets, and F is the value of the optimisation parameter determined by the filtered force diagram. The reproducibility standard deviation was used for graphical adequate analysis, as follows: (i) the error interval of the measured cutting force parameters are defined in F = E(F) ± r, (ii) a deg = 1 polynomic is fitted to the force data (Fig. 4a), (iii) if the polynomic approximates worse than the E(F) ± r defined interval, then the degree of the polynomial model has to be increased: deg = deg + 1, as illustrated in Fig. 4. The graphical adequate analysis resulted in the same degree of polynomials as the regression analysis.

This section of the present study is comprised of three parts. First, the results of the cutting force of climb milling, then the results of the cutting force of conventional milling experiments are presented. Finally, the climb milling and conventional milling results are compared and discussed in detail, from the point of view chip removal mechanisms associated with different fibre cutting angles.

In the present study, machining experiments were carried out in a hand-laminated, thick, unidirectional CFRP in order to analyse the dependence of cutting force on the fibre orientation. According to the present study, the following conclusions can be drawn: