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Abstract

Drilling of carbon fiber-reinforced polyetheretherketone (CF/PEEK) composites poses challenges such as matrix smearing, burr formation, surface roughness, and delamination due to their thermoplastic properties. This study investigates damage mechanisms during CF/PEEK drilling using carbide drills with helix angles of 20°, 30°, and 40°, and evaluates the influence of machining parameters. The effects of helix angle, cutting speed, and feed rate on thrust force, surface roughness, and delamination factor were analyzed through a full factorial experimental design. Process parameters were optimized using a Taguchi L27 orthogonal array. Findings revealed the lowest thrust force with a 30° helix angle drill at 15 m/min cutting speed and 0.05 mm/rev feed rate. The minimum delamination factor was achieved with a 40° helix angle drill at 15 m/min and 0.075 mm/rev. The lowest surface roughness was observed with a 20° helix angle drill at 15 m/min and 0.05 mm/rev. Optimization, supported by signal-to-noise ratios, confirmed superior hole quality at lower speeds. ANOVA identified the most influential parameters for each response. This pioneering study highlights the critical role of varying helix angle drills in surface roughness, delamination, and cutting forces, offering novel insights into machining thermoplastic composites.

Keywords

CF/PEEK thermoplastic composite drill helix angle Taguchi method ANOVA surface roughness delamination factor thrust force

Introduction

In the pursuit of achieving zero emissions and ensuring environmental sustainability, minimizing material waste during processing, recycling materials post-consumption, and reintegrating them into the economy have become imperative. These approaches not only conserve the planet’s limited resources but also mitigate environmental concerns. As a result, international organizations are increasingly advocating for the use of recyclable materials to promote sustainability and reduce the carbon footprint while imposing restrictions on non-recyclable materials.¹

Industries requiring high-performance and lightweight materials, such as aerospace, have extensively adopted polymer-based composite materials. While thermoset matrix composites have traditionally been the primary choice, growing environmental concerns and the demand for recyclability have spurred extensive research into thermoplastic composite materials, which are gaining prominence in various applications. It is anticipated that thermoplastic polymer composites will eventually replace their thermoset counterparts.²

Fiber-reinforced thermoplastic polymer composites have garnered significant attention in recent years, finding widespread applications in aerospace, automotive, healthcare, and energy sectors.^3,4 A notable example is the Airbus A350 XWB, introduced in 2019, which incorporates a substantial proportion of thermoplastic polymer composite materials.⁵ Compared to thermoset composites, fiber-reinforced thermoplastic composites offer distinct advantages, including superior impact resistance, shorter processing cycles, reduced precision requirements during manufacturing, and increased production efficiency.² Moreover, due to their inherent properties, thermoplastic composites are recyclable and repairable, aligning with sustainability objectives.^6,7 Given their high performance, durability, and recyclability, these materials are expected to experience greater adoption in the coming years, supporting global zero-emission targets.^8,9 Consequently, fiber-reinforced thermoplastic polymer composites are poised to emerge as strong alternatives to fiber-reinforced thermoset composites.¹⁰

Figure 1 illustrates the material distribution in a modern aircraft. Composites, primarily carbon laminates and carbon sandwiches, account for 50% of the structure. Additionally, aluminum (20%), titanium (15%), steel (10%), and other materials (5%) are also utilized. Considering this distribution, it indicates the increasing preference for lightweight composites in aviation to enhance performance and efficiency.¹¹

Figure 1.

Boeing 787 material composition.

However, for polymer composite materials to serve as structural components, they require assembly via mechanical fastening methods such as riveting and bolting. This necessitates extensive hole drilling, which significantly influences the quality and integrity of the joints.¹² In commercial aircraft, the number of drilled holes ranges from 1 to 3 million, while military aircraft require approximately 300,000 assembly holes.^13,14 Studies indicate that nearly 60% of composite plates are discarded due to delamination defects arising during drilling.¹⁵ Given that most polymer composites in use are thermoset-based and non-recyclable, they are often disposed of through environmentally harmful methods.⁶ Thus, minimizing defects and optimizing the drilling process are critical for the sustainable utilization of composite materials.

With their potential to replace thermoset composites, thermoplastic composites have gained significant research interest, particularly concerning their drilling performance. High-performance engineering thermoplastics such as polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyphenylene sulfide (PPS), polyimide (PI), and polyetherimide (PEI) are commonly employed as matrix materials due to their excellent mechanical properties and thermal stability.² Figure 2 shows the classification of thermoplastics and their common areas of application.

Figure 2.

Classification and applications of thermoplastic polymers.

Carbon fiber-reinforced PEEK (CF/PEEK) composites, in particular, have emerged as a leading thermoplastic material for structural aerospace applications. The predominant machining method for assembling these composites is drilling; however, their drilling behavior differs significantly from that of thermoset composites. Owing to their chain structure, thermoplastic composites exhibit ductile characteristics, leading to distinct material responses during machining and varied hole surface morphologies.¹⁶ Furthermore, their lower thermal conductivity compared to thermosets imposes additional challenges in machining. The complex interaction between the drill and thermoplastic matrix remains an area of active investigation.¹⁶

Ensuring the reliable assembly of composite structures necessitates precision drilling to achieve dimensional accuracy and high-quality holes. While most studies focus on the drilling of thermoset composites and special metal alloys, limited research has been conducted on thermoplastic composites, particularly CF/PEEK. Existing machining techniques and cutting tool materials developed for thermoset composites cannot be directly applied to thermoplastic composites without a thorough understanding of their unique machinability characteristics. Early studies by Raman et al.¹⁷ examined the influence of machining parameters on CF/PEEK drilling, identifying tool wear as a function of feed rate, cutting depth, and cutting speed. Davim et al.¹⁸ compared the drilling performance of glass fiber-reinforced PEEK and unreinforced PEEK, reporting that feed rate significantly affects cutting forces, with unreinforced PEEK exhibiting superior surface quality. Chambers et al.¹⁹ analyzed the performance of cemented carbide and polycrystalline diamond (PCD) drills on CF/PEEK composites, concluding that PCD tools yielded better results. Similarly, Mata et al.²⁰ investigated the drilling behavior of unreinforced PEEK, CF/PEEK, and GF/PEEK composites using PCD drills, finding that cutting force increases with feed rate across all materials.

The machinability of CF/PEEK is notably influenced by its high abrasiveness, necessitating optimized tool selection and machining parameters to achieve precision tolerances. Key machinability criteria include cutting force, tool life, tool wear, surface roughness, and delamination factor. Although machinability primarily characterizes the work material, Groover²¹ emphasized that machining performance also depends on cutting parameters, tool geometry, and machining operations. Izamsah et al.²² applied response surface methodology (RSM) to evaluate the effect of milling parameters on the surface roughness of PEEK under dry machining conditions, identifying feed rate as the most influential factor. Their findings also indicated that exceeding a critical cutting speed threshold causes polymer softening. Yuan et al.²³ assessed the impact of drill geometry on CF/PEEK hole quality using helical, brad, and dagger drills, reporting superior performance with helical drills. Du et al.²⁴ compared the drilling performance of CF/PEEK and CF/epoxy composites using drills with varying point angles, concluding that brad drills with the smallest point angle produced the least burr formation. Zhao et al.²⁵ examined the effects of tool geometry, drilling parameters, and thrust force on PEEK hole quality, determining that cutting force and tool temperature significantly influence surface quality. Chang et al.²⁶ found that feed rate has a greater impact on cutting force than cutting speed, with lower feed rates and cutting speeds resulting in reduced cutting forces.

In addition to mechanical parameters, thermal effects generated during drilling of CF/PEEK composites have been reported in the literature to play a decisive role in delamination and surface roughness. The thermal response of these materials during machining has been extensively investigated due to its significant influence on hole quality and material integrity. For instance, Özcan et al.²⁷ confirmed that during slot milling processes, CF/PEEK composites begin to exhibit surface degradation and fiber fracture at temperatures above 120°C, with pronounced matrix deformation observed beyond 145°C. Although the CF/PEEK composite used in this study has a higher glass transition temperature (Tg = 165°C), experimental findings by Liu et al.²⁸ indicated that dimensional instability and machining issues may arise at temperatures exceeding 130°C. Ge et al.²⁹ reported that temperatures approaching the glass transition temperature (Tg ∼ 143°C) lead to matrix softening, which in turn reduces the stiffness and adhesion at the fiber–matrix interface, thereby increasing delamination and surface roughness. Moreover, Chang et al.²⁶ demonstrated that heat accumulation due to prolonged tool–material contact at low feed rates can trigger matrix softening.

In this research, combinations of high feed rate and cutting speed (f = 0.1 mm/rev, Vc = 35 m/min) resulted in thrust forces exceeding 130 N and surface roughness values greater than 2 µm, suggesting that the local temperatures reached or surpassed the matrix softening regime. These findings support the causal relationship between thermal accumulation and process-induced damage mechanisms as described in the literature.^26–29 Building upon these thermal effects and their implications for machining damage, the current study explores the role of tool helix angle in optimizing drilling outcomes.

In this study, a series of drilling experiments was conducted on CF/PEEK composite plates using uncoated carbide drills with three different helix angles. The effects of cutting speed (three levels) and feed rate (three levels) on cutting force, surface roughness, and delamination were analyzed. The objective was to determine optimal machining parameters through experimental design based on the collected data. By investigating the influence of drill helix angles on CF/PEEK drilling performance, this study aims to contribute to the growing body of knowledge on thermoplastic composite machining.

Although extensive research has been conducted on the drilling of CFRP and thermoplastic composites, previous studies have primarily focused on varying cutting speeds and feed rates, with limited attention to the effect of drill helix angle. This study represents the first attempt to systematically analyze the impact of different helix angles (20°, 30°, and 40°) on the drilling performance of CFRP-PEEK composites. The findings contribute to a deeper understanding of the influence of drill geometry on surface roughness, delamination, and cutting forces, which are critical parameters in machining high-performance thermoplastic composites.

Experimental procedure

Composite specimens and cutting tools

In this study, the workpiece used was a carbon fiber-reinforced PEEK thermoplastic plate. The PEEK matrix composite plates were reinforced with carbon fiber fabrics with a plain weave (0°/45°/90°/−45°) fiber orientation angle. The composite plates were sourced from MIR-ARGE, a company based in Istanbul, Turkey. The plates consist of 60% carbon fiber and 40% PEEK thermoplastic. They were formed using a hot press method in a Dwell brand autoclave at a temperature of 385°C for 30 min. During the heating process, the pressure was set at approximately 5 MPa. After being removed from the autoclave, the plates were allowed to cool to room temperature. The prepreg material, composed of approximately 16 layers, had dimensions of 150 × 35 × 5 mm. The properties of the material are provided in Table 1.

Table 1.

Physico-chemical and mechanical properties of the CF/PEEK laminates.

Property	Value
Carbon fiber type	High-modulus PAN-based CF
Composite type	CF/PEEK
Matrix	PEEK (victrex 450G)
Reinforcement	Carbon fiber fabric (0°/45°/90°/−45°)
Fiber volume fraction (vf)	60%
Matrix volume fraction	40%
Lay-up configuration	±45°/0°/90°, 16 layers
Manufacturing method	Hot press (385°C, 5 Mpa), 30 min (autoclave)
Cooling	Ambient air
Final dimensions (mm)	150 × 35 × 5
Cured ply thickness (mm)	0,311
Laminate density (g/cm³)	1,55
Degree of crystallization (%)	40
Melting point (°C)	344
Glass transition temperature (°C)	165
Tensile strength 0°(RT) (MPa)	941
Tensile strength 90° (RT) (MPa)	944
Shear strength (RT) (MPa)	145
Bending strength 0° (RT) (MPa)	1056
Bending strength 90° (RT) (MPa)	941
Compressive strength 0°(RT) (MPa)	600
Compressive strength 90° (RT) (MPa)	574

In addition to the properties outlined in Table 1, the crystallization behavior of PEEK should also be considered, as it is a semi-crystalline polymer and its degree of crystallinity depends on processing parameters such as heating/cooling rates, dwell time, and pressure.^26,30 In this study, CF/PEEK plates were fabricated using a hot-press technique at 385°C under 5 MPa pressure for 30 min, followed by natural cooling to room temperature. The crystallinity level of the laminate was measured using differential scanning calorimetry (DSC), and an average crystallinity of 40% was obtained, which aligns with the values reported in literature for similar processing conditions.³⁰ Furthermore, DSC measurements across the thickness confirmed a homogeneous distribution of crystallinity. Thus, the mechanical and physical properties presented in Table 1 represent a semi-crystalline and structurally uniform CF/PEEK laminate manufactured under controlled processing conditions.

Beyond crystallinity, certain intrinsic material characteristics significantly affect hole quality in CF/PEEK composites. In multilayered composites, the in-plane strength is generally much higher than the through-thickness (Z-directional) strength, which predisposes the material to delamination under the influence of thrust forces during drilling.³¹ Low interlaminar shear strength and weak fiber–matrix bonding further promote layer separation during drilling.³² Moreover, a high fiber volume fraction (60%) increases stiffness but also brittleness, leading to crack formation around the hole edges.³¹ Although the thermoplastic nature of the PEEK matrix provides high toughness, when the drilling temperature approaches or exceeds its glass transition temperature (165°C), matrix softening occurs, resulting in increased surface roughness and dimensional deviations.³³ Additionally, symmetric lay-up configurations (0°/±45°/90°) directly influence chip formation, crack propagation, and delamination resistance.³⁴ Therefore, evaluating the machinability of CF/PEEK composites should include not only cutting parameters but also these inherent material-specific characteristics.

In the experiment, uncoated carbide drills with the same point angle (120°) but three different helix angles (20°, 30°, and 40°) were selected to drill holes on the surface of the CF/PEEK composite material. The properties of the drill bits are provided in Table 2.

Table 2.

Properties of drills used in the drilling process.

Diameter	Coating material	Helical angle	Point angle
8 mm	Uncoated carbide	20^°/30^°/40^°	120°

Figure 3 illustrates the schematic view of the composite plate along with the geometric characteristics of the drills.

Figure 3.

(A) Schematic representation of the layered CF/PEEK plate, (B) views of the drills used.

The composite plate used in this study features a 16-ply quasi-isotropic lay-up sequence [0°/45°/90°/−45°]_s, as depicted in Figure 3. This type of stacking is widely employed in aerospace structures, such as fuselage panels, wing skins, and floor beams, due to its ability to offer mechanically balanced, in-plane quasi-isotropy. Quasi-isotropic laminates are typically composed of plies oriented at 0°, ±45°, and 90°, which simulates in-plane isotropic behavior.^35,36 Furthermore, MIL-HDBK-17, a key design reference standard in aviation, emphasizes the use of symmetric, quasi-isotropic lay-up configurations in drilling and primary structural applications.³⁷

Stacking sequence influences delamination and thrust force during drilling. Previous studies comparing drilling in cross-ply versus quasi-isotropic laminates report that the latter exhibit improved damage resistance and lower delamination factors under similar cutting conditions.³⁸

Experimental setup

The experimental studies were conducted at ambient temperature under dry conditions using a First MCV 300 vertical machining center. The drilling forces generated during the machining of the CF/PEEK composite plate were recorded using a Kistler 9257B piezoelectric dynamometer. The acquired signals were transmitted to the 5697A data acquisition (DAQ) module and subsequently analyzed using DynoWare software. To ensure the accuracy and reliability of the experiments, a support fixture was employed to stabilize the composite specimen. The experimental setup is illustrated in Figure 4.

Figure 4.

Experimental setup for drilling CF/PEEK composite laminates.

Following the drilling tests, a Nikon SMZ800 optical microscope was utilized to evaluate delamination damage. The conventional delamination factor method was adopted for the measurement of delamination.

F_{d} = \frac{D_{\max}}{D_{0}}

Here, D_max represents the maximum diameter of the delaminated region and D₀ is the nominal hole diameter, was adopted in this study due to its practicality in measuring maximum delamination around the hole and its widespread use in the literature. The primary objective of this study is to investigate the effects of drilling parameters on delamination and to perform a parameter optimization. In this context, rather than employing advanced damage mechanics analyses, a method that enables parametric comparisons was considered appropriate and sufficient for the intended purpose.^39–42

The visual data acquired from the optical microscope were subsequently analyzed using the ImageJ software, developed by Fiji, to enhance the visibility of the damaged areas and to calculate the delamination factor.

The Mitutoyo SJ-210 series profilometer was used for surface roughness measurements. This commonly used profilometer conducted a total of eight measurements for each hole to perform precise surface scans. The average of these eight measurements was then taken to determine the optimum surface roughness.

The machining parameters of drills with different helix angles are detailed in Table 3. Cutting speeds were set at three levels: 15 m/min, 25 m/min, and 35 m/min, while feed rates were also adjusted at three levels: 0.05 mm/rev, 0.075 mm/rev, and 0.1 mm/rev. These parameters were selected based on relevant literature studies^16,42–44 and recommendations from tool suppliers.

Table 3.

Machining parameters used in the drilling process.

Factors	Symbols	Level 1	Level 2	Level 3
Helical angle (°)	A	20°	30°	40°
Cutting speed (V_c—m/min)	B	15	25	35
Feed rate (f—mm/rev)	C	0,05	0,075	0,1

For each set of drilling parameters, three holes were drilled to ensure repeatability. To minimize the effect of tool wear, each new drill bit was used to drill only three holes.⁴⁵ Furthermore, in the experimental study, all control factor combinations between input parameters (helix angle, feed rate, cutting speed) and output parameters (surface roughness, cutting force, delamination factor) were evaluated within the framework of the Taguchi L27 design. The optimization of these control factors was carried out using the signal-to-noise (S/N) ratio, employing the “smaller is better” approach. The Taguchi method was preferred due to its ease of interpretation and cost-effectiveness.

The parameters and their levels used in the experimental study are shown in Table 3.

Results and Discussion

In the study, nine drilling experiments were conducted for each helical angle. A total of 27 experiments were performed for three different helical angles. Therefore, the Taguchi orthogonal experimental design (L27) was selected as the most suitable for these levels. The experimental design and output parameters are presented in Table 4.

Table 4.

Experimental results.

Exp number	Helix angle [°]	Cutting speed [Vc] [m.min⁻¹]	Feed rate [f] [mm.rev⁻¹]	Delamination factor [F_d] (entry)	Delamination factor [F_d] (exit)	Surfaces roughness [R_a][µm]	Thrust force [F][N]
1	20	15	0.05	1,021	1,18	0,404	68,69
2	20	15	0.075	1,038	1,12	0,616	88,8
3	20	15	0.1	1,045	1,11	0,74	97,7
4	20	25	0.05	1,035	1,175	1,058	81,87
5	20	25	0.075	1,046	1,21	1,214	93,5
6	20	25	0.1	1,057	1,14	1,328	111,8
7	20	35	0.05	1,043	1,22	1,714	114,1
8	20	35	0.075	1,061	1,181	1,983	122,8
9	20	35	0.1	1,072	1,195	1,898	138,7
10	30	15	0.05	1,013	1,06	0,479	53,19
11	30	15	0.075	1,022	1,07	0,648	67,68
12	30	15	0.1	1,037	1,085	0,75	75,8
13	30	25	0.05	1,03	1,1	1,261	82,7
14	30	25	0.075	1,036	1,125	1,027	88,35
15	30	25	0.1	1,043	1,13	1,047	98,2
16	30	35	0.05	1,041	1,14	1,82	89,8
17	30	35	0.075	1,047	1,16	2,193	95,91
18	30	35	0.1	1,055	1,24	3,032	102,07
19	40	15	0.05	1,018	1,075	0,501	57,92
20	40	15	0.075	1,03	1,04	0,635	67,67
21	40	15	0.1	1,036	1,05	0,747	90,32
22	40	25	0.05	1,022	1,095	0,843	78,36
23	40	25	0.075	1,031	1,09	1,003	92,56
24	40	25	0.1	1,035	1,08	1,105	99,62
25	40	35	0.05	1,028	1,07	2,456	90,97
26	40	35	0.075	1,035	1,1	1,976	100,74
27	40	35	0.1	1,043	1,12	2,024	115,38

Drilling thrust force

During the drilling of composite materials, thrust force occurs on the material surface due to the advancement of the drill bit. This applied load reduces the material’s fatigue strength and can lead to delamination.^16,46,47 The movement of the drill bit and the resulting thrust force during material drilling essentially consists of five stages: (1) approach of the drill to the workpiece, (2) contact of the drill bit with the surface, (3) drilling of the hole, (4) exit from the hole, and (5) withdrawal of the drill.⁴² These stages are illustrated in Figure 5.

Figure 5.

Thrust force generated by drill movement.

During the idle advancement of the drill bit, there is no load on the workpiece, so the graph progresses linearly (1st region). Upon contact of the drill bit with the surface, there is a linear increase in thrust force (2nd region). The increase in thrust force progresses linearly as the drill advances into the material (3rd region). As the thickness of the workpiece decreases, a decrease in thrust force occurs (4th region). Once the drill exits the material, the effect of thrust force diminishes (5th region). Figure 5 illustrates that after reaching its peak value, the Fz force signifies the occurrence of the drilling process. As the thickness of the layer decreases during drilling, the force decreases. Figure 6 illustrates the maximum thrust forces generated using drills with different helix angles at various feed rates and cutting speeds.

Figure 6.

Variation of thrust force values under different feed rates and cutting speeds for drills with 20°, 30°, and 40° helix angles, (a) 15 m/min cutting speed, (b) 25 m/min cutting speed, (c) 35 m/min cutting speed.

In Figure 6, the variations in maximum thrust force encountered during the drilling of CF/PEEK composite laminates using drills with 20°, 30°, and 40° helix angles under different drilling parameters are analyzed. It was observed that cutting speed, feed rate, and drill helix geometry significantly affect thrust force across all tool types. Notably, drills with a 20° helix angle generated higher thrust forces in all tested conditions.

Considering the maximum thrust force values obtained for three feed rates [f] [0.05, 0.075, and 0.1 mm/rev] at each cutting speed [Vc] [15, 25, and 35 m/min], the 20° helix drill produced 19%, 6%, and 29% higher forces than the 30° helix drill, and 13%, 5%, and 22% higher forces than the 40° helix drill, respectively. Similarly, the 30° helix drill generated 6%, 0.43%, and 6.5% more thrust force than the 40° helix drill under the same conditions.

Analyzing the effect of feed rate at each cutting speed level provides a clearer understanding of how each parameter combination contributes to peak thrust force. This insight helps identify critical machining conditions that may promote delamination, thus offering valuable guidance for process optimization.

Overall, at a cutting speed of 25 m/min, a decrease in thrust force was observed across all drill types, indicating more efficient chip evacuation before matrix softening. On average, the 20° helix drill produced 18% and 13% more thrust force than the 30° and 40° helix drills, respectively. Meanwhile, the 30° helix drill generated only 0.04% more thrust force than the 40° helix drill, which is notably lower than the difference observed with the 20° helix drill. These differences in thrust force are closely related to increased delamination observed at the hole exit.

Assessment of delamination damage

After the completion of the drilling experiments, the quality of the holes was examined. The entry and exit morphologies, surface burr formation, fuzzing, and tearing were analyzed using an advanced Nikon SMZ800 Digital Optical Microscope. This analysis enabled the observation of entry and exit delaminations. In Figure 7, the delamination area and the formed burrs can be observed.

Figure 7.

The appearance of the area affected by delamination failure at the hole exit.

The concept of delamination factor (Fd) was used to identify the critical delamination defect threshold in drilling composites, defined by the formula $= \frac{D_{\max}}{D 0}$ , where D_max represents the delamination diameter on the hole and D₀ is the nominal hole diameter. Figure 7 illustrates the delamination diameter (D_max) and the nominal hole diameter (D₀) on the hole surface.

To provide a more comprehensive understanding of the delamination mechanisms, several key factors were considered. First, CF/PEEK composites exhibit continuous chip formation during drilling due to the thermoplastic nature of the matrix, which leads to localized temperature elevation in the cutting zone. The combination of increased temperature and chip congestion raises the thrust force and promotes both surface damage and delamination, particularly at the hole exit.²⁴ Second, due to the anisotropic structure of composite laminates, the through-thickness (Z-directional) strength is significantly lower than the in-plane strength. This disparity facilitates Mode I delamination under the influence of thrust force, especially during the push-out phase.³² Additionally, when drilling is performed near the glass transition temperature (Tg ≈ 165°C for PEEK), matrix softening occurs, reducing interfacial integrity between fiber and matrix, which further promotes delamination and dimensional inaccuracy around the hole.^3,24 Although material properties were not directly included as control factors in the Taguchi-based optimization, their influence was inherently accounted for through consistent manufacturing parameters (e.g., crystallinity level, fiber orientation, and laminate thickness), ensuring homogeneity in mechanical behavior during drilling.

This thermomechanical behavior directly influences how and where delamination initiates and propagates.

As illustrated in Figure 8 and supported by previous studies,^48–50 delamination during drilling typically occurs in two stages: peel-up delamination at the hole entry (associated with Mode I + Mode III failure), and push-out delamination at the hole exit (involving Mode I + Mode II failure). Drilling with low helix angle tools tends to generate higher thrust forces, thereby intensifying Mode I dominated delamination at the exit surface.

Figure 8.

Delamination types and their failure modes: (a) peel up delamination; (b) pushout delamination.^48,49,50

Although interlaminar fracture toughness was not directly measured in this study, the delamination factor (Fd) was used as an indirect metric to evaluate the influence of cutting parameters on interfacial damage. The effects of drilling parameters (including cutting speed, feed rate, and helix angle) on delamination were analyzed using the Taguchi method and ANOVA, enabling a robust understanding of damage evolution under different machining conditions. Figure 8 illustrates delamination damage modes (Modes I, II, and III) occurring at the entry and exit during drilling of composite laminates.

To complement the statistical analysis and deepen the understanding of damage evolution, microscopic examinations of hole entry and exit surfaces were conducted under varying cutting conditions using a drill with a 20°, 30°, 40° helix angles, as shown in Figures 9-11. These observations revealed distinct failure patterns depending on the feed rate and cutting speed. At the hole entrance, delamination (red), tearing (yellow), and burr formation (green) were identified, with damage severity increasing at higher feed rates and cutting speeds. Notably, peel-up delamination was more prominent in low feed–high speed combinations, aligning with Mode I and Mode III fracture mechanisms. At the hole exit, push-out delamination dominated, often accompanied by peripheral burrs and cracks, especially at elevated feed rates. These visual findings support the statistical results derived from Taguchi and ANOVA analyses, confirming that lower helix angles and higher mechanical loads significantly contribute to interfacial damage during drilling of CF/PEEK laminates.

Figure 9.

Hole entry and exit images obtained using a drill bit with a 20° helix angle under varying drilling parameters.

Figure 10.

Hole entry and exit images obtained using a drill bit with a 30° helix angle under varying drilling parameters.

Figure 11.

Hole entry and exit images obtained using a drill bit with a 40° helix angle under varying drilling parameters.

Figure 9 illustrates the entrance and exit surfaces of holes drilled using a 20° helix angle drill. At low feed rates, minimal delamination and tearing are observed at the hole entrance. However, as the feed rate increases, both tearing and the delaminated region expand. With the increase in cutting speed (25 m/min and 35 m/min), burr formation is also observed along with tearing and delamination. In summary, holes drilled with a 20° helix angle drill exhibit a higher tendency for tearing and delamination at the entrance, whereas the primary damage at the hole exit is predominantly burr formation, followed by delamination.

Figure 10 presents the entrance and exit surfaces of holes drilled using a 30° helix angle drill. At low feed rates, a small amount of delamination is observed at the hole entrance. However, as the feed rate increases, both the tearing and the delaminated region expand. With the increase in cutting speed (25 m/min and 35 m/min), burr formation is also observed along with tearing and delamination. In summary, similar to the 20° helix angle drill, holes drilled with a 30° helix angle drill exhibit a higher tendency for tearing and delamination at the entrance. At the hole exit, the primary damage is predominantly burr formation, followed by delamination and localized tearing.

Figure 11 presents the entrance and exit surfaces of holes drilled using a 40° helix angle drill. The analysis of this figure reveals that the primary defects at the hole entrance include burr formation, tearing, and delamination. The severity of these defects varies depending on the feed rate and cutting speed. As the feed rate increases (0.075 mm/rev and 0.1 mm/rev), the affected regions expand, leading to a decline in hole quality. Delamination is the most prevalent defect at the hole entrance, followed by tearing and burr formation. At the hole exit, burr formation is the predominant defect, accompanied by delamination and tearing as additional forms of damage.

When examining the hole quality at both the entrance and exit, it is evident that severe defects occur at the hole entrance during the drilling of CF/PEEK composites. Moreover, at the hole exit, burr formation is observed as a more dominant defect, particularly at lower cutting speeds and lower feed rates. Unlike CF/Epoxy thermoset composites, CF/PEEK composites exhibit significantly higher burr formation during the drilling process.^45,50

Delamination defects typically occur at the entry section as peel delamination, interlaminar fracture, and at the exit of the hole.⁵¹ It is observed that delamination at the exit is more pronounced in the helical 20^°drill bit. With the helical 30^° drill bit, delamination intensity increases noticeably from the first hole onwards. The green-marked area, which enhances delamination visibility at the ninth hole, is attributed to the overly bright surface of the hole as seen under the optical microscope. Delamination defects and burr formation are less noticeable with the helical 40^° drill bit. Burrs are visible at the first and sixth holes. In the other holes, delamination is less pronounced. In the study using three different drill bits with three different cutting speeds and feed rates, burr formation occurred at the exit of almost all holes. Along with burr formation, fiber fractures were also observed.

Table 4 presents the measurement results of delamination factors at the hole exits. Drills with a 40° helix angle exhibited lower burr and tearing rates compared to those with 20° and 30° helix angles. This finding suggests that larger helix angles improve chip evacuation and thermal dissipation, thereby reducing delamination.^52,53

Under various drilling conditions, burr formation was observed to be more dominant than tearing. The drill with a 20° helix angle resulted in the highest delamination factor at the hole exit. This outcome can be attributed to the inefficient chip evacuation in lower helix angle drills, which leads to chip congestion, elevated thrust forces, and subsequently more severe tearing and burr formation.

CF/PEEK thermoplastic composites possess higher thermal toughness and plastic deformation capability compared to thermoset composites such as CF/epoxy. However, literature reports that this behavior may also lead to increased thermal effects during drilling, which in turn intensifies burr formation and delamination.⁵⁸ In the present study, burr formation was particularly prevalent and dominant at lower cutting speeds in CF/PEEK composites. This observation is consistent with the findings of Tsao and Hocheng,⁵² Geier and Szalay.⁵³

At high feed rates (0.1 mm/rev), especially with a 30° helix angle, the delamination factor [Fd][exit] increased from 1.06 to 1.24, indicating that increased feed imposes greater mechanical loads, thereby raising the risk of interlaminar separation. Similar findings have been reported by Du et al.,⁵⁴ who noted that continuous chip formation in CF/PEEK enhances delamination tendencies.

Conversely, increasing the cutting speed was found to reduce delamination to a limited extent. This reduction may be associated with thermal softening of the matrix induced by the elevated cutting temperatures. However, when increased cutting speeds are combined with higher feed rates, delamination tends to rise again. These results highlight that in thermoplastic composites, delamination mechanisms are not solely governed by geometric or mechanical factors but are also strongly influenced by thermo-mechanical interactions.^13,15

When evaluating the hole entrance and exit quality, it is evident that the most severe damage during the drilling of CF/PEEK composites occurs at the hole entrance. At the hole exit, particularly under low cutting and feed rates, burr formation emerges as the dominant damage mechanism. Compared to thermoset composites, burr formation is more prevalent in thermoplastic composites.⁵⁵ As illustrated in Figures 9–11, delamination defects are typically observed as peel-up at the hole entrance and push-out at the hole exit.⁵⁶

The effect of cutting speed, feed rate, and helix angle on both entry and exit delamination factors [Fd] is illustrated in Figure 11. Each group of bars represents a specific cutting speed (15, 25, and 35 m/min), while the colors correspond to the feed rates (Blue: 0.05 mm/rev, Orange: 0.075 mm/rev, Purple: 0.1 mm/rev). The solid portion of each bar indicates the entry delamination factor, whereas the hatched portion represents the exit delamination factor. The Fd values were calculated based on the ratio of the maximum delamination diameter to the nominal hole diameter, and the data were taken from Table 4.

As the feed rate increases, a noticeable rise in Fd values is observed, particularly with the 20° helix angle drill, where Fd values exceed 1.2 at higher feed levels. In contrast, the 40° helix angle drill consistently resulted in the lowest delamination values across all cutting parameters. This indicates that higher helix angles promote more efficient chip evacuation and thermal dissipation.

Although there is no universally defined threshold in the literature, several studies^57,58 have reported that Fd values greater than 1.2 may pose structural risks in aerospace-grade composites. These findings are consistent with the numerical data in Table 4 and the damage mechanisms observed in Figures 9–11. As reported in the literature, during machining operations conducted near the glass transition temperature, the PEEK matrix undergoes thermal softening, which weakens the fiber–matrix interfacial strength and consequently promotes delamination due to thermo-mechanical effects.^45,59 Figure 12 illustrates the entry and exit delamination factors (Fd) for different helix angle drills under varying cutting speeds and feed rates.

Figure 12.

Entry and exit delamination factor (Fd) values for various cutting speeds, feed rates, and drill helix angles.

To better understand the effect of drilling parameters on the delamination factor, an optimization process will be conducted in the following sections. According to the analysis of delamination factors at different feed rates and cutting speeds, burr formation and fiber push-out are the primary damage mechanisms at low feed rates, whereas peel-up damage at the entrance becomes more pronounced at higher feed rates.⁶⁰

Surface roughness

To assess surface roughness, the average surface roughness (Ra) value is used. Surface roughness (Ra) is commonly employed in evaluating the hole quality of composite materials.^16,61–64 For surface roughness measurements, a Mitutoyo SJ-210 device was used to take 8 linear surface roughness measurements from four different points on each hole. The measurements were taken longitudinally along the inner wall of the drilled hole, specifically from four equally spaced circumferential quadrants (0°, 90°, 180°, and 270° positions) at a constant insertion depth of approximately 1.5 mm from the hole entrance. This was done to ensure consistent and repeatable data across all samples. The average values from these 8 measurements were considered as the representative Ra for each hole. Trends in average surface roughness (Ra) are shown in Figure 13.

Figure 13.

Avarage surface roughness (Ra) values with respect to hole number, cutting parameters (feed rate and cutting speed) and measurement locations.

When examining surface roughness values, different outcomes are observed for each drill under varying cutting speeds and feed rates (Figure 13). Specifically, the 20° helix angle drill exhibits the lowest surface roughness (∼0.4 µm) at hole 1. In contrast, at hole 4, the 40° helix angle drill produces a lower surface roughness value (0.84 µm) compared to the others. This indicates the distinct influence of machining parameters and drill helix geometry on surface finish. In general, lower cutting speeds and feed rates result in reduced surface roughness for all drills. This finding aligns with the existing literature, which suggests that low-speed drilling reduces thermal and mechanical stresses, helps preserve the fiber–matrix interface, and minimizes surface damage.^45,59

However, an increase in cutting speed typically worsens surface roughness due to matrix softening and elevated drilling temperatures. This trend is consistent with the findings of Zhang et al.⁵⁹ and Du et al.,⁶⁵ who reported that the thermoplastic nature of CF/PEEK results in continuous chip formation, local heat accumulation, matrix softening, and plastic deformation, all contributing to rougher hole surfaces.

For example, with the 40° drill at a cutting speed of 35 m/min, surface roughness increases at hole seven and continues to rise in holes 8 and 9. This behavior is attributed to matrix softening and insufficient chip evacuation, which leads to material adhesion. Conversely, higher feed rates tend to reduce surface roughness by improving chip evacuation and minimizing thermal effects. This trend is clearly observed in the decreasing Ra values from hole 6 onward when using the 30° drill.

Meanwhile, the 20° drill exhibits a more gradual increase in surface roughness, with a slight decrease at hole 9, suggesting improved chip evacuation and reduced adhesion. Both the 20° and 40° drills show a reduction in Ra values at hole 9, indicating enhanced chip flow at higher feed rates and cutting speeds. However, the sharp increase observed at hole 9 with the 30° drill may be due to local material heterogeneity or microstructural defects.

All average values shown in the graph include standard deviation (σ) error bars calculated from eight repetitions, reflecting the variability of the process. The red dashed line at Ra = 1.2 µm represents a commonly accepted internal surface roughness limit in the aerospace industry, often implemented in AS9100-based quality management systems.^66,67 Surface finishes exceeding this threshold may compromise structural integrity and fatigue life in high-performance applications.¹⁵

Optimization processes

As input parameters in the experimental study, cutting speed [V_c] (15, 25, 35 m/min), feed rate [f] (0.05, 0.075, 0.1 mm/rev), and drill helix angles (20°, 30°, 40°) were considered. Output parameters included surface roughness [R_a], thrust force [F], and exit delamination factor [F_d]. The Taguchi L27 orthogonal array was employed for optimization. The L27 array represents 27 different experimental conditions, each consisting of specific combinations of factors and their levels. For each of the three output parameters:

S / N = ‐ 10 \log (1 / n \sum_{n = 1}^{n} y_{i}^{2})

S/N represents the signal-to-noise ratio and is typically expressed in decibels (dB). In the Taguchi method, it is used to evaluate the stability and quality of measurements. The value n denotes the number of repetitions performed under the same experimental conditions and is important for statistical reliability. y_i, refers to the measurement value obtained in the i-th repetition of the experiment. The ∑ (summation) symbol represents the sum of the squares of the measurement values obtained from repeated trials. Log indicates the base-10 logarithmic operation used in the calculation of the S/N ratio.

Smaller-the-better, signal-to-noise ratio parameter was selected.

Table 5 presents the signal-to-noise (S/N) ratios for thrust force, exit delamination factor, and surface roughness, determined according to the “smaller-the-better” criterion. According to the formula used in this criterion, smaller measurement values result in higher S/N ratios (as the negative contribution decreases). Therefore, in the “smaller-is-better” approach, a higher S/N ratio indicates a more desirable outcome.Analysis of the table data reveals that the optimal values for surface roughness are achieved at level 1 with a 20° helix angle drill (−0,6816), a cutting speed of 15 m/min (4,4262), and a feed rate of 0.05 mm/rev (0,1736). Examination of the delta values indicates that cutting speed exerts the most significant influence (10,8369), followed by feed rate (2,1077), while drill helix angle has the least impact (0,6114).

Table 5.

Response table for signal-to-noise ratios thrust force.

Response table for signal to noise ratios-thrust force
Smaller is better
Level	Drill helix	Cutting speed	Feed rate
1	−39,99	−37,24	−37,82
2	−38,3	−39,22	−39,04
3	−38,74	−40,57	−40,17
Delta	1,69	3,33	2,35
Rank	3	1	2

Response table for signal to noise ratios-delamination factor
Smaller is better
Level	Drill helix	Cutting speed	Feed rate
1	−1,3603	−0,7248	−1,0046
2	−1,0012	−1,0347	−0,9891
3	−0,6664	−1,2683	−1,0341
Delta	0,6939	0,5435	0,045
Rank	1	2	3

Response table for signal to noise ratios-surface roughness
Smaller is better
Level	Drill helix	Cutting speed	Feed rate
1	−0,6816	4,4264	0,1736
2	−1,2934	−0,7436	−0,967
3	−0,7526	−6,4104	−1,9341
Delta	0,6118	10,8369	2,1077
Rank	3	1	2

When evaluating the parameters affecting the exit delamination factor, the smallest delamination occurs with a 40° helix angle drill (−0,6664), a cutting speed of 15 m/min (−0,7248), and a feed rate of 0.075 mm/rev (−0,9891). Among these, helix angle emerges as the most influential parameter, with a delta value of 0,6939. Cutting speed follows with a delta value of 0,5435, while feed rate has the least impact on delamination, exhibiting a delta value of 0,045.

For thrust force, the lowest value is observed with a 30° helix angle drill (−38,3), a cutting speed of 15 m/min (−37,82), and a feed rate of 0.05 mm/rev (−37,82). Cutting speed is identified as the most influential factor on thrust force, with a delta value of 3,33. Feed rate follows with a delta value of 2,35, while drill helix angle exerts the least influence, as indicated by a delta value of 1,69.

Overall, cutting speed is determined to be the most significant parameter affecting both thrust force and surface roughness. In contrast, drill geometry, specifically helix angle, plays a dominant role in influencing delamination.

In summary, cutting speed demonstrates the largest impact on both surface roughness and delamination factors. This underscores the critical importance of optimizing cutting speed for controlling surface quality and reducing delamination. For thrust force, drill helix angles appear as the most impactful factor, highlighting the significant role of selecting the correct helix angle in minimizing thrust force. Feed rate is consistently the least influential factor across all three parameters.

Figure 14 illustrates the main effect plots of the signal-to-noise (S/N) ratio and the relationships among the parameters. This figure presents the influence of input parameters, including drill helix angle, cutting speed, and feed rate, on thrust force, exit delamination factor, and surface roughness in the drilling process.

Figure 14.

Main effect graphs for S/N ratio.

The thrust force graph indicates that the lowest thrust force occurs at a 30° helix angle, 15 m/min cutting speed, and 0.05 mm/rev feed rate. As cutting speed and feed rate increase, thrust force also increases. Additionally, interaction plots reveal that the 30° drill consistently produces lower thrust forces across all cutting speeds when combined with the lowest feed rate, confirming its suitability for minimizing drilling-induced forces.

The delamination graph reveals that higher helix angles result in lower delamination. The minimum delamination is observed at a 40° helix angle, 15 m/min cutting speed, and 0.075 mm/rev feed rate. In contrast, the highest delamination occurs at 35 m/min cutting speed and 0.1 mm/rev feed rate. Interaction plots further indicate a sharp increase in delamination under high-speed and high-feed conditions, emphasizing the need to avoid such parameter combinations for maintaining composite integrity.

The surface roughness graph shows that the effects of 20° and 40° helix angles are similar. The lowest surface roughness is achieved at 15 m/min cutting speed, 0.05 mm/rev feed rate, and a 20° helix angle. Increasing cutting speed and feed rate leads to higher surface roughness. Notably, interaction plots show more pronounced variation with the 30° drill, which may be attributed to inconsistent chip evacuation or localized material irregularities.

Overall, the trends observed in Figure 14 are consistent with the values presented in Table 5, confirming the reliability of the obtained results. In general, a cutting speed of 15 m/min and a feed rate of 0.05 mm/rev yield the most favorable S/N ratios across all output responses, supporting their recommendation for high-quality drilling of CF/PEEK composites.

Figure 15 presents the three-dimensional surface plots illustrating the effects of cutting speed [Vc] and feed rate [f] on three key output parameters, exit delamination factor [Fd], surface roughness [Ra], and thrust force [F], for different drill helix angles (20°, 30°, and 40°), displayed separately.

(a) Exit delamination factor [Fd], for all helix angles, delamination tends to increase with higher cutting speeds and feed rates. The lowest Fd value is observed at the 40° helix angle under low to moderate cutting conditions. This suggests that higher helix angles facilitate chip evacuation and thereby reduce laminate damage. In contrast, the highest delamination values are recorded at the 30° helix angle, particularly under aggressive parameters such as 0.1 mm/rev and 35 m/min, likely due to insufficient chip evacuation associated with this geometry.

(b) Surface roughness [Ra] increases with both cutting speed and feed rate for all helix angles. The lowest Ra value is obtained at the 20° helix angle under the lowest parameter settings (0.05 mm/rev, 15 m/min), indicating that lower helix angles may provide smoother cutting and improved surface finish. Conversely, surface roughness significantly increases at the 30° and 40° helix angles under high cutting parameters, indicating surface degradation.

(c) Thrust force [F] increases with cutting speed and feed rate across all helix angles. However, the influence of helix angle varies. The 30° helix angle produces the lowest thrust force, especially at 0.05 mm/rev and 15 m/min. On the other hand, the 20° and 40° helix angles yield higher thrust forces under the same conditions. This suggests that the 30° helix angle provides more efficient cutting force distribution and chip removal.

Figure 15.

Three-dimensional (3D) surface plots illustrating the effects of cutting speed and feed rate on (a) delamination factor, (b) surface roughness, and (c) thrust force.

In conclusion, each helix angle offers distinct advantages: 40° is more effective in minimizing delamination, 20° provides better surface finish, and 30° results in the lowest thrust force. These findings indicate that optimizing drill geometry based on specific performance priorities is crucial to improving hole quality in CF/PEEK drilling applications.

Analysis of Variance (ANOVA)

According to Table 6, (a) the most influential parameter on thrust force is cutting speed (51.30%), which is statistically significant (F = 72.05, p < .0001). The next influential parameters are feed rate (25.16%) and helix angle (16.43%). Both of these parameters also show statistically significant effects (p < .0001). The model’s coefficient of determination (R² = 92.88%) indicates that the model fits the data well and is reliable. Additionally, the adjusted R² (90.75%) and predicted R² (87.03%) values show that the model has high generalizability. Cutting parameters, particularly cutting speed and feed rate, are critical in controlling the thrust force generated during drilling.

(b) The most significant factor affecting delamination is the drill’s helix angle (47.04%), which is statistically significant (F = 19.85, p < .0001). Helix angle is followed by cutting speed (29.03%, F = 12.25, p < .001). However, the effect of feed rate is very low and statistically insignificant (0.22%, p = .91). The model’s R² value of 76.39% provides a moderate level of explanation, with the adjusted R² (69.19%) and predicted R² (56.96%) values suggesting that the model’s reliability is borderline. To reduce delamination, the optimization of helix angle and cutting speed should be prioritized, while the effect of feed rate can be neglected.

(c) Regarding surface roughness, cutting speed is the sole determining parameter, contributing 86.91% (F = 85.67, p < .0001). Feed rate and helix angle have low contributions and are statistically insignificant (p > .1). The model’s R² value (89.86%) is high, with adjusted R² (86.81%) and predicted R² (81.51%) indicating that the model is strong and generally applicable. To improve surface quality, cutting speed should be carefully optimized, while the effects of other factors are minimal.

Table 6.

Analysis of Variance (ANOVA) results for (a) thrust force, (b) delamination factor, and (c) surface roughness in the drilling of CF/PEEK composite laminates.

(a)	Analysis of Variance-thrust force
Source	DF	Seq SS	Contribution	Adj SS	Adj MS	F-value	p-value
Cutting speed	2	5095	51,30%	5095	2547,48	72,05	7E-10
Feed rate	2	2499	25,16%	2499	1249,48	35,34	2,723E-07
Drill helix angle	2	1631,4	16,43%	1631,4	815,72	23,07	0,000006388
Error	20	707,1	7,12%	707,1	35,36
Total	26	9932,5	100,00%

Model summary-thrust force
S	R-sq	R-sq (adj)	PRESS	R-sq (pred)
5,94601	92,88%	90,75%	1288,69	87,03%

(b)	Analysis of variance-delamination factor
Source	DF	Seq SS	Contribution	Adj SS	Adj MS	F-value	p-value
Cutting speed	2	0,022506	29,03%	0,022506	0,011253	12,25	0,00034
Feed rate	2	0,000172	0,22%	0,000172	0,000086	0,09	0,91092
Drill helix angle	2	0,036467	47,04%	0,036467	0,018233	19,85	0,00002
Error	20	0,018372	23,70%	0,018372	0,000919
Total	26	0,077517	100,00%

Model summary-delamination factor
S	R-sq	R-sq (adj)	PRESS	R-sq (pred)
0,0303086	76,39%	69,19%	0,0334834	56,96%

(c)	Analysis of variance-surface roughness
Source	DF	Seq SS	Contribution	Adj SS	Adj MS	F-value	p-value
Cutting speed	2	10,6738	86,91%	10,6738	5,33692	85,67	0,000
Feed rate	2	0,2603	2,12%	0,2603	0,13014	2,09	0,149
Drill helix angle	2	0,1016	0,83%	0,1016	0,05079	0,82	0,456
Error	20	1,2459	10,14%	1,2459	0,0623
Total	26	12,2816	100,00%

Model summary-surface roughness
S	R-sq	R-sq (adj)	PRESS	R-sq (pred)
0,249594	89,86%	86,81%	2,27072	81,51%

Regression model and predicted response validation

According to the regression analysis presented in Table 7, the drill helix angle [°], cutting speed [Vc], and feed rate [f] exert varying degrees of influence on thrust force [F], delamination factor [Fd], and surface roughness [Ra]. However, direct comparisons of the regression coefficients may be misleading due to the different units and magnitudes of the variables. Therefore, to ensure accurate interpretation of the regression results, it is appropriate to evaluate them in conjunction with normalized criteria such as the Signal-to-Noise (S/N) ratios.

Table 7.

Regression equation for (1) thrust force, (2) delamination factor, and (3) surface roughness in the drilling of CF/PEEK composite laminates.

(1)	Regression equation-thrust force
Drill helix angle
20°	Thrust force [F][N]	=	28,19 + 1,703 cutting speed + 461,0 feed rate
30°	Thrust force [F][N]	=	10,91 + 1,703 cutting speed + 461,0 feed rate
40°	Thrust force [F][N]	=	9,01 + 1,703 cutting speed + 461,0 feed rate
(2)	Regression equation-delamination factor
Drill helix angle
20°	Delamination factor [Fd ][mm]	=	1,0759 + 0,003533 cutting speed + 0,078 feed rate
30°	Delamination factor [Fd ][mm]	=	1,0292 + 0,003533 cutting speed + 0,078 feed rate
40°	Delamination factor [Fd ][mm]	=	0,9858 + 0,003533 cutting speed + 0,078 feed rate
(3)	Regression equation-surface roughness
Drill helix angle
20°	Surfaces roughness [Ra ][µm]	=	−1,024 + 0,07542 cutting speed + 4,74 feed rate
30°	Surfaces roughness [Ra ][µm]	=	−0,880 + 0,07542 cutting speed + 4,74 feed rate
40°	Surfaces roughness [Ra ][µm]	=	−0,987 + 0,07542 cutting speed + 4,74 feed rate

As the helix angle increases from 20° to 40°, the constant term in the thrust force regression equation significantly decreases (20°, 28.19 N; 30°, 10.91 N; 40°, 9.01 N). This indicates that a 40° helix angle improves chip evacuation efficiency, thereby reducing cutting resistance and consequently the thrust force. In addition, S/N analysis reveals that cutting speed is the most influential parameter on thrust force (Delta: 3.33), highlighting the critical importance of selecting an appropriate cutting speed to control thrust forces effectively.

In the delamination factor regression equations, the constant term also decreases with increasing helix angle (20°, 1.0759; 30°, 1.0292; 40°, 0.9858), suggesting that higher helix angles reduce delamination damage. This outcome implies that drills with a 40° helix angle induce less exit damage in the laminate. These results are consistent with the S/N analysis, which identifies the helix angle as the most influential parameter for delamination (Delta: 0.6939).

In terms of surface roughness, both 20° and 40° helix angles yield similarly low initial Ra values (1.024 and 0.987, respectively), while a relatively higher value is observed at 30° (0.880). This trend indicates that surface quality does not follow a linear relationship with helix angle. Moreover, both regression and S/N analyses confirm that cutting speed is the most dominant factor affecting surface roughness (Delta: 10.8369), indicating that higher cutting speeds tend to degrade surface quality.

In summary, drills with a 40° helix angle, when used at low cutting and feed rates, provide the most favorable results in terms of minimizing thrust force, delamination, and surface roughness. These findings underscore the critical importance of jointly optimizing drill geometry and cutting parameters to achieve high-quality holes in CF/PEEK composite drilling applications.

Figure 16 demonstrates a strong correlation between the experimental data and the predicted values. The thrust force versus predicted thrust force plot (a) exhibits a linear relationship, while the delamination factor versus predicted delamination factor (b) and the surface roughness versus predicted surface roughness (c) plots reveal second-order polynomial relationships. These plots indicate high statistical validity of the models, with R² values calculated as 99.1%, 97.4%, and 94.8%, respectively, implying that a significant portion of the variance in the dependent variables is explained by the models. The low standard error (S) values indicate minimal prediction error, while the narrow bands of the 95% confidence interval (CI) and 95% prediction interval (PI) around the regression lines confirm the models’ strong internal and external validity.

Figure 16.

Modeling of parameter–response relationships using predictive regression analysis.

In conclusion, the regression models provide high accuracy and reliable confidence levels, demonstrating that the experimental data are statistically well-represented by the developed predictive models.

Conclusions

In this study, the effects of input parameters (helix angle, cutting speed, and feed rate) on output responses (thrust force, delamination damage, and surface roughness) during the drilling of thermoplastic CF/PEEK plates were experimentally and statistically analyzed.

On average, the drill with a 20° helical angle increased the thrust force by approximately 18% compared to the drill with a 30° helical angle, and by 13.3% compared to the drill with a 40° helical angle. The drill with a 30° helical angle, on the other hand, generated 4.3% higher thrust force compared to the 40° helical drill. The lowest thrust force was observed when using the 30° helical drill at a cutting speed of 15 m/min and a feed rate of 0.05 mm/rev.

Regarding delamination, all three drills exhibited similar levels of influence. According to the Signal-to-Noise (S/N) ratio graph, the minimum delamination occurred with the 40° helical drill at a cutting speed of 0.05 m/min and a feed rate of 0.075 mm/rev. The graph also indicated that delamination damage decreases as the helical angle increases. Conversely, an increase in cutting speed leads to greater delamination damage, while the effect of feed rate appears to be inconsistent.

In terms of surface roughness, the drill with a 30° helical angle produced surface roughness values that were 14% and 11% higher compared to the 20° and 40° helical angle drills, respectively. The lowest surface roughness value was obtained using the 20° helical drill at a cutting speed of 15 m/min and a feed rate of 0.05 mm/rev.

Based on the ANOVA analysis, the most influential parameters on thrust force were cutting speed (51.30%), feed rate (25.16%), and helical angle (16.43%). For delamination damage, the influential parameters were helical angle (47.04%), cutting speed (29.03%), and feed rate (0.22%). In terms of surface roughness, the percentage contributions of the parameters were: cutting speed (86.91%), feed rate (2.12%), and helical angle (0.83%).

In conclusion, the drilling behavior of thermoplastic composites differs significantly from that of other materials. Experimental findings revealed that each helix angle has a distinct effect on thrust force, delamination damage, and surface roughness. The lowest thrust force and surface roughness values were achieved using the 20° helix angle drill at a cutting speed of 15 m/min and a feed rate of 0.05 mm/rev. In contrast, the minimum delamination occurred with the 40° helix angle drill at a cutting speed of 15 m/min and a feed rate of 0.075 mm/rev. Therefore, drill geometry and cutting parameters should be carefully selected based on the specific output parameter to be optimized. This study provides practical recommendations for determining suitable drilling conditions in the machining of CF/PEEK composites. However, it is recommended that further experimental validation be conducted for different stacking sequences and laminate thicknesses.

Footnotes

Acknowledgements

We acknowledge the use of AI-based tools, Grok and ChatGPT, for assistance with language editing and translation during the preparation of this manuscript. These tools aided in enhancing the clarity and accuracy of the text; however, all content was thoroughly reviewed and validated by the authors.

Author contributions

All authors contributed to the investigation, conceptualization, and analysis of the information in this manuscript, and were involved in the writing process.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Ethical considerations

This article does not contain any studies with human or animal participants.

ORCID iDs

Necdet Yakut

Orhan Çakır

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Investigation of drilling cf/peek thermoplastic composites with varying helix angle drills and optimization of cutting parameters

Abstract

Keywords

Introduction

Experimental procedure

Composite specimens and cutting tools

Experimental setup

Results and Discussion

Drilling thrust force

Assessment of delamination damage

Surface roughness

Optimization processes

Analysis of Variance (ANOVA)

Regression model and predicted response validation

Conclusions

Footnotes

Acknowledgements

Author contributions

Declaration of conflicting interests

Funding

Ethical considerations

ORCID iDs

References