Effect of Drilling Parameters on Surface Roughness and Delamination of Ramie–Bamboo-Reinforced Natural Hybrid Composites

Abstract

Plastics reinforced with glass fiber have a significant likelihood of being replaced by natural fiber hybrid composites (NFHCs). Making holes helps in part assembly, which is a crucial activity in the machining of composite constructions. As a result, choosing the right drill bit and cutting parameters is crucial to creating a precise and high-quality hole in composite materials. The present study employs the Taguchi approach to examine the delamination behavior and hole quality of ramie–bamboo composite laminates consisting of epoxy and nano-fillers (SiC, Al₂O₃) with feed, spindle speed, and three distinct drill bit types. Surface roughness and delamination are significantly influenced by feed and spindle speed, as indicated by the results of the analysis of variance. It was found that the spindle speed had a major impact on the delamination factor and surface roughness, while the feed and drill bit type had a minor influence. The surface roughness (76.5%) and delamination factor (66.7%) are significantly affected by the spindle speed.

1. Introduction

Natural fiber composites (NFCs) are materials made by combining at least one key element derived from a natural source. Structures reinforced with natural fibers have demonstrated significant potential to substitute synthetic fiber composites like carbon fiber-reinforced plastics (CFRP). They are cheap, biodegradable, lightweight, and less expensive compared to synthetic fibers. Replacing synthetic fibers with NFCs will reduce carbon emissions and help in controlling global warming. Natural fibers are derived from leaves, animals, seeds, minerals, etc. [1]. Among all these, ramie [2] and bamboo [3] fibers show exceptional physical properties that make them suitable replacements for synthetic materials. The performance of NFCs can be enhanced by adding nanofillers into the resin. Some such nanofillers are aluminum oxide (Al₂O₃), silicon carbide (SiC) [4], walnut powder [5], titanium dioxide (TiO₂) [6], etc. Even after adding nanofillers, some factors like agglomeration might cause a reduction in the composite strength [7].

Drilling is a process of making precise holes in materials using tools that are rotating at high speeds. Holes are made to assemble components. The drilling of composites is a little different when compared to the drilling of metals as they are anisotropic and non-uniform. While machining composites, various factors such as delamination, surface roughness, and thrust force may affect the physical properties of the same [8,9]. Many researchers have been investigating different methods to reduce damage like fiber pullout and debonding caused by drilling and to maintain the structural integrity of NFCs. This damage can be observed using various imaging techniques, for instance, optical microscopes and x-ray-based imaging techniques [10]. While the drill bit is rotating at higher speeds, the area in contact may become uneven and lead to fiber pullout. This irregularity can be reduced by selecting the right factors like feed rate, spindle speed, drill type, drill diameter, etc. [11,12]. It was observed that with a larger diameter of the drill bit and at a higher feed rate, the delamination factor was minimal on various natural fiber composites made of epoxy resin [13]. Similar results were observed by Xu et al. [14] where feed rate became the dominant factor. It was concluded that a low feed rate resulted in reduced delamination. In another study [15], it was found that with a decrease in feed rate, there is a decrease in thrust force for all types of drill bits. This study was carried out on sisal-reinforced natural fiber composites with epoxy and polypropylene as matrices. The presence of irregularities like burrs is not only a sign of improper structural integrity but also increases machining cost as the irregularities need to be removed [16]. Molded holes showed better performance than drilled holes in the case of jute/epoxy composites. Variable results were observed using 4 mm and 8 mm drill bits [17]. This again proved that drill diameter is one of the critical factors.

It was found that the combination of low feed and high speed gave the best results for the parabolic drill bit, which minimized torque and thrust force [18]. During the drilling process, due to rough surface interactions and dissipation of friction energy in the form of heat, debonding within the composite may be caused. In particular, some NFCs may burn due to their low fire resistance. This causes degradation of physical properties and hence, wet drilling conditions are used [19]. After investigating the drilling behavior of hemp/glass hybrid composites, it was observed that plexi point and parabolic drills showed lower delamination size and low thrust force [20]. Recent studies on drilling glass–bamboo hybrid composites have revealed that utilizing a center drill with increased spindle speeds and reduced feed rates significantly minimizes delamination [21]. Similarly, studies on drilling ramie fiber woven composite showed that there is close correlation between thrust force value and delamination damage [22]. Experimental studies on natural fiber-based composites showed that there is a direct correlation between delamination factor and feed rate [23,24].

From the above literature, it can be concluded that drilling-induced damage in fiber-reinforced composites, such as delamination and burr formation, can be significantly reduced by optimizing machining parameters and tool geometry. Proper selection of these factors is crucial in minimizing defects and improving the quality of the drilled holes. By carefully adjusting these variables, CFRP components’ structural integrity and performance can be enhanced. Therefore, this study investigates the impact of cutting parameters such as feed rate, spindle speed, and the type of drill bit on the drilling performance of woven ramie–bamboo hybrid natural fiber composites. The primary output variables analyzed in this research are the delamination factor and surface roughness. The Taguchi technique is used to develop a design of experiments with different parameters.

2. Materials and Methods

Composites were manufactured using woven ramie, bamboo fibers as reinforcements, LY-556 epoxy as resin (Diglycidyl ether of bisphenol-A), and HY-991 as the polymer matrix. To check fiber strength, single-fiber tensile tests for both ramie and bamboo were conducted at Konkan Specialty Poly Products Pvt Ltd., in Mangalore, the results of which are included in

Table 1. General properties of fiber mats [25,26].

Material	Tensile Strength (MPa)	Young’s Modulus (GPa)	Density (g/cm3)	Diameter (mm)	Cellulose (%)	Lignin (%)	Type
Ramie	180–210	1.3	1.5	0.28	68.6	0.6	Woven
Bamboo	180	14.2	0.3–0.5	0.34	73.8	10	Woven

, along with the other physical properties of the materials used in Table 1 and

Table 2. Properties of materials used in laminates [27].

Material	Density (g/cm3)	Size (nm)	Color
Epoxy Resin	1.2	-	Transparent
Al2O3	3.9	20–30	White
SiC	3.2	50	Grey

. Five composites were made with different fiber stacking sequences namely TS1, TS2, TS3, TS4, and TS5, the compositions of which are given in

Table 3. Composition of laminates.

Test Specimen Configuration	Laminate Configuration	Fiber (wt%)	Epoxy (wt%)	Nano Filler (wt%)
TS1	RRRRRRRR	30	70	-
TS2	BBBBBBBB	30	70	-
TS3	RBRBBRBR	30	70	-
TS4	RBRBBRBR + Al2O3	30	68	2
TS5	RBRBBRBR + SiC	30	68	2

R: Ramie; B: Bamboo.

. Each composite specimen consisted of eight layers stacked using the hand layup technique with a thickness of 5 ± 0.5 mm and underwent compression molding for 24 h. The TS1 composite consists of only ramie fibers alternatively stacked in perpendicular directions while TS2 composite consists of only bamboo fibers stacked in a similar manner. TS3 composite consists of ramie and bamboo fibers alternatively stacked in perpendicular directions. TS4 and TS5 composites are composed of nano fillers, namely Al₂O₃ and SiC, respectively added to ramie and bamboo fibers alternatively stacked in perpendicular directions. Drilling was performed using a computer numerical control vertical machining center for all composites, with a maximum spindle speed of 6000 rpm. All the drilling experiments were carried out under dry conditions.

The specimen was fixed steadily with a plate on the top which is illustrated in Figure 1. For the drilling of ramie–bamboo-reinforced hybrid composites, three different types of drill bits were used, which are: twist drill, step drill, and core drill. All the drill bits are of a constant diameter of 8 mm and are shown in Figure 2. The machining parameters used for drilling are shown in

Table 4. Machining parameters for drilling.

Sl. No.		Parameters
	Spindle Speed (rpm)	Feed (mm rev−1)	Drill Type
1	1500	0.01	Twist
2	3500	0.02	Step
3	5500	0.03	Core

. These parameters were used to develop a Taguchi design in Minitab V15 software. The L27 orthogonal array for delamination factor and surface roughness are shown in

Table 5. L27 orthogonal array for delamination factor.

Spindle Speed (rpm)	Feed (mm rev−1)	Drill Type	TS1	TS2	TS3	TS4	TS5
1500	0.01	Twist	1.173	1.564	1.312	1.384	1.534
1500	0.01	Twist	1.215	1.334	1.265	1.325	1.524
1500	0.01	Twist	1.164	1.367	1.298	1.497	1.597
1500	0.02	Step	1.274	1.514	1.343	1.398	1.454
1500	0.02	Step	1.306	1.437	1.365	1.464	1.398
1500	0.02	Step	1.401	1.370	1.298	1.347	1.354
1500	0.03	Core	1.474	1.217	1.556	1.520	1.435
1500	0.03	Core	1.456	1.500	1.465	1.365	1.354
1500	0.03	Core	1.465	1.410	1.508	1.424	1.414
3500	0.01	Step	1.253	1.245	1.165	1.268	1.217
3500	0.01	Step	1.302	1.267	1.205	1.245	1.204
3500	0.01	Step	1.268	1.234	1.254	1.154	1.165
3500	0.02	Core	1.345	1.497	1.240	1.325	1.198
3500	0.02	Core	1.348	1.504	1.345	1.268	1.335
3500	0.02	Core	1.388	1.540	1.404	1.324	1.354
3500	0.03	Twist	1.409	1.400	1.380	1.405	1.314
3500	0.03	Twist	1.485	1.350	1.400	1.385	1.334
3500	0.03	Twist	1.412	1.427	1.354	1.304	1.375
5500	0.01	Core	1.154	1.110	1.170	1.137	1.145
5500	0.01	Core	1.163	1.190	1.114	1.178	1.198
5500	0.01	Core	1.168	1.1500	1.190	1.194	1.214
5500	0.02	Twist	1.203	1.245	1.135	1.197	1.235
5500	0.02	Twist	1.263	1.226	1.165	1.267	1.227
5500	0.02	Twist	1.308	1.260	1.184	1.204	1.287
5500	0.03	Step	1.265	1.324	1.274	1.298	1.254
5500	0.03	Step	1.303	1.298	1.285	1.247	1.264
5500	0.03	Step	1.408	1.356	1.298	1.245	1.214

and

Table 6. L27 orthogonal array for surface roughness (µm).

Spindle Speed (rpm)	Feed (mm rev−1)	Drill Type	TS1	TS2	TS3	TS4	TS5
1500	0.01	Twist	3.85	3.55	3.42	3.41	3.82
1500	0.01	Twist	3.59	3.76	3.51	3.32	3.6
1500	0.01	Twist	3.31	3.49	3.19	3.63	3.14
1500	0.02	Step	3.11	2.78	2.84	3.94	3.63
1500	0.02	Step	3.28	2.97	2.68	3.51	3.84
1500	0.02	Step	2.91	2.57	2.46	3.8	3.75
1500	0.03	Core	3.76	3.31	3.47	4.42	3.57
1500	0.03	Core	3.98	3.42	3.63	4.25	3.77
1500	0.03	Core	4.19	3.54	4.63	3.84	3.95
3500	0.01	Step	2.68	2.58	2.24	3.1	2.91
3500	0.01	Step	2.48	2.19	2.13	2.79	2.85
3500	0.01	Step	2.82	2.43	2.47	3.03	2.72
3500	0.02	Core	3.56	3.16	3.64	3.62	3.89
3500	0.02	Core	3.79	3.24	3.77	3.71	4.05
3500	0.02	Core	3.92	3.36	3.88	3.97	3.95
3500	0.03	Twist	3.18	3.12	3.46	3.42	3.66
3500	0.03	Twist	3.24	3.29	3.53	3.61	3.42
3500	0.03	Twist	3.41	3.39	3.62	3.55	3.12
5500	0.01	Core	1.21	1.12	1.23	1.17	1.19
5500	0.01	Core	1.24	1.28	1.26	1.23	1.29
5500	0.01	Core	1.35	1.35	1.34	1.31	1.35
5500	0.02	Twist	1.51	1.41	1.45	1.54	1.56
5500	0.02	Twist	1.62	1.52	1.56	1.67	1.63
5500	0.02	Twist	1.73	1.64	1.67	1.72	1.76
5500	0.03	Step	2.27	2.17	2.21	2.33	2.31
5500	0.03	Step	2.36	2.27	2.32	2.43	2.42
5500	0.03	Step	2.58	2.49	2.53	2.66	2.57

respectively. The composite strips were cut into 250 × 25 mm using an abrasive water jet cutting machine. A total of 135 holes were drilled, with 27 holes on each composite. The measured output variables were the delamination and surface roughness. To calculate the delamination factor, Equation (1) was used [28].

$$\mathrm{Delamination}\mathrm{factor}=D_{max}/D_{nom}$$

(1)

where $D_{max}$ is the maximum diameter (mm), $D_{nom}$ is the nominal hole diameter (mm).

The image of the drilled holes was captured and then imported into ImageJ V 1.8.0 software and output variables were measured by setting the right threshold frequency. Main effect plots and optimization plots were obtained using Minitab V15 software.

Nikon Stereo Zoom microscope (SMZ74T series) as shown in Figure 3 was used to obtain the magnified image of the damage caused by drilling. The surface roughness of the hole was measured using a Mitutoyo SJ-210 contact surface profilometer, featuring a 2.4 mm traversing length and a 0.8 mm cutoff value. The average roughness was determined from four measurements taken at different positions parallel to the hole’s axis. Figure 4 illustrates the surface roughness measuring instrument.

3. Results and Discussion

3.1. Surface Roughness Analysis

Surface roughness is an important factor in predicting the quality of the hole. It depends on several factors like cutting speed, feed rate, drill type, point angle, etc. The Main Effects Plot for Means (Figure 5 and Figure 6) sheds important light on the effects of spindle speed, feed rate, and drill type in the examination of surface roughness during drilling. A decrease in the mean surface roughness is observed as the spindle speed is increased from 1500 to 5500 rpm. This is due to built-up edge (BUE) which forms at low speeds and disappears [29], and roughness reduces, demonstrating the advantageous effect of higher speeds on producing smoother finishes. On the other hand, different feed rates show that lower values, especially at 0.01 mm/rev, significantly reduce surface roughness, highlighting the critical role that feed rate plays in maximizing surface quality [30]. Regarding drill bit performance, the step drill bit consistently shows the lowest mean roughness when compared to twist and core drill bits.

In the case of bamboo composite (TS2), lowest surface roughness was noted at a spindle speed of 5500 rpm and a feed rate of 0.01 mm/rev. On the other hand, for hybrid composite (TS3), maximum surface roughness was obtained at a spindle speed of 1500 rpm and a feed rate of 0.03 mm/rev. Though both maximum and minimum were seen in the case of a core drill bit, the step drill showed noticeable results in this drilling process.

In the current experiment, the goal was to reduce surface roughness and delamination factor; the “smaller is better” setting was used [31]. The S/N ratio is given by Equation (2).

$${\displaystyle \frac{S}{N}}=-10\mathrm{l}\mathrm{o}\mathrm{g}[{\displaystyle \frac{1}{n}}\sum _{i=1}^n{y_i}^2]$$

(2)

where, y = responses for the given factor level combination and n = number of responses. Generally, the greater the S/N ratio, the better it is. According to the main effects plot for S/N ratios, a higher spindle speed of 5500 rpm leads to better results in the surface finish. The lower feed rates of 0.01 mm/rev and core drills are the most important conditions for getting a smoother surface finish. This is because the S/N ratio, which normally exhibits lower values for rougher surfaces, clearly favors step drills over twist and core drills when the feed rate increases. A minor rise in the S/N ratio suggests that there may be less roughness at higher speeds (5500 rpm), but the effect seems less substantial.

From

Table 7. Response table for S/N ratios for surface roughness.

Level	Spindle Speed (rpm)	Feed (mm rev−1)	Drill Type
1	−10.957	−7.112	−8.550
2	−10.159	−8.566	−8.753
3	−4.558	−9.991	−8.370
Delta	6.399	2.885	0.383
Rank	1	2	3

, one can observe that the spindle speed has more effect on the surface roughness in this experiment and is ranked 1, followed by feed and drill type. A step drill consists of two different diameters from the start to end; in this case, the primary drill is 4 mm and the secondary drill is 8 mm in diameter. The primary one has a low chisel edge and makes a pilot hole initially, and then the secondary drill enters the already drilled hole, reducing workpiece interaction and helping in heat dissipation. Hence, the damage is reduced, which has been proven in several studies [32,33].

S/N ratios are at three different levels for drill type, feed rate, and spindle speed. The difference between the maximum and minimum S/N ratios for each component is represented by the delta values, which are as follows: spindle speed has the highest delta value (6.399), suggesting a significant effect on the response, followed by feed rate (Delta = 2.885), while drill type has the least impact (Delta = 0.383). The ranking demonstrates that spindle speed is the most important factor followed by feed rate and drill type.

The ANOVA results, which show how much each factor contributed to the variation in the results, are shown in

Table 8. Analysis of variance for S/N ratios for surface roughness.

Source	DF	Seq SS	Adj SS	Adj MS	F	p Value
Spindle speed (rpm)	2	72.9587	72.9587	36.4793	7.87	0.113
Feed (mm rev−1)	2	12.4839	12.4839	6.2420	1.35	0.426
Drill type	2	0.2203	0.2203	0.1102	0.02	0.977
Residual Error	2	9.2723	9.2723	4.6361
Total	8	94.9352

. The overall variability explained by each factor is shown by the adjusted sum of squares (Adj SS) and the sequential sum of squares (Seq SS). The spindle speed appears to account for the most variation, as seen by its highest Seq SS (72.9587) and Adj SS (72.9587). Figure 7 shows the effect of spindle speed and feed rate on surface roughness. They illustrate that higher spindle speed and lower feed rate result in decreased surface roughness.

3.2. Delamination Analysis

During the drilling of composites, delamination typically occurs at the entry and exit points of the drill. The entry delamination is caused by the initial contact of the drill bit with the composite surface, while exit delamination occurs as the drill bit exits the material. Both types of delamination can lead to an increase in the diameter of the hole and potential weakening of the composite structure [34]. In this study, exit delamination was measured and the results are shown in Figure 8a,b. The main effects plots are shown in Figure 9 and Figure 10. The spindle speed, feed rate, and drill type have a major impact on the quality of drilled holes in natural fiber hybrid composites according to the main effects plots for delamination. By increasing the spindle speed from 1500 rpm to 5500 rpm, the mean delamination factor decreased significantly from around 1.45 to 1.20. Higher spindle speeds may reduce cutting pressures and heat output, which could protect the composite material from damage, according to this trend. On the other hand, when the feed rate increased from 0.01 mm/rev to 0.03 mm/rev, the mean delamination factor increased from around 1.275 to 1.325. At higher feeds, inter-layer crack propagation increases, promoting more delamination [35]. This suggests that higher feed rates cause more forces and vibrations, which in turn causes greater delamination.

In terms of drill type, the step drill outperforms twist and core drills. The twist and core drills have mean delamination factors of approximately 1.32, while the step drill has a lower mean delamination factor, suggesting that it is more effective in reducing delamination. This can be explained by the geometry of the step drill, which probably generates less stress. The S/N ratios also support these conclusions, demonstrating better performance at higher spindle speeds and lower feed rates, and identifying the step drill as the more advantageous option.

Minimum delamination factor of 1.11 was observed in bamboo composite (TS2) at a speed of 5500 rpm and feed rate of 0.01 mm/rev, which also aligns with the minimum value of surface roughness. The maximum delamination factor of 1.597 was in Al₂O₃ composite (TS5) at 1500 rpm spindle speed and 0.01 mm/rev feed. This result aligns with various studies [36,37].

S/N ratios at various spindle speeds, feed rates, and drill type levels are shown in

Table 9. Response table for S/N ratios for delamination factor.

Level	Spindle Speed (rpm)	Feed (mm rev−1)	Drill Type
1	−3.042	−1.970	−2.460
2	−2.438	−2.478	−2.321
3	−1.774	−2.807	−2.473
Delta	1.268	0.837	0.152
Rank	1	2	3

, which is the S/N ratio response table. The range of S/N ratios for each factor throughout the levels is represented by the Delta numbers. With the largest Delta (1.268), spindle speed is the factor that affects S/N ratios the most, suggesting its importance. The drill type has the least effect (Delta = 0.152), followed by the feed rate (Delta = 0.837). The spindle speed is confirmed as the most important factor based on the ranking depending on the Delta values, followed by the feed rate and drill type. Quantifying each factor’s statistical significance is demonstrated in

Table 10. Analysis of variance for S/N ratios for delamination factor.

Source	Df	Seq SS	Adj SS	Adj MS	F	p Value
Spindle speed	2	2.41203	2.41203	1.20602	25.19	0.038
Feed	2	1.06569	1.06569	0.53284	11.13	0.082
Drill type	2	0.04247	0.04247	0.02123	0.44	0.693
Residual error	2	0.09574	0.09574	0.04787
Total	8	3.61593

. The variance attributable to each factor is given by the sequential sum of squares (Seq SS) and adjusted sum of squares (Adj SS). The highest Seq SS (2.41203) and Adj SS (2.41203) are found for spindle speed, indicating that it is mainly responsible for the variation in the S/N ratios. Figure 11 shows the interaction between spindle speed and feed rate in the case of delamination.

4. Conclusions

In this study, the delamination and surface roughness of five different ramie–bamboo-based natural fiber composite laminates incorporating epoxy and/or nano-fillers (SiC, Al₂O₃) for different machining parameters such as feed rate, spindle speed, and drill bit types were investigated. The following are the conclusions drawn from the current experimental study.

• Surface roughness is mostly affected by spindle speed, followed by feed rate and drill type. Low chisel edge and progressive multi-step cutting action of step drill bits decrease burr formation, all of which contribute to smoother surfaces.
• It is observed that as spindle speed increases, the delamination decreases and also step drills cause less delamination than twist and core drills among drill types.
• Step drill bits are ideal for natural fiber hybrid composites due to their softness and non-abrasiveness, which result in less tool wear and smoother finishes. Synthetic composites, being abrasive, cause rapid tool wear, making them less suitable for step drill bits.
• From observations, to achieve minimum delamination size and good surface roughness when drilling ramie–bamboo-based hybrid laminates, the optimal combination is using a step drill bit, a spindle speed of 5500 rpm, and a feed rate of 0.01 mm/rev.