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Abstract

Drilling is a secondary material removal and usually carried out to facilitate fastening of parts together. Drilling of composite materials is not usually a problem-free process. Issues related to delamination composite laminates need to be addressed because it introduces the stress concentration point on the composite. This study focussed on the influence of process parameters such as spindle speed, feed rate, type of drill bits and geometry on the extend of delamination experienced by the composite during the drilling process of kenaf-glass fibre-reinforced unsaturated polyester composite, and the delamination measurements were taken under a microscope. Taguchi methods and analysis of variance were employed to find the optimal parameters. From the results, the most significant parameter was the feed rate. The minimum delamination was achieved when the feed rate was 0.05 mm/rev and spindle speed was 700r/min using both types of drill bits. The quality of the drill hole using the twist drill bit has been proven to be better than the brad drill bit.

Keywords

Delamination machining parameters hybrid taguchi analysis analysis of variance

Introduction

Due to both considerations of developing an environmentally friendly material and reducing the dependency on synthetic fibres, natural fibre reinforced composites have raised large interests among material researchers, in recent years.¹ To develop composite materials from natural fibres, plant-derived fibres are combined with polymers to manufacture eco-friendly materials.² Material scientists have triggered big interest in exploring nonpetroleum fibres resources³ to obtain nonabrasive materials with minimum energy consumption. Added to that, using natural fibres will generate rural/agriculture-based economy income. Furthermore, because of their significant other characteristics such as low density and cost, availability, biodegradability, as well as good mechanical properties, that encouraged designers to use them in the aerospace and automotive industries.⁴ Car manufacturers have been interested in combining natural fibre composites in both internal and external parts. This presents a number of goals for the companies to reduce the overall weight of vehicles, to improve fuel efficiency and to improve the sustainability of their manufacturing process.⁵ Compared with milling, turning, etc., the drilling process is largely used in composite materials and it is not usually problem-free, as similar as problems of holes in metals,⁶ which affects composites’ integrity and surface quality, with particular reference to delamination damage generation. The technological method of composite materials drilling differs from metal drilling when connecting assemblies with screws, rivets, and bolts; this one has several tool-specific characteristics. Therewith, the composite material drilling is accompanied by the apparition of some particular defects that appears at the entrance of the hole, at the surface of the hole and at the hole issue. In addition, delamination is a major defect of the composites materials drilling processes due to the stress concentration point on the composite.⁷ Generally, studying the drilling effects and methods have significant influences on the composite properties and performances,⁸ such as the hole’s quality (very special criteria in the case of the composite process), the hole’s accuracy, damage of surface layers and drill life.

Natural fibre reinforced unsaturated polyester composites have been proven suitable for a variety of purposes. Natural fibres contents have a critical impact on the strength characteristics of the composite where the composite strength gets generated linearly with the increasing fibers content. Various types of natural fibres give several effects to the composite behaviour, and some natural fibres could affect oppositely to the composite strength properties.⁹ Due to the great attributes and ecological attention, kenaf fibre-reinforced composites investigation has increased highly. Generally, the use of kenaf fibre-reinforced composites could help to generate employment in urban and rural areas while helping to decrease waste and contribute to the healthier environment.¹⁰ Kenaf fibres are found in many engineering applications because of their low cost and biodegradability as well as a high percentage of cellulose. The aim of this study is to evaluate the usage of kenaf fibres for hybrid laminated with glass composites and suitable kenaf fibres content as alternatives based on certain criteria. Kenaf-glass fibre-reinforced unsaturated polyester composite has attracted researchers in investigating mechanical properties based on previous studies.¹¹

In the last years, due to the requirements of the material industry, there are some new challenges to build the development of products such as machining techniques. It requires the need to better understand the process of cutting with precision and efficiency. Although the near net shape process has progressed with much attention, more complex products need to be machined twice to get the required accuracy. Delamination was caused in both the entrance and exit plane pieces of work. This delamination may be associated with the core drill during entry and exit. While drilling holes in composite materials, delamination occurred. Several attempts have been made to understand the causes of delamination base on the input response and response variables such as point angle, spindle speed, feed rate, thrust force and torque. Drill geometry also will affect delamination. In industries, drilling is frequently used because of the need for component assembly in mechanical structures.¹² The drilling operation (using twist drill) is the most generally applied technique to create holes.¹³ To investigate the influence of input variables which include spindle speed, feed rate, tool type, tool material and tool geometry on output variables, a lot of experiments have been carried out.¹⁴ Delamination is a major concern during the drilling operation of the laminated composites because it reduces the laminates’ structural integrity. Added to that, it causes long-term performance deterioration such as bad assembly tolerance.¹⁵

Composite laminates have been considered as difficult for machining that can bring low efficiency of drilling causing undesirable delamination. Therefore, it is necessary to improve the economic existing process of drilling and develop a more innovative drilling process for composite laminates. Improvement and development of the drilling process will be a benefit from a comprehensive literature review on drilling composite laminates. This study focussed on the influence of the most significant drilling parameters such as spindle speed, feed rate, type of drill bits and geometry on the extend of delamination experienced by the composite during the drilling process. Furthermore, to investigate the drilling parameters that affect whole composites' performance, life and efficiency, and give an overview of the main challenges related to the drilling of fibre -reinforced plastic composite materials. These experiments aim to study the effect of machining parameters on delamination in drilling kenaf-glass fibre reinforced unsaturated polyester (KGFRP) composites, glass fibre-reinforced unsaturated polyester (GFRP) composites, and kenaf fibre-reinforced unsaturated polyester (KFRP) composites, and to compare drilling quality between two types of drill bits. It shows that from the previous researches,¹⁶ there are a few experiments with natural fiber-composite and not specifically with KGFRP composite to conduct the drilling experiment with different fibre content. The Taguchi method has been used to optimize and calculate the signal noise to ratio (SNRA) and to find the optimum value of parameters. The ANOVA (analysis of variance) method also has been used to find the most significant values.

Materials and Methods

Materials

There are four materials used to fabricate KGFRP which are kenaf fiber, glass fiber (E-Glass), unsaturated polyester and hardener. The unsaturated polyester used in this experiment is unsaturated polyester asapes 9509 with MEKP as the hardener for reinforcement. A mould is fabricated with a dimension of 300 mm × 200 mm × 6 mm. The KGFRP composite was fabricated using the hand lay-up technique. The kenaf-glass fiber ratio was 10:90, with unsaturated polyester resin, with keeping a percentage of fiber to polymer 40:60. The other two fabricated materials were KFRP and GFRP composite which also have been made by this method. Figure 1 shows the process of preparing the specimen. The most commonly used technique for the fabrication of fibre-reinforced composites is hand lay-up; the specimen is fabricated in stacked layers, and each layer is oriented to achieve maximum utilization of its properties. Resin is impregnated by hand into the fibre, usually accomplished by rollers, to force the resin into the fabric through rotating rollers. Composite laminates are then allowed to cure under normal atmospheric conditions and dry under for more than 7 days to reach the solid point of the composite. There are several processing steps for the hand lay-up technique which are mould preparation, gel coating, hand lay-up and then finishing. Mould is fabricated with a square profile using a 1 mm sheet plate with a dimension of 300 mm × 200 mm × 6 mm.

Figure 1.

Preparing the specimen.

Cutting operation that was used, a drill bit, to cut a circular hole across the section in the solid material is called drilling. There are many processes used for drilling GFRP such as conventional drilling, drilling assisted by vibrations and ultrasonic-assisted drilling to obtain the required accuracy and maintain the integrity of the material. The cutting edge has a large impact on the machining process.¹⁷ Computer numerical control (CNC) milling machine has been selected to minimize the limitation which makes it able to do repeated work with high accuracy and high precision.¹⁸ The CNC machine programme or G-code needs to be built first before drilling the composites. Two various drill bit types (uncoated HSS twist and uncoated HSS brad) with a 6 mm diameter were used to drill the holes. Machining parameters were selected for spindle speeds of 700 r/min, 1400 r/min and 2100 r/min. Feed rates were set at 0.05 mm/rev, 0.12 mm/rev and 0.20 mm/rev. Before the drilling process begins, the fabricated composite had been cut into the specimen size dimension of 140 mm × 60 mm × 6 mm as illustrated in Figure 2. Based on the 27th Edition Machinery Handbook on the topic limits and fits, clearance fits and limits for cylindrical parts for the hole or shaft apply in American and British standards. Referring to American National Standard Preferred Hole Basis Metric Clearance Fits (ANSI B4.2 1978 (R1999)), the basic size for no-go pin gauge used is 6 mm H7 (hole) with a maximum tolerance of 6.012 mm and a minimum tolerance of 6.000 mm. The most common test used to measure hole diameter is using no-go pin gauge test. The use of this fixed limit gauge is the simplest form of pass/fail attribute inspection. If the go fits and the no-go not, the hole is considered intolerant and as passed. When inspecting a cylindrical hole, if the go does not enter, the hole is too small (undersize) and fails. If the no-go enters, the hole is considered too large (oversize) and also fails.

Figure 2.

Cutting and drilling process.

The selected machining parameters for this research were the spindle speed and the feed rate. This experiment also used two types of drill bits to compare drilling quality on specimens. All data are collected and have been analysed according to the results of the delamination factor and comparing two types of drill bits. Table 1 shows all the values of the parameters.

Table 1.

Selected spindle speed and feed rate.

Parameter	Unit	Level 1	Level 2	Level 3
Spindle speed	r/min	700	1400	2100
Feed rate	mm/rev	0.05	0.12	0.20

The drilled holes on the specimen were inspected and measured by the tool maker microscope with Motic Image Plus software. Figure 3 below illustrates inspection through the microscope, and Fd (the delamination factor) is calculated as a measure of delamination damage

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

(1)

where Dmax is the maximum diameter of delamination and D is the diameter of the drill.

Figure 3.

Inspection of delamination using a microscope.

The two different types of drill bits, as shown in Figure 4, used are twist drill and brad drill. Both drills are 6 mm diameters. Brad drill has a specific point geometry causing the fibres tensioning prior to cutting thus enabling a ‘clean-cut’ of the fibres. As a consequence, machined surfaces are smoother. The helix profile was maintained in order to compare the cutting performance of the drill with the twist or brad drill. The drilled hole’s quality is determined by using no-go pin gauge (6 mm) with 6.012 mm maximum tolerance and 6.000 mm minimum tolerance. This is to measure the hole oversize in terms of drilled hole quality.

Figure 4.

Types of drill bits: (a) Twist drill bit and (b) brad drill it.

After all experiments have been done, Taguchi analysis (for three parameters (material, spindle speed and feed rate and three levels)) is used to analyse orthogonal arrays experiments which give much-reduced variance for the experiment with optimum settings of control parameters.

From the results of the experiment, SNRA values were calculated for each level/factor. The most suitable SNRA was calculated using the condition ‘the smaller the better’

SNRA (η) = - 10 \log_{10} \frac{1}{n} \sum_{i = 1}^{n} y_{i}^{2}

(2)

where

y_{i}

= observed value and

η

= number of replications.

Consequently, ANOVA was conducted to obtain the factor that has the greatest influence in machining parameters.

Results and Discussion

After drilling experiments were performed, it was necessary to assess the damage inspection of machining damage, using the microscope and image analysis software to measure the extent delamination size, as shown in Figures 5 and 6.

Figure 5.

Measurement specimen for the twist drill bit.

Figure 6.

Measurement specimen for the brad drill bit.

The maximum diameter, Dmax, was measured. The results for Dmax (entry and exit) are tabulated in Table 2 for the twist drill bit. It shows the process parameters that are feed rate and spindle speed. The types of drill bit used also give an effect on the maximum delamination. The maximum diameter is created due to delamination around the hole. The higher the value for Dmax, the more the delamination at the drill hole. Based on Table 2 (twist drill bit), the highest value for Dmax (entry) is from experiment number 4 (8.142 mm) with feed rate 0.05 mm/rev and spindle speed 1400 r/min followed by experiment number 6 (8.134 mm) with feed rate 0.2 mm/rev and spindle speed 1400 r/min and the third is experiment number 5 (7.643 mm) with feed rate 0.12 mm/rev and spindle speed 1400 r/min. The lowest value of the maximum diameter is from experiment number 13 (6.032 mm) with 0.05 mm/rev feed rate and 1400 r/min spindle speed. The highest Dmax for exit (twist drill bit) is from experiment number 9 (11.107 mm) with feed rate 0.2 mm/rev and spindle speed 2100 r/min followed by experiment number 21 (10.921 mm) with feed rate 0.2 mm/rev and spindle speed 700 r/min and the third is experiment number 26 (10.905 mm) with feed rate 0.12 mm/rev and spindle speed 2100 r/min. The lowest value of the maximum diameter is from experiment number 10 (6.072 mm) with 0.05 mm/rev feed rate and 700 r/min spindle speed.

Table 2.

D_max value (entry and exit) for the twist drill bit.

Experiment No.	Material	Spindle speed (r/min)	Feed rate (mm/rev)	D_max (entry)	D_max (exit)
1	GFRP	700	0.05	6.387	9.270
2			0.12	6.484	9.850
3			0.20	6.806	10.051
4		1400	0.05	8.142	8.392
5			0.12	7.643	10.720
6			0.20	8.134	10.608
7		2100	0.05	6.734	9.907
8			0.12	7.200	10.656
9			0.20	6.967	11.107
10	KFRP	700	0.05	6.250	6.072
11			0.12	7.103	6.105
12			0.20	6.862	6.072
13		1400	0.05	6.032	6.757
14			0.12	6.097	8.263
15			0.20	6.048	7.643
16`		2100	0.05	6.040	6.685
17			0.12	6.089	8.610
18			0.20	6.403	9.238
19	KGFRP	700	0.05	6.129	8.626
20			0.12	6.347	9.890
21			0.20	6.749	10.921
22		1400	0.05	6.524	9.705
23			0.12	6.138	10.204
24			0.20	6.282	10.430
25		2100	0.05	6.073	9.898
26			0.12	6.194	10.905
27			0.20	6.170	10.438

GFRP: Glass fiber reinforced unsaturated polyester, KFRP: Kenaf fiber reinforced unsaturated polyester, KGFRP: Kenaf glass fiber reinforced unsaturated polyester.

The results for D_max (entry and exit) are tabulated in Table 3 for the brad drill bit. The highest value for Dmax (entry) is from experiment number 6 (11.557 mm) with feed rate 0.20 mm/rev and spindle speed 1400 r/min followed by experiment number 9 (11.356 mm) with feed rate 0.2 mm/rev and spindle speed 2100 r/min and the third is experiment number 8 (11.203 mm) with feed rate 0.12 mm/rev and spindle speed 2100 r/min. The lowest value of maximum diameter is from experiment number 19 (6.564 mm) with 0.05 mm/rev feed rate and 700 r/min spindle speed.

Table 3.

D_max value (entry and exit) for the twist drill bit.

Experiment No	Material	Spindle speed (r/min)	Feed rate (mm/rev)	D_max (entry)	D_max (exit)
1	GFRP	700	0.05	8.054	8.360
2			0.12	9.866	17.155
3			0.20	8.465	19.539
4		1400	0.05	7.136	10.848
5			0.12	8.151	15.117
6			0.20	11.557	22.028
7		2100	0.05	8.481	13.338
8			0.12	11.203	18.564
9			0.20	11.356	19.789
10	KFRP	700	0.05	7.338	7.410
11			0.12	7.619	19.040
12			0.20	7.555	20.336
13		1400	0.05	7.184	12.266
14			0.12	7.200	18.098
15			0.20	7.362	18.726
16		2100	0.05	6.943	15.681
17			0.12	7.354	19.974
18			0.20	7.039	24.033
19	KGFRP	700	0.05	6.564	11.260
20			0.12	7.072	18.476
21			0.20	7.241	19.410
22		1400	0.05	6.693	13.611
23			0.12	6.790	21.134
24			0.20	6.927	23.140
25		2100	0.05	6.765	18.274
26			0.12	6.766	23.687
27			0.20	7.023	24.138

GFRP: Glass fiber reinforced unsaturated polyester, KFRP: Kenaf fiber reinforced unsaturated polyester, KGFRP: Kenaf glass fiber reinforced unsaturated polyester.

The highest Dmax for exit (brad drill bit) is from experiment number 27 (24.138 mm) with feed rate 0.20 mm/rev and spindle speed 2100 r/min followed by experiment number 18 (24.033 mm) with feed rate 0.20 mm/rev and spindle speed 2100 r/min and the third is experiment number 26 (23.687 mm) with feed rate 0.12 mm/rev and spindle speed 2100 r/min. The lowest value of the maximum diameter is from experiment number 10 (7.410 mm) with 0.05 mm/rev feed rate and 700 r/min spindle speed.

Delamination Factor

Both peel-up and push-down delamination occurred at the entrance plane of the workpiece. At the beginning of the contact, cutting force acting in the perpendicular direction is the agent of delamination. It produces a peeling force in the axial direction through the drill flute that tends to separate the entry laminas thus forming a delamination zone. Push-down delamination occurs at the exit of the material as the drill approaches it. When the drill reaches the end, the thickness of the uncut chip is smaller and reduces the resistance to deformation. At one point, the load exceeds the interlaminar bond strength and delamination occurs. The damage occurs at the entry and exit during the drilling (GFRP, KFRP and KGFRP). The Fd is calculated by using equation (1) after measuring D_max (the maximum diameter in the damage around each hole). The computed results at the entry and exit specimen from the conducted experiments are presented in Table 4 for using the twist drill bit and Table 5 for the brad drill bit. The results denote that with an increase indrill tool diameter, the delamination factor increased. On the contrary, the increase in spindle speed decreases the delamination factor Fd_u in drilling processes. It could be noticed that drill tool diameter has a great effect on the drilling of composites. An increase in drill tool diameter increases the load on the tool as well as contact of the workpiece and tool, which leads to delamination increment. In addition, as the spindle speed increases, the delamination factor decreases. At the drilling site, the matrix is softening due to heat generated by tool friction resulting in easier removal of the matrix. These findings are in agreement with Thongkaew et al.’s investigation.¹⁵ Noticeably, when the feed rate increasing, caused increased in delamination factor. This is due to the contact between the workpiece hole and load on the tool, which in turn increases the delamination factor. The minimum delamination factor for GFRP (1.064) appeared on feed rate 0.05 mm/rev under spindle speed 700 r/min using twist drill bit at entry delamination. The maximum delamination factor of GFRP is 3.298 spotted at feed rate 0.20 mm/rev under spindle speed 2100 r/min using brad drill bit at exit delamination. There is a sudden increase in the results when drilling using the brad drill bit. For the kenaf fiber composite (KFRP), the minimum delamination factor is 1.005 at 0.05 mm/rev under spindle speed 1400 r/min using the twist drill bit at entry delamination. The maximum delamination factor of KFRP (4.006) was detected at feed rate 0.20 mm/rev under spindle speed 2100 r/min using the brad drill bit at exit delamination. The results on drilling this kenaf fiber show that the factor which increases the delamination factor is feed rate but the one increasing the spindle speed did not show any rapid increase in delamination for both drill bits. From data shown, it can be seen that increasing spindle speed decreases the entry delamination factor for KFRP. For the fiber composite of this experiment (KGFRP), the minimum delamination factor is 1.012 acting on feed rate 0.05 mm/rev under spindle speed 2100 r/min using the twist drill at entry delamination. These results indicated that the delamination factor decreases with the increase in spindle speed. The maximum delamination factor for this composite is 4.006 with 0.20 mm/rev under spindle speed 2100 r/min using the brad drill at exit delamination as the results using the brad drill bit gives a higher value of delamination factor compared to the twist drill bit.

Table 4.

Entry and exit Fd (delamination factor) for the twist drill bit.

Experiment No	Material	Spindle speed (r/min)	Feed rate (mm/rev)	Entry delamination Fd_u	Exit delaminationFd_b
1	GFRP	700	0.05	1.064	1.545
2			0.12	1.081	1.642
3			0.20	1.134	1.675
4		1400	0.05	1.357	1.399
5			0.12	1.274	1.787
6			0.20	1.356	1.768
7		2100	0.05	1.122	1.651
8			0.12	1.200	1.776
9			0.20	1.161	1.851
10	KFRP	700	0.05	1.042	1.012
11			0.12	1.184	1.018
12			0.20	1.144	1.012
13		1400	0.05	1.005	1.126
14			0.12	1.016	1.377
15			0.20	1.008	1.274
16		2100	0.05	1.007	1.114
17			0.12	1.015	1.435
18			0.20	1.067	1.540
19	KGFRP	700	0.05	1.022	1.438
20			0.12	1.058	1.648
21			0.20	1.125	1.820
22		1400	0.05	1.087	1.617
23			0.12	1.023	1.701
24			0.20	1.047	1.738
25		2100	0.05	1.012	1.650
26			0.12	1.032	1.818
27			0.20	1.028	1.740

GFRP: Glass fiber reinforced unsaturated polyester, KFRP: Kenaf fiber reinforced unsaturated polyester, KGFRP: Kenaf glass fiber reinforced unsaturated polyester.

Table 5.

Entry and exit delamination factor (Fd) for the brad drill bit.

Experiment No.	Material	Spindle speed (r/min)	Feed rate (mm/rev)	Entry delamination Fd_u	Exit delamination Fd_b
1	GFRP	700	0.05	1.342	1.393
2			0.12	1.644	2.859
3			0.20	1.411	3.257
4		1400	0.05	1.189	1.808
5			0.12	1.358	2.520
6			0.20	1.926	3.671
7		2100	0.05	1.413	2.223
8			0.12	1.867	3.094
9			0.20	1.893	3.298
10	KFRP	700	0.05	1.223	1.235
11			0.12	1.270	3.173
12			0.20	1.259	3.389
13		1400	0.05	1.197	2.044
14			0.12	1.200	3.016
15			0.20	1.227	3.121
16		2100	0.05	1.157	2.614
17			0.12	1.226	3.329
18			0.20	1.173	4.006
19	KGFRP	700	0.05	1.094	1.877
20			0.12	1.179	3.079
21			0.20	1.207	3.235
22		1400	0.05	1.115	2.269
23			0.12	1.132	3.522
24			0.20	1.154	3.857
25		2100	0.05	1.128	3.046
26			0.12	1.128	3.948
27			0.20	1.170	4.023

GFRP: Glass fiber reinforced unsaturated polyester, KFRP: Kenaf fiber reinforced unsaturated polyester, KGFRP: Kenaf glass fiber reinforced unsaturated polyester.

The suitable technique for drilling composites is the twist drill bit compared to the brad drill bit. It means that types of drill and geometry have an important effect in the delamination factor. If the majority is taken, the appropriate feed rate can be chosen as 0.05 mm/rev but not for spindle speed. From Table 6, conclusion can be made that the delamination factor is high at exit delamination for both drill bits in line increase of feed rate. This situation can be explained such that the higher feed rates affect thrust force which results in serious delamination.

Table 6.

The suitable drilling composites technique.

Composite	Drill bit	Spindle speed, r/min	Feed rate	Type delamination
GFRP	Twist	700	0.05 mm/rev	Entry (1.064)
KFRP	Twist	1400	0.05 mm/rev	Entry (1.007)
KGFRP	Twist	2100	0.05 mm/rev	Entry (1.012)

GFRP: Glass fiber reinforced unsaturated polyester, KFRP: Kenaf fiber reinforced unsaturated polyester, KGFRP: Kenaf-glass fiber reinforced unsaturated polyester.

Main Effects Plot for SNRA

After calculating delamination factors (Table 4 for the twist drill bit and Table 5 for the brad drill bit), the next step is to compute the SNRA. SNRA for each datum was calculated by applying equation (2). The SNRA response graph for twist-entry delamination and twist-exit delamination is shown in Figure 7 (a) and (b). A greater SNRA corresponds to a better performance, the factor level with the highest SNRA is the optimum level.¹⁹ Therefore, the optimum parameters combination level identified for the present experiment in the drilling process for Twist-Entry is KGFRP with spindle speed at 2100 r/min and feed rate at 0.05 mm/rev. Meanwhile at Twist-Exit, the optimal parameters combination level was KFRP with spindle speed at 700 r/min and feed rate at 0.05 mm/rev.

Figure 7.

(a) Main effects plot for SNRA (Twist-Entry), (b) main effects plot for SNRA (Twist-Exit).

Based on graph at Figure 8 (a) for drilling process using the brad drill bit, the optimal machining parameters for Brad-Entry is KGFRP with spindle speed at 1400 r/min and feed rate at 0.05 mm/rev. Figure 8 (b) shows GFRP, spindle speed at 700 r/min and feed rate at 0.05 mm/rev are the optimal value for Brad-Exit.

Figure 8.

(a) Main effects plot for SNRA (Brad-Entry), (b) main effects plot for SNRA (Brad-Exit).

ANOVA for SNRA

ANOVA analysis and p tests were carried out to prove the importance of each factor. This analysis was conducted for the significance level of α = 0.05 (95% confidence level). If p factor exceeds p ≤ 0.05 value, this factor contribution is insignificant. F-ratio is the tool to compare each the sources of variation. The higher the F-ratio, the more significant the factor is. Table 7 and Table 8 show ANOVA for the SNRA model twist drill bit for Twist-Entry and Twist-Exit. Focussing on machining parameters from Table 7, ANOVA for SNRA twist-entry, p value of spindle speed and feed rate is more than 0.05. This means that it is insignificant or does not contribute to the evaluation of the response. From Table 8, the most significant factor is feed rate at twist-exit based on p and F-value. ANOVA for SNRA using brad drill bit was carried out in Table 9 and Table 10 and it was found that feed rate was a significant value. Feed rate was found as the most significance factor for this experiment.²⁰

Table 7.

ANOVA for SNRA (twist-entry).

Sources	Df	Seq SS	Adj SS	Adj MS	F	p
Materials	2	7.0772	7.0772	3.5386	10.40	0.001
Spindle speed (r/min)	2	0.7702	0.7702	0.3851	1.13	0.342
Feed rate (mm/rev)	2	0.4444	0.4444	0.2222	0.65	0.531
Residual error	20	6.8034	6.8034	0.3402	—	—
Total	26	15.0952	—	—	—	—

ANOVA: analysis of variance, SNRA: signal noise to ratio.

Table 8.

ANOVA for SNRA (twist-exit).

Sources	Df	Seq SS	Adj SS	Adj MS	F	P
Materials	2	51.256	51.256	25.6282	59.28	0.000
Spindle speed (r/min)	2	7.126	7.126	3.5629	8.24	0.002
Feed rate (mm/rev)	2	7.838	7.838	3.9192	9.07	0.002
Residual error	20	8.646	8.646	0.4323	—	—
Total	26	74.866	—	—	—	—

ANOVA: analysis of variance, SNRA: signal noise to ratio.

Table 9.

ANOVA for SNRA (brad-entry).

Sources	Df	Seq SS	Adj SS	Adj MS	F	p
Materials	2	33.2812	33.2812	16.6406	23.62	0.000
Spindle speed (r/min)	2	0.8163	0.8163	0.4081	0.58	0.569
Feed rate (mm/rev)	2	5.1368	5.1368	2.5684	3.65	0.045
Residual error	20	14.0894	14.0894	0.7045	—	—
Total	26	53.3237	—	—	—	—

Table 10.

ANOVA for SNRA (brad-exit).

Source	Df	Seq SS	Adj SS	Adj MS	F	p
Material	2	13.03	13.03	6.513	4.99	0.017
Spindle speed (r/min)	2	25.74	25.74	12.870	9.87	0.001
Feed rate (mm/rev)	2	125.54	125.54	62.769	48.12	0.000
Residual error	20	26.09	26.09	1.304	—	—
Total	26	190.39	—	—	—	—

ANOVA: analysis of variance, SNRA: signal noise to ratio.

Drilled Hole Quality Comparison

Figures 9 and 10 show the delamination factor versus drilled hole in term comparing using two types of drill bits at entry and exit of the specimen. From the results, we can observe the plotted graph of 135 holes for entry delamination, lower delamination and overall delamination between the twist and brad drill bit. The orange line represents delamination of the drilled hole by the brad drill bit; meanwhile, the blue line represents delamination of the drilled hole by the twist drill bit. Based on all graphs, the value of delamination made by the brad drill bit is higher compared to the value of delamination made by the twist drill bit.²¹ This is particularly noticeable at the exit of the delamination made by the brad drill bit. It is clearly shown that the drilling process by the twist drill bit is better than the brad drill bit. In another word, it can be observed that, the hole made by the twist drill bit (blue) has the lowest delamination factor than the brad drill bit for overall.

Figure 9.

Entry delamination: twist versus brad.

Figure 10.

Exit delamination: twist versus brad.

All specimens also have been tested by the no-go pin gauge test to measure the quality of the drilled hole. 74% from 135 drilled holes made by the twist drill bit is fit and 91% from 135 drilled holes made by the brad drill bit is not fit. Figure 11 illustrate a summary of the no-go specimen test for twist drill bit analysis out of the 135 drilled holes. 100 drilled holes or 74% met the required standard while the rest were found to have not met the standard requirement or the drilled holes being oversize. Based on Figure 12, only 8% or 11 drilled holes meet the requirement, 91% or 123 drilled holes are not fit for the requirement and 1% drilled holes are oversize. By comparing these two graphs, observation can be made that drilled holes made by the twist drill bit have more quality than made by the brad drill bit. The size of the hole is a very important factor because of the close tolerance between the fastener and hole during assembly.²² It is clearly shown from Figure 12 that the drilled hole by using a twist drill bit is of more quality and produces the most dimensionally accurate one. It produces a hole with a diameter that is close to the nominal diameter. Compared to Figure 12, drill holes made by brad drills (91%) are not fit. It may happen because of the peeling, fiber pullout,^23,24 protruded fibers, incomplete removal of the fibers in the existing side, or burrs inside the hole.²⁵

Figure 11.

No-go specimen test (twist drill bit).

Figure 12.

No-go specimen test (brad drill bit).

Conclusions

Based on the findings, the delamination at the entry and exit specimen of GFRP, KFRP and KGFRP have been measured and following conclusions can be drawn from this experiment:

- Feed rate, tool material and cutting speed are the most influential factors in delamination. Thus, hard tool materials and lower feed rates can reduce delamination drilling GFRP.

- The optimal value of the machining parameters is feed rate. Based on the SNRA, the lowest feed rate and lowest spindle speed have the lowest value of the delamination factor.

- Both spindle speed and the feed rate have the highest influence on the delamination damage of the specimen.

- The results of ANOVA revealed the most significant value of the machining parameters, and also feed rate. The damage increases with an increase in both machining parameters, and it means that the composite damage is bigger for high feed rate and higher spindle speed.

- The delamination factor made by the twist drill bit showed a superior result, and it means the twist drill bit produces less delamination at the GFRP, KFRP and KGFRP composite (entry and exit) than the brad drill bit.

- The quality of drilled holes made by the twist drill bit has met the requirements higher than using the brad drill bit.

Footnotes

Acknowledgments

Authors would like to thank the Ministry of Higher Education and Scientific Research/Iraq and to Mustansiriyah University, College of Engineering/Mechanical Engineering Department, for their scientific assistance. Appreciation also extends to all technicians working in the Mechanical Department Laboratory/UPM, for their support. Further appreciate goes to Facilities Maintenance Engineering team, UniKL Malaysian Institute of Industrial Technology (MITEC), for supporting.

Declaration of conflicting interests

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

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Suhad D. Salman

Z. Leman

S. M. Sapuan

References

Tang

Zhang

Tang

, et al. Polymer matrix wave-transparent composites: A review. J Mater Sci Technol 2021; 75: 225–251.

Ishak

Leman

Sapuan

, et al. Mechanical properties of kenaf bast and core fibre reinforced unsaturated polyester composites. IOP Conf Ser Mater Sci Eng 2010; 11: 012006.

Lin

Zhang

. Random copolymer membrane coated PBO fibers with significantly improved interfacial adhesion for PBO fibers/cyanate ester composites. Chin J Aeronautics February 2021; 34(Issue 2): 659–668.

Salman

Leman

. Physical, mechanical and ballistic properties of kenaf fiber reinforced poly vinyl butyral and its hybrid composites. In: Sapuan

Ismail

Zainudin

(eds) Natural fibre reinforced vinyl ester and vinyl polymer composites. Cambridge: Woodhead Publishing, 2018, pp. 249–263.

Salman

. Partial replacement of synthetic fibres by natural fibres in hybrid composites and its effect on monotonic properties. J Ind Textiles 2019; 51: 258–276. DOI: 10.1177/1528083719878843.

Devitte

Cristiano

Candido Souza

Souza

, et al. Optimization for drilling process of metal-composite aeronautical structures. Journal Sci Eng Compos Mater 2021; 28(1): 264–275. DOI: 10.1515/secm-2021-0027.

, et al. Influence of machining parameters on delamination in drilling of GFRP-armour steel sandwich composites. Proced Eng 2013; 51: 758–763. DOI: 10.1016/j.proeng.2013.01.108.

Wei

Ming

, et al. Effect of drilling parameters and tool geometry on drilling performance in drilling carbon fiber–reinforced plastic/titanium alloy stacks. Adv Mech Eng 2016; 8(9). DOI: 10.1177/1687814016670281.