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The effect of pilot hole on delamination when core drill drilling composite materials
ARTICLE IN PRESS
International Journal of Machine Tools & Manufacture 46 (2006) 1653–1661 www.elsevier.com/locate/ijmactool
The effect of pilot hole on delamination when core drill drilling composite materials C.C. Tsao Department of Automatic Engineering, Ta-Hua Institute of Technology, Hsinchu 307, Taiwan, ROC Received 6 May 2005; received in revised form 15 August 2005; accepted 22 August 2005 Available online 3 November 2005
Abstract Removal of chips is a serious problem when core drill drilling polymer composites. As the chip is formed it moves to the inner hole of core drill. A hole is pre-drilled to eliminate the thrust caused by the removal chip, thus the threat for delamination is significantly reduced. The diameter of the pre-drilled hole is set equal to the inner hole of core drill. A smaller diameter of pilot hole cannot solve the problem of removal chips, while a larger one tends to cause undesired delamination during pre-drilling. Although valuable efforts have been made for the analysis of drilling-induced delamination, little has been reported on the effect of pilot hole diameter on delamination for core drills. The design of drill tools can be improved using obtained results. r 2005 Elsevier Ltd. All rights reserved. Keywords: Core drill; Delamination; Thrust force; Pilot hole
1. Introduction Drilling is by far the most used secondary machining of fiber-reinforced composite laminates, while the delamination occurs frequently during machining. Drilling-induced delamination occurs both at the entrance and the exit planes of the workpiece, it has been correlated with the thrust force during exit of the drill [1–5]. A rapid increase in feed rate at the end of drilling will cause cracking around the exit edge of the hole [6]. It was also stated that the larger the feeding load, the more serious the cracking. The drill geometry is also considered the most important factor that affects drill performance [7–10]. Jain and Yang designed a hollow grinding drill and tested it [3]. Mathew et al. used a trepanning tool to reduce the thrust force and torque during drilling glass fiber reinforced plastic (GFRP) laminates [11]. The cutting action of the trepanning tool starts from the periphery of the cutting edge that puts the fibers in tension during the entire cutting operation [12]. In industrial experiences, core drills show better drilling quality. Based on the earlier proposed concentrated-load model, the thrust force is the main cause to delaminate the workpiece [1–5]. However, the central concentrated loading is a special case of the radial circular loading. Utilizing the radial distributed force the allowable critical thrust force and the drilling quality will be increased. With a pilot hole, delamination can be reduced significantly. Recently, Won and Dharan investigated the effect of the chisel edge on the thrust force, and an innovative process model was developed to predict the advantage of a specimen with pre-drilled pilot hole [13]. Tsao and Hocheng studied the effect of chisel length and associated pilot hole on delamination when twist drill drilling composite materials [14]. However, removal of chips is a serious problem for the core drill in drilling. As the chip is formed it moves to the inner hole of core drill. A hole is pre-drilled to eliminate the thrust caused by the removal chip, thus the threat for delamination is significantly reduced. The diameter of the pre-drilled hole is set equal to the inner hole of core drill. Smaller diameter of pilot hole cannot solve the problem of removal chips, while larger one tends to cause undesired delamination during pre-drilling. Although valuable efforts have been made for the analysis of Corresponding author. Tel.: +886 3 5927700; fax: +886 3 5921047.
E-mail address: [email protected]. 0890-6955/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2005.08.015
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Nomenclature E FA FCR GIC H M U X a b c dA dU
Young’s modulus thrust force, N thrust force of uniformly distributed along the pre-drilled pilot hole critical crack propagation energy per unit area in mode I workpiece thickness flexible rigidity of the plate stored strain energy displacement delamination crack radius, mm pilot hole radius, mm core drill radius, mm increase in the area of the delamination crack infinitesimal strain energy
dX da h q r s t b g n Z Z*
movement distance infinitesimal delamination crack uncut depth under tool uniformly distributed load of intensity radius at the selected point of the drill ratio of the drill radius to delamination radius, mm/mm thickness of core drill ratio of the thickness to core drill radius, mm/ mm ratio of pilot hole radius to inner uncut portion radius, mm/mm Poisson’s ratio ratio of pilot hole radius to core drill radius, mm/mm critical ratio of pilot hole radius to core drill radius, mm/mm
drilling-induced delamination, little has been reported on the effect of pilot hole diameter on delamination for core drill. The design of drill tools can be improved using obtained results. 2. Model of delamination analysis During drilling-induced delamination, the drill movement distance dX is associated with the work done by the thrust force F A , which is used to deflect the plate as well as to propagate the interlaminar crack. The energy balance equation gives GIC dA ¼ F A dX dU,
(1)
where dU is the infinitesimal strain energy, dA is the increase in the area of the delamination crack, and GIC is the critical crack propagation energy per unit area in mode I. The value of GIC is assumed a constant to be a mild function of strain rate by Saghizadeh and Dhahran [15]. Fig. 1(a) depicts the schematics with a pre-drilled central hole. The diameter of the pilot hole is selected equal to the inner hole of core drill, in order to eliminate the disadvantage of the inner hole induced thrust force and avoid the threat of creating large delamination by large pre-drilled hole. Fig. 1(b) depicts the schematics of a core drill and the induced delamination. F CR is the thrust force, X is the displacement, H is the workpiece thickness, h is the uncut depth under tool, 2c is the diameter of core drill, 2b is the diameter of pilot hole, t is the thickness of core drill, and a is the radius of existing delamination. It provides more rigorous approach than the previous work [10,16]. The isotropic behavior and pure bending of the laminate are assumed in the model. In Eq. (1), one notes that dA ¼ 2pa da.
(2)
A mathematical model of a plate subjected to symmetrical bending by thrust force F CR uniformly distributed along the circular inner edge of a hole is shown in Fig. 2. The deflection of the circular plate is given [17] (i) 0prpc (inner portion) C r2 F CR 2 r r 1 X ðrÞ ¼ r ln 1 þ þ C 2 ln þ C 3 , a a 8pM 4 where F CR ¼ pq½c2 b2 ð1 þ gÞ2 , 9 8 bð1 þ gÞ 2 2 2 2 > 2 > > þ ð1 nÞ > a ð1 nÞð2 þ k1 Þ þ b ð1 þ gÞ ð1 þ nÞk1 2b ð1 þ gÞ 2ð1 þ nÞ ln = F CR < a , C1 ¼ > > 8pM > a2 ð1 nÞ þ b2 ð1 þ gÞ2 ð1 þ nÞ > ; :
(3)
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2b
H
(a)
Pre-drilled hole of diameter 2b
FCR Drill
c
n ~
H q h γb
t
2b
Delamination
a X
(b) Drilling hole of diameter 2b in a pre-drilled laminate Fig. 1. Circular plate model for delamination analysis of a pre-drilled specimen. (a) Pre-drilled hole of diameter 2b. (b) Drilling hole of diameter 2b in a pre-drilled laminate.
a FCR c
q
b
b
X
Fig. 2. A circular plate with a circular hole and distributed forces along inner edge subject to clamped boundary condition [17].
2
3 bð1 þ gÞ 1 F CR a b ð1 þ gÞ 6 7 a C2 ¼ 4 2 5, 4pM a ð1 nÞ þ b2 ð1 þ gÞ2 ð1 þ nÞ 2 2
2
ð1 þ nÞ ln
9 bð1 þ gÞ > þ ð1 nÞ > a2 ð1 nÞð2 þ k1 Þ þ b2 ð1 þ gÞ2 ð1 þ nÞk1 2b2 ð1 þ gÞ2 2ð1 þ nÞ ln = F CR a , C3 ¼ k2 > 32pM > a2 ð1 nÞ þ b2 ð1 þ gÞ2 ð1 þ nÞ > > ; : 8 > > a2 <
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4b2 ð1 þ gÞ2 c c þ b ð1 þ gÞ bð1 þ gÞ c2 , ln k1 ¼ 4 ln 2 2 a c a2 b ð1 þ gÞ 1 c2 2
2
k2 ¼ 6
2
3½c2 þ b2 ð1 þ gÞ2 2r2 ½c2 þ b2 ð1 þ gÞ2 2a2 a4
2 2 2 b ð1 þ gÞ 4r 2½c þ b ð1 þ gÞ c 2b ð1 þ gÞ bð1 þ gÞ 4r c bð1 þ gÞ c2 þ 2 ln , þ ln ln ln 2 2 2 2 2 a a c a c a b ð1 þ gÞ b ð1 þ gÞ 2 2 a 1 a 1 c2 c2 2
2
2
2
2
2
where g ¼ ðc t bÞ=b is the ratio between pilot hole and inner uncut portion of core drill, E is Young’s modulus, M ¼ Eh3 =ð12ð1 n2 ÞÞ ¼ flexural rigidity of the plate and n is Poisson’s ratio. (ii) cprpa (outer portion) F CR r r r2 ½c2 þ b2 ð1 þ gÞ2 2 2 2 2 2 2 2 2 2 X¼ 2a 2r þ 2½c þ b ð1 þ gÞ ln þ 4r ln þ ½c þ b ð1 þ gÞ .ð4Þ a a a2 32pM Differentiation of Eq. (4) with respect to da yields dX F CR a ½1 þ Zð1 þ gÞs 2 2 ¼ k3 þ k4 ½1 þ Zð1 þ gÞ s þ k5 ln , da 16pM 2
(5)
where k3 ¼
6
½1 þ Zð1 þ gÞ2 þ 4½1 þ Z2 ð1 þ gÞ2 2 s þ ½1 þ Zð1 þ gÞ2 ½1 þ Z2 ð1 þ gÞ2 s4 2
2ð1 nÞ½k1 þ 1 Z2 s2 ð1 þ gÞ2 þ Z2 s2 ð1 þ gÞ2 ½ð1 þ nÞk1 þ 4 ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ
þ ð1 nÞ
k4 ¼
ð1 nÞ½k1 þ 2 2Z2 s2 ð1 þ gÞ2 þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞfk1 4 ln½Zsð1 þ gÞg , ½ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ2
1 ð1 nÞfk1 þ ½1 þ Z2 ð1 þ gÞ2 s2 g þ Z2 s4 ð1 þ gÞ2 ð1 þ nÞ½1 þ Z2 ð1 þ gÞ2 þ 2 4½ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ ð1 nÞ
k5 ¼
ð1 nÞ½k1 þ 2 2Z2 s2 ð1 þ gÞ2 þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞfk1 4 ln½Zsð1 þ gÞg , 4½ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ2
4Z2 s2 ð1 þ gÞ2 f2ð1 þ nÞ ln½Zsð1 þ gÞ þ ð1 þ nÞg 8Z2 s2 ð1 þ gÞ2 ð1 nÞfð1 þ nÞ ln½Zsð1 þ gÞ þ 1g . ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ ½ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ2
The stored strain energy is (i) 0prpc (inner portion) # Z c " d2 X 1 dX 2 U1 ¼ p M þ r dr dr2 r dr b ¼
pMC 21 c2 F 2CR c2 F CR C 1 c2 k6 þ ð2 ln s 1Þ Z2 ð1 þ gÞ2 ½2 ln Zsð1 þ gÞ 1 þ 1 Z2 ð 1 þ g Þ 2 , 16pM 4 2
where k6 ¼ ð2 ln2 s 2 ln s þ 1Þ Z2 ð1 þ gÞ2 f2 ln2 ½Zsð1 þ gÞ 2 ln½Zsð1 þ gÞ þ 1g.
ð6Þ
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(ii) cprpa (outer portion) # Z c " d2 X 1 dX 2 r dr U2 ¼ p M þ dr2 r dr b F 2CR a2 1 s2 ½1 þ Z2 ð1 þ gÞ2 2 4 ½1 þ Z2 ð1 þ gÞ2 2 6 2 2 4 2 2 ¼ 2s ln s þ s . s þ ½1 þ Z ð1 þ gÞ s ln s 16pM 2 2 8 8
ð7Þ
The total strain energy is F 2CR c2 F CR C 1 c2 pMC 21 c2 k6 þ fð2 ln s 1Þ Z2 ð1 þ gÞ2 ½2 ln Zsð1 þ gÞ 1g þ ½1 Z2 ð1 þ gÞ2 16pM 4 2 F 2 a2 1 s 2 ½1 þ Z2 ð1 þ gÞ2 2 4 ½1 þ Z2 ð1 þ gÞ2 2 6 2s2 ln2 s þ þ CR s þ ½1 þ Z2 ð1 þ gÞ2 s4 ln s s 16pM 2 2 8 8
U¼
ð8Þ
and dU F2 a ¼ CR da 16pM
k7 k9 k9 ð1 nÞk10 ð1 nÞk7 k10 k7 þ k11 ½1 Z2 ð1 þ gÞ2 1 þ k 8 s2 þ . 2 2 2 2
ð9Þ
k7 ¼ 2s2 ð1 2 ln sÞ 2Z2 s2 ð1 þ gÞ2 f1 2 ln½Zsð1 þ gÞg, k8 ¼
ð1 nÞðk1 þ 2Þ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞk1 2Z2 s2 ð1 þ gÞ2 f2ð1 þ nÞ ln½Zsð1 þ gÞ þ ð1 nÞg , ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ
k9 ¼
ð1 nÞfk1 þ ½1 þ Z2 ð1 þ gÞ2 s2 g þ Z2 s4 ð1 þ gÞ2 ð1 þ nÞ½1 þ Z2 ð1 þ gÞ2 , ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ
ð1 nÞ½k1 þ 2 2Z2 s2 ð1 þ gÞ2 þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞfk1 4 ln½Zsð1 þ gÞg , ½ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ2 ½1 þ Z2 ð1 þ gÞ2 2 4 ½1 þ Z2 ð1 þ gÞ2 2 6 2 4 2 4 2 2 2 s 2½1 þ Z ð1 þ gÞ s ln s þ s . ¼ 1 þ 4s ln s ½1 þ Z ð1 þ gÞ s 4 2
k10 ¼
k11
Substituting Eqs. (2), (5) and (9) into Eq. (1), one obtains the thrust force of the core drill with pre-drilled pilot hole at the onset of crack propagation as 8 > > <
91=2 > > =
32G IC M F CR ¼ p > > k k ð1 nÞk k ½1 þ Zð1 þ gÞs k9 ð1 nÞk10 7 9 7 10 > : k3 k 7 þ ; þ ½1 þ Zð1 þ gÞ2 k4 s2 þ ½1 Z2 ð1 þ gÞ2 1 þ k11 þ k5 ln k 8 s2 > 2 2 2 2 2
,
(10) where s ¼ c=a
and
Z ¼ b=c.
3. Experimental setup 3.1. Specimen preparation Composite laminates are made from woven WFC200 fabric carbon fiber prepregs with the stacking sequence of [01/ 901]12S. The laminates are cured in an autoclave at 150 1C and 600 KPa. The plates are cut into coupon specimens of 60 mm by 60 mm. Twenty-four lamina make the plate thickness 6 mm. The fiber volume fraction is 0.55, the Young’s modulus, Poisson’s ratio and strain energy release rate are measured 18.4 GPa, 0.3 and 140 J/m2, respectively [18]. 3.2. Drilling test Drilling tests were carried out on a vertical machining center. The thrust forces during drilling were measured with a Kistler 9273 piezoelectric dynamometer and Kistler 5019 charge amplifiers and were stored on a TEAC DR-F1 digital
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Spindle Chuck
Drill
Workpiece
Fixture
Dynamometer Kistler 9273 Recorder TEAC DR-F1 Amplifier Kistler 5019 Drill machining table
Fig. 3. Schematic of experimental setup.
recorder subsequently. The amplifiers have to stabilize for at least an hour. The drilling set-up used in data collection is shown in Fig. 3. The core drills are 10 mm in diameter plated with diamond of a #60 grit size at front end. The thickness of core drill is 0.75 mm and 1.2 mm, respectively. All drilling tests were conducted coolant free at a spindle speed of 1000 rpm and feed rates of 8, 10, 11 and 12 mm/min. 4. Results and discussion Eq. (10) indicates that the critical thrust force for specimens with pre-drilled pilot hole is a function of the ratio of the drill diameter to delamination diameter (s) and the ratio of the pilot hole to drill diameter (Z). In the Tsao-Hocheng model [19] without a pilot hole, the critical thrust force is given by
FR ¼ p
8 > > > < > > > :1
91=2 > > > =
32G IC M 2 2 > 3b 4ð1 bÞ ð6 12b þ 11b2 5b3 þ b4 Þ 2ð1 bÞ2 ð2 2b þ b2 Þ > 4 4b þ þ lnð1 bÞ s2 þ þ lnð1 bÞ s4 > ; 2 bð2 bÞ 2 bð2 bÞ
,
(11) where b is the ratio between thickness and radius of core drill (namely, b ¼ t=c). To evaluate the effect of pre-drilled pilot holes on the critical thrust force value, the critical thrust force predicted by Eq. (10) was compared with that of Eq. (11) at b ¼ 0.2 in Fig. 4 much clearer than the early reference [13–14]. It shows that the critical thrust force decreases slightly for specimens with pre-drilled pilot holes. On the other hand, one realizes the drilling thrust force will be significantly reduced by removal of the chip contact in drilling. The critical thrust force ratio without and with pre-drilled pilot hole for b ¼ 0.2 and Z ¼ 0.2 at various Poisson’s ratio is shown in Fig. 5(a). Note that when s increases the critical thrust force ratio increases. The presence of larger the Poisson’s ratio leads to an increase in the critical thrust force ratio at sX0:5. From Fig. 5(b), it is known that a smaller value of b is necessary if a higher critical thrust force ratio for n ¼ 0:3 and Z ¼ 0:4 is required. For the relationship between the pilot hole to diameter ratio (Z) and the critical thrust force ratio is shown in Fig. 5(c). It illustrates that the lower the pilot hole to diameter ratio (lower Z), the larger is the critical thrust force ratio. The critical thrust force with pre-drilled pilot hole has a maximum at a certain s approach to 1. Fig. 6(a) shows the variation of the critical thrust force as a function of with pre-drilled pilot hole to diameter ratio (Z) for various Poisson’s ratio at g ¼ 0.2 and s ¼ 1.0. It is evident that the higher the Poisson’s ratio (larger n), the lower is the Z*. The thrust force of core drill with pre-drilled pilot hole remains little different at sX0:5. Fig. 6(b) illustrates the variation of the critical thrust force with Z for different ratio of pilot hole to inner uncut portion of core drill (g) at n ¼ 0.3 and s ¼ 1.0. It can be found that the critical thrust force with pre-drilled pilot hole decreases with the increasing g. These results
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Critical Thrust Force [ ( 32GICM ) 0.5 ]
6 Without Pilot Hole With Pilot Hole, η = 0.4
5
4
3
2
1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Drill Diameter to Delamination Diameter Ratio ( s = c / a )
Fig. 4. Comparison of critical thrust force between Eq. (10) and Eq. (11) at b ¼ 0:2.
2.5 2 1.5 1 0.5 0
(a)
2.5 ν = 0.4 ν = 0.3 ν = 0.2
Critical Thrust Force Ratio ( FR / FCR )
Critical Thrust Force Ratio ( FR / FCR )
3
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Drill Diameter to Delamination Diameter Ratio ( s = c / a )
2 1.5 1 0.5 0
(b)
β = 0.2 β = 0.3 β = 0.4
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Drill Diameter to Delamination Diameter Ratio ( s = c / a )
Critical Thrust Force Ratio ( FR / FCR )
2.5 2 1.5 1 0.5 0
(c)
η = 0.2 η = 0.4 η = 0.6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Drill Diameter to Delamination Diameter Ratio ( s = c / a )
Fig. 5. Critical thrust force ratio for core drill without and with pre-drilled pilot hole. (a) b ¼ 0:2 and Z ¼ 0:2. (b) n ¼ 0:3 and Z ¼ 0:4. (c) n ¼ 0:3 and b ¼ 0:2.
indicate that g has a significant influence on the critical thrust force with pre-drilled pilot hole and can be easily understood. And with the decreasing g, the larger is the Z . When n ¼ 0:3, g ¼ 0:2 and s ¼ 1:0, the Z is found to be 0.505. Fig. 7 indicated the thrust force increases with feed rate. Experimental results of thrust force are shown in Table 1. Without pre-drilled pilot hole (e.g. Z ¼ 0:85), the thrust force during drilling lies quite well above the critical thrust force, which implies the occurrence of delamination at commonly used feed rate. With pre-drilled pilot hole (e.g. Z ¼ 0:85),
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0.5 Critical Thrust Force ( FCR ) [ (32GICM ) 0.5 ]
Critical Thrust Force ( FCR ) [ (32GICM ) 0.5 ]
0.55
0.5
0.45
0.4
0.35 ν = 0.2, With Pilot Hole ν = 0.3, With Pilot Hole
0.3
ν = 0.4, With Pilot Hole
0.25
0.45
0.4
0.35
0.3 γ = 0.2, With Pilot Hole
γ = 0.6, With Pilot Hole
0.2 0
0.1
0.2
(a)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
γ = 0.4, With Pilot Hole
0.25
1
0
0.1
0.2
(b)
Pilot Hole to Drill Ratio ( = b / c )
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Pilot Hole to Drill Ratio ( = b / c )
Fig. 6. Critical thrust force with pre-drilled pilot hole to drill ratio. (a) g ¼ 0:2 and s ¼ 1:0. (b) n ¼ 0:3 and s ¼ 1:0.
60 η = 0.76, Without Pilot Hole η = 0.85, Without Pilot Hole η = 0.76, With Pilot Hole η = 0.85, With Pilot Hole
55
Thrust Force ( N )
50 45 40 critical thrust force at = 0.85
35 30 critical thrust force at = 0.76
25 20 7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
Feed Rate ( mm / min )
Fig. 7. Effects of pre-drilled pilot hole on thrust forces (drill diameter ¼ 10 mm, Z ¼ 0.76 and 0.85).
Table 1 Experimental results of thrust force Pilot hole to drill ratio (Z)
Feed rate (mm/rev)
With pilot hole thrust force (N)
Without pilot hole thrust force (N)
0.85
0.008 0.010 0.011 0.012
23.6 24.4 26.3 28.4
31.7 36.2 44.7 48.5
0.76
0.008 0.010 0.011 0.012
29.4 30.6 32.7 35.9
33.4 40.1 49.6 55.2
however, the thrust force during drilling can be lower than the critical value at large range of feed rates, which indicate the delamination-free drilling is achievable. As shown in Fig. 7, Z ¼ 0:85 with pre-drilled pilot hole allows for the largest drilling feed rate, while Z ¼ 0:76 without pre-drilled pilot hole has to be operated at the lowest drilling feed rate to avoid the
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delamination. The results agree well with both the analytical findings in Eq. (10) and the industrial experience. The safety margin of the process window in use of core drill with pre-drilled pilot hole is therefore wider and higher drilling efficiency is achieved. The use of pre-drilled pilot hole is thus illustrated. Based on these experiments, the removal chip for core drill is a major contributor to the thrust force. However, when large feed rate is used, the experimental thrust force increases. By setting properly the drilling conditions, thickness and predrilled pilot hole, one can produce delamination-free hole. The current analysis explains how to use the pre-drilled pilot hole to enhance quality drilling of composite material using core drill. 5. Conclusions An analytical approach to identifying the pilot hole between drill diameter (Z) and inner uncut portion (g) for delamination-free drilling based on linear elastic fracture mechanics is derived in this study. The prediction of the model agrees quantify with the experimental results. Due to the different pre-drilled pilot hole, the core drill shows different level of the drilling thrust force varying with the feed rate. The advantageous pre-drilled pilot hole is explained based on the theoretical models that illustrate the thrust force exerted by core drill. Fig. 7 has shown the verification and validation of the proposed method. Experimental results indicate the critical thrust force is reduced with pre-drilled pilot hole, while the drilling thrust is largely reduced by removal of chip effect. Controlling the ratio of pre-drilled pilot hole relative to drill diameter, one can conduct medium to large hole of composite laminates drilling at higher feed rate without delamination damage. This approach can provide a useful design chart for industrial engineers. Acknowledgement The research is partially supported by National Science Council, Taiwan, ROC, under contract NSC94-2212-E-233-001. References [1] H. Hocheng, C.K.H. Dharan, Delamination during drilling in composite laminates, Transactions of the ASME, Journal of Engineering for Industry 112 (1990) 236–239. [2] S. Jain, D.C.H. Yang, Effects of feedrate and chisel edge on delamination in composite drilling, Transactions of the ASME, Journal of Engineering for Industry 115 (1993) 398–405. [3] S. Jain, D.C.H. Yang, Delamination-free drilling of composite laminates, Transactions of the ASME, Journal of Engineering for Industry 116 (1994) 475–481. [4] J.P. Jung, G.W. Kim, K.Y. Lee, Critical thrust force at delamination propagation during drilling of angle-ply laminates, Composite Structures 68 (2005) 391–397. [5] G.W. Kim, K.Y. Lee, Critical thrust force at propagation of delamination zone due to drilling of FRP/metallic strips, Composite Structures 69 (2005) 137–141. [6] A. Kobayashi, Machining of Plastics, McGraw-Hill, New York, 1967. [7] D.F. Galloway, Some experiments on the influence of various factors on drill performance, Transactions of the ASME 79 (1957) 191–237. [8] J.P. Davim, P. Reis, C.C. Anto´nio, Experimental study of drilling glass fiber reinforced plastics (GFRP) manufactured by hand lay-up, Composites Science and Technology 64 (2) (2004) 289–297. [9] C.C. Tsao, H. Hocheng, Taguchi analysis of delamination associated with various drill bits in drilling of composite material, International Journal of Machine Tools & Manufacture 44 (10) (2004) 1085–1090. [10] H. Hocheng, C.C. Tsao, Comprehensive analysis of delamination in drilling of composite materials with various drill bits, Journal of Materials Processing Technology 140 (2003) 335–339. [11] J. Mathew, N. Ramakrishnan, N.K. Naik, Investigations into the effect of geometry of a trepanning tool on thrust and torque during drilling of GFRP composites, Journal of Materials Processing Technology 91 (1999) 1–11. [12] J. Mathew, N. Ramakrishnan, N.K. Naik, Trepanning on unidirectional composites: delamination studies, Composites Part A 30 (8) (1999) 951–959. [13] M.S. Won, C.K.H. Dharan, Chisel edge and pilot hole effects in drilling composite laminates, Transactions of the ASME, Journal of Manufacturing Science and Engineering 124 (2002) 242–247. [14] C.C. Tsao, H. Hocheng, The effect of chisel length and associated pilot hole on delamination when drilling composite materials, International Journal of Machine Tools & Manufacture 43 (11) (2003) 1087–1092. [15] H. Saghizadeh, C.K.H. Dharan, Delamination fracture toughness of graphite and aramid epoxy composites, Transactions of the ASME, Journal of Engineering Materials and Technology 108 (1986) 290–295. [16] H. Hocheng, C.C. Tsao, Analysis of delamination in drilling composite materials using core drill, Australasian Journal of Mechanical Engineering 1 (1) (2003) 49–53. [17] S. Timoshenko, S. Woinowsky-Keiger, Theory of Plates and Shells, second ed., McGraw-Hill, New York, 1959. [18] The Center for Aviation & Space Technology of Industrial Technology Research Institute, Private communication regarding material test, 2004. [19] C.C. Tsao, H. Hocheng, Effects of exit back-up on delamination in drilling composite materials using a saw drill and a core drill, International Journal of Machine Tools & Manufacture 45 (11) (2005) 1261–1270.
International Journal of Machine Tools & Manufacture 46 (2006) 1653–1661 www.elsevier.com/locate/ijmactool
The effect of pilot hole on delamination when core drill drilling composite materials C.C. Tsao Department of Automatic Engineering, Ta-Hua Institute of Technology, Hsinchu 307, Taiwan, ROC Received 6 May 2005; received in revised form 15 August 2005; accepted 22 August 2005 Available online 3 November 2005
Abstract Removal of chips is a serious problem when core drill drilling polymer composites. As the chip is formed it moves to the inner hole of core drill. A hole is pre-drilled to eliminate the thrust caused by the removal chip, thus the threat for delamination is significantly reduced. The diameter of the pre-drilled hole is set equal to the inner hole of core drill. A smaller diameter of pilot hole cannot solve the problem of removal chips, while a larger one tends to cause undesired delamination during pre-drilling. Although valuable efforts have been made for the analysis of drilling-induced delamination, little has been reported on the effect of pilot hole diameter on delamination for core drills. The design of drill tools can be improved using obtained results. r 2005 Elsevier Ltd. All rights reserved. Keywords: Core drill; Delamination; Thrust force; Pilot hole
1. Introduction Drilling is by far the most used secondary machining of fiber-reinforced composite laminates, while the delamination occurs frequently during machining. Drilling-induced delamination occurs both at the entrance and the exit planes of the workpiece, it has been correlated with the thrust force during exit of the drill [1–5]. A rapid increase in feed rate at the end of drilling will cause cracking around the exit edge of the hole [6]. It was also stated that the larger the feeding load, the more serious the cracking. The drill geometry is also considered the most important factor that affects drill performance [7–10]. Jain and Yang designed a hollow grinding drill and tested it [3]. Mathew et al. used a trepanning tool to reduce the thrust force and torque during drilling glass fiber reinforced plastic (GFRP) laminates [11]. The cutting action of the trepanning tool starts from the periphery of the cutting edge that puts the fibers in tension during the entire cutting operation [12]. In industrial experiences, core drills show better drilling quality. Based on the earlier proposed concentrated-load model, the thrust force is the main cause to delaminate the workpiece [1–5]. However, the central concentrated loading is a special case of the radial circular loading. Utilizing the radial distributed force the allowable critical thrust force and the drilling quality will be increased. With a pilot hole, delamination can be reduced significantly. Recently, Won and Dharan investigated the effect of the chisel edge on the thrust force, and an innovative process model was developed to predict the advantage of a specimen with pre-drilled pilot hole [13]. Tsao and Hocheng studied the effect of chisel length and associated pilot hole on delamination when twist drill drilling composite materials [14]. However, removal of chips is a serious problem for the core drill in drilling. As the chip is formed it moves to the inner hole of core drill. A hole is pre-drilled to eliminate the thrust caused by the removal chip, thus the threat for delamination is significantly reduced. The diameter of the pre-drilled hole is set equal to the inner hole of core drill. Smaller diameter of pilot hole cannot solve the problem of removal chips, while larger one tends to cause undesired delamination during pre-drilling. Although valuable efforts have been made for the analysis of Corresponding author. Tel.: +886 3 5927700; fax: +886 3 5921047.
E-mail address: [email protected]. 0890-6955/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2005.08.015
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Nomenclature E FA FCR GIC H M U X a b c dA dU
Young’s modulus thrust force, N thrust force of uniformly distributed along the pre-drilled pilot hole critical crack propagation energy per unit area in mode I workpiece thickness flexible rigidity of the plate stored strain energy displacement delamination crack radius, mm pilot hole radius, mm core drill radius, mm increase in the area of the delamination crack infinitesimal strain energy
dX da h q r s t b g n Z Z*
movement distance infinitesimal delamination crack uncut depth under tool uniformly distributed load of intensity radius at the selected point of the drill ratio of the drill radius to delamination radius, mm/mm thickness of core drill ratio of the thickness to core drill radius, mm/ mm ratio of pilot hole radius to inner uncut portion radius, mm/mm Poisson’s ratio ratio of pilot hole radius to core drill radius, mm/mm critical ratio of pilot hole radius to core drill radius, mm/mm
drilling-induced delamination, little has been reported on the effect of pilot hole diameter on delamination for core drill. The design of drill tools can be improved using obtained results. 2. Model of delamination analysis During drilling-induced delamination, the drill movement distance dX is associated with the work done by the thrust force F A , which is used to deflect the plate as well as to propagate the interlaminar crack. The energy balance equation gives GIC dA ¼ F A dX dU,
(1)
where dU is the infinitesimal strain energy, dA is the increase in the area of the delamination crack, and GIC is the critical crack propagation energy per unit area in mode I. The value of GIC is assumed a constant to be a mild function of strain rate by Saghizadeh and Dhahran [15]. Fig. 1(a) depicts the schematics with a pre-drilled central hole. The diameter of the pilot hole is selected equal to the inner hole of core drill, in order to eliminate the disadvantage of the inner hole induced thrust force and avoid the threat of creating large delamination by large pre-drilled hole. Fig. 1(b) depicts the schematics of a core drill and the induced delamination. F CR is the thrust force, X is the displacement, H is the workpiece thickness, h is the uncut depth under tool, 2c is the diameter of core drill, 2b is the diameter of pilot hole, t is the thickness of core drill, and a is the radius of existing delamination. It provides more rigorous approach than the previous work [10,16]. The isotropic behavior and pure bending of the laminate are assumed in the model. In Eq. (1), one notes that dA ¼ 2pa da.
(2)
A mathematical model of a plate subjected to symmetrical bending by thrust force F CR uniformly distributed along the circular inner edge of a hole is shown in Fig. 2. The deflection of the circular plate is given [17] (i) 0prpc (inner portion) C r2 F CR 2 r r 1 X ðrÞ ¼ r ln 1 þ þ C 2 ln þ C 3 , a a 8pM 4 where F CR ¼ pq½c2 b2 ð1 þ gÞ2 , 9 8 bð1 þ gÞ 2 2 2 2 > 2 > > þ ð1 nÞ > a ð1 nÞð2 þ k1 Þ þ b ð1 þ gÞ ð1 þ nÞk1 2b ð1 þ gÞ 2ð1 þ nÞ ln = F CR < a , C1 ¼ > > 8pM > a2 ð1 nÞ þ b2 ð1 þ gÞ2 ð1 þ nÞ > ; :
(3)
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2b
H
(a)
Pre-drilled hole of diameter 2b
FCR Drill
c
n ~
H q h γb
t
2b
Delamination
a X
(b) Drilling hole of diameter 2b in a pre-drilled laminate Fig. 1. Circular plate model for delamination analysis of a pre-drilled specimen. (a) Pre-drilled hole of diameter 2b. (b) Drilling hole of diameter 2b in a pre-drilled laminate.
a FCR c
q
b
b
X
Fig. 2. A circular plate with a circular hole and distributed forces along inner edge subject to clamped boundary condition [17].
2
3 bð1 þ gÞ 1 F CR a b ð1 þ gÞ 6 7 a C2 ¼ 4 2 5, 4pM a ð1 nÞ þ b2 ð1 þ gÞ2 ð1 þ nÞ 2 2
2
ð1 þ nÞ ln
9 bð1 þ gÞ > þ ð1 nÞ > a2 ð1 nÞð2 þ k1 Þ þ b2 ð1 þ gÞ2 ð1 þ nÞk1 2b2 ð1 þ gÞ2 2ð1 þ nÞ ln = F CR a , C3 ¼ k2 > 32pM > a2 ð1 nÞ þ b2 ð1 þ gÞ2 ð1 þ nÞ > > ; : 8 > > a2 <
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4b2 ð1 þ gÞ2 c c þ b ð1 þ gÞ bð1 þ gÞ c2 , ln k1 ¼ 4 ln 2 2 a c a2 b ð1 þ gÞ 1 c2 2
2
k2 ¼ 6
2
3½c2 þ b2 ð1 þ gÞ2 2r2 ½c2 þ b2 ð1 þ gÞ2 2a2 a4
2 2 2 b ð1 þ gÞ 4r 2½c þ b ð1 þ gÞ c 2b ð1 þ gÞ bð1 þ gÞ 4r c bð1 þ gÞ c2 þ 2 ln , þ ln ln ln 2 2 2 2 2 a a c a c a b ð1 þ gÞ b ð1 þ gÞ 2 2 a 1 a 1 c2 c2 2
2
2
2
2
2
where g ¼ ðc t bÞ=b is the ratio between pilot hole and inner uncut portion of core drill, E is Young’s modulus, M ¼ Eh3 =ð12ð1 n2 ÞÞ ¼ flexural rigidity of the plate and n is Poisson’s ratio. (ii) cprpa (outer portion) F CR r r r2 ½c2 þ b2 ð1 þ gÞ2 2 2 2 2 2 2 2 2 2 X¼ 2a 2r þ 2½c þ b ð1 þ gÞ ln þ 4r ln þ ½c þ b ð1 þ gÞ .ð4Þ a a a2 32pM Differentiation of Eq. (4) with respect to da yields dX F CR a ½1 þ Zð1 þ gÞs 2 2 ¼ k3 þ k4 ½1 þ Zð1 þ gÞ s þ k5 ln , da 16pM 2
(5)
where k3 ¼
6
½1 þ Zð1 þ gÞ2 þ 4½1 þ Z2 ð1 þ gÞ2 2 s þ ½1 þ Zð1 þ gÞ2 ½1 þ Z2 ð1 þ gÞ2 s4 2
2ð1 nÞ½k1 þ 1 Z2 s2 ð1 þ gÞ2 þ Z2 s2 ð1 þ gÞ2 ½ð1 þ nÞk1 þ 4 ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ
þ ð1 nÞ
k4 ¼
ð1 nÞ½k1 þ 2 2Z2 s2 ð1 þ gÞ2 þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞfk1 4 ln½Zsð1 þ gÞg , ½ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ2
1 ð1 nÞfk1 þ ½1 þ Z2 ð1 þ gÞ2 s2 g þ Z2 s4 ð1 þ gÞ2 ð1 þ nÞ½1 þ Z2 ð1 þ gÞ2 þ 2 4½ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ ð1 nÞ
k5 ¼
ð1 nÞ½k1 þ 2 2Z2 s2 ð1 þ gÞ2 þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞfk1 4 ln½Zsð1 þ gÞg , 4½ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ2
4Z2 s2 ð1 þ gÞ2 f2ð1 þ nÞ ln½Zsð1 þ gÞ þ ð1 þ nÞg 8Z2 s2 ð1 þ gÞ2 ð1 nÞfð1 þ nÞ ln½Zsð1 þ gÞ þ 1g . ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ ½ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ2
The stored strain energy is (i) 0prpc (inner portion) # Z c " d2 X 1 dX 2 U1 ¼ p M þ r dr dr2 r dr b ¼
pMC 21 c2 F 2CR c2 F CR C 1 c2 k6 þ ð2 ln s 1Þ Z2 ð1 þ gÞ2 ½2 ln Zsð1 þ gÞ 1 þ 1 Z2 ð 1 þ g Þ 2 , 16pM 4 2
where k6 ¼ ð2 ln2 s 2 ln s þ 1Þ Z2 ð1 þ gÞ2 f2 ln2 ½Zsð1 þ gÞ 2 ln½Zsð1 þ gÞ þ 1g.
ð6Þ
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(ii) cprpa (outer portion) # Z c " d2 X 1 dX 2 r dr U2 ¼ p M þ dr2 r dr b F 2CR a2 1 s2 ½1 þ Z2 ð1 þ gÞ2 2 4 ½1 þ Z2 ð1 þ gÞ2 2 6 2 2 4 2 2 ¼ 2s ln s þ s . s þ ½1 þ Z ð1 þ gÞ s ln s 16pM 2 2 8 8
ð7Þ
The total strain energy is F 2CR c2 F CR C 1 c2 pMC 21 c2 k6 þ fð2 ln s 1Þ Z2 ð1 þ gÞ2 ½2 ln Zsð1 þ gÞ 1g þ ½1 Z2 ð1 þ gÞ2 16pM 4 2 F 2 a2 1 s 2 ½1 þ Z2 ð1 þ gÞ2 2 4 ½1 þ Z2 ð1 þ gÞ2 2 6 2s2 ln2 s þ þ CR s þ ½1 þ Z2 ð1 þ gÞ2 s4 ln s s 16pM 2 2 8 8
U¼
ð8Þ
and dU F2 a ¼ CR da 16pM
k7 k9 k9 ð1 nÞk10 ð1 nÞk7 k10 k7 þ k11 ½1 Z2 ð1 þ gÞ2 1 þ k 8 s2 þ . 2 2 2 2
ð9Þ
k7 ¼ 2s2 ð1 2 ln sÞ 2Z2 s2 ð1 þ gÞ2 f1 2 ln½Zsð1 þ gÞg, k8 ¼
ð1 nÞðk1 þ 2Þ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞk1 2Z2 s2 ð1 þ gÞ2 f2ð1 þ nÞ ln½Zsð1 þ gÞ þ ð1 nÞg , ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ
k9 ¼
ð1 nÞfk1 þ ½1 þ Z2 ð1 þ gÞ2 s2 g þ Z2 s4 ð1 þ gÞ2 ð1 þ nÞ½1 þ Z2 ð1 þ gÞ2 , ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ
ð1 nÞ½k1 þ 2 2Z2 s2 ð1 þ gÞ2 þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞfk1 4 ln½Zsð1 þ gÞg , ½ð1 nÞ þ Z2 s2 ð1 þ gÞ2 ð1 þ nÞ2 ½1 þ Z2 ð1 þ gÞ2 2 4 ½1 þ Z2 ð1 þ gÞ2 2 6 2 4 2 4 2 2 2 s 2½1 þ Z ð1 þ gÞ s ln s þ s . ¼ 1 þ 4s ln s ½1 þ Z ð1 þ gÞ s 4 2
k10 ¼
k11
Substituting Eqs. (2), (5) and (9) into Eq. (1), one obtains the thrust force of the core drill with pre-drilled pilot hole at the onset of crack propagation as 8 > > <
91=2 > > =
32G IC M F CR ¼ p > > k k ð1 nÞk k ½1 þ Zð1 þ gÞs k9 ð1 nÞk10 7 9 7 10 > : k3 k 7 þ ; þ ½1 þ Zð1 þ gÞ2 k4 s2 þ ½1 Z2 ð1 þ gÞ2 1 þ k11 þ k5 ln k 8 s2 > 2 2 2 2 2
,
(10) where s ¼ c=a
and
Z ¼ b=c.
3. Experimental setup 3.1. Specimen preparation Composite laminates are made from woven WFC200 fabric carbon fiber prepregs with the stacking sequence of [01/ 901]12S. The laminates are cured in an autoclave at 150 1C and 600 KPa. The plates are cut into coupon specimens of 60 mm by 60 mm. Twenty-four lamina make the plate thickness 6 mm. The fiber volume fraction is 0.55, the Young’s modulus, Poisson’s ratio and strain energy release rate are measured 18.4 GPa, 0.3 and 140 J/m2, respectively [18]. 3.2. Drilling test Drilling tests were carried out on a vertical machining center. The thrust forces during drilling were measured with a Kistler 9273 piezoelectric dynamometer and Kistler 5019 charge amplifiers and were stored on a TEAC DR-F1 digital
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Spindle Chuck
Drill
Workpiece
Fixture
Dynamometer Kistler 9273 Recorder TEAC DR-F1 Amplifier Kistler 5019 Drill machining table
Fig. 3. Schematic of experimental setup.
recorder subsequently. The amplifiers have to stabilize for at least an hour. The drilling set-up used in data collection is shown in Fig. 3. The core drills are 10 mm in diameter plated with diamond of a #60 grit size at front end. The thickness of core drill is 0.75 mm and 1.2 mm, respectively. All drilling tests were conducted coolant free at a spindle speed of 1000 rpm and feed rates of 8, 10, 11 and 12 mm/min. 4. Results and discussion Eq. (10) indicates that the critical thrust force for specimens with pre-drilled pilot hole is a function of the ratio of the drill diameter to delamination diameter (s) and the ratio of the pilot hole to drill diameter (Z). In the Tsao-Hocheng model [19] without a pilot hole, the critical thrust force is given by
FR ¼ p
8 > > > < > > > :1
91=2 > > > =
32G IC M 2 2 > 3b 4ð1 bÞ ð6 12b þ 11b2 5b3 þ b4 Þ 2ð1 bÞ2 ð2 2b þ b2 Þ > 4 4b þ þ lnð1 bÞ s2 þ þ lnð1 bÞ s4 > ; 2 bð2 bÞ 2 bð2 bÞ
,
(11) where b is the ratio between thickness and radius of core drill (namely, b ¼ t=c). To evaluate the effect of pre-drilled pilot holes on the critical thrust force value, the critical thrust force predicted by Eq. (10) was compared with that of Eq. (11) at b ¼ 0.2 in Fig. 4 much clearer than the early reference [13–14]. It shows that the critical thrust force decreases slightly for specimens with pre-drilled pilot holes. On the other hand, one realizes the drilling thrust force will be significantly reduced by removal of the chip contact in drilling. The critical thrust force ratio without and with pre-drilled pilot hole for b ¼ 0.2 and Z ¼ 0.2 at various Poisson’s ratio is shown in Fig. 5(a). Note that when s increases the critical thrust force ratio increases. The presence of larger the Poisson’s ratio leads to an increase in the critical thrust force ratio at sX0:5. From Fig. 5(b), it is known that a smaller value of b is necessary if a higher critical thrust force ratio for n ¼ 0:3 and Z ¼ 0:4 is required. For the relationship between the pilot hole to diameter ratio (Z) and the critical thrust force ratio is shown in Fig. 5(c). It illustrates that the lower the pilot hole to diameter ratio (lower Z), the larger is the critical thrust force ratio. The critical thrust force with pre-drilled pilot hole has a maximum at a certain s approach to 1. Fig. 6(a) shows the variation of the critical thrust force as a function of with pre-drilled pilot hole to diameter ratio (Z) for various Poisson’s ratio at g ¼ 0.2 and s ¼ 1.0. It is evident that the higher the Poisson’s ratio (larger n), the lower is the Z*. The thrust force of core drill with pre-drilled pilot hole remains little different at sX0:5. Fig. 6(b) illustrates the variation of the critical thrust force with Z for different ratio of pilot hole to inner uncut portion of core drill (g) at n ¼ 0.3 and s ¼ 1.0. It can be found that the critical thrust force with pre-drilled pilot hole decreases with the increasing g. These results
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Critical Thrust Force [ ( 32GICM ) 0.5 ]
6 Without Pilot Hole With Pilot Hole, η = 0.4
5
4
3
2
1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Drill Diameter to Delamination Diameter Ratio ( s = c / a )
Fig. 4. Comparison of critical thrust force between Eq. (10) and Eq. (11) at b ¼ 0:2.
2.5 2 1.5 1 0.5 0
(a)
2.5 ν = 0.4 ν = 0.3 ν = 0.2
Critical Thrust Force Ratio ( FR / FCR )
Critical Thrust Force Ratio ( FR / FCR )
3
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Drill Diameter to Delamination Diameter Ratio ( s = c / a )
2 1.5 1 0.5 0
(b)
β = 0.2 β = 0.3 β = 0.4
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Drill Diameter to Delamination Diameter Ratio ( s = c / a )
Critical Thrust Force Ratio ( FR / FCR )
2.5 2 1.5 1 0.5 0
(c)
η = 0.2 η = 0.4 η = 0.6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Drill Diameter to Delamination Diameter Ratio ( s = c / a )
Fig. 5. Critical thrust force ratio for core drill without and with pre-drilled pilot hole. (a) b ¼ 0:2 and Z ¼ 0:2. (b) n ¼ 0:3 and Z ¼ 0:4. (c) n ¼ 0:3 and b ¼ 0:2.
indicate that g has a significant influence on the critical thrust force with pre-drilled pilot hole and can be easily understood. And with the decreasing g, the larger is the Z . When n ¼ 0:3, g ¼ 0:2 and s ¼ 1:0, the Z is found to be 0.505. Fig. 7 indicated the thrust force increases with feed rate. Experimental results of thrust force are shown in Table 1. Without pre-drilled pilot hole (e.g. Z ¼ 0:85), the thrust force during drilling lies quite well above the critical thrust force, which implies the occurrence of delamination at commonly used feed rate. With pre-drilled pilot hole (e.g. Z ¼ 0:85),
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0.5 Critical Thrust Force ( FCR ) [ (32GICM ) 0.5 ]
Critical Thrust Force ( FCR ) [ (32GICM ) 0.5 ]
0.55
0.5
0.45
0.4
0.35 ν = 0.2, With Pilot Hole ν = 0.3, With Pilot Hole
0.3
ν = 0.4, With Pilot Hole
0.25
0.45
0.4
0.35
0.3 γ = 0.2, With Pilot Hole
γ = 0.6, With Pilot Hole
0.2 0
0.1
0.2
(a)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
γ = 0.4, With Pilot Hole
0.25
1
0
0.1
0.2
(b)
Pilot Hole to Drill Ratio ( = b / c )
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Pilot Hole to Drill Ratio ( = b / c )
Fig. 6. Critical thrust force with pre-drilled pilot hole to drill ratio. (a) g ¼ 0:2 and s ¼ 1:0. (b) n ¼ 0:3 and s ¼ 1:0.
60 η = 0.76, Without Pilot Hole η = 0.85, Without Pilot Hole η = 0.76, With Pilot Hole η = 0.85, With Pilot Hole
55
Thrust Force ( N )
50 45 40 critical thrust force at = 0.85
35 30 critical thrust force at = 0.76
25 20 7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
Feed Rate ( mm / min )
Fig. 7. Effects of pre-drilled pilot hole on thrust forces (drill diameter ¼ 10 mm, Z ¼ 0.76 and 0.85).
Table 1 Experimental results of thrust force Pilot hole to drill ratio (Z)
Feed rate (mm/rev)
With pilot hole thrust force (N)
Without pilot hole thrust force (N)
0.85
0.008 0.010 0.011 0.012
23.6 24.4 26.3 28.4
31.7 36.2 44.7 48.5
0.76
0.008 0.010 0.011 0.012
29.4 30.6 32.7 35.9
33.4 40.1 49.6 55.2
however, the thrust force during drilling can be lower than the critical value at large range of feed rates, which indicate the delamination-free drilling is achievable. As shown in Fig. 7, Z ¼ 0:85 with pre-drilled pilot hole allows for the largest drilling feed rate, while Z ¼ 0:76 without pre-drilled pilot hole has to be operated at the lowest drilling feed rate to avoid the
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delamination. The results agree well with both the analytical findings in Eq. (10) and the industrial experience. The safety margin of the process window in use of core drill with pre-drilled pilot hole is therefore wider and higher drilling efficiency is achieved. The use of pre-drilled pilot hole is thus illustrated. Based on these experiments, the removal chip for core drill is a major contributor to the thrust force. However, when large feed rate is used, the experimental thrust force increases. By setting properly the drilling conditions, thickness and predrilled pilot hole, one can produce delamination-free hole. The current analysis explains how to use the pre-drilled pilot hole to enhance quality drilling of composite material using core drill. 5. Conclusions An analytical approach to identifying the pilot hole between drill diameter (Z) and inner uncut portion (g) for delamination-free drilling based on linear elastic fracture mechanics is derived in this study. The prediction of the model agrees quantify with the experimental results. Due to the different pre-drilled pilot hole, the core drill shows different level of the drilling thrust force varying with the feed rate. The advantageous pre-drilled pilot hole is explained based on the theoretical models that illustrate the thrust force exerted by core drill. Fig. 7 has shown the verification and validation of the proposed method. Experimental results indicate the critical thrust force is reduced with pre-drilled pilot hole, while the drilling thrust is largely reduced by removal of chip effect. Controlling the ratio of pre-drilled pilot hole relative to drill diameter, one can conduct medium to large hole of composite laminates drilling at higher feed rate without delamination damage. This approach can provide a useful design chart for industrial engineers. Acknowledgement The research is partially supported by National Science Council, Taiwan, ROC, under contract NSC94-2212-E-233-001. References [1] H. Hocheng, C.K.H. Dharan, Delamination during drilling in composite laminates, Transactions of the ASME, Journal of Engineering for Industry 112 (1990) 236–239. [2] S. Jain, D.C.H. Yang, Effects of feedrate and chisel edge on delamination in composite drilling, Transactions of the ASME, Journal of Engineering for Industry 115 (1993) 398–405. [3] S. Jain, D.C.H. Yang, Delamination-free drilling of composite laminates, Transactions of the ASME, Journal of Engineering for Industry 116 (1994) 475–481. [4] J.P. Jung, G.W. Kim, K.Y. Lee, Critical thrust force at delamination propagation during drilling of angle-ply laminates, Composite Structures 68 (2005) 391–397. [5] G.W. Kim, K.Y. Lee, Critical thrust force at propagation of delamination zone due to drilling of FRP/metallic strips, Composite Structures 69 (2005) 137–141. [6] A. Kobayashi, Machining of Plastics, McGraw-Hill, New York, 1967. [7] D.F. Galloway, Some experiments on the influence of various factors on drill performance, Transactions of the ASME 79 (1957) 191–237. [8] J.P. Davim, P. Reis, C.C. Anto´nio, Experimental study of drilling glass fiber reinforced plastics (GFRP) manufactured by hand lay-up, Composites Science and Technology 64 (2) (2004) 289–297. [9] C.C. Tsao, H. Hocheng, Taguchi analysis of delamination associated with various drill bits in drilling of composite material, International Journal of Machine Tools & Manufacture 44 (10) (2004) 1085–1090. [10] H. Hocheng, C.C. Tsao, Comprehensive analysis of delamination in drilling of composite materials with various drill bits, Journal of Materials Processing Technology 140 (2003) 335–339. [11] J. Mathew, N. Ramakrishnan, N.K. Naik, Investigations into the effect of geometry of a trepanning tool on thrust and torque during drilling of GFRP composites, Journal of Materials Processing Technology 91 (1999) 1–11. [12] J. Mathew, N. Ramakrishnan, N.K. Naik, Trepanning on unidirectional composites: delamination studies, Composites Part A 30 (8) (1999) 951–959. [13] M.S. Won, C.K.H. Dharan, Chisel edge and pilot hole effects in drilling composite laminates, Transactions of the ASME, Journal of Manufacturing Science and Engineering 124 (2002) 242–247. [14] C.C. Tsao, H. Hocheng, The effect of chisel length and associated pilot hole on delamination when drilling composite materials, International Journal of Machine Tools & Manufacture 43 (11) (2003) 1087–1092. [15] H. Saghizadeh, C.K.H. Dharan, Delamination fracture toughness of graphite and aramid epoxy composites, Transactions of the ASME, Journal of Engineering Materials and Technology 108 (1986) 290–295. [16] H. Hocheng, C.C. Tsao, Analysis of delamination in drilling composite materials using core drill, Australasian Journal of Mechanical Engineering 1 (1) (2003) 49–53. [17] S. Timoshenko, S. Woinowsky-Keiger, Theory of Plates and Shells, second ed., McGraw-Hill, New York, 1959. [18] The Center for Aviation & Space Technology of Industrial Technology Research Institute, Private communication regarding material test, 2004. [19] C.C. Tsao, H. Hocheng, Effects of exit back-up on delamination in drilling composite materials using a saw drill and a core drill, International Journal of Machine Tools & Manufacture 45 (11) (2005) 1261–1270.