116926941 drilling at high speed

7/29/2019 116926941 Drilling at High Speed

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Effect of drill geometries in drilling of glass fiber reinforced plastic (GFRP) athigh spindle speed

V Krishnaraj*, Department of Mechanical Engineering, PSG College of Technology,Coimbatore 641 004, India

Abstract

High speed machining is now recognised as one of the key manufacturing

technologies for higher productivity and throughput. Drilling experiments were

conducted with drill geometries, namely standard twist drill, double cone drill, Zhirov-

point drill, and multi facet drill, using wide range of spindle speed, and feed rate. Thrust

force, delamination and surface roughness were measured and studied in the test

trials. The analysis of variance (ANOVA) is employed to investigate the drilling

characteristics. From the experiments it is found that standard twist drill and double

cone could be used successfully at high spindle speed and low feed rate since the

cutting force is less (thrust force and torque recorded a very low value). The special

geometry improves the quality of the hole further, especially Zhirov point drill (with

surface finish values of 4-5m). Multifacet drill is found superior as for as the

delamination value is concerned

Keywords:High speed drilling; Drill geometries; Thrust force; Delamination; Surface

Roughness; ANOVA

*Corresponding author:[email protected]
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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1. Introduction

Drilling is one the major machining operations which is carried out on fiber-

reinforced composite materials owing to the need for components assembly in

mechanical structures. For example, over 100,000 holes are made for a small single

engine aircraft, in a large transport aircraft millions of holes are made mostly for

fasteners like rivets, bolts etc. The quality of the drilled hole can be critical to the life of

the joints for which the holes are used. Aspects of the hole such as

waviness/roundness of its wall surface, axial straightness and roundness of the hole

cross-sections can cause high stress on the joints, leading to its failure [1]. There are

many problems encountered when drilling fiber-reinforced composites. These

problems include delamination of the composite, rapid tool wear and fiber pullout [2-4].

The delamination of composites is main concern and its presence will reduce the

strength against fatigue, results in a poor assembly tolerance and affects the

composites structures integrity [5]. Cheng and Dharan [6] used fracture mechanics

approach to analyze the delamination of fiber-reinforced materials. They cited that

thrust force is the main cause for delamination and predicted the critical thrust force

above which delamination is initiated. Tagliaferri, Caprino and Diterlizzi [7,8] studied

the effect of machining parameter and tool conditions on the damage, finish and

mechanical properties of fiber-reinforced composite materials and cutting mechanism

in drilling. Chen [9] carried experimental investigation on carbon/epoxy composite and

recommended that the high speed and low feed rate are key factors for producing

delamination free and good surface finish holes. Increasing the cutting speed will

certainly increase production rate. Another possible benefit of increasing the cutting

speed is the reduction of cutting forces. It has already been found that increase of

cutting speed may decrease the cutting force when cutting aluminum [10]. If increasing


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the cutting speed can reduce the thrust force, the delamination may be overcome.

Mustapha Elahachimi, Serge Torbaty, and Pierre Joyot [11, 12] developed a

theoretical model to predict thrust force and torque in high speed drilling in terms of

geometric features of the drill, cutting conditions and the properties of the machined

material. Lin et al. [13, 23] studied the effects of increasing drilling speed on the thrust

force as well as other drilling characteristics on carbon/epoxy composites and

unidirectional glass fiber-reinforced composites. They concluded that drill wear is the

major problem at high spindle speed. Piqute et al. [14] carried out a study of drilling

thin carbon/epoxy laminates with two types of drills, a helical drill and a drill of special

geometry and concluded that both drills lead to damage at the entrance in wall and exit

of the hole, with the exception of special geometry drill which is possible to cause a

significant reduction in the final damage. Delamination is one of the serious concerns

in drilling holes in composite materials at the bottom surface of the workpiece [22].

Quite a few references of the drilling of fiber reinforced plastics report that the quality

of cut is strongly dependent on drilling parameter as well as drill geometry.

Some of the key solutions for successfully machining composites include high

spindle speeds, light or shallow cuts. In this work, an attempt is made to study the

effects of higher spindle speed in drilling of woven fiber GFRP with different drill

geometries. The results of the experiments are presented in this paper.

2. Experimental procedure

2.1. Work piece material and cutting tool

Woven E-glass fiber reinforced epoxy laminates were prepared by hand lay-up

process. The laminate consisted of 35 layers with a nominal thickness of 9.5 mm and

fiber content (Vf) of 45% by volume was used in this study. Due to glass fiber content,


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the work piece material is very abrasive to cutting tools and chips produced by

machining are hazardous and irritating to the skin and lungs.

Due to high wear resistance while drilling fiber-reinforced materials, micro-grain

carbide ( 10 mm) was used in this investigation [1, 15]. Four different drill

geometries are used to study the effect of high speed on GFRP laminates. One is a

standard twist drill (Fig.1-a) with 118 point angle and 30 helix angle. The second type

is a double cone drill (Fig.1-b), which has two-point angles. The first section is short

and has an included angle of 70 -75 while the second is a longer section with point

angle of 116 -118. The first portion having an included angle 70-75reduces the

chip thickness and improves surface finish. The third type is Zhirov point drill (Fig. 1-c),

which has a triple lip at each cutting edge, an extra rake ground on the face of the lips,

and a split point. The shortest lip has an included angle of 55, the intermediate lip has

a point angle of 70, and the largest lip has the standard point angle of 118.The chisel

edge has been reduced by a slot or groove. Therefore, extrusion action is replaced by

cutting action. This design results in reduced feed thrust compared to conventional

drill, permitting a higher feed rate with an acceptable drill life. The drill life also

improved due to the presence of triple lips. More dimensionally accurate holes can be

produced because of less spindle deflection by the reduction of thrust force [16]. The

fourth type is the multifacet drill (Fig. 1-d). The chisel edge length is only 0.3 mm to

reduce the thrust force. In order to strengthen the reduced chisel edge length, the tip

height is designed to be 3mm with an inner point angle of 135 (2). The large value

of inner point angle is also required to avoid high-temperature generation by heat

transfer in the reduced chisel edge. An arc cutting edge with a radius of 1.0 mm is


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made to increase the rake angle. The arc cutting edge is effective in dividing chip and

drill centering [17].

2.2 Experimental set up

Experiments were conducted using Acumac high-speed spindle (5kW) mounted

on a vertical CNC machine. Fig. 2 shows the experimental set-up. Machining of

laminates was carried out for the following conditions.

Spindle speed: 14,000, 16,500 & 19,000 rpm

Feed rate: 0.01, 0.03, 0.05, 0.08 mm/rev

Geometry: Standard twist drill, Double Cone, Zhirov and multifacet [16, 17]

A Syscon two-component drilling dynamometer (model: SI-674) of strain gauge

type was used to measure the axial thrust force and torque. The proportional charge

output from the dynamometer was fed to a Syscon amplifier (model SI-223D), thus

producing a scaled voltage output signal proportional to the applied load. The thrust

force was continuously monitored and recorded using a digital storage Oscilloscope

(Tektronix Model: TDS 210 with 60 MHz bandwidth, 1GS/s sample rate and 2500

points record length for each channel). Delamination was measured using a scanner

as suggested by Khashaba [21]. Surface roughness of the drilled hole was measured

using surface profilometer with ruby crystal probe (model of Taylor Hobson pneumo

Surtronic 3+).

3. Results and discussion

3.1. Influence of cutting parameters on thrust force

Drilling parameters cause change in cutting forces, which lead to difference in

quality of the holes in terms of surface finish, circularity, delamination, fiber pull out,

matrix cratering, etc. From the experiments it was found that increasing spindle speed

and feed increase the thrust force, especially feed rate, this is because the larger the


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feed rate, the larger the cross sectional area of the undeformed chip will be, the

greater the resistance of chip formation and consequently the greater the axial thrust

force and torque. As can be seen in the Fig.3 Zhirov point drill can be drilled with lower

thrust force for the same operating conditions when compared to other geometries.

This is because in the Zhirov drill the chisel edge has been replaced by a slot,

therefore extrusion action is replaced by cutting action. The Zhirov-point drill also

produces more dimensionally accurate holes (hole deviation within 10m) because of

less deflection in the spindle through a reduction of the thrust force.

At lower feed rate (0.01 mm/rev) standard twist drill, double cone and multifacet

generated more or less same thrust force (around 20 N). This value is very less when

compared to drilling at normal spindle speed (around 50 N). For all the drill geometries

and cutting parameters the torque values are between 0.1 Nm to 0.2 Nm. Not much

variation in the torque values are recorded within the range examined.

Table 1 shows the results of the analysis of variance with the thrust force in

GFRP material by considering drill geometry as a factor [18-20]. From the analysis of

Table 1, we can observe that the feed rate factor (P= 63.13 %) has statistical and

physical significance on the trust force followed by drill geometries (P=27.98 %). The

spindle speed factor (P=2.76%) on thrust force does not present percentage of

physical significance of contribution because P(percentage of contribution) < Error

associated. Notice that the error associated to the table ANOVA for the thrust force is

approximately 6.13%.

3.2. Influence of cutting parameters on delamination factor

Delamination near the exit side is introduced as the tool acts like a punch,

separating the thin uncut layer from the remainder of the laminate. The entry hole


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produced was neat for all the geometries. However, the fiber pull out at exit was more

in the case of twist drill and Zhirov drill. Multifacet drill produced clean cut holes at the

exit side of the laminate. This is because the cutting mechanism of a multifacet at the

last ply is like a trepanning with knife-edge. Therefore, exit hole was neat and fuzzy

free. A button like chip was ejected at the exit side of the laminate while drilling using

multifacet drill.

The delamination was evaluated in terms of delamination factor. The

delamination factor is the ratio of maximum diameter (Dmax) of the damaged zone to

the actual hole diameter (D). Fig. 4 shows the relationship between the delamination

factor and drilling parameters. It is concluded that delamination factor increases with

feed rate and spindle speed. Fig. 5 shows the hole machined in the drilling process for

standard twist drill, double cone, Zhirov, and multifacet drill respectively. Multifacet drill

presents better performance than other drill geometries. The special characteristic of

the drill is the extreme sickle-form design of the cutting edges. This pre-stresses the

fibers in the direction of pull and separates them in the direction of thrust. This results

in a clean cut with a smooth surface. The delamination is less compared to other drill

geometries.

From the analysis of Table 2, we can observe that the feed rate (P=91.14%)

and the drill geometry (P=5.85) followed by spindle speed have statistical and physical

significance on the delamination factor obtained, especially feed rate factor.

3.3 Influence of cutting parameters on surface roughness

After the drilling test, the quality of hole at entry and exit has been examined.

The surface roughness (Ra) was evaluated as per ISO 4287/1. For each test 3

measurements over drilling surfaces were made. Fig. 6 shows the effect of drill


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geometry on surface finish. The value of surface roughness increases with the feed

rate, and decreases with the cutting speed. Zhirov drill produced good surface finish

(4-5m) at lower feed rate and the circularity of the hole was also good (measured

using CMM, the values were within 10 m). The outer most lip produced thin chip

which improves the finish of the hole. Multifacet and double cone also generated good

surface finish at lower feed rate when compared to standard twist drill.

From the analysis of Table 3, we can observe that the feed rate (P=45.27%)

and the drill geometry (P= 28.33) have statistical and physical significance on the

surface finosh obtained, especially feed rate factor. The spindle speed factor (P=2.52

%) on surface roughness does not present percentage of physical significance of

contribution because P (percentage of contribution) < Error associated. Notice that the

error associated to the table ANOVA for the surface finish is approximately 23.88%

[18-20].

4. Conclusions

In drilling of composites, high spindle speed and low feed rate improves the

machinability aspects within the range examined. The cutting force is less (thrust force

and torque both recorded a very low value). The special geometry improves the quality

of the hole further, especially Zhirov point drill. Standard and double cone was found

suitable for producing more number of holes at high spindle speed and low feed rate.

At a spindle speed of 16,000rpm and feed rate of 0.01mm/rev its performance is

comparable with (Cutting force, delamination and surface finish) Zhirov and multifacet

drills. Fifty holes were drilled for each geometry and found that the force values are

stable after the initial increase. When the geometries of the different drills were

examined, the chisel edge of the multifacet was found damaged, and chip off was

found in the lip of the Zhirov, where as in standard and double cone uniform wear on


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the flank was found. When the standard drill was subjected to drill wear study it lasted

up to 250 holes.

The feed rate is the cutting parameter, which has influence on thrust force

(63.13%) followed by drill geometry (P=27.98 %). The special geometries contribute to

the thrust force.

The feed rate contribution on delamination is (P=91.14%) high followed by drill

geometry (P=5.58%). The feed rate and drill geometry have contributions on surface

roughness (Ra) (P=45.27% and P=28.33%).

Acknowledgement

The authors gratefully acknowledge Department of Science and Technology,

Govt. of India, (Grant No: III.5(75)/2001-SERC-Engg) for funding this research work.

References

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GFR/epoxy composites, Composite Structure, 63 (2004), 329-338.

[2].S.K.Malhotra, Some studies on drilling of fibrous composites, Journal of Material

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[3].H.Ho-Cheng, H.Y.Puw, On drilling characteristics of fiber-reinforced thermosets and

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583-592.

[4]. Ranga Komanduri, Machining fiber-reinforced composites, Mechanical

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[6].H.Ho-Cheng, C.K.H.Dharan, Delamination during drilling in composite laminates,

ASME Journal of Engineering for Industries, 112 (1990) 236-239.


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[7].V.Tagliaferri, G.Caprino, A.Diterlizzi , Effect of drilling parameters on the finish and

mechanical properties of GFRP composites, International Journal of Machine Tool &

Manufacture, 30(1)(1990) 77-84.

[8].A.Dillio, V.Tagliaferri, F.Veniali, Cutting mechanism in drilling of aramide

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[9].Wen-Chou Chen, Some Experimental investigation in the drilling of carbon fiber

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speed drilling 1: predicting torque and thrust, International Journal of Machine Tool &

Manufacture, 39(1999) 553-568.

[12]. Mustapha Elahachimi, Serge Torbaty, Pierre Joyot, Mechanical modelling of high

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Manufacture, 39(1999) 569-581.

[13].S.C.Lin, I.K.Chen, Drilling carbon fiber-reinforced composite material at high

speed, Wear 194(1996) 156-162.

[14].Piquet R,Ferret B, Lachaud F, Swinder P, Experimental analysis of drilling

damage in thin carbon/epoxy laminate using special drills, Composites part A: Applied

Science & Manufacturing 31(10) (2000)1107-1115.


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[15].K.Sakuma, Y.Yakoo, M.Seto, Study on drilling of fiber-reinforced plastic-relation

between tool material and wear behavior, Bulletin of the JSME,27(228) (1984) 1237-

1244

[16].A. Bhattacharya, Metal cutting theory and practice, New central book agency,

Calcutta, India, 2000, pp.603-605.

[17].S.M.Wu, Multifacet drills, in: Robert I. King (Ed), Handbook of High-speed

Machining Technology, Chapman and Hall, 1985, pp.305-316.

[18] J.Paulo Davim, Pedro Reis, Study of delamination in drilling fiber reinforced

plastics (CFRP) using design experiments, Composite Structures 59 (2003) 481-487.

[19] CC Tsao, H Hocheng, Taguchi analysis of delamination associated with various

drill bits in drilling of composite material, International journal of machine tools and

manufacture, 44(2004) 1085-1090.

[20] J Paulo Davim, Pedro Reis, C Conceicao Antonio, Experimental study of drilling

glass fiber reinforced plastics (GFRP) manufactured by hand lay-up, Composite

Science and Technology, 64(2)(2004)287-297

[21] UA Khashaba, Delamination in drilling GFR-thermoset composite structures,

Composite structures, 63(3) (2004) 313-327.

[22] CC Tsao, H.Hocheng, Effect of exit back-up on delamination in drilling composite

materials using saw drill and core drill, International Journal of Machine Tools &

Manufacture, 54 (2005) 1261-1270.

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materials at high speed, Journal of Composite Materials 33(9) 1999.


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(a) (b)

(c) (d)

Fig.1 View showing geometry data of: (a) Standard twist drill (b) Double cone

drill (c) Zhirov-point drill (d) Multifacet drill


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Fig. 2 Photographic view of the experimental set-up

High speed spindle

Strain gauge

dynamometer

Oscilloscope

Dynamometer

out put

AC Inverter

GFRP laminate


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Feed Vs Thrust force for std. twist drill

0

10

20

30

40

50

60

70

80

0.01 0.03 0.05 0.07

Feed rate (mm/rev)

Thrustforce(N)

14000 rpm

16500 rpm

19000 rpm

Feed Vs Thrust force for double cone drill

0

10

20

30

40

50

60

70

80

0.01 0.03 0.05 0.07Feed rate (mm/rev)

Thrustforce(N)

14000 rpm

16500 rpm

19000 rpm

(a) (b)

Feed rate Vs Thrust force for Zhirov drill

0

20

40

60

80

0.01 0.03 0.05 0.07

Feed rate (mm/rev)

T

hrustforce(N)

14000 rpm

16500 rpm

19000 rpm

Feed rate Vs Thrust force for multifacet

drill

0

20

40

60

80

0.01 0.03 0.05 0.07Feed rate (mm/rev)

Thrustforce(N)

14000 rpm

16500 rpm

19000 rpm

(c) (d)

Fig. 3 Effect of feed rate & spindle speed on thrust force: (a) Standard drill

(b) Double cone drill (c) Zhirov-point drill (d) Multifacet drill


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Feed rate Vs Delamination factor for

Standard twist drill

1.07

1.09

1.11

1.13

1.15

1.17

1.19

1.21

0.01 0.03 0.05 0.07Feed rat (mm/rev)

Delaminationfactor

14000 rpm

16500 rpm

19000 rpm


Double cone drill

1.07

1.09

1.11

1.13

1.15

1.17

1.19

1.21

0.01 0.03 0.05 0.07Feed rate (mm/rev)

Delaminationfactor

14000 rpm

16500 rpm

19000 rpm

(a) (b)


Zhirov drill

1.07

1.09

1.11

1.13

1.15

1.17

1.19

1.21

0.01 0.03 0.05 0.07Feed rate (mm/rev)

Delaminati

onfactor

14000 rpm

16500 rpm

19000 rpm


Multifacet drill

1.07

1.09

1.11

1.131.15

1.17

1.19

1.21

0.01 0.03 0.05 0.07

Feed rate (mm/rev)

Delaminatio

nfactor

14000 rpm

16500 rpm

19000 rpm

(c) (d)

Fig. 4 Effect of feed rate & spindle speed on delamination: (a) Standard twist

drill; b) Double cone drill (c) Zhirov point drill; (d) Multifacet drill


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(a)

(b)

(c) (d)

Fig. 5 Effect of drill geometry on delamination: (a) Standard twist drill; (b) Double

cone drill (c) Zhirov point drill; (d) Multifacet drill


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Feed Vs Surface roughness for standard

twist drill

0

5

10

15

20

0.01 0.03 0.05 0.07Feed rate (mm/rev)

Surfaceroughness(m)

14000 rpm

16500 rpm

19000 rpm

Feed Vs Surface roughness for double

cone drill

0

5

10

15

20

0.01 0.03 0.05 0.07Feed rate (mm/rev)

Surfaceroughness(m)

14000 rpm

16500 rpm

19000 rpm

(a) (b)

Feed rate Vs Surface roughness for Zhirov

drill

0

5

10

15

20

0.01 0.03 0.05 0.07Feed rate (mm/rev)

Surfaceroughness(m) 14000 rpm

16500 rpm

19000 rpm

Feed rate Vs Surface roughness for

multifacet drill

0

5

10

15

20

0.01 0.03 0.05 0.07

Feed rate (mm/rev)

Surfaceroughness(m)

14000 rpm

16500 rpm

19000 rpm

(c) (d)

Fig. 6 Effect of feed rate & spindle speed on surface roughness: (a) Standard

twist drill (b) Double cone drill (c) Zhirov point drill; (d) Multifacet drill


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Table 1ANOVA for thrust force

SourceSum ofsquares

DOF VarianceVarianceratio(F)

F=0.5%Pure sum ofsquares(S')

% ofcontribution

Spindle speed 444.4 2 222.2141 11.562 3.239 405.99 2.76

Feed rate 9352 3 3117.393 162.212 2.846 9294.53 63.13

Drill Geometry 4178 3 1392.614 72.464 2.846 4120.19 27.98

Error 749.5 39 19.21793 903.24 6.13

Total 14724 47 14723.95 100.00


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Table 2 - ANOVA for delamination factor

SourceSum ofsquares



% ofcontribution

Spindle speed 0.0013 2 0.00065 33.80 3.239 0.001264 1.76

Feed rate 0.0657 3 0.02190 1137.20 2.846 0.065649 91.14

Drill Geometry 0.0043 3 0.00142 73.85 2.846 0.00421 5.85

Error 0.0007 39 1.93E-05 0.000905 1.25

Total0.0720

470.072027

100.00


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Table 3 - ANOVA for surface roughness

SourceSum ofsquares



% ofcontribution

Spindle speed 30.203 2 15.101 3.477 3.239 21.517 2.52

Feed rate 399.941 3 133.314 30.697 2.846 386.912 45.27

Drill Geometry 255.115 3 85.038 19.581 2.846 242.086 28.33

Error 169.373 394.343 204.117

23.88

Total 854.632 47 854.632 100.00


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FIGURE CAPTIONS

Fig.1 View showing geometry data of: (a) Standard twist drill (b) Double cone drill (c)Zhirov-point drill (d) Multifacet drill

Fig. 2 Photographic view of the experimental set-up

Fig. 3 Effect of feed rate & spindle speed on thrust force: (a) Standard drill (b) Double conedrill (c) Zhirov-point drill (d) Multifacet drill

Fig. 4 Effect of feed rate & spindle speed on delamination: (a) Standard twist drill; b) Double

cone drill (c) Zhirov point drill; (d) Multifacet drill

Fig. 5 Effect of drill geometry on delamination: (a) Standard twist drill; (b) Double cone drill(c) Zhirov point drill; (d) Multifacet drill

Fig. 6 Effect of feed rate & spindle speed on surface roughness: (a) Standard twist drill (b)Double cone drill (c) Zhirov point drill; (d) Multifacet drill

TABLE CAPTIONS

Table 1-ANOVA for thrust force

Table 2-ANOVA for delamination factorTable 3-ANOVA for surface roughness

116926941 drilling at high speed

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