experimental studies on hole quality and machinability characteristics in drilling of unreinforced...

JOURNAL OFC O M P O S I T EM AT E R I A L SArticle

Experimental studies on hole quality andmachinability characteristics in drilling ofunreinforced and reinforced polyamides

VN Gaitonde1, SR Karnik2, Juan Carlos Campos Rubio3,Wanderson de Oliveira Leite4 and JP Davim5

Abstract

In this study, experimental studies on hole quality and machinability in drilling of unreinforced polyamide (PA6) and

reinforced polyamide with 30% of glass fibers (PA66-GF30) using cemented carbide (K20) tool have been carried out.

The experiments have been planned as per full factorial design of experiments. The effects of spindle speed, feed rate,

and point angle on hole quality such as hole diameter and circularity error; the machinability characteristics such as

thrust force and specific cutting coefficient have been analyzed by developing response surface methodology based

second-order mathematical models. The parametric analysis shows that the quality of holes can be improved by

proper selection of cutting parameters. The analysis also indicates the influence of reinforced fiber on proposed machin-

ability characteristics during drilling of polyamides.

Keywords

PA6 and PA66-GF30 polyamides, drilling, hole quality, machinability, design of experiments

Introduction

Generally, engineering polymeric materials are used inmaking various machine parts because of lightweightand superior specific strength as compared to metallicmaterials. The material cost of engineering plastics iscompetitive and the machinability of these materials isfairly good.1 The polyamide is a polymer-containingmonomers of amides joined by peptide bonds. Theycan occur both naturally (i.e. proteins such as wooland silk) or can be made artificially (i.e. nylons, ara-mids, and sodium polyaspartate). In general, poly-amides present good compromise between toughnessand strength with low coefficient of friction and highthermal resistance.2

The polyamides are thermoplastic polymer compos-ites, widely used in numerous engineering fields such asaircrafts, automobiles, robots, and machines due tooutstanding property profile and hence replaced manyconventional metallic materials. The encouraging prop-erties include high specific strength and stiffness, wearresistance, dimensional stability, low weight, and direc-tional properties. The polyamides physically vary interms of melting point, glass transition temperature,

crystallinity, and tensile modulus, among the otherthings. The PA66 polyamide has a melting point of262�C, which is higher than that of PA6 at 219�C; itsglass transition temperature is 65�C against 52�C forPA6; the crystal structure of PA66 is triclinic, whereasPA6 has a monoclinic structure and its tensile modulusis 2.9GPa, while it is a little lower for PA6.

The addition of short fibers to polyamides enhancesthe properties over unreinforced polyamides. The

1Department of Industrial and Production Engineering, B.V.B. College of

Engineering and Technology, Hubli, Karnataka, India2Department of Electrical and Electronics Engineering, B.V.B. College of

Engineering and Technology, Hubli, Karnataka, India3Department of Mechanical Engineering, Federal University of Minas

Gerais, Brazil4Department of Production Engineering, Federal University of Minas

Gerais, Brazil5Department of Mechanical Engineering, University of Aveiro, Aveiro,

Portugal

Corresponding author:

VN Gaitonde, Department of Industrial and Production Engineering,

B.V.B. College of Engineering and Technology, Hubli 580 031,

Karnataka, India.

Email: [email protected]

Journal of Composite Materials

2014, Vol 48(1) 21–36

! The Author(s) 2012

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DOI: 10.1177/0021998312467552

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stiffness, strength, hardness, thermal stability, and fric-tional properties of reinforced plastics are much super-ior than that of unreinforced thermoplastics.3 Theinclusion of fibers can often provide a helpful increasein service temperature. The glass fibers are the commonreinforcements, which appreciably condense the expan-sion rate and enhance the flexural modulus of PA6.Alternatively, glass fiber reinforced polyamide is enor-mously abrasive when machined and brings out manyundesirable results such as rough surface finish, rapidtool wear, and faulty subsurface layer with cracks anddelaminations.

Even though the polyamides are produced to nearnet shaped, the machining has to be performed duringthe final production stage to get the finished compo-nents. Further, the mechanism of machining of poly-amide composites has been recognized as a processdifferent from that of homogeneous metal removal ofconventional materials. Hence, the successful perform-ance of machining operation is significantly affected bywork material properties. As a result of superior prop-erties and potential applications of unreinforced andreinforced polyamides, there is a need to understandthe manufacturing processes, essentially in machiningof these composites.4

Among the most widely used machining processes,drilling is perhaps the most important conventionalmechanical process associated with the manufactureof components made of fiber-reinforced plastic (FRP)composites. Drilling is essential to install the fastenersfor assembly of the composite laminates. Further, inaircraft industries, the drilling of these composites iscarried out for the purpose of joining using rivets,bolts, and nuts. Drilling of such materials is a challen-ging task to manufacturing engineers because of differ-ential machining properties.

Some researchers have performed experimentalinvestigations on machining of unreinforced and rein-forced polyamides. Mata et al.5 carried out an experi-mental study on unreinforced polyamide (PA6) andreinforced polyamide with 30% of glass fibers (PA66-GF30) turning using polycrystalline diamond tool.They analyzed the influence of glass fiber reinforcementon friction angle, shear plane angle, normal and shearstresses, and chip deformation under cutting condi-tions. The experimental model has also been comparedwith Merchant’s theoretical model. Davim and Mata6

conducted turning experiments on PA6 and PA66-GF30 polyamides with cemented carbide (K15) tools.They observed the effect of glass fiber reinforcement onfriction angle, shear plane angle, normal and shearstresses, and chip deformation under the prefixed cut-ting conditions. The experimental physical model wasalso compared with Merchant’s equation. Gaitondeet al.4 studied various machinability aspects such as

machining force, cutting power, and specific cuttingpressure during PA6 and PA66-GF30 turning usingcemented carbide (K10) tool. Gaitonde et al.7 alsoused Taguchi’s quality loss function for minimizingpower and specific cutting force during turning ofPA6 and PA66-GF30 polyamides. The artificialneural networks models were developed by Gaitondeet al.8 to analyze the influence of work material, toolmaterial, cutting speed, and feed rate on machiningforce, power, and specific cutting force during turningof PA6 and PA66-GF30 polyamides. They reportedthat the machinability is poor in reinforced polyamidewhen compared to unreinforced polyamide turning.

Several papers have been published aiming to evalu-ate the effect of cutting parameters and drill geometryon the machinability of FRP composites such as epoxyor polyester resins reinforced with aramid, glass, orcarbon fibers.9–11 Among the damages induced by dril-ling, delamination is probably the most severe andtherefore, a number of works has been concentratedon quantifying the damage level at the entrance andexit of the drill.12–17 Palanikumar et al.18 used designof experiments (DOE) to examine the effect of processparameters on surface roughness during turning ofglass fiber reinforced plastics composites. The researchfindings indicated that feed rate is the main factoraffecting surface roughness, followed by the cuttingspeed.

According to Guu et al.19 conventional machiningtechniques damage the workpiece through chipping,cracking, delamination, and high wear on cuttingtool. The structure for aerospace and automotive appli-cations contains holes for various purposes such asbolted and riveted joints and these joints are used totransfer load within the structure. As reported byPersson et al.,20 the quality and accuracy of the holesgreatly affect the joint strengths. The influence of highspeed (9550–38,650 r/min) on thrust force, torque, toolwear, and hole quality for multi-faceted and twist drillsduring drilling of carbon fibre reinforced plastic(CFRP) composites has been studied by Lin andChen.21 An increase in cutting speed decreases the cut-ting force, which in turn minimizes the delaminationand thus increasing the production rate. The relation-ship between the thrust force and the amount ofdamage was also developed in their experimental inves-tigations. The delamination study in high-speed drillingof CFRP composites was performed by Gaitondeet al.22 and Karnik et al.23 considering the speed,feed, and point angle as the process parameters. Theyproposed that the delamination can be minimized byemploying the higher speed with lower values of bothfeed rate and point angle.

Krishnaraj et al.24 carried out an investigative studyon thin CFRP laminates using K20 carbide drill by

22 Journal of Composite Materials 48(1)



varying the machining parameters to determine theoptimum cutting conditions. The hole diameter, circu-larity, and delamination tendency were analyzed. Tsaoand Hocheng25 presented an exhaustive study on thrustforce and surface roughness during drilling of compos-ite material using candlestick drill. The experimentalresults showed that the feed rate and the drill diameterare the major factors affecting the thrust force, whilethe feed rate and spindle speed contribute the most tothe surface roughness. An experimental study on dril-ling of CFRP/aluminum stack with carbide drills (K20)to analyze the influence of diameter, spindle speed, andfeed rate on thrust force, torque, surface finish, andhole diameters was carried out by Zitoune et al.26

Their experimental results revealed that the quality ofholes could be improved by proper selection of cuttingparameters. Campos Rubio et al.11 studied the surfaceroughness of the walls of holes generated by drillingpolyamide reinforced with glass whiskers (PA66-GF30) obtained from extruded bars. They affirmedthat the temperatures developed during machiningmight severely damage the workpiece, leading to melt-ing near the cutting edge path, damage finish, and dis-tortion of the component.

As seen from the literature, only few works havebeen carried out on cutting aspects of unreinforcedand reinforced polyamides drilling. Further, the rela-tionship among the influencing factors and their effectson hole quality and machinability are not known. Thus,the current study attempts to fill the gap by reportingan experimental study on drilling of polyamide withand without 30% glass fibers reinforcing (unreinforcedPA6 and PA66-GF30 reinforced) using cemented car-bide (K20) drill to study the influence of spindle speed,feed rate, and point angle on hole quality such as holediameter and circularity error; machinability aspectssuch as thrust force and specific cutting coefficient.

The traditional method involves the variation of oneparameter at a time, while other parameters are kept atfixed levels and hence, time consuming. Moreover, theconventional method not only requires huge number ofexperiments to be performed but also does not includethe interactive effects among the process parameters.The mathematical model development by response sur-face methodology (RSM) is a convenient method,which requires less number of experiments and thusreducing the cost and time.27 Hence, an attempt hasbeen made in this investigation to construct theRSM-based mathematical models of hole quality andmachinability characteristics with reduced number ofexperiments. The experiments have been planned asper full factorial design (FFD), which allows thestudy of interactions among the process parameters.The effects of spindle speed, feed rate, and pointangle on hole diameter, circularity, thrust, and specific

cutting coefficient have been analyzed by developingsecond-order mathematical models. The parametricanalysis clearly indicates the influence of reinforcedfiber on proposed machinability characteristics duringdrilling of polyamides.

Response surface methodology

The RSM is a mathematical modeling tool used fordeveloping the relationship between the process param-eters and the desired response(s). The RSM is useful fordeveloping, improving, and optimizing the process,which provides an overall perspective of the systemresponse within the design space.27 Using FFD ofexperiments and applying the regression analysis, themathematical modeling of any desired criteria to sev-eral input process parameters can be obtained. TheRSM is used to describe and identify the influence ofinteractions of the process parameters on characteristicwhen they are varied simultaneously. This type ofdesign is more practical, economical, and relativelyeasy to use when curvature is suspected in the responsesurface.27 The RSM design procedure includes the care-ful selection of DOE for the adequate and reliablemeasurement of response.

In several occasions, it is possible to represent theindependent process parameters in quantitative formand the response in terms of process parameters canbe expressed as27

Y ¼ �ðx1, x2, x3, . . . , xkÞ ð1Þ

where Y is the response, x1, x2, x3, . . . , xk the quantita-tive factors, and � is the response function. It can beapproximated within the experimental region by a poly-nomial when the mathematical form of response func-tion is unknown. Higher the degree of polynomialbetter the correlation, but the experimentation costswill increase.

Experimental details

Planning of experiments

The planning of experiments is crucial for developingthe mathematical models based on RSM. The mathem-atical modeling provides reliable equations obtainedthrough DOE. In the current investigation, spindlespeed (N), feed rate (f), and point angle (�) are identi-fied as the process parameters to evaluate the hole qual-ity such as hole diameter (D) and circularity error (")and two aspects of machinability such as thrust force(Ft) and specific cutting coefficient (Kf). Since, the par-ameters identified are multi-level variables and theiroutcome effects are not linearly related as per the

Gaitonde et al. 23



authors’ preliminary investigations, it has been decidedto use multi-level tests for the cutting parameters. Threelevels for each of the three parameters were selected andthe effects of process parameters on hole quality andmachinability are tested through a set of plannedexperiments based on FFD to explore the quadraticresponse surface.27 Thus, 27 trials based on FFDwere planned.27 The process parameters and theirlevels are illustrated in Table 1 and the experimentallayout plan as per FFD for the present investigationis given in Table 2.

Work material and tool material

Unreinforced polyamide (PA6) and polyamide rein-forced with 30% glass whiskers (PA66-GF30) producedby extrusion were used as the work materials and thesematerials were supplied by ‘ERTA�’ company. Thedimension of the glass whisker is 5 mm in diameterand 50 mm long, randomly distributed into the poly-meric matrix. Figure 1 shows a scanning electronmicrograph (SEM) of work material and the procedureto prepare the machining samples with 20mm diameterand 5mm thickness. Table 3 gives the composition ofPA66-GF30 material and Table 4 lists the mechanicaland thermal properties of tested work materials.

During reinforced polyamide machining, hard andabrasive glass fibers result in high tool wear andhence it is essential to employ the appropriate cuttingtool. Thus, the carbide tool has been selected as thecutting tool material. Tungsten carbide twist drills(ISO grade K20) with 5mm diameter with 25� helixangle and different point angle values manufacturedby ‘Guhring oHG’ company were used as cuttingtools. Figure 2 shows the helical point drills ofcode WN 11 RN, DIN 6539 RN, and DIN 8038 RNwith 85�, 115�, and 135� point angles used in theexperiments.

Experimentation, hole quality, andmachinability evaluation

The drilling experiments were conducted as per FFD ona machining center, which is equipped with 7.5 kW

spindle power and maximum spindle speed of6000 r/min. Figure 3 shows the experimental setupand an appropriate clamping system devised to fix thesamples in the machining center. A fresh drill was usedin every trial for each of the materials tested and hencetool wear is negligible. The trials were randomizedin order to remove the effects of any factorsunaccounted for.

The coordinate measuring machine (CMM)(Figure 4) was used for measuring the hole quality,i.e. hole diameter and circularity error at the middleof the thickness of the wafers PA6 and PA66-GF30composites. The circularity is a two-dimensional geo-metric tolerance that permits how much a feature candeviate from a perfect circle. Seven points were mea-sured and the difference between the maximum diam-eter of the circle and the minimum diameter of the circleis determined. The roundness is the condition where allpoints are in a circle. To be considered round, the mea-sured circle must fall within a specified tolerance zoneformed by two concentric circles (circularity error).Figure 5 shows the deviation from the calculatedcircle to each of the measured points for circle diameteranalyzed (mean diameter and circularity error). Themeasurement mode in CMM is as follows: unidirec-tional repeatability—0.75 mm; calibration and errorcompensation—0.001mm; palpeur tridimensionnel—-bille rubis Ø 2mm and number of points for circle—7.Figure 6 shows the photographs of the roundness meas-urement of drilled holes in polyamide composites.

In the present investigation, the machinability wasassessed by two parameters, namely, thrust force andspecific cutting coefficient related to thrust. Thrust forceis the reaction force against the advancement of drillinto the workpiece material. The drilling fixture wasmounted on the dynamometer and the thrust forcewas measured using a strain gage drilling dynamom-eter. Each trial was repeated twice and the averagewas taken as the process response. The specific cuttingcoefficient related to thrust (Kf) is calculated from thefollowing equation28

Kf ¼2Ft

f� dð2Þ

where Ft is the thrust force, f the feed rate, and d thedrill diameter. The measured values of hole diameter(D), circularity error ("), thrust force (Ft), and the com-puted values of specific cutting coefficient related tothrust (Kf) for each of the materials tested, i.e. unre-inforced polyamide (PA6) and reinforced polyamidewith 30% of glass fibers (PA66-GF30) drilling are sum-marized in Table 2. This experimental database is thenused to develop the RSM-based mathematical models.

Table 1. Process parameters and their identified levels for

drilling of polyamide composites

Parameter

Level

1 2 3

Spindle speed, N (r/min) 1500 3000 6000

Feed rate, f (mm/rev) 0.05 0.10 0.15

Point angle, � (�) 85 115 135




Table 2. Experimental layout plan along with the responses for polyamide composites

Trial

no.

Levels of process parameter settings Work material: PA6 Work material: PA66-GF30

Spindle

speed

(r/min)

Feed rate

(mm/rev)

Point

angle

(�)

Hole

diameter

(mm)

Circularity

(mm)

Thrust

(N)

Specific

cutting

coefficient

(MPa)

Hole

diameter

(mm)

Circularity

(mm)

Thrust

(N)

Specific

cutting

coefficient

(MPa)

1 1500 0.05 85 4.9965 0.0105 108.93 871.44 4.9815 0.0125 23.175 185.4

2 1500 0.1 85 4.9775 0.011 117.765 471.06 4.985 0.022 29.585 118.34

3 1500 0.15 85 4.9675 0.0165 119.815 319.51 4.9905 0.0245 37.345 99.59

4 3000 0.05 85 5.025 0.0125 89.32 714.56 4.9805 0.0175 20.795 166.36

5 3000 0.1 85 5.0275 0.018 93.55 374.2 4.994 0.0195 26.345 105.38

6 3000 0.15 85 4.9725 0.0245 102.55 273.47 4.999 0.0235 31.37 83.65

7 6000 0.05 85 5.064 0.0245 57.33 458.64 4.993 0.008 19.865 158.92

8 6000 0.1 85 5.079 0.073 64.765 259.06 4.997 0.0145 24.7 98.8

9 6000 0.15 85 5.1055 0.121 71.62 190.99 5.012 0.0165 29.405 78.41

10 1500 0.05 115 4.939 0.021 45.67 365.36 4.987 0.0215 19.12 152.96

11 1500 0.1 115 4.966 0.03 45.405 181.62 4.992 0.0225 22.77 91.08

12 1500 0.15 115 4.9955 0.0375 45.425 121.13 4.9945 0.0325 26.165 69.77

13 3000 0.05 115 5.009 0.012 41.25 330 4.9895 0.0205 18.03 144.24

14 3000 0.1 115 4.9585 0.0155 42.85 171.4 4.991 0.025 20.59 82.36

15 3000 0.15 115 4.972 0.0185 46.415 123.77 5.002 0.0205 23.545 62.79

16 6000 0.05 115 5.028 0.0095 31.825 254.6 4.994 0.0155 16.15 129.2

17 6000 0.1 115 4.985 0.016 32.935 131.74 4.9945 0.0245 19.435 77.74

18 6000 0.15 115 4.9735 0.0155 37.575 100.2 4.9925 0.019 21.62 57.65

19 1500 0.05 135 4.9905 0.0355 66.16 529.28 5.057 0.03 29.175 233.4

20 1500 0.1 135 5.04 0.0315 75.64 302.56 5.0665 0.0245 38.515 154.06

21 1500 0.15 135 5.057 0.0275 90.455 241.21 5.0825 0.017 48.53 129.41

22 3000 0.05 135 5.0265 0.035 59.385 475.08 5.0925 0.017 24.01 192.08

23 3000 0.1 135 5.03 0.0225 70.905 283.62 5.0515 0.023 36.99 147.96

24 3000 0.15 135 5.029 0.0145 81.785 218.09 5.047 0.03 46.565 124.17

25 6000 0.05 135 5.032 0.009 54.435 435.48 5.068 0.0165 23.96 191.68

26 6000 0.1 135 5.061 0.015 66.11 264.44 5.0555 0.022 35.83 143.32

27 6000 0.15 135 5.0405 0.012 75.15 200.4 5.037 0.03 44.985 119.96

Extruded blank(Polyamide)

Wafer

Drillingspindle

Aluminumsupport

Figure 1. MEV microstructure of polyamide PA6 hole’s wall (magnification 15�).

Gaitonde et al. 25



Development of RSM-based

mathematical models

In the present investigation, each parameter was inves-tigated at multi-levels to analyze the non-linearityeffects of process parameters. Hence, second orderRSM-based mathematical models for hole diameter(D), circularity error ("), thrust force (Ft), and specificcutting coefficient related to thrust (Kf) have been devel-oped with spindle speed (N), feed rate (f), and pointangle (�) as process parameters. The RSM-based math-ematical model is of the form27

Y ¼ b0 þ b1Nþ b2 f þ b3� þ b11N2 þ b22f

2 þ b33�2

þ b12Nfþ b13N� þ b23f�

ð3Þ

where Y is the response, i.e. D, ", Ft, and Kf; b0, . . . , b23the regression coefficients of quadratic models are to bedetermined for each of the responses.

The values of regression coefficients of the quadraticmodel are determined by27

B ¼ ðXTXÞ�1XTY ð4Þ

where B is the matrix of process parameter estimates; Xthe calculation matrix, which includes linear, quadratic,and interaction terms; XT the transpose of X; and Y thematrix of response.

The mathematical models as determined by regres-sion analysis to predict hole diameter (D), circularityerror ("), thrust force (Ft), and specific cutting coeffi-cient related to thrust (Kf) for PA6 and PA66-GF30polyamides drilling are given by:

Hole diameter

DðPA6Þ ¼ 5:8196644þ 0:0000557646N� 0:521396f

� 0:017184� � 0:0000494444Nf

� 0:000000400877N� þ 0:0072588f�

þ 0:00000000063786N2 � 0:5666667f2

þ 0:000082111�2 ð5Þ

DðPA66GF30Þ ¼ 5:5330742þ 0:0000197788N

þ 0:62984649f� 0:0126251�

� 0:0000350794Nf

� 0:000000130618N� � 0:0062325f�

� 0:000000000201646N2 þ 1:02222222f2

þ 0:0000686111�2 ð6Þ

Circularity

"ðPA6Þ ¼ �0:024787þ 0:000023636Nþ 0:94841374f

� 0:0008434� þ 0:0000748413Nf

� 0:000000382999N� � 0:0093465f�

þ 0:000000000179835N2 � 0:177778f2

þ 0:000013074�2

ð7Þ

Table 4. Mechanical and thermal properties of PA6 and PA66-GF30 polyamides

Properties PA6 PA66-GF30

Tensile modulus (MPa) 1400 3200

Rockwell hardness M85 M76

Charpy impact resistance (kJ/m2) Without fracture 50

Tensile strength (MPa) 76 100

Melting temperature (�C) 220 255

Density (g/cm3) 1.14 1.29

Coefficient of thermal expansion (<150�C) (m/m�K) 90 � 10�6 50 � 10�6

Coefficient of thermal expansion (>150�C) (m/m�K) 105 � 10�6 60 � 10�6

Table 3. Composition of PA66-GF30 composite laminates

Material Type

Matrix Polyamide 66

Reinforcement E-glass fiber (Ø 5 mm and length 50 mm)

Modulus of elasticity 2.15 GPa

Yield strength 82.55 MPa




"ðPA66GF30Þ ¼ �0:021156� 0:00000451932N

þ 0:22339181fþ 0:00060412�

þ 0:000010873Nf

þ 0:0000000267544N� � 0:0009035f�

� 0:0000000000740741N2 � 0:5f2

� 0:00000224074�2 ð8Þ

Thrust force

FtðPA6Þ ¼ 959:39844� 0:028398613N� 78:506433f

� 15:250244� þ 0:003688889Nf

þ 0:000180447N� þ 1:5377193f�

þ 0:000000314774N2 þ 116:555556f2

þ 0:06387463�2 ð9Þ

FtðPA66GF30Þ ¼ 250:043� 0:003854363N� 15:561769f

� 4:3434961� � 0:003293651Nf

þ 0:00000686257N� þ 1:61662281f�

þ 0:000000329136N2 � 126:88889f2

þ 0:019767593�2 ð10Þ

Specific cutting coefficient

Kf ðPA6Þ ¼ 5454:9135� 0:171586316N� 13935:805f

� 72:548828� þ 0:310409524Nf

þ 0:000897992N� þ 35:2589474f�

þ 0:0000017228N2 þ 29862:4444f2

þ 0:289527407�2 ð11Þ

Figure 3. Experimental setup and appropriate clamping system mounted inside machining center.

Figure 2. Helical point drills WN 11 RN (85�) and DIN 8038 RN (135�) and with thinned web DIN 6539 RN (115�) used in the

drilling experiments.

Gaitonde et al. 27



Kf ðPA66GF30Þ ¼ 1172:3785� 0:021130428N

� 2491:4999f� 16:918679�

þ 0:032560317Nfþ 0:000018868N�

þ 0:44482456f� þ 0:00000161918N2

þ 7590:22222f2 þ 0:079958148�2

ð12Þ

where N in r/min; f in mm/rev; � in degree; D in mm;" in mm; Ft in N, and Kf in MPa.

Results and discussion

Adequacy checking of mathematical models

The statistical testing of the proposed quadratic modelswas checked through Fisher’s test (F-test) for the ana-lysis of variance (ANOVA).27 The ANOVA consists ofsum of squares and degrees of freedom and the funda-mental technique is a partitioning of total sum ofsquares and mean square into components such asdata regression and its error. The mean square is theratio of sum of squares to degrees of freedom. As perANOVA, the calculated value of F-ratio of the devel-oped model should be more than F-table for the modelto be adequate. Table 5 presents the summary ANOVA

results of the proposed models and it is found that thedeveloped models are significant at 95% confidenceinterval as F-ratio of all the models is greater than2.49 (F-table(9, 17, 0.05)).

The adequacy of the proposed mathematical modelsis also verified through the coefficient of determination(R2).27 The R2 quantity provides a measure of variabil-ity in observed values of response and can be explainedby the controlled process parameters and their inter-actions. The R2 -values are presented in Table 5,which show the good correlation between the experi-mental and the predicted values of the proposed holequality and machinability parameters.

The comparison of the predicted and experimentalvalues for the experimental data of FFD during drillingof PA6 and PA66-GF30 polyamides is displayed inFigures 7 and 8, respectively. As seen from these fig-ures, there exists close relationship between the experi-mental and the predicted values and also found thatthere are no abnormal variations between the experi-mental and the predicted values. Hence, the developedRSM-based quadratic models can be used for the pre-diction of the hole quality and machinabilitycharacteristics.

Parametric analysis of cutting parameters

The proposed mathematical models, i.e. equations (5)to (8) are used to predict the hole quality and equations(9) to (12) are used to predict the machinability bysubstituting the values of spindle speed (N), feed rate(f), and point angle (�) within the ranges of the processparameters selected. The effects of process parameterson hole quality and machinability for the two materialstested are shown in Figures 9 to 12. In this analysis, theresponses are plotted as a function of spindle speedwith hold values of three different combinations offeed rate and point angle. It is quite evident fromthese figures that there exists significant interactioneffects between the process parameters, namely, spindlespeed, feed rate, and point angle on hole quality andmachinability (indicated by non-parallelism of responselines) for both the materials tested.

Figure 4. CMM used for the measurement of hole diameter

and circularity.

CMM: coordinate measuring machine.

Figure 5. Measurement circle in CMM: mean diameter and

circularity error.

CMM: coordinate measuring machine.




Hole quality. Figures 9 to 10 show the comparison ofhole quality variations in drilling of unreinforced(PA6) and reinforced with 30% glass fibers (PA66-GF30) polyamide materials. As observed from

Figure 9, an increased spindle speed results in increasedhole diameter for the combinaions of low feed rate withlow point angle (low condition) as well as medium feedrate with medium pont angle (intermediate condition)

Figure 6. Roundness measurement of the drilled holes in polyamide materials.

Table 5. Summary of ANOVA and R2 -values for proposed mathematical models

Response

Sum of squares Degrees of freedom Mean square

F-ratio R2Regression Residual Regression Residual Regression Residual

Material: PA6

Hole diameter 0.0336768 0.0099809 9 17 0.0037419 0.0005871 6.37 0.7714

Circularity 0.0111683 0.0028386 9 17 0.0012409 0.0001670 7.43 0.7973

Thrust 16,621.6 527.5 9 17 1846.8 31 59.52 0.9692

Specific cutting coefficient 820,612 32,904 9 17 91,179 1936 47.11 0.9615

Material: PA66-GF30

Hole diameter 0.0303974 0.0016757 9 17 0.0033775 0.0000986 34.26 0.9478

Circularity 0.00050464 0.00034303 9 17 0.00005607 0.00002018 2.78 0.5953

Thrust 2001.57 135.52 9 17 222.4 7.97 27.90 0.9368

Specific cutting coefficient 51,890.9 517.2 9 17 5765.7 30.4 189.52 0.9901

ANOVA: analysis of variance.

Gaitonde et al. 29



for both the composites tested. On the other hand,in case of PA6 polyamide, the hole diameter isinsensitive to spindle speed variations when feedrate and point angle are kept at higher values(high condition), while the hole diameter decreaseswith increased spindle speed in case of PA66-GF30polyamide drilling. It is interesting to note that incase of PA66-GF30 material, there is always a poorhole quality for any specified combination of spindlespeed, feed rate, and point angle. However, the holequality can be improved by utilizing higher spindlespeed values with low values of both feed rate andpoint angle in drilling of both unreinforced and rein-forced polyamide materials.

Figure 10(a) shows the circularity variation of PA6polyamide drilling, which clearly exhibits highly non-linear behavior with spindle speed for all values of feedrate and point angle combinations, while it is linear incase of PA66-GF30 polyamide drilling (Figure 10(b)).It is observed that for unreinforced PA6 drilling, a com-bination of lower values of spindle speed, feed rate, andpoint angle will result in minimum circularity error. Onthe other hand, in case of reinforced PA66-GF30

drilling, it is desirable to select higher spindle speedwith low feed rate and point angles.

Machinability. The machinability characteristics of unre-inforced (PA6) and reinforced with 30% glass fibers(PA66-GF30) polyamide drilling is evaluated in termsof thrust force and specific cutting coefficient and areshown in Figure 11 and 12. It is evident from thesefigures that in case of both PA6 and PA66-GF30 poly-amides drilling, an increased spindle speed results bettermachinability (lower values of both thrust force andspecific cutting coefficient) under all combinations offeed rate and point angle. In addition, for both thematerials tested, moderate values of both feed rate(0.1mm/rev) and point angle (110�) along with highspindle speed value (6000 r/min) are necessary for mini-mizing both thrust force and specific cutting coefficient,resulting in better machinability.

From the above discussion, it is seen that low tomedium values of both feed rate and point angle isnecessary to improve the hole quality and machinabilityof unreinforced PA6 and reinforced PA66-GF30

1 4 7 10 13 16 19 22 25 274.9

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Figure 7. Comparison of experimental and predicted values of hole quality and machinability characteristics for PA6 work material.




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Figure 8. Comparison of experimental and predicted values of hole quality and machinability characteristics for PA66-GF30 work

material.

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(a) (b)

Figure 9. Interaction effects of process parameters on hole diameter.

Gaitonde et al. 31



polyamides drilling. Further, it is also noticed thatalthough higher spindle speed values result in bettermachinability but with increased circulatity error andhence poor hole quality. A comparison plots of PA6and PA66-GF30 polyamides drilling on hole quality

and machinability characteristics indicate that holequality in unreinforced PA6 drilling is better as com-pared to reinforced PA66-GF30 drilling, even thoughthe machinability is poor. Hence, it can be inferredfrom the present analysis that drilling of reinforced

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Figure 11. Interaction effects of process parameters on thrust.

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Figure 10. Interaction effects of process parameters on circularity.




with 30% glass fibers (PA66-GF30) polyamide materialresults in better machinbility as compared to unre-inforced (PA6) polyamide but at the cost of inferiorhole quality.

Discussion

The polyamides play crucial role in modern industriesas well as in wide variety of engineering applications.The fibers carry bulk of load and matrix serves as amedium for transfer of load to the fibers in fiber com-posites. The cutting mechanism in fiber composites isdue to the combination of plastic deformation,

shearing, and bending rupture and depends on flexibil-ity, orientation, and toughness of the fibers, which inturn constitute surface texture on the workpiece.29 Thepresence of glass fibers in the polymer matrix increaseshardness as well as strength. The drilling process ofbrittle thermosets and fibers results in a series ofmini-fiber fractures, fiber pullouts, and matrix crackinginto pieces.30 Due to anisotropic and heterogeneousnature of polyamides, it is very difficult to predicthole quality and machinability.

Figure 13 shows the SEM of the drilled unreinforcedPA6 and PA66-GF30 reinforced polyamides for thecutting conditions of 3000 r/min, 0.1mm/rev with

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Figure 12. Interaction effects of process parameters on specific cutting coefficient.

Figure 13. SEM of work materials: (a) unreinforced PA6 polyamide; (b) PA66-GF30 reinforced polyamide.

SEM: scanning electron micrograph.

Gaitonde et al. 33



115� point angle drill. The heat generation due to fric-tion between the cutting edges and the work materialleads to the softening of the polymeric matrix, thusimpairing surface finish. Figure 13(a) clearly indicatesthe influence of cutting temperature in the unreiforcedPA6 polyamide drilling. In case of PA66-GF30 poly-amides drilling, the glass fiber reinforcement provides abetter surface finish and the observation of the fibersafter cutting without great influence of cutting tempera-ture in the matrix is evident in Figure 13(b).

In the current investigation, it is observed fromFigures 11 and 12 that in case of both PA6 andPA66-GF30 polyamide drilling, machinability (i.e.thrust force and specific cutting coefficient) is better atlow to medium values of both feed rate and point anglealong with high spindle speed value. As reported byBasavarajappa et al.,31 in general, at low feed ratethere is a smaller resistance to drill in the feed direction,resulting in less vibration and hence reduced thrustforce. On the other hand, at larger feed rates, thework material offers more resistance in the directionof drilling there by increase in friction leading tohigher thrust forces. Also, with increased feed rate,the contact area between the work material and drillincreases, which in turn increases the thrust force. Thedecrease in thrust force with increased spindle speed isattributed to reduced work–tool contact length.31

Further, at high-speed values, thermal softening oftool material occurs,32 which in turn remove the built-up edge formed on the drill and hence chips form rap-idly. As a result, the chips easily leave the material andsubsequently reduce the thrust force. Mohan et al.9 andPalanikumar et al.18 also noticed that an increase inspindle speed and decrease in feed rate promote lowerthrust force in drilling of composites. Our investigationon polyamides drilling also support this finding. Atlower point angle, the thrust force decreases due tolower stresses and lower shear area.11 Also, at lowerfeed rates with lower point angles, the thrust forcedecreases mainly due to decrease in cross-sectionalarea of undeformed chip.22,23,33 Hence, due to the com-bined effect of all three, i.e. increased spindle speed withreduced feed rate and point angle, the thrust force nat-urally decreases.

The low value of feed rate indicates that shear modelcould not fit adequately the chip formation process asthe material is subjected to lower strain rates and hencethe specific cutting coefficient increases.31 Further athigher feed rates, the number of fibers to be shearedwill be reduced and hence reduced specific cutting coef-ficient.28 As can be observed from Figure 9, under thesimilar cutting conditions, machinability of reinforcedPA66-GF30 polyamide drilling is found to be betterwhen compared to unreinforced PA6 polyamide. Thereason might be at higher spindle speed with lower feed

rates, the chips consist of less deformed matrix materialand cut fibers in reinforced polyamide, which results inreduced thrust when compared to unreinforced poly-amide. On the other hand, at low spindle speeds andhigh feed rates, there is an increase in self-generatedfeed angle which significantly reduces the workingclearance angle of the drill resulting in rubbing againstthe work material causing higher thrust force.26,34,35

From Figure 9, it is observed that for unreinforcedPA6 polyamide drilling, at high spindle speed with lowfeed rate and point angle, the hole diameter is morethan the nominal diameter of 5mm. This is becauseat high spindle speed with low feed rate, the cuttingtemperature goes up because of frictional heating,which results in higher hole diameter.24 Krishnarajet al.24 also reported that the specific cutting resistanceincreases at lower feeds due to smaller uncut chip thick-ness resulting in higher shear forces, which in turnincrease the vibration. It also results in larger holesize at lower feeds and decreases at higher feeds.

From our investigation, as displayed in Figure 10,the circularity error can be minimized by employinglower feed rate with low point angle. The possiblereason might be at low feed, the contact time of drillwith the workpiece is less and the related heat generatedis sufficient for the penetration of the tool with lessthrust force. Krishnaraj et al.24 reported that a highspindle speed creates less circularity error in high-speed drilling of CFRP laminates, which is also truein our case of reinforced PA66-GF30 polyamide dril-ling. This is because the rotational stability of the drillis better at high speeds when compared to low speeds.Because of the stability of the drill at higher speed, thematrix and glass fibers are pushed out easily for easypenetration of the tool, thereby producing roundnesswith minimum circularity error. However, Krishnarajet al.24 also reported that a low feed rate creates greatercircularity in high-speed drilling of CFRP laminates,which does not support our findings on unreinforcedand reinforced polyamide drilling.

Conclusions

The effects of speed, feed rate, and point angle on holequality such as hole diameter and circularity error,machinability aspects, namely, thrust force and specificcutting coefficient have been analyzed during drilling ofunreinforced polyamide (PA6) and reinforced poly-amide with 30% of glass fibers (PA66-GF30) withcemented carbide (K20) tool using RSM-basedsecond-order mathematical models. Based on theparametric analysis, the following conclusions aredrawn within the ranges of the process parametersselected.




1. For the combinations of low feed rate with low pointangle and medium feed rate with medium pointangle, the hole diameter increases with increase inspindle speed for both unreinforced and reinforcedpolyamides drilling.

2. A combination of lower values of spindle speed, feedrate, and point angle results in minimum circularityerror for unreinforced PA6 drilling, while it is bene-ficial to select higher spindle speed with low feed rateand point angles in case of reinforced PA66-GF30polyamide drilling.

3. The hole quality could be improved by employinghigher spindle speed values with low values of feedrate and point angle during drilling of both unre-inforced and reinforced polyamides.

4. An increased spindle speed results better machinabil-ity under all combinations of feed rate and pointangle during drilling of both unreinforced and rein-forced polyamides.

5. The feed rate of 0.1mm/rev and point angle of 110�

along with high spindle speed of 6000 r/min areessential for minimizing both thrust force and spe-cific cutting coefficient during drilling of both unre-inforced and reinforced polyamide.

Funding

The authors thank CNPq and Fapemig (Brazil) and FCT

(Portugal) for sponsoring this research study.

Acknowledgments

The authors thank Machining and Automation Laboratoryat Federal University of Minas Gerais (Brazil) for the experi-

mental setup and assisting in the experimental work.

Conflict of interest

None declared.

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