Damage and dimensional precision on milling carbon

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Journal of Materials Processing Technology 160 (2005) 160167

Damage and dimensional precision on milling carbonfiber-reinforced plastics using design experiments

J. Paulo Davim, Pedro ReisDepartment of Mechanical Engineering, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal

Received 24 June 2002; received in revised form 10 May 2004; accepted 8 June 2004

Abstract

Milling composite materials is a rathercomplex task owingto its heterogeneityand thenumber of problems, such as surface delamination,

that appear during the machining process, associated with the characteristics of the material and the cutting parameters. With the purpose ofunderstanding and reducing these problems, this paper presents a study that evaluates the cutting parameters (cutting velocity and feed rate)

under the surface roughness, and damage in milling laminate plates of carbon fiber-reinforced plastics (CFRPs). A plan of experiments,

based on the Taguchis method, was established considering milling with prefixed cutting parameters in an autoclave CFRP composite

material. An analysis of variance (ANOVA) was performed to investigate the cutting characteristics of CFRP composite material using

cemented carbide (K10) end mills. The objective was to establish a model using multiple regression analysis between cutting velocity and

feed rate with the surface roughness and damage in a CFRP composite material.

2004 Elsevier B.V. All rights reserved.

Keywords: Milling; Carbon fiber-reinforced plastics (CFRPs); Dimensional precision; Taguchis method; Orthogonal arrays; Analysis of variance (ANOVA)

1. Introduction

1.1. Milling fiber-reinforced plastics (FRPs)

Milling is the machining operation most frequently used

in manufacturing parts of fiber-reinforced plastics, because

components made of composite materials are commonly

produced by net-shape that often require the removal of ex-

cess material to control tolerances, and milling is used as

a corrective operation to produce a well defined and high

quality surfaces [1].

The machinability of fiber-reinforced plastics is strongly

influenced by the type of fiber embedded in the composite

and by its properties. Mechanical and thermal properties

have an extremely importance on machining FRP. The fiber

used in the composites has a greater influence in the selection

of cutting tools (cutting edge material and geometry) and

machining parameters. It is fundamental to ensure that the

tool selected is suitable for the material. The knowledge

of cutting mechanisms is indispensable in view of cutting

mechanics and machinability assessment in milling [1,2].

Corresponding author. Tel.: +351-234-370830;

fax: +351-234-370953.

E-mail address: [email protected] (J.P. Davim).

Composite materials such as carbon fiber-reinforced plas-

tics (CFRPs) made by using carbon fibers for reinforcingplastic resin matrices, such as epoxy, are characterised by

having excellent properties as light weight, high strength

and high stiffness. These properties make them especially

attractive for aerospace applications [2].

Surface roughness is a parameter that has a greater influ-

ence on dimensional precision, performance of mechanical

pieces and on production costs. For these reasons, research

developments have been carried out with the purpose of op-

timising the cutting conditions to reach a specific surface

roughness [3,4]. For achieving the desired quality of the ma-

chined surface, it is necessary to understand the mechanisms

of material removal, the kinetics of machining processes af-

fecting the performance of the cutting tools [5].

The works of a number of authors [612], when reporting

on milling of FRP, have shown that the type and orientation

of the fiber, cutting parameters and tool geometry have an

essential paper on the machinability.

Everstine and Rogers [6] presented the first theoretical

work on the machining of FRPs in 1971, since then the

research made in this area has been based on experimental

investigations.

Koplev et al. [7], Kaneeda [8] and Puw and Hocheng [9]

concluded that the principal cutting mechanisms correlate

strongly to fiber arrangement and tool geometry.

0924-0136/$ see front matter 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmatprotec.2004.06.003


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J.P. Davim, P. Reis / Journal of Materials Processing Technology 160 (2005) 160167 161

Santhanakrishman et al. [10] and Ramulu et al. [11] car-

ried out a study on machining of polymeric composites and

concluded that an increasing of the cutting speed leads to a

better surface finish.

Hocheng et al. [12] studied the effect of the fiber orienta-

tion on the cut quality, cutting forces and tool wear on the

machinability.In summary, it can be noticed that the works carried out

on the machinability of FRP, are basically related on the

wear of cutting tools and the quality on the surfaces, as a

function of the cutting conditions, the distribution of staple

fibers in the polymeric matrix and the angle of inclination

of staple fibers.

The current paper investigates the influence of cutting

parameters (cutting velocity and feed rate) on the surface

roughness (Ra), delamination factor (Fd), and international

dimensional precision (IT), on CFRP composite material us-

ing cemented carbide end mills, with the purpose to establish

a empirical relationship between cutting parameters (V and

f) and surface roughness (Ra) and delamination factor (Fd).

1.2. Autoclave process

The autoclave process is widely used to produce

high-performance laminates usually with fibers reinforced

epoxy systems. Composite materials manufactured by auto-

clave are particularly important for aerospace applications.

This process uses a pressurised vessel to apply pressure and

heat to both parts that have been sealed in a vacuum bag.

Next it can be seen the several stages of this process.

On the first stage, the prepreg carbon-fiberepoxy mate-

rial is carefully laid out on a table to ensure that fiber ori-entation meets the design requirement, where the prepreg

material consists of unidirectional long carbon fibers in a

partially cured epoxy matrix. On the second stage, pieces

of the prepreg material are cut out and placed on top of

each other on a shaped tool to form a laminate. The layers

could be placed in different directions to produce the desired

Fig. 1. Laminate plate (CFRP composite material) produced by autoclave with a fiber orientation of 0/90.

strength pattern since the highest strength of each layer is

in direction parallel to the fibers.

After the required number of layers has been prop-

erly placed, the tooling and the attached laminate are

vacuum-bagged, for removing the entrapped air from the

laminated part. Finally, the vacuum bag and the tooling is

put into an autoclave for the final curing of the epoxy resin.After removed from the autoclave, the composite material

is ready for further finishing operations [2,13].

2. Experimental procedure

2.1. Method and materials

In order to reach the objective of this experimental work,

mainly the establishment of the correlations between cut-

ting parameters (V and f) and surface roughness (Ra) and

delamination factor (Fd), machining issues were performed

under different cutting conditions on the CFRP composite

material.

The composite material used in the tests (epoxy matrix

reinforced with 55% of carbon fiber), supplied by INEGI,

was produced by autoclave with a fiber orientation of 0/90,

as can be observed in Fig. 1.

The experiments have been carried out in a laminate plate,

made up with 16 alternating layers of fibers with 4 mm of

thickness, using two cemented carbide (K10) end mills, pre-

sented in Fig. 2, with 6 mm of diameter. Both cemented car-

bide end mills, two-flute (R216.32-06030-AC10P-1020) and

six-flute (CCT-GSR-D0635), were manufactured according

to ISO. The two-flute end mill presents the following geom-

etry: a helix angle of 30, a rake angle of 1030, a clearanceangle of 9 and a flute length of 10 mm. The six-flute end

mill presents a neutral helix and 20 mm of flute length. The

depth of the cut on CFRP composite material was 2 mm.

A milling machine LC-11/2VS First with 2.2 kW spin-

dle power and a maximum spindle speed of 2500 rpm was

used to perform the experiments.


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162 J.P. Davim, P. Reis / Journal of Materials Processing Technology 160 (2005) 160167

Fig. 2. (a) Two-flute cemented carbide (K10) end mill, (b) six-flute cemented carbide (K10) end mill.

Fig. 3. Fixation of the laminate plate in the press of jaw of the milling

machine.

The fixation of the composite material (plate) was made

as observed in Fig. 3, to make sure that vibrations and dis-

placement did not exist.

The surface roughness was evaluated (according to ISO

4287/1) with a Hommeltester T1000 profilometer, as can be

observed in Fig. 4.

For each test five measurements were made over milling

surfaces, according to Fig. 5. Considering the number of

measurements to be carried out, a programmable technique

was used, by previously selecting a roughness profile, the

cut-off (0.8 mm) and the roughness evaluator parameter (Ra)

according to ISO. Data acquisitions were made through pro-

Fig. 4. Surface roughness evaluated with a Hommeltester T1000 pro-

filometer.

Fig. 5. Diagram of the five measurements that were made for each

palpation over milling surfaces.

filometer, by interface RS-232 to PC using the software

Hommeltester Turbo-Datawin.

The damage caused on the composite material was mea-

sured with a shop microscope, Mitutoyo TM 500, with 30

magnification and 1m resolution.

2.2. Plan of experiments

Taguchis method has been widely used in engineering

analysis and consists of a plan of experiments with the ob-

jective of acquiring data in a controlled way, in order to ob-

tain information about the behaviour of a given process. The

Taguchis method for two factors at three levels was used for

the elaboration of the plan of experiments. Table 1 indicates

the factors studied and the assignment of the corresponding

levels. By levels is meant the values taken by the factors.

The orthogonal array L9 (24), was selected as shown in

Table 2, which has nine rows corresponding to the number

of tests (8 degrees of freedom) with two columns at three

levels. The factors and the interactions are assigned to the

columns.

The plan of experiments was made of nine tests (array

rows), where the first column was assigned to the cutting

velocity (V) and the second column to the feed rate (f) and

Table 1

Assignment of the levels to the factors

Level Cutting velocity V (m/min) Feed rate f (mm/min)

1 28 200

2 38 410

3 47 860


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Table 2

Orthogonal array L9 (24) of Taguchi [15]

L9 (24) test 1 2 3 4

1 1 1 1 1

2 1 2 2 2

3 1 3 3 3

4 2 1 2 35 2 2 3 1

6 2 3 1 2

7 3 1 3 2

8 3 2 1 3

9 3 3 2 1

Linear graph L9 (24) [15].

the remaining were assigned to the interactions. The outputs

studied were surface roughness (Ra) and delamination factor

(Fd), in the CFRP composite material.

The treatment of the experimental results was based on

the analysis average and the analysis of variance (ANOVA)

[1417].An analysis of variance of the data with the surface rough-

ness and delamination factor, on the CFRP composite mate-

rial was done with the objective of analysing the influence

of the cutting velocity, and the feed rate on the total variance

of the results.

3. Results and discussion

The results of milling tests allowed the evaluation of the

CFRP composite material manufactured by autoclave, using

two cemented carbide (K10) end mills. The machinability

was evaluated by surface roughness (Ra), delamination fac-

tor (Fd) and international dimensional precision (IT).

3.1. Influence of the cutting parameters on the surface

roughness

The surface roughness (Ra) was evaluated with a Hom-

meltester T1000 profilometer, according to ISO 4287/1.

Tables 3 and 4 show the results of the surface roughness

(Ra) as a function of the cutting parameters, for both end

mills, two- and six-flute, respectively.

Table 3Values of Ra as a function of the cutting parameters, for the two-flute end mill

Test V (m/min) f (mm/min) Test 1 Test 2 Test 3 Test 4 Test 5 Surface roughness (Ra)a (m)

1 28 200 1.20 1.35 1.30 1.29 1.55 1.34

2 28 410 1.50 1.90 1.65 1.30 1.80 1.63

3 28 860 2.78 2.86 2.74 2.77 2.80 2.79

4 38 200 0.95 1.25 1.12 1.15 1.32 1.16

5 38 410 1.55 1.45 1.65 1.60 1.50 1.55

6 38 860 2.30 2.45 2.48 2.50 1.90 2.32

7 47 200 1.15 1.25 1.20 1.23 1.10 1.19

8 47 410 1.50 1.34 1.15 1.28 1.17 1.29

9 47 860 1.90 1.58 2.14 1.63 2.00 1.90

a Average of five measurements.

In Fig. 6, the evolution of the surface roughness (Ra) can

be seen with the feed rate, for the different cutting speed

values. From Fig. 6, it can be realised that the value of

Ra increases with feed rate and decreases with the cutting

velocity, i.e. with a higher cutting velocity and a lower feed

rate it is possible obtain a better surface finish. It can also be

observed that the two-flute end mill provides a better surfacethan the six-flute end mill.

Table 5 shows the results of the analysis of variance with

the surface roughness (Ra) for both end mills. This analysis

was carried out for a level of significance of 5%, i.e. for a

level of confidence of 95%. The last column of the previously

shown tables indicates the percentage of contribution (P) of

each factor on the total variation indicating then, the degree

of influence on the result.

From Table 5, it can be realised that the feed rate factor ( P

= 77.5%), have statistical and physical significance on the

obtained surface roughness (Ra), for two-flute end mill. The

factor cutting velocity (P= 9.5%) does not present statistical

and physical significance on the surface roughness, becauseTest F < F = 5% and P (percentage of contribution) F = 5% and P (percentage of

contribution) > error associated. Notice that the error asso-

ciated to the table ANOVA for the Ra was approximately1.7%.

The value of international dimensional precision (IT) can

be obtained by the following empirical equation according

to UNI ISO 3963/2:

IT = 30Ra (1)

Ra being the surface roughness in m.

Table 6 shows the results of the dimensional precision

(IT), obtained by Eq. (1), as a function of the cutting pa-

rameters.


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Table 4

Values of Ra as a function of the cutting parameters, for the six-flute end mill

Test V (m/min) f (mm/min) Test 1 Test 2 Test 3 Test 4 Test 5 Surface roughness (Ra)a (m)

1 28 200 1.21 1.64 1.54 1.50 1.68 1.51

2 28 410 1.86 2.00 1.79 1.60 1.59 1.77

3 28 860 2.60 2.83 2.90 2.00 2.65 2.59

4 38 200 1.38 1.44 1.64 1.75 1.55 1.555 38 410 1.79 1.90 1.86 1.98 1.73 1.85

6 38 860 2.86 2.79 2.49 3.00 2.64 2.80

7 47 200 1.60 1.23 1.46 1.22 1.58 1.42

8 47 410 1.45 1.50 1.68 1.52 1.43 1.52

9 47 860 2.86 2.47 2.28 2.24 2.51 2.47

a Average of five measurements.

Fig. 6. Surface roughness (Ra) as a function of cutting parameters.

In Fig. 7 the evolution of the dimensional precision (IT)

can be seen with the feed rate, for the different cutting speed

values. According to the graph, it is evident that the IT

increases with the feed rate, and decreases with the cutting

speed.

It can also be observed that for both end mills (two- and

six-flute) the surface presents ITs between 35 and 80 m,

and 40 and 80m, respectively, i.e. it is possible to get sur-

faces of quality of mechanics current construction, nomi-

nated qualities of IT 9 and 11.

Table 5

Table ANOVA for the surface roughness (Ra) for both K10 end mills

Source of variance SDQ d.f. Variance Test F F = 5% P (%)

Two-flute K10 end mill

V (m/min) 0.318 2 0.159 3.96 6.94 9.5

f (mm/min) 2.009 2 1.005 25.03 6.94 77.5

Error 0.161 4 0.040 12.9

Total 2.487 8 100

Six-flute K10 end mill

V (m/min) 0.105 2 0.052 11.06 6.94 4.2

f (mm/min) 2.140 2 1.070 225.51 6.94 94.1

Error 0.019 4 0.005 1.7

Total 2.264 8 100

SDQ, sum of squares; d.f., degrees of freedom; P, percentage of contribution.

Fig. 7. International dimensional precision (IT) as a function of the cutting

parameters.

3.2. Influence of the cutting parameters on the

delamination factor

The damage caused on the laminate plate (CFRP com-

posite material) was measured perpendicular to the feed rate

with a shop microscope Mitutoyo TM 500, as can be ob-

served in Fig. 8. The composite material (laminate plate)

was positioned and fixed on the XY stage glass of the mi-

croscope, then the alignment of an initial measuring point

with one of the cross-hairs was made on the machined fea-


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Table 6

Values of international dimensional precision (IT) as a function of the cutting parameters

Test V (m/min) f (mm/min) International dimensional precision (IT) (m)

Two-flute end mill Six-flute end mill

1 28 200 40.2 45.3

2 410 48.9 53.1

3 860 83.7 77.74 38 200 34.8 46.5

5 410 46.5 55.5

6 860 69.6 84.0

7 47 200 35.7 42.6

8 410 38.7 45.6

9 860 57.0 74.1

ture. Moving the XY stage glass by turning the micrometer

head with a Digital Counter to the final point with the same

cross-hair has been measured the damage (maximum width).

After the measurement of the maximum width of dam-

age (Wmax) suffered by the material, the damage normallyassigned by delamination factor (Fd) was determined. This

factor is defined as the quotient between the maximum width

of damage (Wmax), and the width of cut (W).

The value of delamination factor (Fd) can be obtained by

the following equation:

Fd =Wmax

W(2)

Wmax being the maximum width of damage in m and W

the width of cut in m

Table 7 shows the results of the delamination factor (Fd)

for the two end mills, obtained as Eq. (2) as a function ofthe cutting parameters.

In Fig. 9 the evolution of the delamination factor (Fd) can

be seen with feed rate, for different cutting speed values.

From Fig. 9, it can also be noticed that the Fd increases with

the feed rate. It can also be observed a significant variation

on the delamination factor (Fd) for the six-flute end mill with

the increase of the cutting speed. Finally, it can be realised

that two-flute end mill presents a lower delamination factor

than the six-flute end mill, i.e. the two-flute end mill leads

to a smaller damage on the CFRP composite material.

Fig. 8. Diagram of the measurement of the width of maximum damage

with a shop microscope Mitutoyo TM 500.

Table 7

Values of delamination factor (Fd) as a function of the cutting parameters

Test V (m/min) f (mm/min) Delamination factor (Fd)

Two-flute end mill Six-flute end mill

1 28 200 1.007 1.083

2 410 1.012 1.096

3 860 1.027 1.093

4 38 200 1.007 1.131

5 410 1.012 1.152

6 860 1.022 1.154

7 47 200 1.010 1.126

8 410 1.018 1.137

9 860 1.022 1.153

Table 8 shows the results of the analysis of variance

with the delamination factor (Fd) for both end mills. From

Table 8, it can be realised that the feed rate factor (P= 83.9%), have statistical and physical significance on the

obtained delamination factor (Fd), for two-flute end mill.

The factor cutting velocity (P = 0.6%) does not present sta-

tistical and physical significance on the delamination factor

(Fd), because Test F< F = 5% and P (percentage of con-

tribution) < error associated. Notice that the error associated

to the table ANOVA for the Fd was approximately 16.7%.

Equally from Table 8, it can be inferred that the feed rate

factor (P = 85.9%), and the cutting velocity (P = 9.8%)

have statistical and physical significance on the delamina-

Fig. 9. Delamination factor (Fd) as a function of the cutting parameters.


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Table 8

Table ANOVA for the delamination factor (Fd) for both K10 end mills

Source of variance SDQ d.f. Variance Test F F = 5% P (%)

Two-flute K10 end mill

V (m/min) 1.45E05 2 7.27E06 0.85 6.94 0.6

f (mm/min) 3.59E04 2 1.79E04 21.10 6.94 83.9

Error 3.40E05 4 8.51E06 16.7

Total 4.08E04 8 100

Six-flute K10 end mill

V (m/min) 6.66E04 2 3.33E04 10.00 6.94 9.8

f (mm/min) 5.33E03 2 2.67E03 80.02 6.94 85.9

Error 1.33E04 4 3.33E05 4.3

Total 6.13E03 8 100.0

SDQ, sum of squares; d.f., degrees of freedom; P, percentage of contribution.

tion factor (Fd) obtained, especially the feed rate factor, for

six-flute end mill.

The factors (Vand f) present a statistical and physical sig-nificance, because Test F > F = 5% and P (percentage of

contribution) > error associated. Notice that the error asso-

ciated to the table ANOVA for the Fd was approximately

4.3%.

3.3. Multiple regression analysis (MRA)

The correlation between factors (cutting velocity, feed

rate) and surface roughness (Ra) and delamination factor

(Fd), for both end mills on the CFRP composite material

were obtained by multiple linear regression with a sample

size (n) of 9. The equations obtained for both end mills were

as follow:Two-flute end mill:

Ra = 1.76 2.40 102V+ 1.71 103f, R = 0.93

(3)

Fd = 1.00 6.58 105V+ 2.34 105f, R = 0.89

(4)

Six-flute end mill:

Ra = 1.76 7.70 103V+ 1.75 103f, R = 0.94

(5)

Fd = 1.01 2.58 103V+ 2.73 105f, R = 0.67

(6)

V being the cutting of velocity in m/min, and f the feed rate

in mm/min.

4. Conclusions

Based on the experimental results presented, the fol-

lowing conclusions can be drawn from milling carbon

fiber-reinforced plastics manufactured by autoclave using

cemented carbide end mills:

for both end mills, it was possible obtained surfaces be-tween 1 and 3m of surface roughness (Ra), as function

of the cutting parameters used;

the surface roughness (Ra) and International dimensional

precision (IT) increases with feed rate and decreases with

cutting velocity;

as function of cutting parameters used, was possible to get

surfaces qualities (dimensional precision) of mechanics

current construction, IT 9 and 11, for both end mills, on

the CFRP composite material;

the delamination factor increases lightly with the feed rate,

for both end mills. For the six-flute end mill, the increase

of cutting velocity leads to an increase as well on thedelamination factor;

the two-flute end mill produces less damage on the CFRP

composite material than the six-flute end mill, i.e. the

delamination factor (Fd) is smaller;

feed rate is the cutting parameter that present the high-

est statistical and physical influence on surface roughness

(94.1 and 77.5%), and on delamination factor (83.9 and

85.9%), for both end mills, respectively.

Acknowledgements

The authors acknowledge to Professor Antnio Torres

Marques, from INEGI/FEUP, for providing the CFRP com-

posite material used on the experimental tests.

They also acknowledge to the graduate in Mechanical En-

gineer Pedro Madaleno for their participation in the experi-

mental work.

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Damage and dimensional precision on milling carbon

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