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International Conference on Advanced Manufacturing Engineering and Technologies
Optimization of trimming operations for machining
carbon fibre reinforced thermoplastic composite
Petr Masek1, Petr Kolar1, Pavel Zeman1
1Czech Technical University in Prague, Research Center of Manufacturing
Technology, Prague, Czech Republic
Corresponding author; e-mail:P.Kolar@rcmt.cvut.cz
ABSTRACT
This paper focuses on machining of thermoplastic composite PPS (polyphenol
sulfide) with woven carbon fibers. PPS/C is a commonly used material in the
aerospace and aircraft industry for internal panel design and also for structural
parts etc. Trimming using milling technology is a possible technology for finishing
of the workpiece after prefabrication. In this paper two cutting tools are compared
based on the criterion of the final quality of the machined surface edge and cutting
forces. The Taguchi method design of experiment (DOE) was used. The main
criterion was cutting forces evaluated by S/N ratio. The significance of factors and
optimal adjusting of control factors was determined by this method. Secondary
criteria were surface roughness and size of burr area.
KEYWORDS: sSide milling, thermoplastic composite, PCD cutting tools
1. INTRODUCTION
Most of the already published papers have been focusing mainly on machining of
composite materials with thermoset resin [1, 2, 3] etc. However, various modern composite
parts are very often made of composites of thermoplastic matrix today. Machinability of
thermoplastic composites and possibilities of increasing the productivity of these materials
have not been studied sufficiently yet. Hence, this paper focuses on machining of
thermoplastic matrix PPS (polyphenol sulphide) with woven carbon fibres. PPS/C is
commonly used in the aerospace and aircraft industry for internal panel design etc. Trimming
using milling technology is a possible technology for finishing of the workpiece after
prefabrication. In this paper two cutting tools are compared based on the criteria of the final
quality of the machined surface edge and cutting forces.
An earlier published paper written by König et al. [4] described the benefits of up-
milling. The authors recommended using the up-milling process for any cutting conditions and
any types of cutting tools. The average surface roughness Ra was found higher for any
adjustment of the experiment for graphite epoxy laminate if down milling was used. Then
Colligan and Ramulu [1] tested the same material and they studied delamination during up and
down milling for various surface ply orientations. They found out that the down milling is the
International Conference on Advanced Manufacturing Engineering and Technologies
best solution for the minimisation of the delamination but the difference between up and
down-milling was only 2%. On the grounds of the two contradictory effects of the milling
strategy on the machined surface, which were observed by König and Colligan, the milling
strategy was used as one of the factors in DOE. Thermoplastic composites are not inclinable to
delamination but it was predicted as possible at the beginning of the experiment.
The influence of surface ply orientation on the delamination was found out by Colligan and
Ramulu. The orientation of the surface ply could have influence on the creation of burr as
well. PW and PX orientation was chosen as levels of the ply orientation factor.
Another significant factor was chosen on the grounds of Davim’s paper [3]. He and his
colleagues used orthogonal arrays and the ANOVA test to choose the most significant factors
for GFRP materials with different thermoset resin. These factors were feed rate and cutting
speed. They discovered that feed rate and cutting speed are statistically significant and that
feed rate has a higher influence on surface roughness than cutting speed. Davim also presented
the same results for cutting force. Feed rate and cutting speed was chosen as the third and the
fourth factor in the experiment. It was predicted that for thermoplastic matrix the feed rate will
also be more important than cutting speed in the case of surface roughness.
Puw and Hocheng [2] predicted cutting force for machining unidirectionally reinforced
PEEK/C. They noted that the cutting force consists of specific cutting force, thickness of chip,
which is equal to feed per tooth, and depth of cut. The equations that they designed agree with
experimental results for ABS/C and PEEK/C. In addition, it was presented by Sheikh-Ahmad
[5] that depth of cut should have an influence on surface roughness either alone or in
interaction with feed rate. It is possible that PPS/C will behave differently. The radial depth of
cut was taken into account as the fifth control factor in the experiment.
This paper deals with experimental research into the influence of the cutting conditions
during side milling on the quality of the machined surface of the PPS/C material. The cutting
tools with various geometries were used for the tests. The main results are values of surface
roughness, size of burr area and resultant force for two PCD cutting tools. The control factors
feed rate per tooth, cutting speed, radial depth of cut, strategy and surface ply orientation were
chosen based on the literature review. The influence of control factors on the values of the
main results are expressed in the experiment as values of the S/N ratio. The S/N ratio
compares the magnitude of the desired signal to undesired signal, in other words called noise
[6]. The experiment for this paper was designed according to the Taguchi method. It is an
efficient way how to reduce the number of experiments to the minimum. Taguchi uses
orthogonal arrays for this purpose. The arrays are compiled for the given number of tested
parameters and levels of these parameters.
Two PCD cutting tools were tested. The main task was to compare different cutting
geometries. One of the tools had a modified face of cutting edge. A chip breaker was made by
laser into a hard and solid PCD tip. This modification increased the positivity of the rake
angle. The more positive rake angle should improve the quality of the machined surface and
decrease cutting forces. The optimal cutting conditions were determined for both tools and a
direct comparison between these two tools was made. The outputs of the experiment and
criteria of comparison for the tested PCD tools were the size of burr, surface roughness and
cutting forces.
2. SET-UP OF THE EXPERIMENT
The two cutting tools which were compared were almost the same but the second one
had a small modification of the rake angle made by laser. The diameter of the cutting tools
International Conference on Advanced Manufacturing Engineering and Technologies
was 8 mm. The milling cutters can be seen in Fig. 1. Both cutters are commercial cutting
tools, which were designed for machining polymer composite with fibres.
Fig. 1. PCD cutters (above with a chip breaker).
The geometry of the PCD cutters is shown in Table 1. It can be seen that the angles of
the cutting edge are the same. Only the rake angle is different for both tools due to the chip
breaker on the PTW08 tool. The rake angle was varying from the face of the cutting tool to the
end of PCD cutting tip in regard to flatness of the PCD cutting tip. A significant rake angle,
that has an influence on the cutting forces and surface quality, is the rake angle in the area of
contact between cutting tool and machined material. These cutting angles were measured and
they are shown in Table 1.
Table 1. Measured cutting edge angles on the tested cutters.
PCD08
PTW08
Rake angle*
0°
20°
Rake angle
-15,58°
5,06°
Clearance angle
17,84°
17,16°
Helix angle
-5,13°
-5,09°
*theoretical value of the rake angle near by the cutting tool face
The tested material was PPS/C. This is a high performance carbon fiber composite with
a thermoplastic matrix. It had a woven structure of fibers. This material is used for internal
panels of planes. The panels are usually produced by neer-net-shape technology, but in order
to achieve dimensions with specific tolerances it is necessary to usetrimming operations.
Small specimens of PPS/C with dimensions 53 x 53 mm were prepared and two different
orientations were machined in this experiment. The orientation PW and PX (Fig. 2) could
have a significant influence on the magnitude of the response. The orientation of plies was one
of the control factors which were included in the experiment.
Fig. 2. The schematic illustration of PW and PX orientation of composite.
Five control factors were chosen. These five factors have the main influence on the
quality of the machined surface and cutting forces. The optimal adjustment of these factors is
unknown for tested cutting tools for now. The chosen factor was feed per tooth ft, cutting
International Conference on Advanced Manufacturing Engineering and Technologies
velocity vc, radial depth of cut ae, ply orientation in composite and strategy of milling. Two
levels were chosen for each factor (Table 2).
The Taguchi design of experiment (DOE) was proposed (Table 3). This design of
experiment is one of the partial designs which are based on the orthogonal arrays designed by
Genichi Taguchi [7]. The Taguchi DOE unites interactions with factors but it is possible to use
it for cases where the factors are independent. Its advantage consists in the shortening of the
experiment and saving costs. The Taguchi DOE is mostly evaluated by S/N ratio that is a
powerful instrument for identifying the most significant control factors and an optimal
adjustment of the control factors.
Table 2. Control factors and their levels.
Parameters /
Factors
Unit
Symbol
Levels
Level 1
Level 2
feed rate
mm
ft
0.05
0.1
cutting speed
m/mint
vc
150
300
depth of cut
mm
ae
1
3
strategy
-
-
Up milling
Down milling
ply orientation
-
-
PW
PX
Table 3. Design of experiment.
Run ID
A (ft)
B (vc)
AB
C (ae)
AC
D (strat.)
E (ply orient.)
1
1
1
1
1
1
1
1
2
1
1
1
2
2
2
2
3
1
2
2
1
1
2
2
4
1
2
2
2
2
1
1
5
2
1
2
1
2
1
2
6
2
1
2
2
1
2
1
7
2
2
1
1
2
2
1
8
2
2
1
2
1
1
2
Interaction AB and AC was included into DOE. Interaction AB was chosen on the basis
of the preliminary tests, in which the ANOVA was used for identifying probably significant
interactions. Interaction AC was chosen on the basis of heredity. Factor A had high
significance and factor C was also very significant. In regard to the high significance of these
two factors there was a possibility of interaction AC being significant.
International Conference on Advanced Manufacturing Engineering and Technologies
Fig. 3. Clamping device on machine tool.
A clamping device, which provided stiff clamping of the specimen without the
possibility of specimen vibration, was designed for this experiment (Fig. 3). Eliminating
vibration is necessary for correct measuring of the cutting forces. The clamping device was
bolted directly to the dynamometer. The machine tool was a horizontal CNC milling machine
tool with a wide range of revolutions and possibility of exhaustion.
3. EVALUATION PARAMETERS
The cutting forces were measured by a Kistler 9255B dynamometer. Three components
of the cutting force were measured: Fx, Fy, Fz. The active cutting force was calculated
according to equation (1). Next, the resultant cutting force was calculated according to
equation (2). The resultant cutting force was a response of the experiment. It was calculated
for 200 engagements. Next, the average resultant force was calculated.
(1)
(2)
The surface roughness was measured by Surtronic 3+ from Tylor Hobson. This
measuring device is a contact device. The surface roughness is picked up by a solid stylus tip.
The Rz parameter was measured. This parameter gives a more real view of the surface
condition than e.g. the Ra parameter. The Rz parameter is calculated according to equation
(3).
The last measuring response was the size of burr which was measured along the whole
length of the specimen. A photograph was taken of the top and bottom side of the specimen
and this photograph was modified by a special program which was created in the Matlab
software. The contrast of the photograph was increased and then the machined edge of
composite was identified. The rest of the picture was cut off from the burrs. Next, the area of
International Conference on Advanced Manufacturing Engineering and Technologies
the burr was calculated in square millimetres. The procedure of the burr calculation is shown
in Fig. 4.
Fig. 4. Procedure of burr measuring from photograph.
4. RESULTS AND DISCUSSION
The evaluation of the resultant force was made by S/N ratio. The S/N ratio is a ratio
between signal factors which are desirable and noise factors which are undesirable. It is
possible to identify weak and strong factors from this statistical approach. The weak factors
are those which have a small influence on the response. Optimal values of the machining setup
can be achieved using this method.
The smaller-the-better (STB) (4) method was used for calculating S/N ratio. The reason
for this was that the optimal adjustment of the control factors is for minimal resultant cutting
forces.
Fig. 5. Main effects plot for SN ratios of PTW08 tool (with chipbreaker).
International Conference on Advanced Manufacturing Engineering and Technologies
Table 4. Response Table for Signal to Noise Ratios - Smaller is better – PTW08.
Level
ft
vc
ae
Milling
strategy
Ply
orientation
1
-39.30
-40.82
-41.06
-39.97
-41.01
2
-42.83
-41.31
-41.07
-42.16
-41.11
Delta
3.54
0.50
0.01
2.18
0.10
Rank
1
3
4
2
5
If the PTW08 cutting tool with chipbreaker was used, the most significant factor was the
feed per tooth as can be seen in Table 1. It means that the resultant cutting force is the most
influenced by this factor. The second one is the milling strategy. There were only two
possibilities for the choice of milling strategy: up milling and down milling. Up milling was
better than down milling according to Fig. 5. The engagement of teeth with the material is
smoother than for down milling because the tooth goes from minimal to maximum chip. It is
necessary to choose small feed per tooth in order to decrease the resultant cutting forces (0.05
mm for the case of this experiment). It makes sense because the cutting force is usually mainly
dependent on the chip cross-section which is a multiple of the feed per tooth and radial depth
of cut. The third most significant factor was cutting velocity. But its significance is much
smaller than ft and milling strategy. The radial depth of cut and the ply orientation were almost
insignificant.
For comparison, the same analysis was performed for cutting tool PCD08 without
chipbreaker. The results were almost the same but the ply orientation was less significant than
radial depth of cut this time (Table 5. Response Table for Signal to Noise Ratios - Smaller is
better – PCD08.Table 5 and Fig. 6). The significance of both these factors was very small as
in the case of PTW08. The results showed that cutting velocity was slightly more significant.
This could be caused by the higher difference between the results of all runs.
Fig. 6 Main effects plot for SN ratios of PCD08 tool (without chipbreaker).
International Conference on Advanced Manufacturing Engineering and Technologies
Table 5. Response Table for Signal to Noise Ratios - Smaller is better – PCD08.
Level
ft
vc
ae
Milling
strategy
Ply
orientation
1
-44.23
-45.30
-45.97
-45.13
-45.61
2
-47.31
-46.24
-45.56
-46.41
-45.92
Delta
3.08
0.94
0.41
1.28
0.31
Rank
1
3
4
2
5
The direct comparison showed that resultant cutting forces were by approximately 30%
to 40% lower if the PTW08 was used (Fig. 1). The higher positivity of the rake angle on the
cutting edge had a beneficial influence on decreasing of cutting forces.
Fig. 7. Direct comparison of resultant cutting force results.
The statistical evaluation of surface roughness was made for both tools. The differences
between results in runs were too small and this was the reason why the S/N ratio failed (see
Fig. 8). We were not able to decide which factor was more significant than others. It was also
impossible to decide how beneficial the chipbreaker was in the case of surface roughness. The
differences between cutting tools were smaller then differences between runs and both were
smaller than the measuring device uncertainty of measurement. The average surface roughness
was only slightly smaller for cutting tool PTW08. The average surface roughness for PTW08
was 4.84 µm and for PCD08 5.27 µm.
Fig. 8. Direct comparison of surface roughness.
0
50
100
150
200
250
300
1 2 3 4 5 6 7 8
Resultant cutting force
[N]
Runs of the experiment
PCD08 PTW08
0,00
1,00
2,00
3,00
4,00
5,00
6,00
7,00
1 2 3 4 5 6 7 8
Surface roughness Rz [µm]
Runs of the experiment
PCD08 PTW08
International Conference on Advanced Manufacturing Engineering and Technologies
The statistical comparison fails for measuring of the burrs as well. The method of
measuring used is very influenced by the place of cutting on the specimen and the definition
of the photograph. The resolution ability of this method is decreased by this. It was not
possible to apply the S/N ratio method in regard to small differences between results in the
experiment runs for PTW08 and PCD08. But there was an obvious difference between the
results for both tools (Fig. 9). The tool with the chipbreaker created hardly any burrs as can be
seen in Fig. 10. The higher positivity of the rake angle caused elimination of the burr creation.
Fig. 9. Direct comparison of burr area.
Fig. 10. Comparison of burr for tool PCD08 and PTW08.
5. CONCLUSION
The comparison of the PCD tools with and without a chipbreaker was the main goal of
this article. The near-net-shape composite parts with thermoplastic resin have to be machined
in order to obtain the right dimensions in specific tolerances and a neat composite part edge
without burrs. The right choice of the cutting tool and cutting conditions is paramount. The
cutting tool has to withstand the high abrasion of the carbon fibres and also create surfaces
without burrs. The PCD tools usually produced without a chipbreaker create a large amount of
burrs due to the zero or negative rake angle. The positive rake angle can be produced on the
face of the cutting edge by laser. Subsequently, the cutting process creates smaller cutting
forces and burr-free surface. The disadvantage of the tool with a lasered chipbreaker lies in the
0
2
4
6
8
10
12
1 2 3 4 5 6 7 8
Area of burr [mm2]
Runs of the experiment
PCD08 PTW08
International Conference on Advanced Manufacturing Engineering and Technologies
fact that it is almost twice as expensive. Carbide tools are an alternative solution to obtaining a
machined shape without burrs due to easier production of the high positivity of cutting edge.
However, the life time of uncoated carbide tools is up to 10 times shorter [8]. It can be the
reason why the machining cost will be higher if a carbide tool is used instead of the PCD tool
with a chipbreaker.
The optimal adjustment of cutting conditions was determined for the case of minimizing
the resultant cutting forces. This adjustment is independent on the cutting tool geometry. The
feed per tooth was the most significant control factor and its optimal value was 0.05 mm. The
second one was the milling strategy. It is much better to use down milling for machining of
PPS/C composite than up milling. The third most significant factor was cutting velocity,
whose optimal value was 300 m/min in this case. Other factors were insignificant for PTW08.
Some significance of other control factors was only identified for cutting tool PCD08. The
results showed that the fourth most important factor was radial depth of cut with an optimal
value of 1 mm. The least significant factor was orientation of plies where the optimal position
was PX.
REFERENCES
[1]
K. Colligan and M. Ramulu, Delamination in surface plies of graphite/epoxy caused by the edge trimming
process, Processing and manufacturing of composite materials - ASME,vol.27, 1991.
[2]
H. Y. Puw and H. Hocheng, Machinability test of carbon fiber-reinforced plastics in milling, Hsinchu, Taiwan:
Materials and manufacturing processes, 8(6), 1993, pp. 717-729.
[3]
P. J. Davim, P. Reis and C. C. Antonio, A study on milling of galss fiber reinforced plastics manufactured by
hand-lay up using statistical analysis (ANOVA), Porto, Portugal: Elsevier - Composite structures, vol. 64, 2004,
pp. 493-500.
[4]
W. König, P. Wulf, P. Graß and H. Willerscheid, Machining of fibre reinforced plastics, Annals of the CIRP, vol.
34, 1985, pp. 537-548.
[5]
J. Y. Sheikh-Ahmad, Machining of polymer composites, New York: Springer Science + Business Media, 2009, p.
320.
[6]
S. Maghsoodloo, G. Ozdemir, V. Jorda at. al, “Strenghts and limitations of Taguchi's contributions to quality,”
Journal of Manufacturing Systems, vol. 23, no. 1, pp. 73 - 126, 2005.
[7]
G. Taguchi and S. Konichi, “Taguchi methods, Orthogonal arrays and linear graphs, tools quality engeneering,”
MI: ASI Press, 1987.
[8]
R. Teti, Machining of composite materials, Elsevier : CIRP Annals - Manufacturing Technology, 2002, pp. 611-
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