O. BÍLEK et al.: CUTTING-TOOL PERFORMANCE IN THE END MILLING OF CARBON-FIBER-REINFORCED PLASTICS
819–822
CUTTING-TOOL PERFORMANCE IN THE END MILLING OF
CARBON-FIBER-REINFORCED PLASTICS
ZMOGLJIVOST REZILNEGA ORODJA PRI REZKANJU PLASTIKE,
OJA^ANE Z OGLJIKOVIMI VLAKNI
Ondøej Bílek, Soòa Rusnáková, Milan @aludek
Faculty of Technology, TBU Zlín, Nám. T.G.M 5555, 760 01 Zlín, Czech Republic
bilek@ft.utb.cz
Prejem rokopisa – received: 2015-06-30; sprejem za objavo – accepted for publication: 2015-09-09
doi:10.17222/mit.2015.153
Carbon-fiber-reinforced plastics (CFRPs) are no longer the exclusive domain of aerospace industries and nowadays find more
applications in the automotive and consumer industries. The growing volume of these materials increases the need for their
efficient machining. However, the machinability of CFRPs is a rather complex task due to the heterogeneity and the consider-
able number of parameters influencing the cutting process. The cutting tool has long been recognized as an important factor
influencing both surface quality and dimensional accuracy. Hence, in this paper, tool performance is investigated in the end
milling of CFRPs. The effect of tool geometry and coatings on the cutting forces, dimensional accuracy and surface quality were
experimentally examined for side and slot milling.
Keywords: CFRP, composites, end mill, surface quality, cutting forces
Plastika, oja~ana z ogljikovimi vlakni (CFRP), dosedaj ekskluzivna domena letalske industrije, se v zadnjem ~asu bolj pogosto
uporablja tudi v strojni{tvu, v avtomobilski industriji in v potro{ni{tvu. Z nara{~anjem uporabe teh materialov, nara{~a tudi
potreba po u~inkoviti strojni obdelavi. Vendar pa je strojna obdelava CFRP-a precej kompleksna naloga zaradi heterogenosti in
{tevilnih parametrov, ki vplivajo na proces rezanja. @e dolgo je znano, da je rezilno orodje pomemben faktor, ki vpliva na
kvaliteto povr{ine in dimenzijsko natan~nost. Zato je v ~lanku preiskovana zmogljivost orodja pri rezkanju plastike, oja~ane z
ogljikovimi vlakni. Pri bo~nem in utorovnem rezkanju je bil preiskovan vpliv geometrije orodja in povr{inske prevleke na sile
rezanja, dimenzijsko zanesljivost in na kvaliteto povr{ine.
Klju~ne besede: CFRP, kompoziti, ~elni rezkar, kvaliteta povr{ine, sile rezanja
1 INTRODUCTION
Milling is the key manufacturing operation for the
successful machining of fiber-reinforced plastic parts,
which ensures dimensional and qualitative requirements.
Milling is used, as a rule, as an edge-trimming operation,
or a slotting/routing process to produce complex con-
tours.
Composite materials such as carbon fiber-reinforced
plastics (CFRPs) are heterogeneous materials composed
of dissimilar constituents, where the resulting mechani-
cal properties are more than the sum of the characte-
ristics of the individual components. A plastic matrix of
CFRPs is reinforced by carbon fibers entirely, resulting
in a high specific strength and stiffness throughout the
lifetime of the components, together with electrical and
magnetic resistance. This makes them interesting for
applications in the automotive and aircraft industries, in
railway and ship building, medical applications and the
space industry.1,2
CFRP composites replace traditional metallic
materials; however, their machinability is fundamentally
different and the cutting mechanism is still under inve-
stigation. The machining properties are influenced by the
heterogeneity and anisotropy of the composite material.
The mechanical properties of the matrix are strongly
Materiali in tehnologije / Materials and technology 50 (2016) 5, 819–822 819
UDK 620.168:621.926:678.077 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 50(5)819(2016)
Table 1: Characteristics of the end mill
Tabela 1: Zna~ilnosti steblastih rezkarjev
Tool cutting zone Cutting diameter (mm) Helix angle (°) Rake angle (°) Coating
T1 Down-cut spiral with chip-breaker 5.81±0.07 15 0 PCD
T2 Four flute up-cut spiral 5.96±0.02 10 6 PCD
T3 Cross-pitch spiral 5.92±0.01 25 and 27 7 PCD
T4 Cross-pitch spiral 5.91±0.02 25 and 27 7 uncoated
T5 Up-cut spiral with chip-breaker 5.96±0.03 47 10 Multilayered
T6 Right- and left-hand spiral 5.94±0.02 13 30 Multilayered
T7 Cross-pitch spiral 5.97±0.02 10 0 PCD
T8 Three flute upcut spiral 5.91±0.03 15 30 PCD
temperature dependent, while the carbon fibers can
withstand temperatures up to 3000 °C, and during
machining cause considerable tool wear.3
Machining CFRPs brings with it certain difficulties,
such as layer delaminations, fiber pull-out, uncut fibers,
degradation and burning of the plastic matrix.4–7 Tool
manufacturers and scientists recommend carbide tools
with a polycrystalline diamond (PCD) coating for CFRP
material machining. These tools ensure high productivity
with a sufficient tool life. Unfortunately, the cost of PCD
tools is six times higher than uncoated tools, whereas
tool life is only three times longer.8 End mills usually
have a very special design suitable for the milling of
CFRPs.9 Helical end mills with two flutes are suitable for
rigid or back-supported parts.10 Compression end mills
with two sets of left-hand and right-hand flutes limit the
delamination and burrs on the cut edges.11 Furthermore,
the machining of flexible parts by multi-tooth end mills
eliminates the cutting force along the Z-axis, thereby re-
ducing the deflection and vibration of the part.8 In-bet-
ween are helical mills with a chip-breaker geometry12
that shears fibers and shortens chips with the simulta-
neous reduction of vibrations.
In this work we compare the performance of end
mills on the resulting surface quality, dimensional accu-
racy and cutting forces in the slotting and side milling of
CFRPs.
2 EXPERIMENTAL PROCEDURE
The composite samples used in the experimental
investigation were produced by vacuum infusion techno-
logy. The carbon fiber and the epoxy were supplied by
KORDCARBON CZ. The fabric was Toray HS 3K 200
tex, a balanced twill bidirectional (0–90°) weave and
380 μm thick. The epoxy used in the fabrication was the
commercially available Havel L285 epoxy system.
Fourteen layers of fabric were used to fabricate the
work samples with an average thickness of 4 mm. Over-
all, the CFRP composite average flexure strength was
501 MPa, the modulus of elasticity was 43,300 GPa and
the tensile strength was 378 MPa.
The experiment was carried out by using eight car-
bide end mills acquired from different producers (SECO,
WNT, KTOOLS). The required tool cutting diameter
was 6 mm. In this article the end mills are referred to as
T1-T8 and were concerned for special applications such
as the milling of composites with a wide range of com-
positions. Their geometry is summarized in Table 1.
Except for the T4 tool, the end mills were coated to with-
stand the machining of a highly abrasive work material.
A CNC milling machine C-442 HWT with a 1.0-kW
spindle power and high-speed spindle of 416.16 Hz was
used to perform the experiments. Part of the program
was created to ensure the repeatability and accuracy of
the experiment. The machining was divided into two
consecutive methods, first slot milling (Figure 1a) and
next upcut side milling (Figure 1b) of the CFRP work
material. The machining conditions are given in Table 2
for both methods. A coolant was not used and the tool
cutting length was 150 mm in both methods. A tool-wear
evaluation was carried out at the end of every set of
experiments. First, each tool was milling a slot and
subsequently a side of work material, so that the tool dis-
placement towards the bottom surface of the work
material was 2 mm.
Table 2: Cutting conditions
Tabela 2: Pogoji rezanja
Method
Cutting
speed
vc (m/min)
Feed rate
vf (mm/min)
Depth of
cut
ap (mm)
Width of
cut ae (mm)
Slot milling 100 550 1.25 ae = D
Side milling 100 550 2.0 4.0
Cutting forces were measured on the strain-gauge
dynamometer in the direction of the feed-rate vector
(force Ff) and perpendicularly to Ff (force Ffn). The
signal from dynamometer was processed by a Spider8
data-acquisition system and the evaluations were made
through a PC with Conmes Spider software, as illus-
trated in Figure 2.
The dimensional accuracy of the machined slot was
evaluated using a Mitutoyo caliper. The surface quality
was measured with a portable surface-roughness tester
Mitutoyo SJ-301, according to ISO 4287 on the high-
O. BÍLEK et al.: CUTTING-TOOL PERFORMANCE IN THE END MILLING OF CARBON-FIBER-REINFORCED PLASTICS
820 Materiali in tehnologije / Materials and technology 50 (2016) 5, 819–822
Figure 1: a) slot milling and b) side milling
Slika 1: a) rezkanje utora in b) bo~no rezkanje
lighted areas at the bottom of the slot (Figure 1a) and
the side surface (Figure 1b). The measurements repeated
were 15 times at different surface locations in the
direction of the feed-rate vector that was identified as the
direction of highest roughness. The roughness parameter
Ra was considered as an evaluation parameter for the
surface quality according to ISO.
3 RESULTS AND DISCUSSION
3.1 Cutting forces
All the force measurements were worth a maximum
of 4.86 % of variance. The results were graphically
summarized in Figure 3 and 4. There is a significant
difference between the end mills, moreover not the only
one is suitable both as side and slot milling. Tool T6 with
right- and left-hand spirals reached the lowest values Ff
when slot milling (Figure 3) and the second best value
as for T4 tool without coating. Higher values of Ff and
Ffn were achieved by the same T3 tool with the coating.
This may be due to increased adhesion of work material
to the PCD coating material.
Tools T1, T3, T8 keep along the average. Conversely,
the highest values of Ff achieved tool T2 and T7,
although with different flute geometries, but with the
same helix angle of 10°, despite the normal force Ffn
was the lowest one. The helix angle probably improves
the chip removal at the bottom of the slot; however, the
tool is more power loaded in the feed direction. In the
case of higher cutting depths we can expect a higher tool
bend, but without a buckling effect.
When side milling, the non-significant Ffn force was
not evaluated, yet was mentioned in Figure 4. The force
Ff is more than 2 times higher than when slot milling.
The smallest Ff value comes from the T4 tool without
coating, the worst was the T6 tool with a right- and
left-hand hand spiral. The reason for this may be in the
compression effect of contra-rotating spirals, becoming
for these tools. Although the flutes’ geometry for the T1,
T2, T5 and T8 tools was dissimilar, the feed-force cha-
racteristics were almost identical and quite balanced.
3.2 Surface roughness
The dynamometric investigation was followed by
surface-roughness measurements of the Ra parameter.
O. BÍLEK et al.: CUTTING-TOOL PERFORMANCE IN THE END MILLING OF CARBON-FIBER-REINFORCED PLASTICS
Materiali in tehnologije / Materials and technology 50 (2016) 5, 819–822 821
Figure 4: Cutting forces for side milling of CFRP work material
Slika 4: Sile rezanja pri bo~nem rezkanju CFRP obdelovanca
Figure 2: Experimental set up for milling CFRPs
Slika 2: Eksperimentalni sestav za rezkanje CFRP
Figure 5: Surface roughness Ra for slot and side milling
Slika 5: Hrapavost povr{ine Ra pri rezkanju utora in pri bo~nem
rezkanju
Figure 3: Cutting forces for slot milling of CFRP work material
Slika 3: Sile rezanja pri rezkanju utora v CFRP obdelovanec
The results are shown in Figure 5 for the side and slot
milling.
The lowest average value of Ra = 1.38±0.40 μm was
on the side surface made by the uncoated T4 tool and the
same value Ra = 1.38±0.29 μm is at the bottom of the slot
by the T8 tool. The T5 tool had the highest and most
unsatisfactory effect for side milling, where the rough-
ness value was Ra = 6.19±1.57 μm. It is the only tool
with an upcut spiral in the experiment and is actually a
production tool recommended for rough machining.
Approximately half values of Ra = 3.45±1.30 μm, but
highest for slot milling, was made with the cross-pitched
T7 tool with a 0° rake angle.
3.3 Dimensional accuracy
Since end mills should have a 6 mm diameter, it was
expected that the resulting dimension of the slot would
be given with a certain accuracy close to that value. As it
turned out, this was not entirely true for slot milling of
CFRP work material, as can be seen in Figure 6.
In most cases, the resulting dimension of the slot is
smaller than the desired nominal size. On the one hand,
it could be given by the true tool diameter (Table 1) that
was smaller than the producer specification. In contrst,
for the T5 tool with an upcut spiral we found that the slot
was about 0.13 mm wider. This excludes the eventuality
that a smaller tool diameter was made on purpose, taking
into account the CFRP material’s behavior. Finally, the
T8 tool made it possible to achieve a superior slot
accuracy, but otherwise lagged behind in terms of cutting
efficiency and surface quality.
4 CONCLUSIONS
In this work the cutting performance of the end mills
in the CFRPs milling has been presented, taking into
account the force, surface roughness and dimensional
measurements. After the testing of eight end mills seve-
ral conclusions can be drawn:
• an upcut spiral can cause considerably worse surface
quality together with an increment of the slot dimen-
sion,
• PCD-coated tools for CFRPs machining has a lower
cutting efficiency due to a different friction charac-
teristic than the uncoated tools,
• tools with left-hand and right-hand spirals recom-
mended for side milling, result in up to twice the
cutting force,
• to ensure compliance with the required dimensions,
accuracy and stability it is necessary to include an
offset (by part program, cutting conditions, thermal
field) for the CFRPs tool individually.
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O. BÍLEK et al.: CUTTING-TOOL PERFORMANCE IN THE END MILLING OF CARBON-FIBER-REINFORCED PLASTICS
822 Materiali in tehnologije / Materials and technology 50 (2016) 5, 819–822
Figure 6: Dimensional accuracy of the width of the slot
Slika 6: Dimenzijska natan~nost {irine utora