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Peck drilling of CFRP/titanium stacks: effect of tool wear on hole dimensional

and geometrical accuracy

Article in Production Engineering · August 2019

DOI: 10.1007/s11740-019-00915-1

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Production Engineering (2019) 13:529–538
https://doi.org/10.1007/s11740-019-00915-1

MACHINE TOOL

Peck drilling of CFRP/titanium stacks: effect of tool wear on hole

dimensional and geometrical accuracy
Umberto Prisco1 · Filomena Impero1 · F. Rubino2

Received: 18 June 2019 / Accepted: 5 August 2019 / Published online: 27 August 2019
© German Academic Society for Production Engineering (WGP) 2019

Abstract
The effect of tool wear on dimensional and geometrical accuracy of holes machined by peck drilling in carbon fibre reinforced
plastic (CRFP) and titanium (Ti) stacks is studied. Coated and uncoated tungsten carbide drills of both fine and ultra-fine
microstructures are employed to assess the importance of grain size and coating on hole accuracy. Hole profiles show two
maxima: one at the hole entry and the other at the CFRP/Ti interface. Hole cylindricity as function of tool wear shows a
minimum. It firstly decreases due to flank wear and subsequent reduction of the drill diameter. Then the rise of tool instability
prevails with the result that an increase of the cylindricity with tool wear is brought about. Less wear-resistant drills attain
this minimum in a shorter time of cutting.

Keywords Drilling · Cylindricity · Diameter · Accuracy · Burr · Tool wear

1 Introduction the CFRP/Ti stack to increase the productivity. For the Ti

sheet, a peck drilling strategy is usually preferred to standard
Due to its low specific strength and exceptional corrosion drilling when the stack thickness exceeds three times the
resistance, Ti-6Al-4 V has several uses in the chemical, drill diameter considering that the Ti alloy sheet, placed at
medical, automotive, and aerospace industries [1]. In aero- the bottom of the stack (see below), is a very difficult-to-
space structures, Ti-6Al-4 V is frequently stacked with car- machine material [2, 4].
bon fibre reinforced plastics (CFRP) to build multi-material The quality of the holes, also in terms of requirements
composite parts endowed with superior physical and chemi- for tight geometrical and dimensional tolerances, is critical
cal properties as high stiffness, high strength-to-weight ratio, especially for aerospace applications [5]. Besides the eco-
low density, and good damping capacity [2, 3]. The con- nomical losses due to rejections or reworking of holes out of
nection between the composite laminate and the titanium tolerance, poor hole quality can seriously weaken the fatigue
sheet is normally performed through mechanical fastening strength of bolted joint spoiling the aircraft reliability and
which requires the drilling of a numerous holes into the air- safety [6]. Increasing the drilling performance in terms of
craft structure. These holes are drilled in one pass through hole quality can then have an excellent economical payback.
However, drilling stacks of different material like CFRP
laminates and metal alloys with the aim at achieving close
* Umberto Prisco tolerances is a difficult job owing to the dissimilar machin-
umberto.prisco@unina.it
http://www.docenti.unina.it/umberto.prisco ing properties of the involved materials. This difficulty is
coupled to the fact that the drilling of CFRP and Ti-6Al-4 V
Filomena Impero
filomena.impero@unina.it alone is already known to be very challenging. Due to its
low thermal conductivity, high chemical reactivity at high
F. Rubino
f.rubino@derby.ac.uk temperatures, high hot strength and hardness, Ti-6Al-4 V is
one of the most difficult-to-machine alloys [7–9]. Difficulties
1
Department of Chemical, Materials and Production arise from the tendency of Ti alloys to form large burr and
Engineering, University of Napoli Federico II, Piazzale built-up edge as a result of the high temperature reached by
Tecchio 80, 80125 Naples, Italy
the cutting tool [8, 10]. This in turn results in a rapid tool
2
Institute for Innovation in Sustainable Engineering (IISE), wear, especially flank wear, and a fast increase of the cutting
University of Derby, Quaker Way, Derby DE1 3HD, UK

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530 Production Engineering (2019) 13:529–538

forces. Severe tool wear spoils the hole quality both directly, CFRP layer. As the drill progressed into the Ti layer, the
impairing its dimensional tolerances, and indirectly, through temperature increased to a very high value of 745 °C at the
the higher level of vibrations aroused by the increased cut- bore exit. Tashiro et al. [20] compared the drilling of CFRP/
ting forces. On the other side, CFRP shows problems com- Ti6Al4 V laminated stack using TiAlN-coated cemented car-
ing from the brittle nature and the elastic spring back of bide drills under dry and water-mist-cooling condition. They
the carbon fibres [2, 11]. Possible results are peel-up and found that water-mist-cooling reduces tool abrasion for small
push-out delamination, delamination at the hole surface, and time of cutting. However, if the cutting time increases, the
fibre pull-out [12]. Furthermore, the abrasive character of dry condition provides a longer tool life than the water-mist-
the carbon fibres increases the rate of tool wear. No need cooling. Fernández-Pérez et al. [21] analyzed the perfor-
to say that these phenomena also can seriously spoil the mance of diamond-coated carbide tools when drilling CFRP/
dimensional and cylindrical tolerance of the drilled hole. To Ti stacks under conditions of minimum quantity lubrication.
this regard, Rawat et Attia [13] demonstrated that tool chip- The main wear mechanism observed was diamond-coating
ping and abrasion were the main mechanisms controlling detachment, followed by chipping of the main cutting-edge.
the deterioration of WC bits in drilling woven carbon fibre Moreover, an important adhesion of titanium (mainly on the
composites. During the primary and secondary wear stages, secondary cutting-edge) and a thermal softening effect on
wear on the flank face of main cutting edges was found to the workpiece were observed.
be dominant, while adhesion of carbon was found to occur Lately, a low-frequency vibration-assisted drilling was
along with abrasion in the tertiary zone. developed for CFRP/Ti6Al4 V compound material [22].
For all these problems, there has been an intensive It was demonstrated that with this new method tool life is
research activity on the drilling of CFRP/Ti stacks so far. For increased by more than 300% [23] compared to conventional
example, Kim et al. [14] studied the hole defects resulting drilling while the cutting temperatures is reduced by more
from the drilling of CFRP-Ti stacks using tungsten carbide than 40% [24]. Consequently, thermal damage of the matrix
(WC) and polycrystalline diamond (PCD) twist drills. They material was not found [24] and a significant improvement
found that CFRP entry delamination is greatly affected by in hole quality [25] was reported when applying the low
the tool wear and Ti adhesion. The PCD tool experienced the frequency assisted vibration drilling.
least amount of tool edge rounding and therefore preformed About the hole quality, most of the research focuses
the best with respect to entry delamination. Furthermore, for mainly on the hole diameter and roundness as function of
the WC drills, large flank wear and margin wear occurred drill geometry [26, 27] or machining parameters as cutting
at high spindle speed, resulting in a reduction of hole size speed and feed rate [18]. Other authors studied the mecha-
and an increase of hole roundness and CFRP-Ti interface nism of burr formation at the hole exit [28] and possible
damage. At low spindle speed, tool geometry was changed methods to minimize the burr height [18, 29]. Some other
due to the large edge rounding. This resulted in large fibre studies examined the issue of damage at the CFRP/Ti inter-
pull-out at the CFRP hole surface. The conclusion was that face [14, 30]. However, so far, only one research dealt with
when compared with the WC tool at the same speed condi- the influence of the tool wear on hole quality [14]. The
tion, the PCD drill maintained relatively small hole defects. parameters adopted to measure the hole quality were the
Park et al. [15] studied the wear mechanisms of WC and hole diameter and roundness along the entire profile of the
PCD drills when drilling CFRP/Ti stacks. They demon- hole.
strated that Ti adhesion was a predominant tool wear factor The main aim of this research is to study the influence
for the WC drills. PCD drills showed less titanium adhe- of the tool wear on the hole quality specified in terms of
sion, but had a significant amount of cutting edge chipping. hole dimension and cylindricity. In particular, the wear of
Higher torque and thrust values were observed at the higher the drill and the subsequent increase of the cutting forces
spindle speeds which caused a significant increase in tool are correlated to the evolution of the hole profile and cylin-
wear due to the higher temperature generated, especially dricity as the wear progresses. The hole profile is expressed
when drilling the Ti plates. measuring the diameters of the hole surface throughout the
SenthilKumar et al. [16] analysed tool wear during the hole depth. In this way, contrary to what has been done so far
drilling of CFRP/Ti6Al4 V stacks using a 118° and a 130° by other studies, the hole quality as function of the tool wear
lip angle drill bits. Other authors studied the effect of cut- is assessed by means of both a local parameter, the diameter
ting speed and feed rate on hole surface integrity [17] and measured along the hole depth, and a global parameter, such
the effect of tool wear on hole quality in single-shot drill- as the cylindricity tolerance of the hole.
ing of metallic-composite stacks with diamond-coated tools
[18]. Sato et al. [19] using an infrared radiation pyrometer
measured the temperature variations observed in the drill-
ing of CFRP/Ti stacks. The temperature was ∼ 95 °C in the

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Production Engineering (2019) 13:529–538 531

2 Experimental procedures angle of 150°, a helix of 26°, a chisel edge angle of 105°,
a rake angle of 6° and a lip relief angle of 18°. To allow
The peck drilling experiments were performed as the most lubrication and cooling of the drill two internal helical
faithful mock-up of the standard assembling procedures channels were built into it. A synthetic ester oil-based
commonly adopted by the aerospace industry for the stacks fluid ­(Coolube® 2210EP, viscosity 18.5 cSt at 40 °C, den-
of fibre reinforced plastic laminates and titanium alloy sity 890 kg/m3 at 20 °C) in regime of minimum quantity
sheets in the final assembly of aircrafts. The drilling setup lubrication was used. The lubricating agent was applied
is characterized by the CRFP laminate positioned over the through the internal channels of the drills as an aerosol
Ti alloy plate with the entry point of the drill bit on the spray at a rate of 60–80 ml/h.
composite side. It is noticed that in this configuration the Different procedures and different machining param-
CFRP laminate and the Ti plate are clamped one on top of eters were adopted for drilling the CFRP and the Ti plate.
the other and to the table only through their lateral sides A standard drilling cycle with 3560 rpm and 630 mm/min
using strap clamps, so that their central parts are loose and of feed rate was employed for the CFRP. A peck drilling
not fixed together. Both the CFRP and the Ti-6Al-4 V used cycle with pecks of 1 mm was chosen for the Ti plate. A
in this study were aerospace-grades. They were supplied as rapid retracts to 1 mm outside the hole was programmed at
100 × 100 mm plates. The CFRP was an orthogonal woven the end of each peck followed by a rapid return to a clear-
carbon fabric in an epoxy matrix, manufactured by resin ance level of 0.1 mm above the depth before reached. The
transfer moulding. The areal density of the fabric was 200 g/ last peck was planned 3 mm deep to achieve a 2 mm break-
m2. The CFRP plate had a thickness of 18 mm with an aver- through. As a consequence the cycle to drill the 10 mm
age ply thickness of 0.2 mm. The Ti alloy plate had a thick- titanium plate consisted of 11 pecks. The drilling cycle
ness of 10 mm. for the Ti plate was performed at 995 rpm and at a feed
All drilling experiments were performed using a com- rate of 70 mm/min. To study the influence of the tool wear
mercial three-axis computer numerical control vertical mill. over the quality of the holes a total of 60 holes were drilled
Four different carbide twist drills with identical geometry with each drill bit.
were employed for the experimental campaign. Two drills The torque and thrust force were measured during the
were made of cemented carbide containing ultra-fine grained drilling of each hole using a plate piezoelectric dynamom-
WC and 4.2 wt% of Co. The material of the other couple eter connected to a data acquisition system. The geometry of
of drill bits was characterized by fine-grained WC and a each hole was measured by a computer measuring machine
Co content equal to 5 wt%. Only one drill within each cou- equipped with a touch probe; this equipment is guaranteed to
ple was provided with a 1.5 μm thick diamond-like carbon produce a maximum 2D form error of 0.8 µm and 3D form
(DLC) coating. The coating was applied by a plasma assisted error of 1 µm, respectively. 36 discrete points were collected
chemical vapour deposition technique. Summarizing two around the internal diameter of each hole at a distance of
drills were made of ultra-fine grained carbide material, but 0.5 mm along the hole axis starting from 0.5 mm below the
just one of them is DLC-coated, while the other two were hole entry. Due to the presence of damage at the CFRP/Ti
made of fine-grained carbide material, with just one of them interface, as Ti entry burr and CFRP delamination at the
DLC-coated. The uncoated ultra-fine and fine grained car- hole exit, it was chosen not to collect data points 0.5 mm
bide bits are denoted as UFG and FG, respectively, while above and below this interface, namely at depths of 17.5 mm
their coated counterparts are denoted as UFG-DLC and FG- and 18.5 mm from the top surface of the CFRP laminate. 54
DLC. Table 1 reports the characteristics of the adopted tool diameters were then measured in total acquiring 36 points on
materials. each diameter; this constitutes a point cloud of 1944 meas-
The geometry of the drills was characterized by a diam- urements for each drilled hole. An optical microscope was
eter of 7.937 mm (5/16″), a flute length of 34 mm, a point used to examine the wear of the tool edge measuring the
variation of the average major flank wear (VB).

Table 1  Tool materials Code WC average Co Density [g/cm3] Hardness ­[HV30] Coating
grain size content
[μm] [wt%]

UFG 0.2–0.5 4.2 15.05 1780 –

UFG-DLC 0.2–0.5 4.2 15.05 1780 DLC, microhardness 24 GPa
FG 0.5–0.8 5 14.94 1640 –
FG-DLC 0.5–0.8 5 14.94 1640 DLC, microhardness 24 GPa

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532 Production Engineering (2019) 13:529–538

3 Results and discussion hole that becomes progressively deeper also plays a role in
the increase of the torque. On the contrary, the thrust force
Figure 1 reports the typical curves of the thrust force and reaches a more or less constant value after two pecks of
the torque registered during the experiment. The signals the drilling in titanium. This is indicative that a steady-state
reported are relative to the drilling of the first hole using the regime for the thrust force is quickly reached during this
UFG bit. The first stage of the drilling in the CFRP is clearly stage of the drilling. The rapid attainment of this regime is
visible in both the plots being characterized by lower values surely favoured also by the particular machining strategy
of the thrust force and torque. In particular, in the proposed adopted [2].
conditions, the thrust force during the drilling of the CFRP The maximum torque and the maximum thrust force reg-
is in average the 40% of the one registered during the drilling istered during the drilling of each hole, in the CFRP and the
of the Ti plate. The torque during the drilling of the CFRP is Ti plate, were registered and their values reported versus
in average the 13% of the torque recorded during the drilling the number of drilled holes, see Fig. 2. Tool wear causes
of the Ti plate. an increase of the cutting forces with the increase of the
This first stage of the drilling is followed by the 11 pecks number of drilled holes. Uncoated tools show a much faster
into the Ti-6Al-4 V plate. After each peck, both the thrust increase of the drilling forces than their coated equivalents.
and the torque go to zero. The torque increases constantly as This indicates that these drills underwent higher wear during
the drills penetrate into the stacks because of the increased the drilling. For example, between the first and last hole, the
length of contact between the minor cutting edge and the thrust forces in titanium increased by 130% in average for
machined surface [11]. The evacuation of the chip from a the uncoated bits while the coated tools displayed a growth
of 70%. The same trends are shown by the thrust force in
CFRP and by the torques in Ti and in CFRP. For example,
the maximum torque in titanium increases by 120% in aver-
age for the uncoated tools while the coated bits present an
increase equal to 85%; uncoated tool present an increase of
the maximum thrust force in CFRP equal to 135% while
coated bits show an increase of the 75% on average. The
curves in Fig. 2 also reveal that the ultra-fine grained tools,
both the coated and the uncoated one, underwent a lower
wear than their fine grained equivalents since their cut-
ting forces rise by a lesser amount during the drilling of
the hole sequence. Summarizing, the results of Fig. 2 show
that the DLC coated drills experienced less tool wear during
the drilling of the stacks. Furthermore, drill bits with finer
microstructure performed better in terms of wear resistance
than the ones characterized by a coarser grain size.
As example, Fig. 3 reports a series of imagines of the
UFG drill to testify the wear progress. The images depict
the flank face, major cutting edge, web, land, and margin
of the tool. The main mechanism of wear for all the studied
drills was observed to be abrasion both of the major flank
and of the cutting edge. Abrasive wear was also observed at
the chisel edge and at the secondary cutting edge, although
abrasion on the flank of the two primary cutting edges was
found to be the more severe wear phenomenon. Indeed, in
the drilling of titanium and woven composites, the abrasive
action of the chip on the rake face is not as aggressive as on
the flank face [13, 31].
Titanium adhesion was also observed on all drills. This is
a very well-known problem in drilling titanium alloys due to
the high chemical affinity of titanium with many tool materi-
als [5]. The built-up edge can strongly spoil the quality of
Fig. 1  a Thrust force and b torque profiles during the drilling of the the hole and can result in tool chipping when the built-up
first hole with the UFG tool bit edge detaches from the rake face. In this study, the uncoated

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Production Engineering (2019) 13:529–538 533

drills presented traces of built-up edge formation after drill-

ing 10–20 holes, while their coated equivalents showed this
phenomenon only starting from the 30th hole. Adhesion of
carbon residues on the flank face during the drilling of the
CFRP cannot be ruled out. As a matter of fact, adhesion
wear of the WC tools was observed in the drilling of carbon
fibre reinforced polymeric composites [13]. However, carbon
residues accumulated on the drill are easily removed during
the subsequent drilling of the Ti plate.
Tool wear was quantified measuring the flank wear (VB)
of the drill bits. Figure 4 reports the average value of these
measurements along with the maximum and minimum reg-
istered values of the flank wear up to the 60th hole. Again,
much lower wear of the flank was observed for the coated
drills. DLC coatings are known to decrease the tool tem-
perature and then slow down the wear progression of the
tool because they act as a thermal barrier and improve the
frictional properties of the rake face [32]. Furthermore, it
has been demonstrated that the friction coefficient of DLC-
coated tool decreases during machining [33]. Chip evacu-
ation also receives beneficial effects from DLC coatings
[4]. Likewise, a finer microstructure imparts a better wear
resistance since the tools with a finer grain size present a
lower increase of the flank wear with the increase of drilled
holes [32].
The evolution of the flank wear was not uniform under
the chosen process conditions and can be divided into two
distinct stages, as shown in Fig. 4. In the first stage, a high
wear rate is observed. Indeed, the sharp corner radius of
the new cutting edges results in a small chip/tool contact
area which brings about a very high contact pressure and a
consequent high wear rate. This stage lasts from the first to
the tenth hole under the adopted process conditions. Then,
due to the rounding of the cutting edges, the contact area
increases while the contact pressure is reduced. As a con-
sequence, the wear rate decreases and comes to be almost
constant with the increase in time. This stage should be fol-
lowed by a region of rapid increase of the wear rate due to
the combined increase of the cutting forces and temperature.
However, this stage is not visible in Fig. 4 being located at
higher values of the hole number.
The profiles of the holes were analysed measuring the
diameter all the way through the entire depth of the hole.
The results for the different drills at the first, the 20th and
the 60th hole are reported in Fig. 5. It is noted that the larger
values of the diameter are registered at the hole entry and
at the CFRP/Ti interface. The sudden increase of the thrust
force experienced by the drill when it first starts penetrating
the CFRP laminate and the titanium plate generates insta-
bility and consequently tool vibrations. Only later, after
Fig. 2  Drilling forces as function of the hole number for the differ- that a larger part of the drill has entered into the hole, it
ent tools: a maximum thrust force, b maximum torque in titanium, c will stabilize. The instability experienced by the tool dur-
maximum torque in CFRP
ing the first two penetrations brings about a run-out which

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534 Production Engineering (2019) 13:529–538

Fig. 3  UFG drill bit after 60

holes: a chisel edge, b cutting
edge, c rake face, d flank face

tool. When the drill starts penetrating the titanium plate,

the CFRP laminate undergoes an axial force which tends
to deflect it towards the spindle so that the gap between the
two plates increases. This situation obviously multiplies the
effect of the tool vibrations and run-out on the exit diameter
of the CFRP laminate.
The first hole is oversized for all the drills, as it is
expected under average shop condition. This can be ascribed
firstly to tool instability, and secondarily to thermal expan-
sion of the tool or to formation of a built-up edge. Discharg-
ing the peak of the curves registered at the CFRP/Ti inter-
face, see Fig. 5, the oversize of the first hole is more or
less equal to 0.026 mm for all the drills meaning that the
experimental campaign was performed under good condi-
tions of repeatability, in terms of rigidity of machine tool,
workpiece and setup, cutting fluid used, etc. The diameters
of the holes decrease, showing a tendency to become under-
sized, as the hole number increases, namely as the wear of
the drill progresses, especially the wear of the flank. How-
ever, as the tool wear progresses, the hole diameter decreases
with a different pace for the different tool materials. The
Fig. 4  Progression of the flank wear during the drilling of the 60 drift towards the undersized condition is quicker for holes
holes for the different drills obtained by uncoated tools and by tools characterized by a
coarser microstructure. Indeed, the UFG-DLC drill never
results in a diameter of the hole larger than the drill’s nomi- gets to produce undersized holes, while the FG-DLC drill
nal diameter. The run-out generated by the penetration into starts to machine undersized holes after the 30th; both the
the titanium is obviously larger considering that the thrust uncoated tools start drilling undersized holes before the
force more than doubles when the tool comes in contact 30th. In particular, excluding again the peak values of the
with the titanium plate. The result is the classical bellmouth diameters around the CFRP/Ti interface, Fig. 5 shows that
shape of the hole entrance observed both in the CFRP and the FG drill produces holes 0.01 mm and 0.018 mm under-
in the Ti [34]. It is noticed that the vibrations caused by the sized at the 30th and 60th hole, respectively. This means that
first penetration into the titanium enlarge the hole at the exit this drill experienced the strongest wear with consequent
from the CFRP laminate. This happens because the plates reduction of the drilled diameter.
are clamped together only through their boundaries while Tool wear deeply affects the cylindricity of the holes,
their central part is free of deflecting under the action of the see Fig. 6. The cylindricity versus the number of drilled

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Production Engineering (2019) 13:529–538 535

Fig. 5  Hole diameter vs. the distance below the surface at various hole number for the different tools

holes first decrease and then, after reaching a minimum, before the coated ones. This is clearly observed in the
returns to growth. The first decrease is the result of the data of Fig. 6: in average, the uncoated tools attain the
beneficial effect arising from the reduction of the drill minimum cylindricity at the 20th hole, while their coated
diameter due to the early wear of margins, minor flank and equivalent only at the 50th hole. This data can also be
minor cutting edge. A reduction in cylindricity is indeed interpreted as a shift of the cylindricity curves relative to
expected when the effective diameter of the tool decreases the coated tools in Fig. 6 towards higher hole numbers. As
if the intensity of vibrations and tool instability is con- a consequence, when the cylindricity starts to rise again
stant, as it is the case in this first stage of the tool wear. after the minimum, the curves relative to the uncoated
This is confirmed by the observation that the uncoated tools are above those relative to the uncoated ones. For
tools, which underwent the fastest wear, as previously example, at the 60th hole, there is a difference of 0.08 mm
proven, present the steeper drop of the cylindricity with in cylindricity in favour of the coated drills, with the fine
hole number in this stage. However, as the wear advances grained drills attaining a worse performance compared to
and the cutting forces build up, the increase in vibrations the ultra-fine grained ones.
and tool instability overcome the beneficial effect coming On the other hand, the difference in initial cylindricity
from a reduced tool diameter, henceforth the cylindricity between coated and uncoated drills can be explained con-
returns to grow. Given this mechanism of variation of the sidering the greater performance of DLC drills compared
cylindricity with the increase of drilled holes, the uncoated to uncoated ones in terms of reduction of the friction coef-
drills are expected to reach the minimum of cylindricity ficient and subsequent control of tool vibrations.

13
536 Production Engineering (2019) 13:529–538

are expected to be an increasing function of the wear pro-

gression, i.e. of the drilled holes. In particular, it was dem-
onstrated that the height of the exit burr is a function of
the flank wear and thrust force [14, 18]. Furthermore, since
the quicker the wear progresses, the faster the burr grows,
the drills presenting higher rate of tool wear, namely the
uncoated or coarser-microstructured ones, are expected to
generate higher burrs. This is clearly shown by the trends in
Fig. 7. An initial exit burr of 35 μm in average was observed
for all the drills; then, uncoated drills reach a height of the
exit burr equal to 176 μm in average at the 60th hole, while
this value was equal to 125 μm for the coated drills. Between
the fine grained and the ultra-fine grained tools, the ultra-fine
grained drills show the lower burr because they have supe-
rior wear resistance and mechanical properties.

4 Conclusions

The effect of tool wear on geometrical and dimensional

Fig. 6  Hole cylindricity versus the hole number for the different drills
accuracy of holes machined by peck drilling in CRFP/Ti
stacks was studied as function of the microstructure fineness
of the tool material and of the coating presence/absence.
All tested drills experienced tool wear in terms of Ti adhe-
sion and abrasion of margins and cutting edges. The most
intense observed phenomenon for all the drills was flank
wear. As expected, the progression of flank wear was slower
for the DLC coated tools, compared to the uncoated ones,
and for ultra-fine grained tools, compared to the fine-grained
ones. In agreement with the evolution of the flank wear,
an increase of the cutting forces was registered, so that the
higher wear resistance was joined to the slower increase
of the cutting forces with holes number. Tool wear was
observed to influence strongly the dimensional and geo-
metrical accuracy of the drilled holes. It was found that:

• The drills underwent a strong instability on their first

penetration into the CFRP laminate and especially into
the Ti plate. This causes an enlargement of the hole at its
entry and, to a greater extent, at the CFRP/Ti interface.
The peak values of the diameter are always registered at
the CFRP/Ti interface for all the drill bits.
• The holes tend to become undersize as fast as the tool
Fig. 7  Exit burr height as function of the number of drilled holes for
the different drills wear progresses faster. This phenomenon is then more
pronounced for uncoated and fine grained tools.
• The hole cylindricity as function of tool wear presents a
Exit burrs were observed at the bottom of the titanium minimum for all tested drills. This minimum is caused
plates for all drills. Figure 7 reports the measured height of by the reduction of the drill effective diameter due to tool
the exit burrs as a function of the hole number for the dif- wear progression. Once the wear has exceeded the value
ferent drills. Considering that the drill wear results in larger related to the attainment of this minimum, however, the
heat production and that the burr development in titanium increased level of vibrations and cutting forces induce an
is a rising function of the heat generation [35], burr height increment of the cylindricity with tool wear. Again, the

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Production Engineering (2019) 13:529–538 537

faster the wear evolution, the quicker is the attainment of 8. Özel T, Olleak A, Thepsonthi T (2017) Micro milling of tita-
this minimum. nium alloy Ti-6Al-4 V: 3-D finite element modeling for predic-
tion of chip flow and burr formation. Prod Eng 11(4):435–444.
• The height of the burr exit increases with the increase of https​://doi.org/10.1007/s1174​0-017-0761-4
tool wear. The rate of burr increase is higher for uncoated 9. Storchak M, Jiang L, Xu Y, Li X (2016) Finite element modeling
and fine-grained drills in comparison with their coated for the cutting process of the titanium alloy Ti10V2Fe3Al. Prod
and ultra-fine grained equivalents. Eng 10(4):509–517. https​://doi.org/10.1007/s1174​0-016-0689-0
10. Oezkaya E, Michel S, Biermann D (2018) Experimental stud-
ies and FEM simulation of helical-shaped deep hole twist
It was observed that the effect of tool wear on thrust force drills. Prod Eng 12(1):11–23. https​: //doi.org/10.1007/s1174​
and hole quality is significant. Monitoring the thrust force 0-017-0779-7
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