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A brief review on micromachining of materials

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98 Adv. Mater. Sci. 30 (2012) 98-102
Rev. P. Cardoso and J.P. Davim

A BRIEF REVIEW ON MICROMACHINING OF MATERIALS

P. Cardoso and J.P. Davim

Department of Mechanical Engineering, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal
Received: December 05, 2011

Abstract. The demand for miniaturized devices with high aspect ratios and superior surfaces
has been rapidly increasing in advanced industries. There is a growing need for fast, direct, and
mass manufacturing of miniaturized functional products from metals, polymers, composites
and ceramics. The current article presents a brief review on micromachining with special emphasis
in micromilling.

1. INTRODUCTION in diameter to produce micro-scale features. Al-

though geometric and material capabilities of
The miniaturization of devices is today demanding
micromachining have been demonstrated by [3]
the production of mechanical components with
industrial application of micromachining has been
manufactured features in the range of a few to a few
hindered by the lack of experience and knowledge
hundred microns in fields that include optics,
on the micro-machinability of materials [4].
electronics, medicine, biotechnology,
communications, and avionics, to name a few.
Specific applications include microscale fuel cells, 2. CHARACTERISTICS OF
fluidic microchemical reactors requiring microscale MICROMACHINING -
pumps, valves and mixing devices, microfluidic MICROMILLING
systems, microholes for fiber optics, micronozzles Micromilling, one of the mechanical micromachining
for high-temperature jets, micromolds, deep X-ray methods, is a process that utilizes end mills that
lithography masks, and many more [1]. typically vary in diameter from 100 to 500 m and
As a response to this demand, various micro- have edge radii that vary from 1 to 10 m.
manufacturing techniques have recently emerged, Additionally, the micromilling process has several
such as X-ray lithography electrodeposition molding salient features that differentiate it from the macro-
(LIGA), deep reactive ion etching, deep UV endmilling process. As the endmilling process is
lithography, electrical discharge machining, laser scaled down from conventional sizes (100 m/tooth
machining and computer numerical controlled (CNC) feed rates, 1 mm depths of cut) to micro-endmilling
micromachining. Most of these techniques require sizes (1 m/tooth feed rates, 100 m depths of cut),
inaccessible, expensive, or time-consuming different phenomena dominate the micro-endmilling
equipment [2], so one of the viable micro- process compared to those typically observed in
manufacturing techniques for creating three- conventional milling [5]. 5TMZ[W MZPgf XKL
dimensional (3D) features on metals, polymers, stated that the fundamental difference between
ceramics, and composites is mechanical micromilling and conventional milling arises due to
micromachining. Micromachining utilizes miniature scale of the operation, in spite of being kinematically
milling, drilling and turning tools as small as 10 m the same. However, the ratio of feed per tooth to
Corresponding author: J. Paulo Davim, e-mail: pdavim@ua.pt

k % 2Pb
MZO PCaPe4 Z 4[ P
A brief review on micromachining of materials 99

Fig. 1. Inputs and influences in micromilling.

radius of the cutter is much greater in micromilling transition between two cutting conditions; where
than conventional milling, which often leads to an chips are produced and where ploughing takes place
error in predicting cutting forces. Also, the runout of [6].
the tool tip, even within microns, greatly affects the
accuracy of micromilling as opposed to the 996 P.3 .04 4
. 8
conventional milling. MATERIALS
The chip formation in micromilling depends upon
a minimum chip thickness and hence the chip is Precision cutting tools and machine tools are critical
not always formed whenever tool and workpiece is to micro-mechanical cutting processes, since the
engaged as opposed to conventional milling. The surface quality and feature size of the micro-
tool deflection in the micromilling greatly affects the structures are dependent on them. Nowadays, the
chip formation and accuracy of the desired surface geometries of micromilling tools are created by
as compared to conventional milling. The tool edge scaling down macro tools but due to the increasing
MPU a eU OMXX
eN c Z q m) and its uniformity miniaturization of components, it is becoming ever
along the cutting edge are highly important as the more complex to produce the required tools. In
chip thickness becomes a comparable in size to addition, several researchers [8-9] have shown that
the cutting edge radius [7]. Since the chip load is micro tools respond to influences in a very different
small compared to the cutting edge radius, the size way than macro tools do.
effect and ploughing forces become significant on Conventional milling tools vary widely in size and
both surface and force generation in micromilling. design for different applications. In end milling, the
Micromilling may result in surface generation with common issues are tool deflection and uneven
burrs and increased roughness due to the ploughing- distribution of cutting force among the cutting edges.
dominated cutting and side flow of the deformed The forces are concentrated on the side of the tool
material when the cutting edge becomes worn and and cause the tool to bend in the direction of the
blunter. workpiece feed. The extent of deflection also
There are several phenomena in micromilling that depends greatly on the rigidity of the tool and the
prevent the results of conventional milling from being distance extended from the spindle. In fact, the
applied to it directly. First, it cannot be assumed deflection is directly proportional to the cube of the
that the microstructure of the workpiece material is extension [10]. Also, the smaller the tool diameter,
homogeneous [5]. As tool size becomes smaller, the more prone it is to deflection and this is even
its effect becomes more important. In this work it more so in micromilling, as the tools diameters are
cM a PMZn%-YY [[XMZPR [ U YX U
OUeTU ever so small.
effect was not assumed. Second, the effect of the Tungsten carbide cutting tools are generally used
cutting edge radius is not negligible: it affects the for the micro-mechanical cutting process, due to
chip forming mechanism. Minimum chip thickness their hardness over a broad range of temperatures.
is a function of this parameter, and determines the In the early 1990s, use of coatings to reduce wear
100 P. Cardoso and J.P. Davim

and friction became more common and most of these m and 101.6 m. To investigate the effects of feed,
coatings are referred to by their chemical composi- speed, and axial depth of cut on the performance of
tion, such as TiN (Titanium Nitride), TiCN (Titanium the tools, a design of experiments study was con-
CarboNitride), TiAlN (Titanium Aluminum Nitride) or ducted on polymethyl methacrylate (PMMA). The
TiAlCrN (Titanium Aluminum Chromium Nitride), investigation was done based on two spindle speeds
among others. Advances in end mill coatings are (50000 and 100000 rpm), two feeds (1, 5 m/flute),
being made, however, with coatings such as Amor- and two axial depths of cut (10, 20 m). They con-
phous Diamond and nanocomposite physical vapour cluded that the spindle speed has the most promi-
deposition (PVD) coatings. In 2006, Arumugam, et nent effect for all force components, and increase
al. [11] investigated the performance of polished CVD in spindle speed caused an increase in forces.
diamond tool carbide inserts in comparison with 2X[U Z %%-5TMZ[W MZPgf X KL R [Y P
unpolished CVD diamond coated carbide tool in- experimental and modelling studies on meso/micro-
serts in the dry turning of A390 aluminum, a silicon milling of AL 2024-T6 aluminum and AISI 4340 steel
hypereutectic alloy and concluded that polished to predict chip formation and temperature fields.
chemical vapour deposition (CVD) diamond tool in- They also studied size effects and minimum chip
serts improve tool life and reduce the cutting forces. thickness. To conduct this study, the authors used
However, the size of micro end mills makes coating 2-flute tungsten-carbide on cobalt matrix WC-Co
deposition challenging especially around the cut- ZPYU X
XcU T(%pT X UdMZSX PU MY -, YY
ting edges. The requirements on the coatings for and 3.175 mm and a fixed spindle speed of 60000
micro machining tools are not only the desirable rpm. Cutting speed used was 22.62 m/min and 59.85
properties such as high hardness, high toughness m/min and feed per tooth varied from 0.265 m to
and high chemical/erosive and abrasive wear resis- 4 m. Large force variations were observed as the
tance, but they must also be dense, have a fine diameter of the cutter decreased and the spindle
microstructure and present a smooth surface to the speed increased.
workpiece, with a reduced coefficient of friction com- In order to study the influence of the tool edge
pared to that of the uncoated tool [7]. condition and the workpiece microstructure, Vogler
and his colleagues [5] in 2004 performed
4. MICROMACHINING OF MATERIALS experiments with 508 m diameter end mills on
P. 482 70 0 workpiece materials with different microstructures
over a range of feed rates. Four materials were
The most important machining parameters in selected for the experimentation; two specially
micromilling are spindle speed, feed rate and feed prepared, single phase materials (pure ferrite and
per tooth. Literature shows that many studies have pearlite) and two multi-phase materials with different
been done to show up to which extent these compositions of the two single phase materials.
parameters influence the quality of the machined They performed 5 mm long full-slot endmilling cuts
parts and the consequences on the tool. In Fig. 1 a under several conditions in order to study the
diagram of the inputs and influences in micromilling interaction between ploughing and process condition
is shown. effects on the surface roughness of the slot floor.
In 2008, Filiz et al. [8] investigated the use of The conditions the authors used can be seen in
the mechanical micromilling process for fabrication Table 1.
of micro-scale piercing element from biocompatible In 2007, Filiz et al. [4] used a miniature machine
materials. The authors used two custom made, tool to perform micromachining experiments on
special geometry, tools with cutting diameters 254 99.99% purity Copper. This machine tool was
equipped with a 160,000 rpm air-turbine, air-bearing
Table 1. Workpiece and machining parameters [5]. spindle with a 3.125 mm precision collet. The
spindle-axis runout was quoted by the manufacturer
Workpiece microstructure Pearlite, Ferrite, to be less than 2 m. The micro end mills used
Ferritic and Pearlitic during the experimentation were micro-grain
tungsten carbide (WC) tools, fabricated by diamond
Cutting edge radius 2.0 and 5.0 m grinding, two-fluted and with a 254 m diameter and
Axial depths of cut 50 and 100 m a 30jhelix angle. This experimental study included
Feed rates 0.25, 0.5, 1, 2 full-immersion (slot) cutting with axial depth of cut
and 3 m/flute of 30 m. Four feed rates (0.75, 1.5, 3, and 6
Spindle speed 120,000 rpm m/flute) and three cutting speeds (40; 80, and 120
A brief review on micromachining of materials 101

(a) (b) (c)

Fig. 2. 4[Y MU[Z[R M SU R [ T MY oY [[TR P M /M O

[Z MZ [b X
M UMX N MMX
XX
spiral and (c) parallel zigzag.

Fig. 3. Surface profile comparison between strategies for the same feed rate of 6 mm/tooth: a) constant
overlap spiral, b) parallel spiral and c) parallel zigzag.

m/min) were considered in this experimentation. The Recently, Cardoso and Davim [12] in order to
range of feed rates was selected to include the perform a comprehensive study on surface
ploughing, indentation, and minimum chip thickness roughness of the machined surfaces, cutting
effects in the data. The spindle speed varied parameters such as feed rate as well as machining
according the feed rates: 50,000 rpm for 0.75 m/ strategies were varied to optimisation micromilling.
flute, 100,000 rpm for 3 m/flute and 150,000 rpm In this research, Al 2011 aluminium alloy was used.
for 6 m/flute. It is an Al-Cu-Bi-Pb age-hardened alloy noted for its
 102 P. Cardoso and J.P. Davim

free-machining characteristics and good mechani- [2] S. Mecomber Justin, Hurd Douglas and
cal properties. The tool used to machine the A. Limbach Patrick // International Journal of
workpiece was a cemented carbide K10, 0,8 mm Machine Tools & Manufacture 45 (2005) 1542.
diameter endmill. Four feed rates (2, 4, 6, and 8 [3] J. Chae, S. S. Park and T. Freiheit //
m/flute) and one spindle speed 6,500 rpm were International Journal of Machine Tools &
considered in this experimentation. Three machin- Manufacture 46 (2006) 313.
ing strategies were used: constant overlap spiral, [4] Filiz Sinan, Caroline M. Conley,
parallel spiral and parallel zigzag. Fig. 2 shows the B. Wasserman Matthew and O. B. Ozdoganlar
comparison between the three different strategies. // International Journal of Machine Tools &
The burrs produced with the second strategy (par- Manufacture 47 (2007) 1088
allel spiral) are much pronounced. The constant [5] Michael P. Vogler, R.E DeVor and S.G. Kapoor
overlap spiral strategy was the one that presented // Journal of Manufacturing Science and
the best result. Surface roughness profiles and the Engineering 126 (2004) 685
value of RzD (Mean peak-to-valley height- DIN 4768) KL 2 5TMZ[W MZPD gf X Int.
comparison are shown in Fig. 3. Also the constant J. Mechatronics and Manufacturing Systems
overlap spiral strategy was the one that presented 1 (2008) 23.
the best result to RzD (Fig. 3a). [7] A. Aramcharoen, P.T. Mativenga, S. Yang,
K.E.Cooke and D.G. Teer // International
5. CONCLUSIONS Journal of Machine Tools & Manufacture 48
(2008) 1578.
The most relevant inputs in the micromachining pro- [8] Filiz Sinan, Xie Luke, E. Weiss Lee and O.B.
cess can be said to be the tools (characteristics Ozdoganlar (2008) // International Journal of
and materials), machine tools (spindle technology, Machine Tools & Manufacture 48 (2008) 459
toolholder, rigidity) and, not least importantly, K.L9 Ai f2 FU f hZ ;4 9 ZMZP fMZP
machining parameters (cutting speed, feed rate and 8af YhZ %%, Journal of Materials
feed per tooth). On the other hand, the issues ad- Processing Technology 190 (2007) 18
dressed with micromachining are the minimum chip [10] R. Jalili Saffar, M. R. Razfar, O. Zarei and
thickness and size effect, the cutting temperatures E. Ghassemieh // Simulation Modelling
and cutting forces, which influence the tool wear Practice and Theory 16 (2008) 378
and its failure, which, in turn, influence the burr for- [11] U. Arumugam Prabhu, A.P. Malshe and S.A.
mation and, consequently, surface quality. Batzer // Surface & Coatings Technology 200
(2006) 3399.
REFERENCES [12] P. Cardoso and J.P. Davim // Materials and
[1] X. Liu, R.E.DeVor, S.G. Kapoor and K.F. Manufacturing Processes 25 (2010) 1125.
Egmann // Journal of Manufacturing Science
and Engineering 126 (2004) 666

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