Micromilling technology: a global review Mikrofresaketa teknologia: ikuspegi orokorra Tecnología de microfresado: revisión general

1. MICROMILLING
TECHNOLOGY:
Global review
Further presentations:
www.symbaloo.com/mix/manufacturingtechnology
Dr. Endika Gandarias Mintegi

2. Chapter : Dr. Endika Gandarias Mintegi
AUTHOR
Dr. ENDIKA GANDARIAS MINTEGI
Mechanical and Manufacturing department
Mondragon Unibertsitatea - www.mondragon.edu
(Basque Country)
www.linkedin.com/in/endika-gandarias-mintegi-
91174653

3. INDEX
1. INTRODUCTION__________________________________ 2
2. HISTORICAL BACKGROUND________________________ 3
3. MICROMILLING MACHINES_________________________ 4
3.1. Machine bed & Architecture
3.2. Spindle
3.3. Guides
3.4. Driving systems
3.5. Workpiece clamping and fixturing devices
3.6. CAD/CAM software
3.7. Control & Monitoring systems
3.8. Small machines & Microfactories
4. MICRO-CUTTING TOOLS IN MICROMILLING___________
12
4.1. Cemented carbide tools
4.2. Diamond tools
5. WORKPIECE MATERIALS IN MICROMILLING__________ 15
6. MICROMILLING APPLICATIONS_____________________ 16
7. PROCESS CUTTING DIFFERENCES BETWEEN MACRO
AND MICRO WORLD______________________________ 21
7.1. Specific cutting energy
7.2. Minimum chip thickness
7.3. Surface roughness
7.4. Burr
7.5. Microstructure effect
7.6. Tool wear
7.7. Modeling
7.8. Sensing & monitoring methods
8. FUTURE CHALLENGES___________________________ 31
9. BIBLIOGRAPHICAL REFERENCES__________________ 33

4. MICROMILLING TECHNOLOGY: Global review
Micro-cutting machining has emerged recently as a vital aspect in meeting the
requirements of technological advancement. The continuing quest for smaller, more
reliable consumer and industrial products is pushing current limits in miniaturization
technology [1
].
1. INTRODUCTION
Micromilling is one of the emerging fabrication technologies. It is characterized by
mechanical interaction of a sharp tool with the workpiece material, causing breakage
inside the material along defined paths, and eventually leading to removal of the
useless part of the workpiece in the form of chips [2
].
Even though it may not be capable of obtaining as small feature sizes as in
lithographic processes, it is important in order to bridge the macro domain and the
nano/micro domains for making functional components [3
-4
]. There exists a wide variety
of important applications in the commercial and defense sectors (e.g. masks for deep
X-ray lithography [5
], asymmetric high precision moulds [6
], etc.), which require high-
strength materials and complex geometries that cannot be produced using current
MEMS fabrication technologies. Micromilling has the potential to fill this void in MEMS
technology by adding the capability of free form machining of complex 3D shapes from
a wide variety and combination of traditional and well-understood engineering materials
(alloys, composites, polymers, glasses and ceramics).
In view of this, the fabrication of micro and meso-scale devices has presented
researchers with new and exciting challenges.
This chapter realises a global review of the micromilling technology. First, the origin
of the micromilling technology is described. Second, the micromilling machine is
studied paying special attention to the elements that differ from macro machines, such
as the machine bed, spindle, guides, driving systems and control & monitoring
systems. Third, most commonly used microtool types and their weaknesses are
described. Fourth, workpiece materials are described. Fifth, currently existing
micromilling application areas are mentioned and some samples are depicted. Sixth,
major cutting process differences between macro and micro world are studied. Finally,
future micromilling challenges are described.
2. HISTORICAL BACKGROUND
by E. Gandarias 4

5. Although wood-working machines have been in use since Biblical times, it was not
until the late 17th Century that clockmakers, builders of scientific instruments, and
furniture and gun makers began the changeover to machines capable of manufacturing
steel. They had a need for a variety of gear cutting, grinding and precise screw-cutting
machines to fabricate their products [7
].
The first practical metalworking lathe was invented in 1800 by Henry Maudslay. It
was simply a machine tool that held the piece of material being worked (workpiece) in a
clamp (spindle), and rotated it so that a cutting tool could machine the surface to the
desired contour. The cutting tool was manipulated by the operator through the use of
cranks and handwheels.
Not much later, in 1818, American Eli Whitney built a pioneering horizontal milling
machine; and in 1861, the American firm of Brown & Sharpe constructed the first truly
universal milling machine, specifically for machining the grooves in twist drills (see Fig.
3.1).
Between the 1860s and the 1960s, there was an extraordinary quantum leap in
humanity’s capacity to transform raw metal into highly complex components. Together
with contemporary developments in chemistry and electrical technology, this revolution
in metalworking formed the industrial backbone of the modern world [8
].
Brown's machine had set the stage for the developments of the twentieth century
that were to follow; greater strength and rigidity, faster cutting speeds, higher precision,
the inclusion of the electric motor and fully automatic operation [9
]. These automatic
aids to proficiency, always adhering to the double principle of accuracy and
productivity, have increased through the years.
A b
Fig. 3.1 First Universal Milling machine appears: (a) the inventor Joseph R. Brown [10
]; (b)
the universal milling machine from 1861 [11
].

6. Fig. 3.2 The development of achievable machining accuracy [12
].
Based on these inventions, watchmaker and jewellers built milling machines
capable of producing reliable and consistently small precision parts for serial
production. New design and forms of components unimaginable until then at smaller
sizes within tighter tolerances became possible for manufacturing. These initial steps
established the origins of the forthcoming micromilling technology.
In 1983, Norio Taniguchi of Tokyo Science University illustrated the evolution that
machine accuracy capabilities had underwent since the early 20th century, and the
predicted the trend of these technologies for the following 30 years (see Fig. 3.2).
Taniguchi would first define the concept of Nanotechnology to refer to the production
technology to get the extra-high accuracy and ultra-fine dimensions [13
].
3. MICROMILLING MACHINES
Size and quality of the micro-products depend on the properties of the used
machine tools to manufacture them, including the overall accuracy and dynamic
performance. Excellent capabilities of the machine tool are vital to such product
requirements as size, accuracy, surface roughness and dimensional repeatability. In
particular, machine bed, spindle, guides and driving systems result in key factors in the
design of a micromilling machine.
Table 3.1 gives a rough approximation to the generic micromilling machine error
sources.
by E. Gandarias 6

7. Spindle run-out
0.025µm
at 15Krpm
0.050µm
at 60Krpm
> 1 µm beyond
100 Krpm
Spindle stiffness
Radial < 70 N/µm
Axial < 35 N/µm
Tool mount offset > 1 µm
Tool geometry tolerances
Precision machining
> 2 µm
FIB
< 1 µm
Linear Axis accuracy
(per axis)
Highly Precise Systems
0.15 µm
Industrial Systems
> 0.5 µm
Rotational Axis run out
(per axis)
> 0.1 µm
Tool-tip deflection < 3-4 µm
Tool expansion * > 1 µm
Workpiece referencing > 1 µm
*This error is after CNC compensation. Tool expansion prior to compensation can be tenths of microns.
Table 3.1 Typical milling machine error sources.
a b c
Fig. 3.3 Commercial ultra precision machine tools: (a) Kern EVO [14
]; (b) Kugler Microgantry
GU [15
]; (c) Fanuc ROBOnano [16
].
Examples of some commercially available precision micromachining centres are
depicted in Fig. 3.3a-b-c.
3.1. Machine bed & Architecture
Micromilling machine bed needs to damp outside and process generated vibrations,
minimize the influence of changes in temperature, and guarantee a certain degree of
stiffness [Error: Reference source not found].
Granite is preferred as the structural material due to the low coefficient of thermal
expansion. Polymer concrete is also chosen in some cases, primarily due to the high
damping characteristic. For increasing rigidity steel frames can be incorporated [17
], and
even alumina ceramic base structure prototypes have been studied [18
].
Conventional milling machines can be found with a wide variety of configurations, as
are dependent upon workpiece and process constraints (see Fig. 3.4).

8. Fixed bed moving column Bed type (Two axis at table) Bed type (one axis at table)
Bridge-type Gantry Vertical machining centre
Fig. 3.4 Most common 3-axis milling machine configurations.
Most of the currently available micromilling machine architectures correspond to the
vertical configuration, for which the vertical direction axis is at the tool side and the
workpiece only has the horizontal degrees of freedom. This configuration is usually
made use of in micromilling machines since gravity results not so critical resulting in
lower risks to fall the components and reduced features distortions.
It is frequent in micromilling machines, as having an additional option, the possibility
to implement a 4th and 5th axis on the basis of the three-axis vertical machining centre.
Process integration seems a usual feature in manufacturing of microproducts.
Indeed, some commercial machines perform more than one machining process
besides milling, such as, turning, grinding or laser.
3.2. Spindle
In micromilling, high revolution speeds are demanded due to the small tool
diameters, and speeds as high as 250000 rpm are currently available. Nevertheless, it
is generally agreed that spindle speeds must still considerably increase in the near
future and cover a broader range of revolutions to achieve a competitive degree of
efficiency compared with other micromachining processes. Currently used spindles
make it frequently impossible to reach the recommended cutting speeds, even after
using air turbine mechanism spindles. Indeed, considerable ongoing research to
improve spindle capabilities is being carried out [19
].
According to the high revolution and reduced thermal expansion requirements,
ceramic bearing spindles or aerostatic spindles are mostly extended.
The spindle run-out and lengthening are other limitations which should be minimized
since they influence the machinable tolerances and possible tool failures. The former
can be originated due to the spindle error motion, tool clamping system or tool inherent
geometrical errors while the latter requires a dimensioned cooling system to keep the
by E. Gandarias 8

9. Spindle
Manufacturer
Type
Body
diameter
(mm)
Weight
(kg)
Max.
Speed
(rpm)
Stiffness Run-out (µm)
Radial
(kg/µm)
Axial
(kg/µm)
Static Dynamic
MicrodyneTM
Air bearing 61.91 3.24 125K 0.59 1.07 5.00 8.00
Airturbine Tools
602JS
Ceramic bearings 39.67 0.68 90K 5.08
Westwind D1733 Air bearing 53.34 1.86 250K 5.00
Precise
SC 1060-OA
Oil-air lubricated
Hybrid ceramic
60.00 2.50 160K 2.50 1.70 2.00
Colibri Spindles
HF80L2S140
Air bearing 80.00 9.00 140K 0.30 0.40 0.20 - 0.50
Table 3.2 Characteristics from the main high-spindle suppliers [20
], [21
], [22
], [23
], [24
].
Air bearings Hydrostatic bearings Rolling bearings
Almost frictionless (no limitation
of motion increments)
Low friction (good resolution in
motion)
Stiction (stick-slip-effect) imposes
lower limit on motion increments
Stiff Very stiff Stiff
Low damping Higher damping
Low damping
(depends on preloading)
Large Larger and heavier Compact, lightweight, tough.
Complicated mounting Must deal with fluid Easy mounting
High cost High cost Low cost
Table 3.3 Comparative between air bearings, hydrostatic bearings and rolling bearings [25
].
temperature constant at very small intervals and minimize the spindle lengthening.
The use of laser measuring systems is recommended in order to measure and take
into consideration these enlargements (may reach values of 35 µm) [26
].
In Table 3.2 the characteristics of some current commercial high-precision high-
speed spindles are shown:
3.3. Guides
Fabrication of microstructures with micron details features and high accuracies is
submitted to an ultra-precision stage (i.e. XY table). In order to maintain constant
process conditions even at these details, the axes need to have a high acceleration as
well as a stiff bearing to compensate process forces.
Next, most common types of bearings used in micromilling machine stages are
compared; these include air bearings, hydrostatic bearings and roller bearings (see
Table 3.3).
Hydrostatic guides are currently the most common systems for linear guides in high-
precision micromilling machines. However, the use of aerostatic guides is increasing
since it offers the highest resolution and great machine consistency due to zero wear.
[Error: Reference source not found].
3.4. Driving systems

10. Another issue is the different kind of driving systems that are used in current
ultraprecision machines. Depending on the degree of accuracy, single or dual actuator
driving systems are found (see Fig. 3.5). Single actuators are friction drives, ball
screws or linear motors, while dual actuators couple two driving systems, a coarse one
such as a ball screw or a linear motor and a fine one such as a piezoactuator (PZT) or
a voice coil motor (VCM).
Fig. 3.5 Resolution of different driving systems vs. working range [27
].
Most commercial micromilling machines use single actuators driving systems. Ball
screw drives provide lower accuracy compared with other options but, still reasonable
resolution and accuracy. Linear motors provide backlash-free, friction-free motion and
higher accelerations, so the attainable resolution is higher compared with ball screws
driving systems [28
]. Friction drives allow very high resolution, about 1nm, although they
have low rigidity, low damping and low load capacity, which overall worsens the
dynamic capacity.
So far, the most suitable equipment is linear drive motor with hydrostatic bearings.
The positioning accuracy at high precision machining tools can be located at 0.15 μm
per axis and around ± 2 μm at the workpiece [29
].
3.5. Workpiece clamping and fixturing devices
The large variety of workpiece materials, geometries, and dimensions applied in
micromilling demand the use of different type of clamping and fixturing devices as there
is no unique standardised technique.
Clamping and fixturing devices need to fulfil requirements such as no deformation
and contamination of components, fast workpiece exchange, high reproducibility and
accuracy, stiff assembly, etc.
Furthermore, other factors such as, workpiece properties (strength, stiffness,
chemical resistance, temperature resistance, magnetization susceptibility), workpiece
dimensions and geometries, manufacturing forces, lot sizes, etc., need to be
considered for an adequate selection of a clamping or fixturing method.
According to the way that the clamping and fixturing devices transfer the forces
between the workpiece and the mount, they can be classified as follows (see Fig. 3.6):
by E. Gandarias 10

11. • Force closure devices: bench vice, brackets, magnetic disk, vacuum
clamping, etc.
• Form closure devices: screw fastening, self-centering clamping systems,
etc.
• Adhesive bond techniques: Gluing, embedding in metal or wax, freezing,
magnetorheologic bonding, etc.
a b c
d e f
Fig. 3.6 Clamping and fixturing devices: (a) brackets [30
]; (b) magnetic chuck [31
]; (c)
vacuum chuck [32
], (d) self-centering chuck/pallet systems [33
]; (e) Gluing lines for
workpiece fixturing [34
]; (f) freezing mounts [Error: Reference source not found].
Lately, the self-centering chuck/pallet systems are the most extended methods as
they allow the production of small series and transfer the workpiece from the machine
to a separate measuring station and back, with high position reproducibility.
3.6. CAD/CAM software
The use of CAD/CAM systems in micromilling operations is widely extended to
machine the small and complex workpiece geometries. However, currently available
CAD/CAM systems do not present an adequate behaviour for micromilling process
characteristics and specific CAD/CAM modules are required (see Fig. 3.7).
To adequately support the micro-milling process, the CAD / CAM software should be
able to [35
]:
• Accurately utilise the highly detailed mathematical model, while maintaining
its level of complexity. Having an integrated CAD/CAM solution is ideal,
since it eliminates any data translations in the process;
• Include high-accuracy, built-in CAD capabilities within the CAD system that
can provide assisting geometry (e.g. capping, extending surfaces, etc.) with
the appropriate accuracy and tangency within the CAM system;
• Support tool path calculation with tolerances down to 0.1micron. This is
especially challenging when machining miniature details in large size parts;
• Support calculation with micro-milling level parameters considering the
constraints of the physical machines. For instance, the CAM system may be
required to provide super-finish results with a tool diameter of 0.1mm, a side

12. step of 0.005mm and a 10 times large round corner radius of 0.05mm. Tool
paths must be created accurate to the fifth decimal point;
• Support machining strategies optimised for micro-milling, such as the
machining of rough, re-rough and finish in the same NC operation;
• Use the knowledge of actual remaining stock throughout the entire process
to adjust feed to actual tool load in order to lower machining time while
protecting the delicate tools from breaking.
a b
Fig. 3.7 Commercial Cimatron CAD/CAM solution for micromilling: (a) CAD/CAM
software image; (b) obtained final part [36
].
Resolution Bandwidth Notes
Capacitive sensors 0.1nm 10 kHz Short measurement range.
Laser
interferometer
<1nm
Sub-nanometre resolution.
Measurement is independent of stage motion.
Expensive and complicated implementation.
Optical encoders >1nm
Resolution range (1-100nm).
Less expensive and easy to implement.
Strain gauge 1nm 5 kHz Very small, low heat generation.
LVDT 10nm 1 kHz Good temperature stability. Bulky.
Table 3.4 Most common position sensor characteristics.
3.7. Control & Monitoring systems
Improved machining productivity, higher workpiece accuracies, and longer tool lives
demand the implementation of precise and reliable control and monitoring systems.
Regarding the positioning systems, most commercial machines use glass scales
(optical encoders) as it is the lowest cost option with a sufficiently high accuracy. A
relatively good feedback control performance can be achieved when using hydrostatic
and ball screws. Laser systems, on the other hand, show a higher positioning
resolution although they are sensitive to environmental influences and make the
machines considerably more expensive. Table 3.4 shows several types of positioning
sensors available in the market.
Besides positioning systems control, there is a wide range of control systems to
achieve better machining accuracy and longer tool life. Tool control is a primary
concern, and so micromilling machine manufacturers have developed various systems
to detect and measure the tool during machining. In that context, commercial products
by E. Gandarias 12

13. for control of specific features have emerged due to the demand of high precision at
high spindle speeds. Examples of these control systems are contact probes and non-
contact laser systems. However, further developments are required in this field, and
chapter 4 analyses this area more extensively.
Thermal control, for instance, is another recently used control system which is
increasing its acceptance amongst micromilling machine manufacturers. Currently, the
highest precision machines, such as, the Fanuc Robonano or Cranfield Nanocentre
provide a thermal control of the air and fluid systems in a very narrow range of about
0.01°C. Furthermore, the Tokyo Institute of Technology has recently developed a novel
temperature control system of the spindle supply air with excellent results, which
consists of cooling the input air while machining (see Fig. 3.8).
a b c
Fig. 3.8 Temperature control system: (a) supplied air temperature; (b) spindle temperature;
(c) spindle displacement.
3.8. Small machines & Microfactories
Micromachine manufacturer trends present two clearly different conceptions
worldwide; while Europe and most institutions and companies in U.S.A. are adapting
current machines to the microtechnologies, Japan and some research centres in
U.S.A. are developing tiny machines in order to reduce inertia and energy
consumption, and to improve resolution.
Several researchers and companies are trying to scale down the machine tools to
produce micro-components [37
, 38
, 39
, 40
, 41
]. Micro-machine tools present several
benefits from this miniaturisation including that of reduction in energy, space, materials
and cost. Fig. 3.9 illustrates some miniature micro-machine tools.
The possibility to use more expensive materials that exhibit better engineering
properties or lower thermal expansion due to a smaller construction, or higher natural
frequencies of the micro-machine due to a smaller mass (regenerative chatter
instability is shifted) are one of those benefits. In addition, smaller machine tools have
lower vibration amplitudes [Error: Reference source not found].
The portability of such systems is beneficial, e.g. during military or space exploration
applications. Microfactory is a concept that has been introduced recently referring to
extreme miniaturization of a manufacturing system. Further, microfactories can have
different cells with different functionalities such as micro-lathe, micro-milling and micro-
press (Fig. 3.9c).
However, there are challenges associated with the development of small size micro-
machine tools. Major development should be the increase in structural rigidity and
reduction of the vibration influences. They require small enough accurate sensors as
well as actuators to implant within the machines.

14. a b c
d e f
Fig. 3.9 Micro-machines: (a) miniature micro-lathe [42
]; (b) miniature micro-extrusion machine [43
]; (c)
Microfactory [Error: Reference source not found]; (d) precision milling machine [44
]; (e) 2nd
generation
3-axis horizontal milling machine [45
]; (f) 5-axis milling machine [Error: Reference source not found].
4. MICRO-CUTTING TOOLS IN MICROMILLING
Normally used microtool types for micromilling operations are end-mill, ball-nose,
drills and engraving tools. These tools are usually made of tungsten carbide, and also
of diamond material when not machining ferrous workpieces.
One of the main limiting factors for the miniaturization of the cutting process is the
tool due to the reduced stiffness of the tiny sizes. Common tool wear in conventional
milling cannot be used as an end tool life criterion in micromilling as premature tool
failures occur frequently. Various authors [46
-47
] state that commonly observed tool
breakages may be due to the following mechanisms:
• Excessive stress-related breakage: It will occur very quickly if the cutting
force increases beyond the strength of the tool. The cutting force might
increase for the following two reasons: first, the cutting edge might lose its
sharpness or the cutting edge is partially damaged. And second, deposition of
chips fills the microtools tiny grooves producing chip clogging. High Speed
Steel (HSS) tools tolerate such chip clogging better than carbide tools since
they are less rigid. However, it is almost impossible to predict chip clogging
ahead of time. The workpiece starts to push the shaft of the tool and it
deflects, increasing the static component of the feed direction force [Error:
Reference source not found];
• Fatigue-related breakage: It may happen if the cutting force and the stress
increase as a result of tool wear, and then stay at that level for an extended
by E. Gandarias 14

15. period of time. The stress on the shaft will change repeatedly while it is
rotating [Error: Reference source not found];
• Increase in the specific energy: Tool failures occur due to the substantial
increase in the specific energy required as the chip thickness decreases. This
means that in the case of micromachining, as the chip gets thinner with
smaller depths of cut, the microtool tip will be subject to greater resistance
when compared with conventional machining [Error: Reference source not
found].
Furthermore, tool run-out is another remarkable problem since it creates drastic
changes in the cutting force profile. Tool run-out is caused by a misalignment of the
axis of symmetry between the tool and the tool holder or spindle [48
]. Due to that, it is
quite common to see that only one cutting edge of a two-flute micro end mill performs
the machining operations involving an increase in the force variation, and as a
consequence, the tool wears out faster raising the tool failure probability [49
] (see Fig.
3.10).
Cutting force in micromilling: Ø 0.5 mm end-mill, tool steel SAE H11 (56 HRC),
Vc =56.55 m/min, az =0.009 mm, ae =0.5 mm, ap =0.08 mm)
Fig. 3.10 Tool run-out effect, one cutting edge cut deeper than the other [50
].
This run-out may create drastic force variations in the cutting forces since some
cutting edges of the microtool perform more than the others during the machining
operations.
Various partially conflictive requirements need to be met with in order to produce
precision components. Depending on the required surface properties and workpiece
material cemented carbide tools (CW) or diamond cutting tools are used (scarcely high-
speed steel tools). Traditionally, diamond cutting tools have been used for ultra-
precision machining operations, but because of the high demand for machining ferrous
materials [51
], carbide tools are considered better suited to achieve these ends.
4.1. Cemented carbide tools
Tungsten carbide cutting tools are generally used for the micro-mechanical cutting
process due to their hardness over a broad range of temperatures and materials (see
Fig. 3.11a-b).
Achievable surface roughness ranges between Rz 0.1-0.3 μm depending on the
workpiece material [Error: Reference source not found] and other parameters. These
tools are commercially available in diameters as small as 5 μm produced by grinding
machining [52
].

16. However, tool diameters below 100 μm are not fully reliable, thereby originating an
unpredictable tool failure. Furthermore, microtools of diameter less than 50 μm need a
zero helix angle to improve their rigidity [Error: Reference source not found, 53
] and to
mitigate the limitations of fabrication techniques. Some attempts have been done on it,
e.g. ultrasonic vibration grinding to produce 11 μm diameter tool carbide [54
], fabrication
of a 25 μm diameter tool carbide using focused ion beam machining [55
] or analysis of
different geometries of microtools (triangular and D-shape bases) [56
] (see Fig. 3.12).
In addition, wear reducing PVD-coatings (TiAlN, TiN, etc.) appear essential for
dropping mechanical, chemical & thermal influences to obtain an economically
reasonable tool life. The formation of droplets is still a major problem and needs to be
addressed since the chip flow is complicated resulting in enlarged cutting forces and
thus, a higher probability of tool rupture [Error: Reference source not found].
a b
Fig. 3.11 Cemented tools; (a) Hardness of cutting tool materials as a function of temperature [57
];
(b) Scanning electron micrograph of Al 6061 machined at a feed rate of 10 mm/minute [Error:
Reference source not found].
a b
Fig. 3.12 Micro-cutting tools: (a) Hand-made microtools SEM micrograph [58
]; (b) Commercially
available 25 µm end-mill image obtained in the JEOL JSM-5600LV Scanning Electronic
Microscope.
4.2. Diamond tools
Diamond cutting tools present the capability to produce a surface roughness Ra in
the range of 10 nm with an almost atomic sharpness of the cutting edge [59
]. These
tools are available down to sizes of 0.1 mm in diameter. However, diamond cutting
tools are restricted to workpiece materials containing no ferrous elements due to the
by E. Gandarias 16

17. high affinity of iron to carbon. Above 700ºC, diamond degenerates to graphite leading
to devastating wear and final tool breakage.
Approaches for this problem are the reduction of the contact time between tool-
workpiece by Elliptical Vibration Cutting [60
], application of a nitrating coating [61
], or use
of other cutting tool materials such as cemented carbide, ceramics or different
monocrystalline materials [62
].
Recently, nanocrystalline diamond coatings on tungsten carbide micro-end-mills
have been investigated with excellent results as an alternative to diamond tools [63
].
9. WORKPIECE MATERIALS IN MICROMILLING
The machinable spectrum of materials by micromilling is broad (metals, polymers or
ceramics) but yet, considering steels, limited to a certain hardness of around 50-55
HRC for commercially available microtools mainly due to the tools reduced stiffness.
When cutting with diamond tools, another restriction for an economical process is that
till date, the workpiece is nonferrous [Error: Reference source not found].
The micro-cutting of steel has recently received strong research interest with the
initiation of miniaturised systems using a variety of materials, especially for biomedical
applications and injection moulds. The most commonly used tool steel is 1.2343 (X38
CrMo V5-1), which can be hardened to 54 HRC.
Chemical reactivity with the cutting tool, crystal structures, defect distribution, and
heat treatment are some factors that could affect the micro-machinability. It should also
be considered that the material combined with the geometry of the part ought to be
rigid enough in order to present a minimum distortion when clamping.
The assumption of homogeneity in workpiece material properties is no longer valid
because micrograin size is often of the same order of magnitude as the cutter radius of
curvature [64
].
10. MICROMILLING APPLICATIONS
Emerging miniaturization technologies are potential key technologies of the future
that will bring about completely different ways people and machines interact with the
physical world.

18. Micromilling technology can meet many of those demands of miniaturised
components in fields that include aerospace, automotive, biomedical, electronics,
information technology, optics, telecommunication industries, jewellery, watch-making,
etc.
Thus, many companies involving a broad field of areas, use micromilling technology
integrated in their production systems [Error: Reference source not found]. Some
companies are listed as example; Bic, Gillette, Bosch, Cerametal, Iscar, Sandvik
Coromant, Seco, Rolex, Bauer Christian, Angiomed, Biacore, Curasan, Microtronic,
Oticon, Phonak AG, Microparts Steag, Acritec, Alcon, Bausch & Lomb, Carrera,
Arilens, Medicontur, Morcher, Star Surgical, Braun, Festo, GKN Sinter Metals, Esser,
Amic, Daimler Chrysler, EADS, Fraunhofer Institute, Philips Research, Pro-micron,
Siemens VDO Automotive, Daimler Chrysler, EADS, Philips Research, Pro-micron,
Siemens VDO Automotive, 3M Unitek, Degussa, Ivoclar vivadent, Metalor dental,
Dedienne, Feinmetal, IBM, Lumberg, Tyco electronics, etc.
Many shortcomings of photolithographic batch-processing techniques can be
overcome using micromilling machining. Direct fabrication of masks for X-ray
lithography by mechanical micromilling is an avenue for manufacturing cost-effectively
low-volume production and prototypes [65
].
In addition, the implementation of replication techniques, like microinjection
moulding or hot embossing, rely on the availability of tooling technologies for
manufacturing of tools and moulds. The mould and die industry in Europe has more
than 5500 companies and total sales of 10000 M€ making an important industry [66
].
It needs to be considered that micromilling is a rather incipient process in
micromachining. Currently, few applications are clearly identified for micromilling such
as X-ray mask making, mould making for micro-replication techniques, electrode
making for electro discharge machining processes or watch making.
However, it is expected that micromilling components and applications will undergo
an exponential growth in the following years [67
].
Next, common micromilling applications are classified by field and then, real
components are illustrated as shown in Fig. 3.13, Fig. 3.14 and Fig. 3.15.
• Biomedical: Microtools for surgery, moulds for medical components (micro-
dosage systems), lab-on-chip, moulds for orthodontics (dental brackets),
moulds for biotechnology applications (microchip electrophoresis devices,
polymeric BIOMEMS devices, accelerating polymerase chain reaction for
modular lab-on-a-chip systems), cataract lenses, retinal micro-tacks, etc.
• Watchmaker and jewellery: manufacturing and engraving of watch base
plates, moulds for rings and pendants, etc.
• Information technology: Test membrane for PC chip manufacturing, etc.
• Telecommunications: mould for easy-assembly multi fibre connector for
single and multimode applications, joining elements, etc.
• Automotive: Injection nozzles, electrodes for cutting inserts, etc.
• Aerospace: Miniature devices for rockets, mould for miniature planetary
gear wheels attached to a turbine, etc.
by E. Gandarias 18

19. • Others: Components for measuring devices, electrodes for toy industry,
electrodes for manufacturing shaving head of electric razors, etc.

20. a
b
c
Analysis
Centrifuge
Device Analysis
Centrifuge
Device
d
Fig. 3.13 Micromilling applications: (a)Lab-on-a-chip [68
]; (b) Microplate with 96 capillarity
electrophoresis system (vacuum hot-embossing plate from micromilled mould) and filling
of one structure [69
]; (c) Medicine microdosage system [70
]; (d)Sweat-stick for collecting
human sweat [71
].
by E. Gandarias 20

21. e f g
h i j
k l m
n
o p q
Fig. 3.14 Micromilling applications (continue): (e) Integrated polymer microfluidic stacks
(chemiluminiscence experiment); (f) Dental brackets [Error: Reference source not found]; (g)
Tissue removal tools for endoscopy [Error: Reference source not found]; (h) Cataract lenses
[Error: Reference source not found]; (i) Fabrication of Ti retinal microtack [72
]; (j) Watch base
plate [Error: Reference source not found]; (k) Engraving watch base plate [73
]; (l) Watch parts
[74
]; (m) Pendant mould [75
]; (n) Multi-fibre connector (micromilled mould for X-ray mask
fabrication and micro-injected connector) [76
]; (o) Joining element for optical fibre connector
[Error: Reference source not found]; (p) Test membrane for computer chip manufacturing
[Error: Reference source not found]; (q) Injection nozzles for diesel engines [Error: Reference
source not found].

22. r s t
u
v w x
y z aa
ab ac ad
Fig. 3.15 Micromilling applications: (r) Electrodes for cutting inserts [Error: Reference source
not found]; (s) Mixing disc of a rocket motor [Error: Reference source not found]; (t) Turbine
wheels for microfluidic pumps [Error: Reference source not found]; (u) Assembled micro
impeller and base block [Error: Reference source not found]; (v) Micromould of a planetary
gear wheel [Error: Reference source not found]; (w) Small size plastic components [77
]; (x) 24
cavity micromould with hot sprue [Error: Reference source not found] ; (y) Electrode for
manufacturing a toy locomotive [Error: Reference source not found]; (z) Electrode for
manufacturing shaving head of electric razor [Error: Reference source not found]; (aa) Glass
extrusion die for halogen lamp [Error: Reference source not found]; (ab) Setting screw for
micrometer [Error: Reference source not found]; (ac) Modular mould insert tool for moulding
housings for a hydraulically driven micromilling cutter [78
]; (ad) Miniature microscope lens
holder [79
].
by E. Gandarias 22

23. 11. PROCESS CUTTING DIFFERENCES
BETWEEN MACRO AND MICRO WORLD
Main principle of micromilling is similar to that of conventional cutting operations, in
particular, the surface of the workpiece is mechanically removed using microtools.
However, unlike conventional macro-machining processes, micromilling displays
different characteristics due to its significant size reduction.
Next, major distinguishing factors between micro and macro milling are studied,
such as, specific cutting energy, minimum chip thickness, surface roughness, burr,
microstructure effect, tool wear, modeling, sensing and monitoring methods.
7.1. Specific cutting energy
Lucca et al. [80
] noticed the specific cutting energy required to machine at very low
chip-thickness values could not be explained by the energy required for shearing, and
for overcoming friction on the rake face of the tool. The significance of ploughing and
elastic recovery under these conditions was used to explain the increase in the cutting
energy.
Lucca and Seo [81
] observed the nominal rake angle and the tool edge profile had
significant effects on the resulting forces and dissipated energy using single-crystal
diamond tool edge geometry. When the uncut chip thickness approached the size of
the edge radius, the effective rake angle determined the resulting forces (see Fig.
3.16a).
Lucca et al. [82
] investigated the transition from a shearing dominated cutting
process to a ploughing-dominated process in diamond turning by studying the angle of
the resultant cutting forces in orthogonal cutting. They found that at uncut chip
thickness values less than the edge radius, the force per unit width in the thrust
direction increases more rapidly than the force per unit width in the cutting direction.
The tool edge condition (tool wear) was found to have a significant effect on the thrust
forces when the depth of cut was below the tool edge radius as shown in Fig. 3.16b.
a b
Fig. 3.16 Specific cutting energy: (a) Direction of resultant force vector for new and worn
tools with the same overall tool geometry [Error: Reference source not found]; (b) Effect of
tool edge condition on thrust force in orthogonal fly-cutting of Al 6061-T6 [Error: Reference
source not found].

24. Taminiau and Dautzenberg [83
] also witnessed an increase in cutting energy while
machining brass with decreased chip thickness. The authors discovered that the
specific cutting forces depended only on the ratio of the uncut chip thickness to the
cutting edge radius when the uncut chip thickness was smaller than the edge radius.
7.2. Minimum chip thickness
The minimum chip thickness (hmin) is defined as the minimum undeformed thickness
of the chip removed from a work surface at a cutting edge under perfect performance
of the metal-cutting system (no system deflection).
In macro-machining, the depth of cut is generally bigger than the cutting tool edge
radius (ρ), and it is assumed that the cutting tools completely remove the surface of the
workpiece and generate the chips [Error: Reference source not found].
However, in micro-machining, the small depth of cut compared to the edge radius of
the tool (tungsten carbide tools ∼1-2 µm, diamond tools ∼ 50nm) causes a large
negative rake angle. The consequence of this is a process, which might be dominated
more by rubbing and compression instead of by cutting [84
]. This phenomenon causes a
rough surface and elastic recovery of the workpiece, increasing the possibilities tool
damages to occur (see Fig. 3.17) [Error: Reference source not found, 85
-86
].
Ikawa et al. [87
-88
] discussed the significance of the minimum thickness of a cut in
diamond turning and it was found to be more strongly affected by the sharpness of the
cutting edge than by tool-work interaction. The authors noted that the minimum
thickness of cut might be of the order of 1/10 of the cutting edge radius.
The minimum chip thickness effect was also observed in the micromilling process.
Weule et al. [89
] were the first to point out the existence of the minimum chip thickness
and its significant influence on the achievable surface roughness in micromilling. The
authors hypothesized that the minimum chip thickness effect was responsible for the
sawtoothlike surface profile (see Fig. 3.18). The minimum chip thickness to edge radius
ratio for micromachining was estimated to be hmin = 0.293ρ, which was much larger
than that of nanometric cutting. They also noticed that the softer material state caused
increased surface roughness and claimed that the minimum chip thickness was
strongly dependent on material properties.
Macro Micro
Fig. 3.17 Cutting edge workpiece interaction in micromilling [Error: Reference source not
found].
by E. Gandarias 24

25. ρ = 5µm
ρ = 2µm
Fig. 3.18 Theoretical surface profile considering the minimum chip thickness effect [Error:
Reference source not found].
Kim et al. [90
] performed an experimental study to prove the existence of the
minimum chip thickness in micromilling. It was found that for very small feed rates, the
measured chip volume is much larger than the nominal chip volume, indicating that a
chip is not formed with each pass of the cutting tooth (see Fig. 3.19a). Further evidence
that a chip is not formed with each tooth pass is obtained by examining the distance
between the feed marks on the machined surface. For a small feed per tooth, the
distance between feed marks was found to be much larger than the feed per tooth,
which also indicates that chips do not form with each pass of the tool. Thus, the tools
had an oscillating movement, and the missing creation of a chip resulted in a deflection
of the tool (see Fig. 3.19b).
Vogler et al. [91
] experimentally studied the effects of minimum chip thickness on the
cutting forces in micromilling. It was found when machining at small feed rates, the chip
thickness accumulated and the force increased with each tool pass, for n tooth passes
until the chip thickness was greater than the minimum chip thickness.
7.3. Surface roughness
Surface finish is very critical in micromilling as the machining operation is usually
intended to produce components with high quality surfaces. Due to this, factors like
vibrations, chip removal, etc. that are not so critical in the macro scale, have significant
influence on the surfaces generated at the ultra-precision scale.
a b
Fig. 3.19 Study of minimum chip thickness existence: (a) Chip volume larger at small feed
rates [Error: Reference source not found]; (b) Bending of the tool due to the minimum chip
thickness effect [Error: Reference source not found].
Fig. 3.20 Effect of tool edge radius on surface roughness for pearlite [Error: Reference

26. source not found].
Vogler et al. [92
] studied the surface generation in the micromilling process and it
was found to be strongly affected by the tool edge radius and the feed rate. The
authors observed that for the 2 µm edge radius, as the feed rate was reduced to a
certain value, the surface roughness started to increase, indicating that an optimal feed
rate exists that will produce the smallest surface roughness value (see Fig. 3.20). The
authors claimed that the existence of the optimal feedrate was due to the trade-off
between the traditional effect of feed marks as the feed rate is increased and the
minimum chip thickness effect resulting in tool passes that do not remove any material
as the feed rate is reduced.
Lee [93
] studied the effect of the feed rate, cutting speed and depth of cut on the
surface roughness when machining aluminium. The author observed the effect of the
cutting speed and depth of cut was by far smaller than the feed rate.
Furthermore, the surface roughness depends on the machining parameters used
and the nature of the workpiece (e.g. grain size) and the tool condition; and it can be
further improved by increasing the rigidity and accuracy of the equipment [94
].
7.4. Burr
Burr is one of the major problems in micromilling. Burr cannot only harm people
handling the machined parts, but it is also a problem in successive assembly
processes (see Fig. 3.21).
a b
Fig. 3.21 Two different kinds of burr: (a) exit burr; (b) top burr [95
].
Fig. 3.22 Definition and shape of burrs in micromilling [Error: Reference source not
found].
by E. Gandarias 26

27. Damazo et al. [96
] reported that burr formation was a major limitation on the
minimum wall size that could be machined (a 25.4 µm thick wall with a 305 µm height
was successfully machined).
In Lee and Dornfeld [Error: Reference source not found], experimental studies on
micro-burr formation were performed in micromilling aluminium and copper. Different
burr formation types were observed; flag-type, rollover-type, wavy-type and ragged-
type (see Fig. 3.22). The burr on tool entrance and exit were found to be proportionally
bigger than in conventional milling processes considering the ratio of burr size to chip
load. The authors attributed this difference to the low cutting speed and large edge
radius-to-chip load ratio in micromilling. In addition, the large tool edge radius-to-chip
load ratio causes rubbing and compression instead of cutting and generates more
burrs. The authors also noted that up-milling produced smaller top burrs than down-
milling.
According to M.Xiao et al. [Error: Reference source not found], the burr is suspected
to be influenced by the cutting edge radius and tool run-out, and be increased with
increasing feed rate and depth of cut.
Litwinski et al. [97
] studied the optimization of tool paths in order to minimize burr
formation and avoid deburring processes from destroying delicate microfeatures.
In addition, deburring is very important because it cannot always be applied on
miniature fabricated parts due to the possibility of damaging the workpiece [98
].
Investigations have succeeded either in applying a removable coating onto the
workpiece before machining (only suitable for soft materials) [Error: Reference source
not found] or using electrochemical polishing.
7.5. Microstructure effect
In micromilling, a typical cutting depth of a few micrometers is common. With such a
small depth of cut, chip formation takes place inside the individual grains of a
polycrystalline material (usually between 100 nm and 100 µm) and hence, material
microstructure effects will play an important role in micromachining.
Moriwaki et al. [99
] found for single-crystal copper that the crystallographic orientation
affects the chip formation process in terms of the magnitude of the shear angle and the
cutting forces.

28. Fig. 3.23 Effects of the crystallographic orientation and the depth of cut on surface
roughness [Error: Reference source not found].
Ueda and Iwata [100
] observed on diamond cutting of β-brass a lamella structure on
the free surface of the chip and reported the formation of discontinuous chips in a
particular range of crystallographic orientations. The authors observed the cutting
forces and surface roughness values depended on material anisotropy.
To et al. [101
] conducted diamond turning of single-crystal aluminium and continuous
chip formation was observed under all cutting conditions. They reported that for the
{110} oriented crystals, the highest cutting forces and worst surface roughness were
produced; whereas for the {111} oriented crystals, the lowest cutting forces; and for the
{100}, the best surface finish were obtained (see Fig. 3.23). According to the authors,
the surface roughness was substantially influenced by the crystallographic orientation,
but not significantly affected by the depth of cut.
Vogler et al. [Error: Reference source not found] studied the effects of a multiphase
microstructure on the micromilling cutting forces. The authors observed high-frequency
energy components in ductile iron experiments but not in the single-phase ferrite and
pearlite materials. This fact evidenced that these high-frequency components are due
to the multiphase.
Vogler et al. [Error: Reference source not found] also investigated the effect of the
multiphase material microstructure on the surface generation process. It was observed
that multiphase microstructure materials had a worse surface roughness than single-
phase material. The increased surface roughness was attributed to interrupted chip
formation that occurs as the cutting edge moves between the multiple phases.
7.6. Tool wear
The small depth of cut in micromilling significantly increases friction between the tool
and the workpiece, resulting in thermal growth and wear. As a result, the increased
radius of the tool decreases the quality of the produced part and increases the rate at
which tools fail [102
-103
]. Whereas tool wear monitoring has been extensively studied on
the macro-scale, very limited successful work has been conducted at the micro-scale
[104
, 105
, 106
, 107
, 108
].
Tansel et al. [Error: Reference source not found, Error: Reference source not found]
have studied the effect of wear in the micromilling process. They have found that,
unlike at conventional sizes, the tool does not gradually wear until it causes
undesirable surface effects, but rather the tool breaks quickly as it becomes worn. The
fracture of the microtools is due to the increased cutting forces with the dulling of the
cutting edges causing the stresses to exceed the strength of the small diameter tools.
Miyaguchi et al. [23] found that tool wear in a micromilling process is partly
responsible of the stiffness of a micro-end-mill tool. They concluded that because of the
spring-back of an end mill under the thrust force, the effect of tool run-out is reduced
and the abrasion of each tool edge tends to be uniform.
Rusnaldy et a. [109
] investigated the tool wear mechanism and the factors that affect
the tool wear analysing the vibration and cutting forces on machining performance. It
was observed that the higher the machining time, the higher was the tool wear,
although the cutting force and tool vibration were not affected by this increasing time.
by E. Gandarias 28

29. 7.7. Modeling
The aforementioned differences between micro and macro machining makes
necessary the development of new models which take into account micromilling
particularities. Furthermore, larger values of feed per tooth are demanded to improve
machining productivity. These aggressive conditions lead to higher stresses on the
microtools, and in consequence, more accurate prediction models are required [110
].
Mostly used methods for the characterisation of microcutting phenomena include
molecular dynamics (MD) simulation, the finite element method and mechanistic
process modeling [Error: Reference source not found]. This latest method, the
mechanistic process modeling, is the more extended one in micromilling.
MD simulation performs analysis at the atomic level based on the atomic interaction
potential and is best suited for nanometric cutting.
The underlying theory in the finite element modeling of machining is macroscale
continuum mechanics and it has been mostly used for investigating orthogonal
microcutting [111
-112
].
The mechanistic modeling approach is directed towards deriving a model that
combines a comprehensive characterization of the cutter and cut geometries to relate
the process inputs to outputs with a small number of experiments.
Bao and Tansel developed a first model for micomilling process based on Tlusty’s
model [113
] that includes the effect of trochoidal nature of the tool path [Error: Reference
source not found], the effect of tool run-out [114
], and the effect of tool wear [115
].
However, these models considered the workpiece material to be homogeneous.
Fig. 3.24 End milling of ductile iron workpiece [Error: Reference source not found].
Vogler et al. [Error: Reference source not found] developed a mechanistic model that
used a mapping technique to represent the microstructure for heterogeneous materials
(i.e. ductile iron) and determined the magnitude and variation of the cutting forces in
micromilling (see Fig. 3.24). The model was shown to be capable of capturing the high
frequency variation of the cutting forces that was observed in experiments when
machining ductile iron. The simulation studies showed that the frequency of the
variation is attributed to the spacing of the secondary phase, and that the magnitude of
the variation is determined by the size of the secondary phase particles.

30. Later, Vogler et al. [116
] extended their previously developed micromilling force model
in order to explicitly account for the cutting edge radius and were the first to incorporat
the minimum chip thickness effect. The cutting forces originating from two
mechanisms, chip removal and ploughing/rubbing were computed separately using
slip-line plasticity and interference volume, respectively. It was noted that the predicted
cutting force signal contained a repeated pattern for every n tooth passes, which
provide an additional evidence of the minimum chip thickness phenomena in
micromilling.
Based on their previous experimental studies [Error: Reference source not found],
Kim et al. [117
] developed a static model of chip formation in micromilling, which is able
to describe the intermittency of the chip formation observed at low feed rates due to the
dominance of the minimum chip thickness effect. The model was validated verifying the
level of periodicity in the cutting forces present at various feed rates.
M. T. Zaman et al. [118
] developed the first three-dimensional analytical cutting force
model for micro-end-milling operations. This model established a new concept to
estimate the cutting force in micro end milling by estimating the theoretical chip area
instead of undeformed chip thickness.
C. Li et al. [119
] developed an enhanced three-dimensional cutting force model for
micro-scale end-milling by considering the combination of exact trochoidal trajectory of
the tool tip, tool run-out and minimum chip thickness effect due to the intermittency of
the chip formation at micro-scale.
X. Liu et al. [120
] developed an analytical model to predict the minimum chip
thickness values, which are critical for the process model development and process
planning and optimization. The model accounted for the influence of cutting velocity
and tool edge radius on the minimum chip thickness.
Fig. 3.25 Relationship of cutting force and chip thickness for perlite [Error: Reference
source not found].
Vogler et al. [Error: Reference source not found] developed a process model for the
prediction of the surface roughness for the slot floor centerline and cutting forces. The
model explicitly accounted for the effect of finite edge radius by incorporating the
minimum chip thickness concept. The model showed to accurately predict the surface
roughness for single-phase materials, i.e. ferrite and pearlite.
by E. Gandarias 30

31. X. Liu, M. B. G. Jun et al. [Error: Reference source not found, 121
-122
] developed a
dynamic cutting force and vibration model of the micromilling process that accounts for
the dynamics of the micro end-mill tool, influences of the stable built-up edge, and the
effects of minimum chip thickness, elastic recovery, and the elastic-plastic nature in
ploughing/rubbing. The authors studied the effects of the minimum chip thickness on
the cutting forces and vibrations as well as the stability of the micromilling process.
Concerning the forces, the rate of increase for both the cutting force and thrust force
was found to be much higher for ploughing/rubbing (when the chip thickness is smaller
than the minimum chip thickness) than for the chip formation process (when the chip
thickness is larger than the minimum chip thickness). Further, a local maximum of the
thrust force at the minimum chip thickness, due to the transition between ploughing and
shearing was observed (see Fig. 3.25). The authors, unlike that of macro cutting
processes, also noted that stability of the process was very sensitive to the feed rate.
A. Dhanorker and T. Özel [123
] developed mechanistic and finite element models for
micromilling to predict forces, stresses and temperature distributions in the presence of
tool edge ploughing (see Fig. 3.26). Large force variations were observed as the
diameter of the cutter decreases and the spindle speed increases. FEM modeling was
found to be a promising method for simulation of chip flow and predictions for forces
and temperature fields for meso end milling.
Fig. 3.26 Prediction of temperature distributions by finite element analysis [Error:
Reference source not found].
a b
Fig. 3.27 Comparison of strain analysis results for two tool designs: (a) conventional micro
end mill; (b) optimised micro end mill [Error: Reference source not found].
D. L. Wissmiller and F. E. Pfefferkorn [124
] gave the initial steps to characterise the
heat transfer in micro-end mill tools during machining operations. Experimental tests
were realised measuring the tool temperature with an infrared camera and a finite

32. element model (FEM) was developed for comparison. The results of the numerical
model were quantitatively in good agreement with the measured temperature values for
the steel cases, although they need to be improved for aluminium cases.
D. L. Wissmiller and F. E. Pfefferkorn [125
] gave the initial steps to characterise the
heat transfer in micro-end mill tools during machining operations. Experimental tests
were realised by measuring the tool temperature with an infrared camera and a FEM
model was developed for comparison. The results of the numerical model were
quantitatively in good agreement with the measured temperature values for the steel
cases, although they need to be improved for aluminium cases.
E. Uhlmann and K. Schauer [126
] carried out a load analysis of current micro end
mills using a FEM model for strain simulation, and an innovative parametric tool design
of micro end mills was developed (see Fig. 3.27). This new design was successfully
verified and a process-reliable increase in endurance of up to 30 % and an aspect ratio
of 5:1 were achieved for a hardness of 62 HRC.
7.8. Sensing & monitoring methods
Proper sensing and monitoring of the micromilling process results in an essential
demand for high accuracy components at micro scale-level that requires a thorough
process control.
Significant research studies have been conducted on the monitoring of macro-
machining processes using various sensors such as spindle motor current and power,
vibration signatures, acoustic emissions, cutting forces, etc. However, a direct
application of these sensing methods to the micromilling process is not feasible
because of their narrow frequency bandwidths, low sensitivity to small disturbances, or
simply their impossibility for milling operations due to the highly intermittent nature of
the chip removal process.
Despite years of research in this area, reliable, versatile and practical sensors are
not yet available for the monitoring and controlling of micromilling processes.
12. FUTURE CHALLENGES
Micromilling, as described previously, is an emerging fabrication technology with a
promising future. It is envisaged as technology of choice to create complex three-
dimensional shapes in hard engineering materials, especially for biomedical
applications and injection moulds.
However, this requires addressing in the near future, the following challenges that
are listed below:
• Increase the knowledge related to micromachining process parameters
for materials different to silicon, e.g. steel, aluminium, ceramics, PMMA, etc.,
and in consequence, increase micromachined components applications;
by E. Gandarias 32

33. • Improve microtools rigidity in order to reduce premature & unpredictable
tool failures, achieve reliable tools on diameters below 100 µm, be able to
machine harder materials (innovation regarding different types of coatings) and
increase the removal rate [127
];
• Investigate new techniques in diamond cutting process making it more
compatible for the machining of ferrous materials, e.g. ultrasonic vibration,
carbon rich gas chamber, cryogenically cooled chamber, etc.;
• Increase the rotational speed of the spindle to achieve recommended
cutting speeds and cover a broad range of revolutions with minimum spindle
run-out and lengthening;
• Reduce tool run-out and increase stage positioning precision since it
creates drastic changes in the cutting force profile and excessive tolerances;
• Improve the structural rigidity and reduce the influence of vibrations of
the microfactories;
• Develop specific models for micromilling, considering factors like
minimum chip thickness, heterogeneity of the material, ploughing & elastic
recovery and different materials;
• Develop specific CAD/CAM modules for micromilling processes with
optimized milling strategies, tights machining tolerances and remaining “micro
stocks” recognition;
• Research on reliable, versatile, economical and practical sensing
methods for monitoring and controlling the micromilling process, in particular
the employed microtools.

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by E. Gandarias 34

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