Qin Y. Micromanufacturing Engineering and Technology

Подождите немного. Документ загружается.

machining increasingly more complex-structured

components and products (e.g. axially asymmetric

surfaces and freeform surfaces) with greater accu-

racy and finer surface finish, and coping with any

newly emerging materials for high throughput and

cost-effective manufacturing. These demands and

requirements have led to the significant develop-

ment of a new generation of machine tools.

Figure 2-11 illustrates a five-axis ultra-preci-

sion micro-milling machine, developed at Brunel

University. A typical ultra-precision machine tool

has five major sub-systems, including a mechani-

cal structure, a spindle and drive system, a control

system, a position-measurement an d feedback

system, and an in-process condition-monitoring

and inspection system. These sub-systems criti-

cally determine the performance of the overall

machine tool system.

Mechanical Structure. The mechanical struc-

ture provides a framework and mechanical sup-

port for all the machine components. It encom-

passes important components such as the machine

base, column, worktable, slide, spindle cases and

carriages. The major factors for machine design

and selection include [21]:

1. Structural configuration;

2. Stiffness and damping;

3. Structural connectivity and interface;

4. Structure dynamics and associated perfor-

mance.

A robust design of mechanical structure should

aim to achieve high structural-loop stiffness,

good damping properties, a symmetrical and

closed-loop structural configuration, minimiza-

tion of heat deformation, long-term stability and

isolation of environmental effects.

Material is also a key factor in determining the

final machine performance. While cast iron and

granite have been widely used for fabricating

machine bases an d slideways, polymer concrete

has become popular for ultra-precision machine

tools where light weight with high damping

capacity and rigidity is required. Structural mate-

rials with a low thermal expansion coefficient and

high dimensional stability have also found appli-

cation, including super-invar, synthetic granite,

ceramics and Zerodur.

Spindle and Drive System. The spindle is a key

component of a precision machine and it has sig-

nificant impact on machined components in terms

of form/dimensional accura cy and surface qual-

ity. Two types of spindles are most commonly

used in precision machine tools, i.e. aerostatic

bearing spindles and oil hydrostatic bearing spin-

dles. They are capable of high rotational speed

with high motion accuracy. Aerostatic bearing

spindles usually have lower stiffness than oil

hydrostatic bearing spindles, but they have lower

thermal deformation than the latter. Aerostatic

bearing spindles are widely used in machine tools

FIGURE 2-11 Schematic figure of a five-axis micro-milling machine [20].

CHAPTER 2 Micro-/Nano-Machining through Mechanical Cutting 31

with medium and small loading capacity, whereas

oil hydrostatic bearing spindles are more suitable

for large and heavily loaded machine tools.

More recently, the groove technique has been

used in the design of bearings. A grooved hybrid

air bearing combines aerostatic and aerodynamic

design principles to optimize ultra-high speed per-

formance.

Several drive mechanisms can be used for ultra-

precision machine tools, including piezoelectric

actuators, linear-motor dire ct drives and friction

drives. Piezoelectric actuators usually have a short

stroke with high motion accuracy and wide

response bandwidth. They have been employed

in fine tool positioning so as to achieve high pre-

cision control of the cutting tool (e.g. a diamond-

cutting tool).

Linear-motor direct drives (AC or DC) usually

have a long stroke and they do not need conver-

sion mechanisms such as lead screws, and racks

and pinions. They offer better stiffness, accelera-

tion, speed, motion smoothness, repeatability and

accuracy, etc. [21], although their applications in

the machin e-tools industry are still relatively new.

Friction drives also have a long stroke and usu-

ally consist of a driving wheel, a flat or round bar

and a supporting back-up roller. They offer low

friction force, smooth motion, and good repeat-

ability and reproducibility due to elastic deforma-

tion induced by the preload.

Control System. Following the invention of

Computer Numerical Control (CNC) in the early

1970s, many companies started to develop their

control systems for machine tools. The control sys-

tem typically includes motors, amplifiers, switches

and the controller. High speed multi-axis CNC

controllers play an essential role in efficient and

precision control of servo drives, error compensa-

tion (thermal and geometrical errors), optimized

tool setting and direct entry of the equation of

shapes [22]. Advanced PC-based control

systems have achieved nanometer or even sub-

nanometer levels of control resolution for ultra-pre-

cision and micro-manufacturing purposes, such

systems also being used commonly in the majority

of commercially available ultra-precision machines.

Position Measurement and Feedback Sys-

tem. Ultra-precision machine tools necessarily

require an ultra-precision position measurement

and feedback system. Laser encoders (laser-

interferometer based) are particularly suitable

because interferometers have an intrinsically high

resolution. Interferometer s also have the ability

to eliminate Abbe errors. They have a typical

resolution of 20 nm (digital), and sub-nanometer

resolution can also be achieved with an analog

system via external interpolation. The installation

may be made simpler by means of fiber-optics

laser launch and integrated interferometer optics.

Some laser holographic-linear scales have a reso-

lution of better than 10 nm.

Another alternative technique is to use ultra-

high resolution optical encoders. They can pro-

vide resolution close to that of laser encoders, but

in a more industrially feasible and simple manner.

There is a trend of more optical encoders being

adopted on industrially used precision and ultra-

precision machines.

In-process Condition Monitoring and Inspec-

tion System. Intelligent and smart machine

tools are an important development for ultra-pre-

cision applications. To meet these requirements,

some sensors are generally required to monitor

the operation of the machine tool and to combine

multi-functionality, reliability, sensitivity and

compactness. Monitoring the machining status

during ultra-precision machini ng is usually diffi-

cult because of the associated very small energy

emissions and cutting forces compared with

the conventional machining processes. Thermal

effects have been known to be the largest source

of dimensional errors. It is therefore imp ortant to

implement online temperature monitoring. Con-

dition monitoring may be also applied to other

parameters or variables, e.g. cutting force, chatter

and vibration. It is desirable to use multiple sen-

sors to realize the smart and intelligent machine

tool. Furthermore, tool wear, tool breakage and

its engaging process demand a great deal of

attentioninmicro-/nano-machining because

of the high precision and fragile micro-tools

involved.

32 CHAPTER 2 Micro-/Nano-Machining through Mechanical Cutting

Micro- and Meso-machines

Traditional ultra-precision machines are cur-

rently the major means for underta king micro-/

nano-cutting because of their feasible perfor-

mance and availability. However, these machines

are generally very expensive, working in a tight

temperature control environment, and they are

energy and resources inefficient. To match the

micro-manufacturing of miniature and micro-

components, the concept of micro- and meso-

machine tools has been proposed by some

researchers, as shown in Fig. 2-12 [4].

Micro- and meso-machines and their integra-

tion for the micro-manufacturing system/micro-

factory are suitable for the fabrication of micro-

products at low cost, requiring the occupation of

less space, and being of low energy consumption

and greater mobility, etc. Compared to a conven-

tional ultra-precision machine, a micro- or meso-

machine has the five esse ntial characteristics of

decreased heat deformation, less material con-

sumption, smaller vibration amplitudes, smaller

footprint and thus smaller space occupation, and

less energy consumption [23].

FIGURE 2-13 Micro- and meso-machines developed at Brunel University: (a) a desktop micro-milling/turning test rig; (b) a

three-axis bench-top diamond-turning machine; and (c) a five-axis bench-top micro-milling machine.

FIGURE 2-12 Typical micro- and meso-machines [4].

CHAPTER 2 Micro-/Nano-Machining through Mechanical Cutting 33

At Brunel University, three meso-machines

have been developed for machining micro-com-

ponents and -products. Figure 2-13(a) shows a

desktop micro- milling/turning test rig which has

a tw o-dimensional piezo-driven stage for vibra-

tion-assisted cutting. Figure 2-13(b) shows a

three-axis bench-top diamond-turning machine,

a mirror surface, as illustrated in Fig. 2-14, being

achievable on this machine. Figure 2-13(c) is a

five-axis bench-top micro-milling machine which

is still under development with the specifications

as follows:

1. Overall size: 1140  910  1865 mm.

2. Working XYZ: 150  150  100 mm.

3. Slides and rotary table: direct drive a nd air

bearings with squeeze film dampers to achieve

a nanometric level of positioning and machin-

ing accuracy.

4. Spindle speed: 250k rpm to cope with small

diameter of tools and micro-features.

5. Mechanical handling and visual inspection

integrated into the machine design.

6. Machining accuracy: <100 nm.

7. Surface roughness: <10 nm.

8. Control system: UAMC five axis.

MICRO-TOOLING

Micro-tooling is the essential enabler for micro-/

nano-cutting processes. It also determines the fea-

ture sizes and surface quality of the miniature and

micro-components machined. Smaller tools have

decreased thermal expansion relative to their size,

increased static stiffness from their compact struc-

ture, increased dynamic stability from their higher

natural frequency, and the potential for decreased

cost due to smaller quantities of material uti-

lized [4].

Single-crystal diamond is the preferred tool

material for micro-/nano-cutting due to its out-

standing hardness, high thermal conductivity

and elastic and shear moduli. The crystalline

structure of the diamond makes it easy to generate

a very sharp cutting edge, e.g. a cutting edge in

tens of nanometers can be achieved. More

recently, CVD (chemical vapor deposition) dia-

mond-cutting tools have become available, as

shown in Fig. 2-15 [24]. They are extremely hard

FIGURE 2-15 Cutting tools manufactured by: (a) CVD diamond cutting tools; (b) diamond cutting tools. (Courtesy:

Contour Fine Tooling Ltd).

FIGURE 2-14 Mirror surface achieved on the three-axis

diamond-turning machine.

34 CHAPTER 2 Micro-/Nano-Machining through Mechanical Cutting

and can be used to cut tungsten carbide with a

cobalt percentage of 6% or greater.

However, diamond is limited to the cutting of

non-ferrous materials because of the high chemi-

cal affinity between diamond and iron. Micro-

tools that are used to machine ferrous materials

are normally made from tungsten carbide. As

shown in Fig. 2-16 [25], these micro-milling tools

are made from tungsten carbi de with a diameter

from 0.2 mm to 1.5 mm.

Commercially available micro-drills are

typically on the order of 50 mm diameter [1].

However, methods for minimizing micro-tools

continue to be developed, such as focused

ion beam, EDM, WEDG and grinding, etc.

Figure 2-17 [26] shows micro-end mills developed

by the focused ion beam process, their diameter

being less than 25 mm.

Other trends in the development of micro-tools

include: the optimization of the geometric and

coating properties of micro-tools for longer tool

life and accuracy enhancement; and characteriza-

tion and condition monitoring of micro-tools.

APPLICATIONS

Micro-/nano-cutting has many a pplications,

which are directly related to the overall applica-

tion markets of micro-products. For instance,

MEMS, as shown in Fig. 2-18, is and will remain

one of major driving forces for micro-cutting. It is

expected that the MEMS/MST market volume

will reach $24 billion in 2009 from $12 billion

in 2004. Moreover, completely new products

such as micro-fuel cells, MEMS memori es, chip

coolers, liqu id lenses for cell-phone zoom and

autofocus will be included in this category.

The EU FP6 4M Network has organized the

major applications into the following three divi-

sions [28], i.e.:

1. Micro optics: telecommuni cation, bio-tech-

nological, instrumentation and medical appli-

cations.

2. Micro-sensors and actuators: medicine, bio-

medical field, health and safety, environment

and process control.

3. Micro fluidics: biological, medical, pharma-

ceutical and chemical engineering applica-

tions.

Micro-/nano-cutting is very promising in the

production of micro-products used in the above

three divisions, such as sensors, accelerometers,

actuators, micro-mirrors, fiber-optics connectors

and micro-display, etc. Some typical application

samples resulting from micro-/nano-cutting are

shown in Fig. 2-19 [29].

FIGURE 2-17 Micro-end mills made by focused ion beam sputtering having: (a) two cutting edges; (b) four cutting edges;

and (c) five cutting edges (scanning electron micrographs).

FIGURE 2-16 Micro-milling tools [25].

CHAPTER 2 Micro-/Nano-Machining through Mechanical Cutting 35

Figure 2-19(a) is a joining element (steel) for

optical-fiber connections. There are 18 drill holes

with a diameter of 0.23 mm  5 mm and a drilling

depth of 1.5 mm. The positional tolerance of the

drill holes is 10 mm. The complete machining time

was 25 minutes.

The part shown in Fig. 2-19(b) is a mixing

disc of a rocket motor, produced by five-axes

FIGURE 2-18 Market for MST/MEMS products in the future [27].

FIGURE 2-19 Micro-product samples (Courtesy: KERN Micro- & Feinwerktechnik GmbH).

36 CHAPTER 2 Micro-/Nano-Machining through Mechanical Cutting

machining. Six tools were used and precision

adjustment on spo t faces was required.

Figure 2-19(c) shows three turbine wheels for

micro-fluid pumps, produced by five-axes

machining. The wheels are made of vespel and

ceramic with diameters of 2–7 mm and a circum-

ferential tolerance of 2 mm.

Figure 2-19(d) shows watch base-plates made

by fully automatic production using 30 tools

including milling, drilling and tapping. The posi-

tional tolerances between the drill holes and drill

depths are 3 mm.

The cataract lens shown in Fig. 2-1 9(e) has a

surface finish of R

< 0.2 mm. Its outside contour

is machined with a milling tool (0.4 mm) and the

positioning holes are produced by drilling.

The indispensable advantage of micro-/nano-

cutting is applicable to the manufacture of 3D

complex-shape/form micro-molds. It is antici-

pated that micro-/nano-cutting will be intensively

used in the fabrication of compression molds and

injection molds. Figure 2-19(f) shows an injec-

tion-molding tool for watch-making, machi ned

by micro-milling. The material of the mold is

hardened steel 54 HRC, its position and form

tolerance is 5 mm, and its surface quality is

> 0.25 mm.

COMPETITIVE TECHNOLOGIES AND

ECONOMIC CONSIDERATIONS

Micro-/nano-cutting brings many potentialities to

the fabrication of miniature and micro-products/

components with arbitrary geometry. The micro-/

nano-cutting process is particularly suitable for

the manufacture of individual personalized com-

ponents rather than large batch sizes, which is

largely indispensable for the current vibrant mar-

kets. With the high level of machine accuracy of

ultra-precision machine tools, good surface finish

and form accuracy can be achieved. Micro-/nano-

cutting is also capable of fabricating 3D free-form

surfaces. The high machining speed of micro-/

nano-cutting is another advantage over other

micro-manufacturing technologies. Moreover, it

can fabricate a huge range of materials, such as

steel, aluminum, brass, plastics, ceramic, poly-

mers, etc. Unlike micro-laser beam machining

and lithographic techniques, it does not require

a very expensive set-up, which enables the fabri-

cation of miniatures at an economically reason-

able cost.

ACKNOWLEDGEMENTS

The authors would like to thank all research col-

leagues in the AMEE research group at Brunel

University for stimulating discussions, and rele-

vant research publications and projects to access

and refer to. Thanks are also extended to the EU

MASMICRO Project Consortium, practically to

its RTD 5 sub-group for frequent meetings and

discussions which are always very helpful.

REFERENCES

[1] D. Dornfeld, S. Min, Y. Takeuchi, Recent advances in

mechanical micromachining, CIRP Annals 55 (2)

(2006) 745–768.

[2] D.A. Lucca, R.L. Rhorer, R. Komanduri, Energy dis-

sipation in the ultraprecision machining of coppers,

CIRP Annals 40 (1) (1991) 69–72.

[3] D.A. Lucca, Y.W. Seo, Effect of tool edge geometry

on energy dissipation in ultraprecison machining,

CIRP Annals 42 (1) (1993) 83–86.

[4] J. Chae, S.S. Park, T. Freiheit, Investigation of micro-

cutting operations, Int. J. Mach. Tools Manuf 46 (3–

4) (2006) 313–332.

[5] X. Liu, R.E. Devor, S.G. Kapoor, An analytical

model for the prediction of minimum chip thickness

in micromachining, J. Manuf. Sci. & Tech., Trans.

ASME 128 (2) (2006) 474–481.

[6] N. Ikawa, S. Shimada, H. Tanaka, Minimum thick-

ness of cut, Nanotechnology 3 (1) (1992) 6–9.

[7] M.P. Vogler, S.G. Kapoor, R.E. Devor, On the

modeling and analysis of machining performance in

micro-endmilling, J. Manuf. Sci. & Tech., Trans.

ASME 126 (4) (2004) 685–705.

[8]S.Shimada,N.Ikawa,H.Tanaka,G.Ohmori,

J. Uchikoshi, H. Yoshinaga, Feasibility study on ulti-

mate accuracy in microcutting using molecular dynam-

ics simulation, CIRP Annals 42 (1) (1993) 91–94.

[9] S. Shimada, N. Ikawa, T. Inamura, N. Takezawa,

H. Ohmori, T. Sata, Brittle-ductile transition phe-

nomena in microindentation and micromachining,

CIRP Annals 44 (1) (1995) 523–526.

[10] D. Li, S. Dong, Y. Zhao, M. Zhou, The influence of

rake of diamond tool on the machined surface of

brittle materials with finite element analysis, Pr o-

ceedings of the 1st International Conference and

CHAPTER 2 Micro-/Nano-Machining through Mechanical Cutting 37

General Meeting of the Eur opean Society for Preci-

sion Engineer ing a nd Nanotechnology, Bremen,

Germany (1999) pp. 338–34 1.

[11] Y. Ichida, Ductile mode machining of single crystal

silicon using a single point diamond tool, Proceedings

of the 1st International Conference and General

Meeting of the European Society for Precision Engi-

neering and Nanotechnology, Bremen, Germany

(1999) pp. 330–333.

[12] F.Z. Fang, H. Wu, W. Zhou, X.T. Hu, A study on

mechanism of nano-cutting single crystal silicon,

J. Mats. Proc. Tech. 184 (1–3) (2007) 407–410.

[13] X. Liu, R.E. Devor, S.G. Kapoor, The mechanics of

machining at the microscale: assessment of the cur-

rent state of the science, J. Manuf. Sci. & Tech.,

Trans. ASME 126 (4) (2004) 666–678.

[14] S. To, W.B. Lee, C.Y. Chan, Ultraprecision diamond

turning of aluminium single crystals, J. Mats. Proc.

Tech 63 (1–3) (1997) 157–162.

[15] X. Luo, High precision surfaces generation: model-

ling, simulation and machining verification, PhD the-

sis, Leeds Metropolitan University (2004).

[16] K.S. Woon, M. Rahman, F.Z. Fang, Investigations of

tool edge radius effect in micromachi ning: a FEM

simulation approach, J. Mats. Proc. Tech. 195 (1–3)

(2008) 204–211.

[17] W.G. Hoover, Molecular dyna mics, in: Lect. Notes

Phys. v ol. 258, Springer-Verlag, Berlin (1986 ) 13.

[18] E.B. Tadmor, M. Ortiz, R. Phillips, Quasicontinuum

analysis of defects in crystals, Philos. Mag. A 73

(1996) 1529–1563.

[19] X. Sun, S. Chen, K. Cheng, D. Huo, W. Chu, Multi-

scale simulation on nanometric cutting of single crys-

tal copper, Proc. Inst. Mech. Eng. B, J. Eng. Manuf

220 (7) (2006) 1217–1222.

[20] D. Huo, K. Cheng, A dynamics-driven approach to

the design of precision machine tools for micro-

manufacturing and its implementation perspectives,

Proc. Inst. Mech. Eng. B, J. Eng. Manuf 222 (1)

(2008) 1–13.

[21] X. Luo, K. Cheng, D. Webb, F. Wardle, Design of

ultraprecision machine tools with application to

manufacture of miniature and micro components,

J. Mats. Proc. Tech.167 (2005) 515–528.

[22] N. Ikawa, R. Donaldson, R. Komanduri, W. K

€

onig,

Ultraprecision metal cutting – the past, the present

and the future, CIRP Annals 40 (2) (1991) 587–594.

[23] L. Alting, F. Kimura, H.N. Hansen, Micro engineer-

ing, CIRP Annals 52 (2) (2003) 635–657.

[24] http://www.contour-diamonds.com/HTML/

CVDdiamonds.html (accessed on Nov. 19, 2007).

[25] T. Dow, E. Miller, K. Garrard, Tool force and deflec-

tion compensation for small milling tools, Prec. Eng

28 (1) (2004) 31–45.

[26] D.P. Adams, M.J. Vasile, G. Benavides, A.N. Camp-

bell, Micromilling of metal alloys with focused ion

beam-fabricated tools, Prec. Eng. 25 (2001) 107–

113.

[27] H. Wicht, J. Bouchaud , NEXUS mark et analysis for

MEMS and m icrosy stems III 2005-2009. http://

www.suframa.gov.br/minapim/news/visArtigo.cfm?

Ident=147&Lang=EN (accessed on Nov. 19,

2007).

[28] S.S. Dimov, A.N. Bramley, W. Eberhardt, et al.,

4M Netw ork of Excellence, process repor t 2004-

2006. 4M2007 Conference on Multi-Material

Micro Manufacture, Bor ovets, Bu lgaria(Oct . 3-5,

2007).

[29] http://www.kern-microtechnic.com/2-Sub-Samples.

html (accessed on Nov. 19, 2007).

38 CHAPTER 2 Micro-/Nano-Machining through Mechanical Cutting

Micro-EDM

Eckart Uhlmann, Markus R

€

ohner and Malte Langmack

INTRODUCTION

Today, micro-electrical discharge machining

(m-EDM) is applied in all kinds of fields concern-

ing micro-machining. Especially in single-part or

small batch productio n m-EDM can be used eco-

nomically to produce micro-tools and micro-

molds. By combining different process variants

of m-EDM with current machining processes such

as micro-milling or laser ablation a cost efficient

mass production can be realized [1–4].

Based on a thermal material removal process,

m-EDM is working almost free of process forces. It

is also independent of mechanical workpiece

properties such as the Young’s modulus or hard-

ness. Bringing together huge geometrical freedom

and precision, m-EDM allows the machining of a

wide range of electro-conductive materials with a

minimal conductivity of k = 0.01 S/cm including

high temperature alloys, cemented carbide, and

electro-conductive ceramics.

WORKING PRINCIPLE

Machining Principle of EDM

EDM is a non-mech anical thermal shaping pro-

cess with which material is removed by spatially

and temporally separated electrical discharges

between a workpiece electrode and a tool elec-

trode. The high frequency discharges cause melt-

ing and vaporization of material on the surface of

both electrodes. To enhance the material removal

EDM operates in a non-conducting fluid, the

dielectric fluid (Fig. 3-1).

During machining, the tool and workpiece

electrode are positioned in such a way that a

working gap s filled with dielectric fluid remains

between them. Applying a voltage – depending

on the working gap width and conductivity of

the dielectric fluid – causes an expanding ener-

getic plasma channel and a current flow after

exceeding the dielectric strength of the fluid. Te m-

peratures above T = 10,000 K [5] inside the

plasma channel effect melting and vaporization

processes of the material on the surfaces of both

electrodes. By switching off the current, melted

material is abruptly erupted due to an implosion

of the plasma channel [6–9].

The result of a single discharge is a crater-

shaped pit on the electrodes’ surfaces, the dis-

charge crater (Fig. 3-2(a)). Depending on the

erosion parameters such as the discharge time,

discharge current or the type of dielectric fluid,

typical crater diameters vary from around

=1mm [10] to around d

=100mm. The

EDM-specific surface topography is caused by a

multitude of overlapping discharge craters

(Fig. 3-2(b)). The removed material mostly con-

sists of ball-shaped particles (Fig. 3-2(c)).

Process Variants

With different variants m-EDM is a very flexible

machining process. Three variants of big

industrial relevance are micro-die sinking (m-die

sinking) (Fig. 3-3(a)), micro-wire electrical dis-

charge machining (m-WEDM) (Fig. 3-3(b)) and

micro-electrical discharge drilling (m-ED drilling)

CHAPTER

(Fig. 3-3(c)). In m-die sinking a shaped tool elec-

trode is used for manufacturing three-dimen-

sional shapes and free-forms. m-WEDM comes

into operation for cutting defined contours by

using a fine wire electrode. m-ED drilling, a special

option of m-die sinking, operates with a rotating

pin electrode to produce micro-holes, e.g. as

needed in fuel injection systems.

Processes with less industrial relevance are

micro-electrical discharge milling (m-ED milling)

FIGURE 3-3 Process variants of m-EDM: m-die sinking (a), m-WEDM (b), m-ED drilling (c), m-ED milling (d), m-EDG (e), and

m-WEDG (f).

FIGURE 3-1 Principle illustration of the EDM process.

FIGURE 3-2 Single discharge crater (a), surface topography (b), and removed particles (c).

40 CHAPTER 3 Micro-EDM