Lopez de Lacalle L.N., Lamikiz A. Machine Tools for High Performance Machining

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324 J. A. Sánchez and N. Ortega

mechanical and thermo-physical properties of this material led to the rapid develop-

ment of new alternatives such as brass and coated wires. Mechanical behaviour is

critical during the WEDM process. Axial force is imposed on the wire by the machine

itself to minimise deformations which can affect accuracy; however excessive axial

force may produce wire breakage, and this is why the ultimate tensile strength is con-

sidered a critical property. Tensile strength is 245

N/mm

for copper, 900

N/mm

for

“hard” brass, and may reach as high as 1,930

N/mm

for molybdenum. Of course,

these values depend not only on the material itself, but also on the operations (succes-

sive drawing operations and intermediate heat treatments) which take place during

the manufacturing process. Wires of low ultimate tensile strength may exhibit high

elongation values of well over 20%. These are considered “soft” wires. However,

elongation is much more reduced (below 2%) in “harder” wires. This is especially

important in taper-cutting, particularly when the part angle exceeds 7º. In this case,

a “soft” wire fits the geometry of the guides better, resulting in a more accurate angle.

Better fitting also ensures a higher stiffness of the system, and therefore deformations

induced by the forces exerted by the process will also be smaller.

The explanation is only complete when one looks at the temperatures of melt-

ing and evaporation, properties that depend upon material composition and coat-

ing. A low melting point is associated with the generation of debris from the wire

that facilitates ignition during discharge. A low evaporation temperature ensures

most of the heat generated is transferred to the workpiece, which is associated to

higher process efficiency.

Brass wire was introduced into the market in 1977. Immediately, cutting speed

was accelerated up to values unknown so far: on a 50

mm thick part cutting speed

was dramatically increased from 12

/min to 25

/min, values still very far

away from current speeds (up to 500

/min, as stated in Sect. 9.2.2). The reason

for this improvement is the presence of zinc in the gap. Current basic brass wires

contain 63% copper and 37% zinc. Melting temperature of the latter is 420ºC, well

below 1,080ºC of copper. Zinc evaporates during cutting, because its evaporation

temperature is 1,000ºC. After coming into contact with a cool dielectric, zinc re-

solidifies in the form of debris, favouring ignition in the next discharge. Following

this, higher zinc contents in the wire composition would be beneficial for the proc-

ess. However, the phase-diagram for brass shows that over 40% content of zinc the

structure of CuZn37 results in a fragile (

) structure which causes very im-

portant problems in the production process for small wire diameters.

Brass wire can be found in different tensile strengths, from 490

N/mm

900

N/mm

, depending upon the production process. An alternative used in Japan

is the CuZn33Al2. Addition of 2% of Al enables tensile strength increase in wire

of up to 1,200

N/mm

Due to the above-mentioned impossibility to manufacture high-zinc content

brass, coated wires were developed in the 1970s. The first trials were made using

a zinc coating and copper as a base material, although the low tensile strength and

large dimensional variations forced the industry to move into other alternatives.

The current industrial solution involves using CuZn20 or CuZn30 as a base mate-

rial, coated with 50% Cu and 50% Zn, or even CuZn37 coated with zinc, which

9 Wire Electrical Discharge Machines 325

becomes zinc oxide during the manufacturing process. The coating is compressed

against the base material during the manufacturing process. The result is a wire

that can be easily drawn and produces very important improvements in cutting

speed. Tensile strength of coated wires may range from 480

N/mm

to 800

N/mm

depending upon the application. Coated wires are more expensive; however, they

result in a more stable process and a higher cutting speed of about 30% with re-

spect to brass wires. Optimum choice between coated and brass wires involves

analysing the following aspects:

• Eroding time, in cutting hours per day.

• Non-eroding times (maintenance, part loading/unloading, etc.), in hours per day.

• Wire wasted in non-eroding times.

• Cost of the wire, per kilogram.

• Wire feeding speed.

• Machine cost, per hour.

Where economic aspects play a determining role, technical recommendations

must also be observed. Therefore, Table 9.1 includes technical recommendations

of use of different wires for WEDM.

Molybdenum wires are a high-cost choice for parts with intricate details such

as small internal fillet radii. In this case, tensile strength is over 1,900

N/mm

and therefore a wire diameter as low as 50

µm can be used confidently. Molyb-

denum wires are not very popular due to their cost, and the fact that the high

melting and evaporation temperatures do not particularly benefit the cutting proc-

ess. The main field of application is in µWEDM, in the range of “high” wire

diameters (around 50

µm). Other alternatives for micromachining applications are

multi-layer wires (steel core, Cu, CuZn50 and Ag), tungsten and molybdenum

carbide.

Table 9.1 Recommendations of use of different types of wire for WEDM

(Base)

coating

Tensile

strength

(N/mm

)

Elongation

(%)

Cutting

speed

Cutting

accuracy

Taper-cutting

7º)

Automatic

threading

CuZn37

900 01 8 99 8 99

CuZn37

500 15 8 9 9 8

CuZn37

400 25 8 9 99 8

(Cu) CuZn50

520 01 99 9 8 8

(CuZn20)

CuZn50

430 30 99 9 99 8

(CuZn20)

CuZn50

800 01 99 99 8 9

(CuMg) Zn

600 03 9 99 9 9

(CuZn37) Zn

900 01 99 99 8 9

(CuZn37) Zn 500 15 99 99 99 8

8: not recommende

space9: recommende

space99: strongly recommende

326 J. A. Sánchez and N. Ortega

9.5 The Wire EDM of Advanced Materials

Nowadays, new industrial products involve the development of new materials

with improved properties. This causes the need to obtain optimum machining

parameters and complete analysis of material behaviour during the electrodis-

charge process. This new situation has become more and more regular, since

aeronautic, electronic, and machining tools industries demand exotic materials.

Aeronautic alloys, sintered carbides, and PCD (polycrystalline diamond) are

some examples of the most widely used new materials.

9.5.1 Aeronautical Alloys

Aeronautic alloys, also called superalloys, are metallic alloys for elevated tem-

perature service, usually based on group VIII elements of the periodic table,

and are generally suitable for elevated temperature applications where resistance

to deformation and stability are primary requirements. The common superalloys

are iron, cobalt, titanium and nickel-based, the latter being best suited for aero-

engine applications. Due to their exceptional properties the conformability and

the machining of these allows have become very difficult tasks. For instance,

the low thermal conductivity, plastic deformation resistance, mechanical hard-

ness, and chemical affinity with tools raise the temperature during conventional

machining. Regarding the acceptance of workpieces, the final user decision is

actually prompted by the apparition of a modified microstructure, cracking

extending from the recast to the bulk material, a pervading tensile stress re-

gime, and others which seriously affect material fatigue resistance. Such is the

case of some aerospace industry components. These types of parts are normally

subjected to strict tolerances and very complex geometry. To prevent this situa-

tion, conservative machining conditions must be applied, reducing the produc-

tivity of the process.

Both wire and sinking EDM can provide an optimum solution for the ma-

chining of these materials. When applying processes with thermal removal

mechanisms, surface integrity is a significant problem due to the formation of a

heat affected zone. This is actually the main limitation of EDM, since subse-

quent manual operations are compulsory to eliminate the recast layer. In fact, a

number of industries ensure their results in machining superalloys are excellent

but no reference to the surface integrity damage is made. The most important

EDM machines manufacturers have actually developed anti-electrolytic genera-

tors which minimise the surface damage of the parts.

One of the most popular aeronautic alloys is Inconel 718 whose applications

are widely extended in aerospace and nuclear industries. This is an example of

an alloy which can be easily machined by EDM, once the machining conditions

are properly developed with the aim of preventing thermal surface damage. One

9 Wire Electrical Discharge Machines 327

of the most common standard tests carried out to analyse the process influence

on the component life service is the fatigue test under constant stress range (as

specified by ASME E 466).

9.5.2 Tungsten Carbide

Tungsten carbide is an important material for tool and die manufacturing because

of its high hardness, strength, superior wear and corrosion resistance over a wide

range of temperatures. Due to these extreme properties tungsten carbide cannot be

easily machined by conventional machining techniques. However, electrical con-

ductivity is high enough to make EDM possible. In particular, wire EDM provides

sufficient accuracy for the manufacturing of precision tooling, such as that used,

for instance, in “high speed stamping” applications.

The major drawback when EDM’ing tungsten carbide is the poor surface in-

tegrity obtained due to the electrolytic and powerful thermal effects which can

cause machine surface cracking, pitting or flaking. These phenomena can drive

the machined part to premature failure [17]. The electrolytic effect is generated

during the phase prior to ignition and when high voltage is applied. The local

chemical property of water on the surface of cemented carbide may be dominated

by the dissolved cobalt ions, dislodging WC grains during the EDM process.

Thus, the amount or concentration of WC grains decreases from the internal

structure of the workpiece to the top surface layer. To sum up, electrochemical

effects appear as a consequence of the type of dielectric [18] and electrical pa-

rameters applied [5].

Regarding dielectric fluids, deionised water is commonly used in conventional

wire EDM applications. However, like most metals, cobalt is electrochemically

very active in aqueous media. To minimise this, wire EDM machines manufactur-

ers recommend users remove the workpiece from the tank immediately after ma-

chining and then dry it in an oven. When using water as a dielectric fluid, two

pieces of advice should be taken into account. The first is related to the rise of

electrical conductivity of water over 5

µS; the second is corrosion prevention us-

ing a sacrifice metal (usually zinc) [12]. The pH factor of water must also be care-

fully controlled.

Much research has been done using alternative dielectric fluids [18]. In these

studies, the material removal rate obtained using oil and oxygen gas as dielectric

fluids is compared. For high accuracy applications, and especially in thin wire

EDM (that will be addressed in Sect. 9.6), oil can be used as dielectric fluid. In

this case, the surface integrity of hard metal is greatly improved.

When it comes to machining parameters, high frequency and very low energy

pulses are recommended to obtain better surface integrity. Most new machines

incorporate anti-electrolysis circuits. Basically, this type of circuits keeps the aver-

age voltage close to zero by working in alternate current mode. The latest devel-

opments [17] include anti-electrolysis circuitry and CPLD-based pulse control

328 J. A. Sánchez and N. Ortega

with a frequency of 500

kHz, discharge duration as short as 150

ns and peak cur-

rent as low as 0.7

A. The experiments demonstrate the fine-finish power supply

can reduce the recast layer, reduce cobalt depletion with respect to that produced

by a standard DC power supply.

9.5.3 Advanced Ceramics and PCD

The limitations imposed to conventional machining of advanced ceramics are

widely known. Most ceramic components are manufactured by sintering at a shape

close to the final geometry (“near-net shaping”) and then ground using diamond

tools. However, since EDM is not dependent upon part hardness, abrasiveness or

brittleness, it can provide a feasible alternative to the manufacturing of ceramic

parts, if the electrical resistivity of the workpiece material is lower than 100

Ω/cm.

This is why some research has been done about EDM engineering ceramics.

Konig et al. [6] classified it as non-conductor, natural conductor and conductor,

which is a result of doping non-conductors with conductive elements. Much re-

search has been devoted to the addition of conductive particles with the aim of

increasing electrical conductivity. Following this methodology, Matsuo et al. [10]

investigated the EDM’ing of ZrO

and Al

doped with quantities of TiC, NbC,

and Cr

. From their investigations it can be deduced that the composition is

a very important factor, there being an optimum carbide content that optimises the

removal rate and surface finish.

Due to their inherent properties, industrial interest has focused on materials such

as boron carbide (B

C) and silicon infiltrated silicon carbide (SiSiC). Boron carbide

has long been known as an excellent abrasive powder; however it is only in recent

years that its optimal properties have been applied to different industrial fields. It

must be taken into account that this is the third hardest material known, just after

diamond and cubic boron nitride, and is currently the hardest material produced in

tonnage quantities. Common applications of B

C are armour and wear protections,

neutron absorbers in the nuclear industry, bearings, nozzles and turbines.

Silicon infiltrated silicon carbide is manufactured by the infiltration of Si into

a matrix of SiC (Fig. 9.16). Its thermal conductivity is four times that of steel, and has

a low thermal expansion coefficient, which is why it is commonly used in manufac-

turing high temperature heat exchangers. Other applications are high-efficiency gas

turbines, seals, components in pumps, bearings, shot blasting nozzles, burner noz-

zles, etc. Sanchez et al. [14] provided a literature survey of EDM advanced ceramics

studying the feasibility of electrical discharge machining B

C and SiSiC.

An innovative method of overcoming the technological limitation of requiring

a minimum electrical resistivity has eventually permitted insulating ceramics to be

machined by EDM [11]. Both wire EDM and sinking EDM have been applied to

the machining of several insulating ceramics materials. In the case of WEDM oil

was used as dielectric fluid combined with long discharge duration and low dis-

charge current. To prevent wire breakage at a very low tension down to ¼ of the

9 Wire Electrical Discharge Machines 329

tension usually applied, affecting the accuracy. Shot peening after machining is

compulsory to remove the conductive layer left on the generated surface.

Polycrystalline diamond (PCD) products usually consist of diamond powder

(1–100

μm grain size) treated at high temperature (1,400–1,600°C) and pressure

(5.0–6.5

GPa) on top of a cemented tungsten carbide backing. PCD products are

widely used in abrasive applications such as cutting tools and rock drill bits. The

introduction of polycrystalline diamond (PCD) on a tungsten carbide substrate has

greatly increased cutting efficiency.

The cobalt in PCD does not act as a binder, but rather as a catalyst for the dia-

mond crystals. In addition, the electrical conductivity of the cobalt allows PCD to

be EDM’ed. When PCD is EDM’ed, only the cobalt between the diamonds crys-

tals is being EDM’ed. This sometime generates a gap between the carbide sub-

strate and PCD because of the higher cobalt concentration in the intermediate

layer. EDM’ing PCD, like EDM’ing carbide, is much slower than cutting steel.

Cutting speed for PCD depends upon the amount of cobalt sintered with the dia-

mond crystals and the PCD particle size. As a general recommendation, large

particles of PCD require very high open circuit voltage.

Machine manufacturers have adapted EDM machines to allow quick precise

profiling of PCD cutting tools. The PCD Edge System is the current Fanuc pro-

posal for wire EDM machines. Available from EDM Methods, the system pro-

posed consists of programming/probing/cutting software, a micro-finish power

supply for cutting PCD, a Hirschmann

rotary axis, a Renishaw

probe (mounted

on the EDM) and a tool-clamping system. On the other hand, Walter

has devel-

Fig. 9.16 Heat exchanger cut by WEDM. The material component is silicon infiltrated sili-

con carbide

330 J. A. Sánchez and N. Ortega

oped the Helitronic product family opening up a gateway into the future market of

sharpening tools. These machines have two functions eroding and grinding. Within

the product family Helitronic Power Diamond offers the possibility to erode rota-

tionally symmetrical PDC and CBN and to sharpen carbide and HSS tools. Other

important manufacturers such as GF AgieCharmilles

or ONA-Electroerosión

S.A. also incorporate technologies for machining PCD in their machines.

9.6 Thin-wire EDM

The origin of microtechnologies, as we know them today, can be found in the

techniques of manufacturing of microsystems developed in the 1990s. Technolo-

gies such as thin-film techniques, photolithography and others became very popu-

lar for the silicon industry; however, they are unable to meet the requirements

imposed by other applications related to micromechanics. To satisfy this growing

demand processes such as micro-milling, laser and micro-EDM have been devel-

oped and launched on the market in recent years.

The micro-EDM role has become prominent due to its intrinsic advantages in

the production world of very small components. Since there is no contact between

tool and part, deformations due to cutting forces and dynamic effects disappear.

Micro-EDM is profitable for short runs and even for the production of single parts,

and it is the ideal partner for other technologies, for instance, in the production of

plastic injection micro-moulds or micro-punches for sheet metal forming.

It is usual to reserve the term micro-EDM for sinking operations with very low

diameter electrodes, while thin-wire EDM is commonly used for micro-WEDM.

Under this concept the whole technology of cutting using wires of diameter ranging

Fig. 9.17 The AC Vertex

model of GF AgieCharmil-

les

for fine wire machin-

ing jobs [20]

9 Wire Electrical Discharge Machines 331

from 15 to 50

µm is included. Thin-wire EDM may be conceptually similar to con-

ventional WEDM in the removal mechanisms, however the technology is radically

different and machine manufacturers have developed absolutely new solutions for

machines to meet the extreme requirements imposed. In fact, absolute tolerances of

about 1

µm, a surface finish as low as Ra 0.05

µm, a minimum wall thickness of

mm and slots of width 40

µm are usual values in this type of product. Of course,

cutting speed is no longer a critical issue, ranging in values of about 1

/min. Part

thickness is usually limited to about 3

mm with wire of 30

µm in diameter, increasing

up to 5

mm when 50

µm diameter wire is used. Typical sectors of application are the

watchmaking and medical industries, specifically in the manufacture of surgical

components. Figure 9.17 shows a popular model of a thin-wire EDM machine for

micromachining applications, the AC Vertex of GF AgieCharmilles

The first point at which one must look is the ratio between wire diameter and

gap width. This ratio is about 10:1 in the case of conventional WEDM, and is

reduced to 5:1 in the case of thin-wire EDM. Implications of this fact include

a lower ability of the wire to stand thermal loads, a lower wire rigidity, and

a lower wire strength to stand variations of axial load. Moreover, aspects such as

machine precision, and the effect and compensation of heat sources are also very

important. Some current solutions to these problems proposed by manufacturers

will be analysed in the following paragraphs.

A new family of discharge generators has been developed for thin-wire EDM.

Discharge energy must be limited to a minimum in order to prevent wire break-

age due to thermal load and wire deformation. Minimum values of discharge en-

ergy in conventional WEDM machines equipped with transistor-type (FET) gen-

erators are about 1.5

µJ, value imposed by pulse on-time, the discharge current

and the voltage, which is excessive for micromachining applications. The alterna-

tive is the use of relaxation-type circuits, in which the energy is controlled by

a condenser. In this case, minimum theoretical values of a discharge energy of

3.5

nJ can be obtained.

Manipulation of such a thin wire is also a matter of critical concern. Axial ten-

sion imposed by the machine, guiding and threading must be carefully studied in

these machines. Conventional systems for application of the axial force on the

wire are usually based on the opposite action of an engine and an electromagnetic

brake. This is a good choice in WEDM, where load variations of load of 20

grams

do not affect process performance. However, in thin-wire EDM this value is unac-

ceptable since, for instance, in a wire of 20

µm diameter the maximum load it can

stand is about 70

grams. Controlled solutions that ensure fluctuations below

grams are used in thin-wire EDM. Important research work is currently being

carried out by machine manufacturers, since this is obviously critical in the occur-

rence of wire breakage. Threading is another aspect requiring improvement. While

in WEDM automatic threading has been completely solved, with reliability over

95%, only a small number of thin-wire EDM machines include this option. Man-

ual threading is cost- and time-consuming, and can be improved by the use of

tungsten wires due to their higher stiffness. However, this is yet another important

field of industrial research.

332 J. A. Sánchez and N. Ortega

Machine manufacturing includes using high precision components, high-

stability structures and systems for thermal compensation. Most manufacturers use

preloaded linear guides, although cross-rollers guides are also used. Precision

preloaded ball screws are used for the linear axes, however linear motors are also

the choice of other manufacturers. These latter provide a high dynamic response

and the suppression of backlash, nevertheless the generation of large quantities of

heat must be seriously considered. Measurement is carried out using high-

resolution (about 5

nm) glass scales. Isostatic structures of a high stability using

polymer concrete and including vibration absorbers can be used. The Monoblock

configuration patented by GF AgieCharmilles

is a good example.

Compensation of external conditions is critical in thin-wire EDM machines.

Thermal isolation of the working volume can be combined with intelligent heat

generation to compensate for internal heat sources and external temperature varia-

tions. The cooling of critical elements, including structural components, is com-

pulsory. These solutions combine with the use of ceramic materials with a very

low thermal expansion coefficient for these elements. Thermal expansion coeffi-

cient in ceramics is two times smaller than that of graphite, three times that of

castings used in precision beads, and five times that of stainless steel.

Acknowledgements The authors wish to thank the University of the Basque Country, the

Basque Government and the Spanish Ministry of Education for their continued support to the

EDM Research Group of the Faculty of Engineering of Bilbao. Special thanks are also given to

ONA-Electroerosion

S.A. for their interest and support to our activities during the last 15 years.

Finally, we would like to thank the companies GF AgieCharmilles

, Sodick

, Fanuc

, System

and Bedra

for the information and pictures provided for this chapter.

References

[1] Dauw DF, Beltrami I (1994) High-precision wire EDM by on-line wire position control.

Ann CIRP 43(1):193–197

[2] Dekeyser WL, Snoeys R (1989) Geometrical accuracy of wire EDM. Proc ISEM IX:

226–232

[3] Ho KH, Newman ST, Rahimifard S et al. (2004) State of the art in wire electrical discharge

machining (WEDM). Int J of Mach Tool Manufact, 44:1247–1259

[4] Huang YH, Zhao GG, Zhang ZR et al. (1986) The identification and its means of servo feed

adaptive control system in WEDM. Ann CIRP, 35(1):121–123

[5] Juhr H, Schulze HP, Wollenberg G et al. (2004) Improved cemented carbide properties after

wire-EDM by pulse shaping. J of Mater Process Technol, 149:178–183

[6] Koenig W, Dauw DF, Levy G et al. (1988) EDM – future steps towards the machining of

ceramics. Ann CIRP 37 (2):623–631

[7] Lazarenko B, Lazarenko N (1943) About the inversion of metal erosion and methods to

fight ravage of electric contacts. Moscow WEI-Institute

[8] Liao YS, Yan MT, Chang CC (2002) A neural network approach foro n-line estimation of

workpiece height in WEDM. J Mater Process Technol 121:252–258

[9] Masuzawa T, Fujino M, Kobayashi K et al. (1985) Study on Micro-Hole Drilling by EDM.

Bull. Japan Soc of Pre Eng, 20(2):117–120

9 Wire Electrical Discharge Machines 333

[10] Matsuo T, Oshima E (1992) Investigation on the Optimum Carbide Content and Machining

Condition for Wire EDM of Zirconia Ceramics. Ann. CIRP 41 (1):231–234

[11] Mohri N, Fukuzawa Y, Tani T et al. (1996) Assisting electrode method for machining

insulting ceramics. Ann CIRP, 45(1):201–204

[12] Obara H, Satou H, Hatano M (2004) Fundamental study on corrosion of cemented carbide

during wire EDM. J of Mater Process Technol, 149:370–375

[13] Rajurkar KP, Yu ZY (2000) 3D Micro-EDM Using CAD/CAM. Ann CIRP 49:127–130

[14] Sanchez JA, Cabanes I, Lopez de Lacalle LN et al. (2001) Development of optimum elec-

trodischarge machining technology for advanced ceramics. Inter J Adv Manuf Technol,

18(12):897–905

[15] Sato M (2002) Adaptive control technology for wire-cut EDM. Mitsubishi Electrical Corpo-

ration, R&D Progress Report

[16] Snoeys R, Dekeyser W, Tricarico C (1998) Knowlegde-based system for wire EDM. Ann

CIRP, 37(1):197–202

[17] Yan MT, Lai YP (2007) Surface quality improvement of wire-EDM using a fine-finish

power supply. Int J of Mach Tool Manufact, 47:1686–1694

[18] Yu Z, Takahashi J, Kunieda M (2004) Dry electrical discharge machining of cemented

carbide. J of Mater Process Technol, 149:353–357

[19] www.fanuc.co.jp/en. Accessed 2007-12-10

[20] www.gfac.com. Accessed 2007-12-10

[21] www.ona-electroerosion.com. Accessed 2007-12-10

[22] www.sodick.com. Accessed 2007-12-10

[23] www.system3r.com. Accessed 2007-12-10