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

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314 J. A. Sánchez and N. Ortega

tems, i.e., low discharge current and short pulse on-time. Roughness values for the

first cut in tool steel are about Ra 2.80

µm. This value can be improved down to

Ra 0.8

µm after two trim cuts. Surface finish is also highly dependent on part ma-

terial. It is common for machine manufacturers to specify minimum values of

surface roughness for hard metal, where using special modules with values below

Ra 0.10

µm can be achieved.

Different aspects affect precision during WEDM cutting. The wire, subjected by

the machine to tensile stress, from a mechanical viewpoint behaves as a beam on

which deformation is induced by the forces acting: dielectric pressure, electrostatic

force, electrodynamic force and electromagnetic force. Modelling the above system

of forces is a classical topic in WEDM research ([1],

[2] and many others). Under

these forces, the wire suffers flexing and vibration that make it lose its vertical equi-

librium position. As a result, wire geometry is not exactly aligned with machine

Fig. 9.8 Geometry of the error in corner cutting. a It can be observed that after the first cut, the

amount of material along the corner is not constant. b The situation is clearly improved after trim

cuts

Fig. 9.9 Twin-Wire technology of the Swiss company GF AgieCharmilles

. The photograph

shows the FI2050TW machine [20]

9 Wire Electrical Discharge Machines 315

guides (which are the points actually programmed for wire path), and errors are

transferred to the machined part. Deviations in part accuracy include the verticality

of faces, the corner geometry (the so-called “back-wheel effect”) and angular errors

in taper cutting. As already mentioned, accuracy can be greatly improved using trim

cuts where low dielectric pressure and low energy systems are used. For vertical

cuts, verticality can be maintained below 5

µm after two trim cuts in a conventional

WEDM machine. Figure 9.8 shows the geometry of a corner with internal radius

0.20

mm after the first cut (a) and after two trim cuts (b). The geometry is sounder,

the maximum error has been reduced down to 7

µm, and the surface finish has been

clearly improved.

Low-diameter wires are commonly used for machining intricate details and

small internal corner radius. This type of wire is used in the concept of twin-wire

machine developed by the Swiss company GF-AgieCharmilles

[20]. Twin-wire

technology incorporates automatic wire change to reduce total machine time and

therefore increase productivity. The first cut is carried out using a low cost wire,

and then trim cuts are performed with wires with diameters as low as 50

µm. This

is a very precise machine, in which excellent roughness, surface finish and dimen-

sional tolerances can be achieved due to the use of thermal stabilisation systems.

Figure 9.9 shows a photograph of the FI2050TW model of this company.

9.3 WEDM Machines

A WEDM machine shares many elements in common with other machine tools.

Structural elements and CNC, for instance, do not differ too much from those of

other manufacturing equipment. However, some systems can only be found in

these machines, and thus they will be separately analysed in the following para-

graphs. These elements can be identified in Fig. 9.10.

• Wire transport and wire thread devices.

• A working tank.

• A spark generator.

• A filtering system.

Structural components, mechanical transmission, guiding and measuring de-

vices do not differ too much from those used in other machine tools. Machine

bead is normally built in iron casting, although proposals of using polymer con-

crete can be found both in industry and in literature. In machines where precision

demands are very strict, ceramic components and thermal stabilisation of the

working area can be found.

The guiding system is responsible for the accuracy and straightness of linear

axis movement. Linear guides of hardened steel, held to machine bead, are com-

monly used. This technology increases element wear resistance, although damping

is committed. Movement is transmitted using preloaded linear recirculating ball

bearings mounted on the moving element.

316 J. A. Sánchez and N. Ortega

Fig. 9.10 Main systems of a WEDM machine. The photograph shows an ONA

AX300 wire

EDM machine [21]

In most machines, linear axis movement is generated by a transmission engine-

ball screw. The dynamic response of ball screws has increased dramatically in

recent years at the sight of the competence of linear motors. In fact, some manu-

facturers use linear motors for their linear axes, although this is not the most

common option on the market nowadays. Linear motors, due to their extremely

high dynamic response, and lack of backlash, provide a good solution for improv-

ing flushing (the so-called natural flushing) in the sinking the electrical discharge

machining of difficult geometries (for instance, high-depth and low-width slots).

A good example of application of linear motors to the WEDM technology is given

by the AQ300L machine by Sodick

[22]. Measuring devices involve precision

glass scales mounted on the axes, and in some cases encoders.

Mechanics of wire electrodischarge machines are, in general, more complex

than those of sinking electrodischarge machines. The machine structure commonly

has 5 axes amongst which 4 are interpolated. WEDM machines can be classified

on the basis of the maximum admissible workpiece size. Figure 9.11 shows the

basic mechanical structure of a WEDM machine.

Workpiece height is critical since stability and cutting speed are dependent on it.

Moreover, the WEDM process is the only one on the market capable of precision

cutting of hard parts of high thickness (Fig. 9.12). Currently, there are machines

capable of machining workpieces with the dimensions 2300

1300

600

mm and

a weight of 10,000

kg. In these large machines, cutting must be carried out in

a submerged mode due to fact that the dielectric has a difficult access to the ma-

chining area. As far as taper cutting is concerned, and depending on the machine,

9 Wire Electrical Discharge Machines 317

Eje Z

Eje X

Eje Y

Eje U

Eje V

V Axis

U Axis

Y Axis

X Axis

Z Axis

Fig. 9.11 Basic diagram of a WEDM machine

Fig. 9.12 ONA

AX-10 for the cutting of large thickness parts [21]

318 J. A. Sánchez and N. Ortega

angles as high as ±30º can be cut in a 400

mm thick workpiece, but the angle is

normally limited for higher part thicknesses.

The numerical control of a WEDM machine incorporates some important dif-

ferences with respect to other conventional machine tools, such as machining

centres and turning centres, for instance. In a WEDM machine the servo system

not only closes loops for position and velocity, but it is also in charge of keeping

a constant gap between the wire electrode and the workpiece. Solutions to this

problem involve acquiring and analysing signals related to gap voltage and/or to

delay time. Moreover, “machine intelligence” is of primary importance in WEDM

machines. Manufacturers include in the NC of the machine technological data for

optimum cutting of different materials and thicknesses, strategies for improvement

of accuracy in corner cutting, the intelligent selection of EDM parameters for

situations of degraded erosion (such as stepped parts, large thickness parts, taper

cutting, etc.), and systems for avoidance of wire breakage, amongst others. Some

of these will be addressed in Sect. 9.3.3.

9.3.1 Wire Transport and Wire Thread Devices

As said before, in most cases wire wear is not considered in WEDM since wire

is continuously fed between pulleys. The wire supplying spool can contain from

1.6 to 45

kg with 3,700

m and 105,000

m respectively, and in the case of wire

diameter 0.25

m. With the aim of assuring wire position, different pulley configu-

rations have been designed by manufacturers providing the wire with the pro-

grammed feed rate and axial tension. Prior to and after the machining zone, the

wire is driven through two guides (the upper and lower guide) corresponding to

the nominal position programmed.

The guides are wear resistant accuracy devices; hence, they are made from sap-

phire or diamond. The guide diameter depends on the wire diameter used. There

are, however, several wire thread systems based on wire drawing which allow

using the same guides for a range of wire diameters.

There are two types of wire guides used by manufacturers on their wire EDM

machines. Round or toroidal shaped wire guides are used by a number of EDM

manufacturers, which may provide a slight advantage when machining larger

tapers. A round wire guiding system may help to produce a slightly better finish in

larger taper angles (greater than 15’). A round guide requires some clearance

(∼5

μm) to thread the wire through the guide. However, a number of manufactur-

ers use V-type wire guides due to their reliability for automatic wire threading.

The nozzles drive the upper and lower flushing pressure jet. It becomes appar-

ent that the location of the nozzles, especially in the lower arm, can be affected by

these higher flushing pressures. Hence, a more rigid mechanical structure can

withstand higher flushing pressures better. Nozzle geometry varies also in the

taper cutting to accommodate the wire deformation shape.

9 Wire Electrical Discharge Machines 319

Once the wire has passed through the lower guide, it enters the second part of

the transport system called the evacuation system. The wire passes through the

evacuation tube to the pinch roller after which the wire is cut to store.

Axial force is imposed on the wire to ensure straightness during cutting. Wire

straightness is critical for precision applications. As explained in Sect. 9.2.2, the

forces exerted during the process, due to the discharge and dielectric jet, tend to

deform the wire. As a result accuracy is lost, since the points where discharges

occur do not exactly match the position of the guides (i.e., the position pro-

grammed in the NC path). The value of the axial force imposed by the machine is

limited by the mechanical strength of the wire and its diameter. Thermal load on

the wire produced by continuous discharges also affects its resistance.

Nowadays, apart from these devices, most of the wire EDM machines have

some kind of automatic wire thread (AWT) system which automatically provides

thread or re-thread through the slot or start hole, with nearly 100% reliability.

Automatic wire threading is much simpler and more reliable than manual thread-

ing, primarily when wires with low tensile stress resistance are used. Most AWT

systems heat, draw and guide the wire by high pressure flushing water or air.

AWT systems provide the guides with much longer life and therefore lower cost

per hour and downtime.

9.3.2 Machine Automation

Automating EDM processes is a key aspect in the way to optimise productivity

and work throughput, running machines continuously and unattended. Thus, major

EDM machines manufacturers have developed new devices with the aim of mini-

mising downtimes. In this section, various devices recently established commer-

cially are presented.

The concept of cylindrical wire EDM is based on a rotary axis added to a con-

ventional two-axis wire EDM machine to enable the generation of a cylindrical

form. As conventional 2D machining, the electrically charged wire is controlled

by the X and Y slides to remove the work material and generate the desired cylin-

drical form. The original idea of using wire EDM to machine cylindrical parts was

first reported by Prof. Masuzawa’s research group at the University of Tokyo in

1985 [9], and this configuration has eventually been industrialised. The Mitshubi-

shi Electric

BA8 submerged WEDM machine, equipped with rotating spindle for

EDM-turning and EDM-grinding applications, and the CNC ONA-W64 (by ONA-

Electroerosion

S.A.) capable of controlling 6 axes are examples of this applica-

tion. Of course, special machines can be customised. The configuration of 6 con-

trolled axes was first aimed at the manufacture of small-diameter pins and shafts

to be used as tools for 3D micro-EDM applications [13]. Nowadays, new fields of

application are emerging, such as tool sharpening, diesel engine injector plungers,

gear wheels with integrated shafts for easy gear assembly, conductive bonded grind-

ing wheel dressing, parts for medical industries and aerospace parts industries.

320 J. A. Sánchez and N. Ortega

No doubt, one of the critical concerns in WEDM application is wire breakage.

The demands for a very high cutting speed and overnight cutting without atten-

dance require wire breakage prevention since this would result in unacceptable

increases of machining times, a decrease in machining accuracy, and the deteriora-

tion of the machined surface. A great amount of research effort into wire breakage

prevention has been carried out which concluded wire breakage could be identi-

fied by symptoms such as high gap voltage, a sudden rise in the total sparking

frequency, and an excessive instantaneous energy rate.

Control strategies to prevent wire breakage are therefore based on pulse dis-

crimination according to the characteristics of voltage waveform during machin-

ing for the purpose of improving machining stability and efficiency. To do so,

the control system analyses each discharge and evaluates ignition delay and

frequency of the pulse current as an indication of arcing. Once instability is de-

tected, the system corrects gap distance recovering a stable state. Modern digital

EDM generators include any pulse monitoring system which means a wire break-

age prevention system in the case of WEDM. Adaptive control systems provide

effective control of the cuttings process, automatically modifying the program-

med parameters of the power supply, ensuring optimum machine performance at

each stage.

There are a number of other topics dealing with productivity and accuracy

which can obtain enormous benefit from adaptive control systems. For instance,

complex pieces like staggered components, parts with wide taper cut angles etc.,

can be EDM’ed resulting in improvements of as much as 30% in the cutting speed

when compared with conventional control systems. Industrial examples of such

systems are the AI (artificial intelligence) workpiece thickness adaptive control

developed by Fanuc

, the Expert Erosion System by ONA Electroerosión

, the

AutoMagic by Mitsubishi Electric

, and the Pilot Expert 3 by Agie-Charmillies

Advanced research work in this field includes the use of knowledge-based control

systems [16], explicit mathematical models [4], neural networks [8], and fuzzy

logic controllers [15].

Intelligence is also introduced in the setting of EDM parameters after restarting

a terminated process. In machines equipped with AWT systems (see Sect. 9.3.2) if

the same parameter setting as that before wire breakage is used, the process tends

CNC

BRAKE SERVO

MOTOR

FEED SERVO

MOTOR

Fig. 9.13 Diagram of Twin Servo Wire Tension Control (patent pending) developed by Fanuc

Co.

9 Wire Electrical Discharge Machines 321

to experience breakage again after restart. Thus an “intelligent” system is usually

called up to change the parameter setting after each wire breakage. It is actually

off-line knowledge about wire breakage.

As already explained, strict control of the axial force imposed by the machine

is critical to ensure wire straightness. A good example of machine intelligence

applied to the process is the Twin-Servo Wire Tension Control system developed

by Fanuc, which uses servo motors to control wire tension and wire feeding. By

applying Fanuc's digital servo technology to the wire feeding system, variation in

tension is reduced to less than one fourth of previous systems, resulting in stable

high speed and high precision machining. This function, represented in Fig. 9.13,

is available in the high speed and precision Wire-Cut EDM Fanuc Robocut α-iD

series [19].

9.3.3 Workpiece Fixturing Systems

Palletising is very closely related to automation. The link between mechanical

interfaces – such as those between a workpiece and a machine tool – that have

interchangeability, modularity and flexibility, is a critical first step toward elimi-

nating the downtime associated with workholding. This became even more impor-

tant in the case of EDM machines due to the long machining times associated to

the process. Many workpiece holders’ manufacturers have developed new palletis-

ing systems compatible among machines with very different configurations. For

instance, this is the case of WEDM machines. In these machines, the work table is

formed by two stainless steel guides with plenty of threaded holes and whose work

surface is in a specific position related to the XY-plane and which is arranged in

the direction of movement of the tool. It is necessary to transport the chuck-

mounted work piece from one processing location to the next without changing

workpiece alignment, e.g., in an orthogonal system.

Standardisation provides a stable reference system. However, it is modularity

which gives a method of dealing with various applications. Modularity is the key

to processing different sizes and shapes of workpiece blanks.

Erowa

, Hirschmann

and System 3R

are probably the best known manufac-

turers for workpiece palletising systems (Fig. 9.14). There are countless possible

combinations in the product range adapted to any single application set up by refer-

ence elements, mounting heads, chuck adapters, rulers, adapter elements, support-

ing elements, and presetting/inspection devices. These devices allow the user to set

up the machine quickly with high-quality repeatability specifications. A repetitive

accuracy (consistency) for these systems is quoted in all the cases at 0.002

mm.

Growing demands on the manufacturing industry must be met with increased

flexibility, increased quality and increased productivity. So, workpiece fixturing

systems’ manufacturers also provide pallet handling robots as effective with one-

off manufacturing as with mass production. Examples of robots such as the

WorkMaster by System 3R

, the Robot Multi ERM by Erowa

, and the Erobot

322 J. A. Sánchez and N. Ortega

4018 Workpiece Changer by Hirschmann

can be cited as examples that provide

a significant impact on productivity, profitability and competitiveness.

9.3.4 Filtering Systems

The filtering system is responsible for supplying the dielectric with suitable con-

ditions into the erosion area. In most applications tap water conductivity is re-

duced to 106

Ω⋅cm, although some new materials such as hard metal must be

machined with less conductivity prevent corrosion. Once tap water is electrically

conditioned, it is used as a dielectric to cool and evacuate the debris from the

machining which increases dielectric conductivity. No matter which filtering

system is used, electrical conductivity is reduced employing special resins. If

accuracy demands must be met, cooling capacity must be ensured using an exter-

nal cooling system.

There are two main types of filtering systems: paper cartridge filters and min-

eral filters. Paper cartridge filters are replaceable or consumable which can be

used for 200–300 working hours depending on the EDM process, water volume,

etc. The filtering media consists of pleated impregnated cellulose base material

which is cured and retained within the inner and outer screens by means of hot

melt glue. The construction feature prevents excessive movement of the cellulose

fibre in humid conditions and maintains an even pleat-spacing thus ensuring the

most effective usage of the available media surface area. Nowadays, filters assure

a consistent EDM fluid quality, and uniform process condition to avoid blockage

of rinsing nozzles, increase resin consumption, sediment in the cooling and supply

system, and to increase corrosion deposits. Flow and particle size are kept constant

(particles over 1–3

μm are kept out) throughout the entire machining process with

Fig. 9.14 Example of duplication of frame systems by System 3R

[23]

9 Wire Electrical Discharge Machines 323

repeatable quality results. In some cases, WEDM machine manufacturers also

manufacture filtering systems.

Mineral filter systems for wire EDMs do not require filter media replacement.

It is a stand-alone unit which takes dirty water from the machine filtering it to

a 3-micron cleanliness level to supply clean water for machining. When the filter

vessel reaches its cleaning capacity, as detected by a pressure switch, a backwash-

ing cycle starts and clean filtered water is supplied to the machine dielectric tank.

With this feature the machine always has a supply of clean water.

9.4 Wires for WEDM

The correct choice of the wire for WEDM (Fig. 9.15) is a critical point in optimis-

ing the process. For each type of wire the user must be aware of the physical and

chemical properties which influence the cutting process, together with the eco-

nomic aspects that surrounding it. The result of a proper choice is a stable cut, eco-

nomical and at maximum speed. It is obvious that the cheapest wire is not always

the optimum solution for a certain application.

The first wires ever used were made of copper. Previous experience in SEDM ma-

chines using this material, and the availability of the technology for wire production

in diameters 0.2

mm and 0.25

mm made copper the first option. However, the poor

Fig. 9.15 Wires for WEDM