Journal of Materials Processing Technology 149 (2004) 579–584

Using ultra thin electrodes to produce micro-parts with wire-EDMF. Klocke∗, D. Lung, D. Thomaidis, G. Antonoglou

Laboratory for Machine Tools and Production Engineering (WZL), University of Technology at Aachen, Steinbachstrasse 53,RWTH Aachen 52074, Germany

Accepted 31 October 2003

Abstract

Small-sized tools and parts are gaining importance and share of the total product rangeMicromachining of Engineering Materials,2002; Ann. CIRP 49 (2) (2000) 473]. An appropriate manufacturing process to cover the growing need for accurate small tools iselectro-discharge-machining with thin wires (W-EDM). Until now only a few scientific works have been dealing with cutting by W-EDMusing wires with a diameter below 50�m. The results of the experiments with ultra thin wires are presented in this paper.

The materials of the wires are tungsten with high tensile strength and melting temperature and brass-coated steel wire. Typical ultra thinwire diameters are 20, 25, 30 and 50�m. The process forces—until now often neglected—are becoming significant because of the smalltension forces on the wire and the low wire weightSingle discharging force and single machining volume of wire EDM, ISEM XI, 1995].A special set-up to use 20 and 25�m wires with an existing machine which can run wires up to 30�m was designed and constructed.Different wire types and diameters as well as electrical parameters were tested in several series of experiments.

The process forces on the thin wires were measured with a special measuring device. The measuring sensor was positioned at the placewhere the discharges take place and was electrically isolated in order to prevent measuring interference. The single discharge craters wereinvestigated on the workpiece surface. An electrical circuit was adapted onto the machine in order to allow just one single discharge tooccur. The craters were located on the pre-ground workpiece surface and their dimensions were measured with an optical microscope.© 2004 Elsevier B.V. All rights reserved.

Keywords: Workpiece; Tools; Electrode; Micro-EDM; EDM

1. Introduction

This paper deals with several aspects of micro-wire-EDM,when the machined parts include small radii or very narrowslots. Geometrical elements with dimensions of some mi-crons are often found in micro-parts and parts with microstructures. Setting the right machining parameters is essen-tial in order to fulfil the accuracy requirements. One of themain machining parameters of the wire-EDM process is thewire itself. The wire dimensions, material and performancecan vary, so choosing the right wire is very important forensuring a stable process.

Usually, the wire diameter is 0.25–0.33 mm for most ap-plications, but it can be even as low as 0.02 mm in the caseof cutting very small concave geometrical elements. Thediameter of the wires depend on the smallest inner radiusof the part (Rgeometry,min), the width of the narrowest slot(s) and the distance between the wire and the machined part

∗ Corresponding author. Tel.:+49-241-807-401;fax: +49-241-8888-293.E-mail address: f.klocke@wzl.rwth-aachen.de (F. Klocke).

during the last cut (gap width) The formula for calculatingthe maximum wire diameter (dwire,max) is

dwire,max = 2Rgeometry,min + gap (a)

or

dwire,max = s + 2 gap (b)

For machining micro-parts containing both concave arcsand narrow slots the wire should have the smallest diameterof (a) and (b) calculated above.

The boundary conditions are changed when cutting withthin wires. The allowed tension force of the thin wires is verylow and depends on the discharge energy and the part accu-racy. The thin wires cannot tolerate large discharge energiesand would consequently break. The tension forces are lowerwhen the cutting power is large. When using thin wires theprocess parameters are chosen either for fast cutting—highenergy and low tension force—or for precision cutting—lowenergy and high tension force.

In this paper the mechanical properties of thin wires willbe discussed after describing short the general experimentset-up. Then, the results of the investigations on the single

0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2003.10.061

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discharges will be shown. This includes the optical measure-ment of the crater dimension and SEM images. Finally, thesingle discharge forces will be analysed for various param-eter settings.

The experimental work described here took placeon a WEDM machines with linear drives in which ahydrocarbon-based dielectric fluid is used as the workingmedium. WEDM cutting in hydrocarbon-based dielectricfluid allows smaller interior contours to be achieved, be-cause the gap width is smaller than in a water-based dielec-tric fluid. This machine is capable of running wires witha diameter of 30�m or more. The friction force would bevery high if the thin wires would run through the wholewire transport system of the machine. What is more, themachine does not have wire guides to run ultra thin wires.An additional device with own guides was built which by-passes some machine pulleys for the wire transport in orderto carry out experiments with wires thinner than 30�m.

2. Preliminary tests

2.1. Mechanical properties of thin wires

Thin wires are manufactured by drawing at many stagesuntil the desired diameter is reached. Different materials areused for producing thin EDM wires. Brass, tungsten, copperand molybdenum are the most common ones. Tungsten isknown for its high fatigue strength, which allows higher cur-rents to be supplied from the pulse generator during EDMmachining without wire breakage. On the other hand, brasshas a lower fatigue strength but proved to be suitable forwire-EDM applications, as it can lead to high material re-moval rates. In order to provide thin brass wires with a higherdurability, wire manufacturers usually add a steel core ofhigh fatigue strength, leaving just a thin film of brass in theprocess significant outer wire layers. For the needs of thistest series, two kinds of wires were used: pure tungsten andbrass-coated steel wires. The wire diameter was 50, 30, 25and 20�m.

Fig. 1. Maximum wire tensile force before breakage without EDM.

During the preliminary tests the wires were pulled apartuntil they broke with the pulse generator kept off. The forceneeded for wire breakage was measured with a force mea-surement device. The given fatigue strength of the wireswas 3340 N/mm2 for tungsten and 2000 N/mm2 for thebrass-coated steel type. The theoretical maximum tensileload was calculated for each wire diameter.

The results comparison between the theoretical maximumwire load is shown inFig. 1. No great differences couldbe observed between the given and the measured maximumwire tensile force.

2.2. Cutting performance of thin wires

The generator was turned on and the discharge currentwas increased continuously, until the wires broke. This se-ries was done only with tungsten wires, as it was intendedto determine the maximum material removal rate for thinwires. The tungsten melting point is higher than the one frombrass and steel. The tungsten wires break at higher dischargeenergies and have a higher fatigue strength than brass. Forthis reason the maximal cutting speed is higher as well.

In this experiment series hard tool materials were investi-gated. Polycrystalline diamond (PCD) and cemented carbidewere used as the workpiece material:

(a) PCD is mainly used for cutting wood and light materi-als (Al, Ti, etc.)[4,5]. The material consists of diamondgrains which are located in a cobalt binding phase ona cemented carbide substrate. The PCD layer is 0.5 mmand the cemented carbide layer 1.1 mm thick. Therewere three grain size types. The grains had an averagediameter of 2, 10 and 25�m.

(b) Cemented carbide consists of about 90% tungsten car-bide grains (grain size 2�m for H40S and 1.5�m forCF-H40S) and 10% cobalt binding phase. It is used forcutting purposes as it features very high hardness andlong tool life [6,7]. The type CF-H40S has an addi-tional chrome layer that makes this material oxidationresistant.

F. Klocke et al. / Journal of Materials Processing Technology 149 (2004) 579–584 581

Fig. 2. Material removal rate for different wire sizes.

The material removal rate decreases for thinner wires asa result of the lower allowed discharge current. Cutting withthe 50�m wire was the fastest among the investigated thinwires. The discharge current could be increased up to 37.2 A,whereas cutting with more than 4.8, 2.4 and 1.2 A for the30, 25 and 20�m wires resulted to wire breakage (Fig. 2).For the same wire thickness the small grain-sized PCD canbe cut faster than the big grain-sized types. Apparently, thebig diamond grains (10 and 25�m) push the wire away andprevent the plasma channel from being built, as the grainsare positioned between the wire and the workpiece. The ce-mented carbide can be cut much faster, as it contains nodiamond grains. The material removal rate can reach up to1.6 mm2/min when cutting cemented carbide with a 50�mwire. The diamond grains could be transformed to graphitedue to the high temperature and become electrically con-ductive. A part of the discharges occur then on the graphitesurface, which does not lead to bonding material removal.

3. Single discharges

3.1. Single discharge experiment set-up description

A special electronic circuit was developed to allow justone discharge to occur. The two poles of the machine (wireand workpiece) were connected to the circuit. Five microsec-onds after detecting a falling edge of the voltage, the circuitwould short the two poles by a power transistor. The ma-chine recognised the short circuit and moved the table back.The workpiece surface was ground in advance so that thecraters could be located easily. Only a single crater wouldappear on the workpiece surface. At high feeding rates therewas a danger of mechanical contact between the wire andthe workpiece. The machine would not move the table back,before it came to a mechanical contact. It would continue

moving the wire towards the workpiece, which could lead tomechanical wear of the craters. The maximal allowed feedrate of the machine was therefore set to 0.1 mm/min to assurethat the machine had time enough to react to the short circuit.

The workpiece used was steel. Both tungsten andbrass-coated wires were used. Different machining param-eters were chosen to investigate their effect on the cratermorphology. The wire cut about 0.5 mm into the workpieceto separate the craters of each wire used. To locate thecraters easily, the single craters were programmed to ap-pear having a distance of 0.1 mm in between. When morethan one parameter was changed, the distance between twogroups of craters with the same parameters was 0.3 mm.First, the diameter of the craters was measured and thenSEM images were taken to examine the crater topography.

3.2. Crater diameter

The crater diameter was measured for different machiningconditions. The idle voltage was 150 and 80 V, the pulse ontime varied from 0.5 to 5�s and three maximum currentvalues were chosen: 1.5, 9 and 15 A. The influence of themachining parameters on the crater diameter is shown inTable 1.

There is no noticeable tendency within these tables. Theonly parameter that influences the crater diameter is the idlevoltage. The average crater diameter is 36.6�m at 150 V idlevoltage and 25.5�m at 80 V idle voltage. This effect can beexplained by means of the gap width. The gap width dependson the idle voltage. If the wire comes close to the workpieceat the time the discharge occurs (this is the case of low idlevoltage), the plasma channel cannot spread much. On theother hand, when the distance between wire and workpieceis great (case of high idle voltage) the plasma channel canspread more, thus causing a bigger crater.

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Table 1Crater diameter for different electrical parameters

Crater diameter (�m), U = 150 V Crater diameter (�m), U = 80 V

ON (�s) I (A) Tu 50 Br 50 Tu 30 Br 30 Tu 25 Br 25 Tu 20 ON (�s) I (A) Tu 50 Br 50 Tu 30 Br 30 Tu 25 Br 25 Tu 20

0.5 1.5 32 30 44 26 31 22 27 0.5 1.5 44 23 23 27 19 18 199 35 37 32 38 34 32 27 9 26 15 22 37 22 24 21

15 33 33 35 27 29 32 34 15 29 32 26 25 20 24 18

1 1.5 29 33 37 27 31 33 26 1 1.5 23 24 27 18 22 20 209 34 39 30 29 28 41 30 9 28 29 20 36 22 22 17

15 53 34 42 30 32 50 31 15 28 20 27 21 26 27 20

2 1.5 34 39 46 26 31 38 26 2 1.5 21 22 27 32 22 19 239 38 28 39 30 40 36 29 9 26 33 23 30 16 20 24

15 36 60 36 36 38 43 31 15 26 25 31 26 20 25 21

3 1.5 32 38 39 32 35 33 26 3 1.5 31 25 23 23 25 20 239 36 60 35 44 29 35 26 9 26 36 36 26 26 21 20

15 50 55 33 36 33 42 36 15 27 43 23 23 15 24 14

4 1.5 27 33 34 31 33 36 31 4 1.5 18 23 20 23 16 23 229 47 42 28 33 30 47 28 9 26 37 22 32 17 20 23

15 42 68 37 30 34 53 46 15 34 39 26 29 23 23 22

5 1.5 36 42 36 36 55 34 32 5 1.5 26 28 22 23 20 23 199 41 64 44 36 40 49 30 9 29 24 34 14 21 26 25

15 44 59 58 42 42 40 27 15 26 30 32 28 20 24 24

3.3. SEM images of craters

Scanning electron microscopy (SEM) images were takento examine the topography of the craters. The machiningparameters which were constant for the whole test series

Fig. 3. Single discharge craters for different wire types and idle voltage.

was the pulse on time (3�s) and the maximum dischargecurrent (15 A). The two wire types, tungsten and brass, wereused. The 20�m brass wire has not been used, becausethe wire was breaking after the discharges, even with verylow discharge energy. On the left hand side ofFig. 3 the

F. Klocke et al. / Journal of Materials Processing Technology 149 (2004) 579–584 583

craters are shown which were produced with low idle voltage(80 V). The height of the craters is bigger than the width.The wire was running vertically and because of the smallgap width the plasma could spread parallel to the workpiece.This phenomenon cannot be found at the craters with a highidle voltage (150 V). In this case, the gap width was greatand the plasma channel could spread more homogeneouslyto all directions.

4. Discharge forces

4.1. Discharge force measurement experiment set-updescription

The maximum discharge forces were measured with avery sensitive force sensor, which was hold horizontally. Atungsten pin with a tip diameter of 50�m attached to themachine’s wire guides was moved closer to the force sensor.In order to avoid current flow through the sensor when dis-charges occur, an isolating plastic plate was clamped ontothe sensor. The sensor was connected to a signal filter andamplifier device. Many measuring parameters can be set:the sensor sensitivity (given by the sensor manufacturer),the mechanical-force-to-voltage-unit ratio (calibrated/set bythe operator after applying known forces on the sensor andmeasuring the output voltage), the low pass filter frequencyand the response time. The output of the amplification stagewas connected to a digital storage oscilloscope capable ofrecording high frequency signals. For further analysis thedata of the oscilloscope were transferred to a PC.

The discharge current was measured to examine the phaseshift between current and force signal. The device usedfor detecting the single discharge was a high frequency(100 MHz) current probe. The current probe was connectedto an amplifier/filter device and the signal was transferred

Fig. 5. Discharge forces for different idle voltages and materials.

Fig. 4. Force measurement set-up.

to the oscilloscope. The positive slope of the current signalwas used as the trigger. A schematic drawing of the forcemeasurement set-up is shown inFig. 4.

4.2. Results

The force began to develop about 5�s and after the risingcurrent pulse and reached its maximum after 3�s. It canbe assumed that the mass inertia of the force sensor plateintroduces this small phase shift. This delay did not dependon the pulse time and the maximum discharge current.

The maximum single discharge forces for different ma-chining parameters were measured as shown inFig. 5.The maximum force depends on the maximum dischargecurrent and the idle voltage. The higher are the maximumdischarge current and the idle voltage the higher are theforces. For each idle voltage setting the maximum force islineary proportional to the maximum current. For the samedischarge current the discharge force depends on the idlevoltage, which in turn influences the discharge voltage andenergy.

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Higher force values could be measured for polycrystallinediamond than for steel. The material area available forEDM-machining is lower when machining PCD because ofthe diamond grains. The diamond grains act like a hard layeron the material surface resulting to higher forces. Unlikediamond, the steel material melts and absorbs a part of theplasma channel forces at the place where the craters form.

5. Conclusions

The results from the investigations described above canbe summarised as follows:

• The more electrically non-conductive particles are con-tained in a material the lower is the cutting speed.

• There is no great influence of the idle voltage, pulseon-time and the discharge current on the crater dimen-sions. The craters do not have the same shape even withthe same parameters.

• At lower idle voltages the craters become more elliptical.• The discharge forces depend strongly on the electrical

parameters and the machined materials. The forces arelineary proportional to the discharge current and the idlevoltage.

There are still some issues that should be investigated anddiscussed in future research works concerning thin wires:

• material volume removed after a discharge;• 3D topography investigations of the craters;• metallographic analysis of the white layer;• force measurement over a longer period of time (many

discharges);• cutting performance (machining speed and surface rough-

ness) with trim cuts.

References

[1] J. McGeough, Micromachining of Engineering Materials, 2002.[2] T. Masuzawa, State of the art of micromachining, Ann. CIRP 49 (2)

(2000) 473–488.[3] H. Ohara, Y. Makino, T. Ohsumi, Single discharging force and single

machining volume of wire EDM, ISEM XI, 1995.[4] S. Appel, Funkenerosive Bearbeitung von polykristallinem Diamant,

Ph.D. Thesis, Produktionstechnisches Zentrum Berlin, 1998.[5] F. Klocke, D. Lung, D. Thomaidis, G. Antonoglou, Micro wire-EDM

investigations using thin wires, in: Proceedings of the First In-ternational Conference for Manufacturing Engineering (ICMEN),2002.

[6] T. Noethe, Funkenerosive Mikrobearbeitung von Stahl und Hartmetalldurch Schneiden mit dünnen Drähten, Ph.D. Thesis, RWTH Aachen,2000.

[7] F. Klocke, D. Lung, T. Nöthe, Micro contouring by EDM with finewires, in: Proceedings of the 13th International Symposium for Elec-tromachining ISEM XIII, 2001.