Journal of Materials Processing Technology 149 (2004) 272–277

Some investigations into the electric discharge machining of hardenedtool steel using different electrode materials

Shankar Singha,∗, S. Maheshwaria, P.C. Pandeyba Division of Manufacturing Process and Automation Engineering, Netaji Subhas Institute of Technology, Dwarka, New Delhi 110075, India

b Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India

Accepted 17 November 2003

Abstract

Electric discharge machining (EDM), a ‘non-traditional machining process’, has been replacing drilling, milling, grinding and othertraditional machining operations and is now a well-established machining option in many manufacturing industries throughout the world.Modern ED machinery is capable of machining geometrically complex or hard material components, that are precise and difficult-to-machinesuch as heat treated tool steels, composites, super alloys, ceramics, etc. This paper reports the results of an experimental investigationcarried out to study the effects of machining parameters such as pulsed current on material removal rate, diameteral overcut, electrode wear,and surface roughness in electric discharge machining of En-31 tool steel (IS designation: T105 Cr 1 Mn 60) hardened and tempered to55 HRc. The work material was ED machined with copper, copper tungsten, brass and aluminium electrodes by varying the pulsed currentat reverse polarity.

Investigations indicate that the output parameters of EDM increase with the increase in pulsed current and the best machining rates areachieved with copper and aluminium electrodes.© 2004 Elsevier B.V. All rights reserved.

Keywords: EDM; MRR; Diameteral overcut; Electrode wear surface roughness; Electrode material

1. Introduction

Electric discharge machining (EDM), an important‘non-traditional manufacturing method’, developed in thelate 1940s, has been accepted worldwide as a standard pro-cess in manufacture of forming tools to produce plasticsmouldings, die castings, forging dies etc. New developmentsin the field of material science have led to new engineer-ing metallic materials, composite materials, and high techceramics, having good mechanical properties and thermalcharacteristics as well as sufficient electrical conductivityso that they can readily be machined by spark erosion[1,2].The recent developments in the field of EDM have pro-gressed due to the growing application of EDM process andthe challenges being faced by the modern manufacturingindustries, from the development of new materials that arehard and difficult-to-machine such as tool steels, compos-ites, ceramics, super alloys, hastalloy, nitralloy, waspalloy,nemonics, carbides, stainless steels, heat resistant steel, etc.being widely used in die and mould making industries,

∗ Corresponding author. Tel.:+91-11-2509950; fax:+91-11-25099022.E-mail address: singhshankar@yahoo.com (S. Singh).

aerospace, aeronautics, and nuclear industries. Many ofthese materials also find applications in other industries ow-ing to their high strength to weight ratio, hardness and heatresisting qualities. EDM has also made its presence felt inthe new fields such as sports, medical and surgical instru-ments, optical, dental and jewellery industries, includingautomotive R&D areas[3].

EDM technology is increasingly being used in tool, dieand mould making industries, for machining of heat treatedtool steels and advanced materials (super alloys, ceramics,and metal matrix composites) requiring high precision, com-plex shapes and high surface finish. Traditional machiningtechnique is often based on the material removal using toolmaterial harder then the work material and is unable tomachine them economically. Heat treated tool steels haveproved to be extremely difficult-to-machine using traditionalprocesses, due to rapid tool wear, low machining rates, in-ability to generate complex shapes and imparting better sur-face finish.

In this paper the authors have investigated in detail thematerial removal rate, electrode wear, surface roughness,and diameteral overcut (dimensional accuracy) produced inEDM on En-31 tool steel.

0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2003.11.046

S. Singh et al. / Journal of Materials Processing Technology 149 (2004) 272–277 273

1.1. EDM process

EDM is a thermo-electrical material removal process, inwhich the tool electrode shape is reproduced mirror wiseinto a work material, with the shape of the electrode definingthe area in which the spark erosion will occur[4]. EDM isaccomplished with a system comprising two major compo-nents: a machine tool and power supply. The machine toolholds a shaped electrode, which advances into the work ma-terial and produces a high frequency series of electrical sparkdischarges. The sparks are generated by a pulse generator,between the tool electrode and the work material, submergedin a liquid dielectric, leading to metal removal from the workmaterial by thermal erosion or vaporisation[5]. The EDMphenomenon, as it is understood, can be divided into threestages namely application of adequate electrical energy, di-electric breakdown, sparking, and expulsions (erosion) ofwork material[6]. The spark erosion of the work materialmakes use of electrical energy, converting them into thermalenergy through a series of repetitive electrical discharges be-tween the tool electrode and the work material electrode[7].The thermal energy generates a channel of plasma betweenthe two electrodes, at a temperature ranging from 8000 to12,000◦C, and as high as 20,000◦C[8–10]. When the pulsedDC supply∼20,000–30,000 Hz, is switched off, the break-down of plasma channel occurs, resulting in a sudden reduc-tion in the temperature, allowing the circulating dielectricfluid to flush away the molten work material from the EDmachined surface in form of microscopic debris[11].

Melting and vaporisation of the work material dominatesthe material removal process in EDM, leaving tiny craterson the surface of the work material. EDM has no contactand no cutting force process, and therefore does not makesdirect contact between tool electrode and the work material.This eliminates the chances of mechanical stress, chatter andvibration problems, as is prominent in traditional machining.

Material removal rate (MRR) for EDM operation issomewhat slower than with traditional machining meth-ods, where chips are produced mechanically. The rate ofmaterial removal is dependent upon the following factors:amount of pulsed current in each discharge, frequency of thedischarge, electrode material, work material and dielectricflushing condition. Diameteral overcut (dimensional accu-racy) becomes important when close tolerance componentsare required to be produced for space application and also intools, dies and moulds for press work, plastic moulding anddie casting. EDM does not induce any mechanical stressesduring EDM thereby providing an additional advantage inthe manufacture of intricate and complex-shaped products[11].

Electrode wear takes place during the EDM operationwhen the electrode (i.e. the tool) gets eroded due to thesparking action. The rate at which the electrode wears is con-siderably less than that of the work material. In EDM, eachelectrical spark discharge produces a tiny spherical crater inthe work material by local melting and vaporisation. With

high sparking frequencies the spark erosion gives substantialmetal removal rates. The depth of the crater defines the sur-face finish which in turn depends on the current, frequency,and finish of the electrode. The metal removal rates and sur-face finish are controlled by the frequency and intensity ofthe spark. It has been found that high frequency and low am-perage settings give the best surface finish. High amperageleaves a larger crater having large diameter and depth in arandom location[12]. Surface finish produced on machinedsurface plays an important role in production. It becomesmore desirable so as to produce a better surface when hard-ened materials are machined, requiring no subsequent pol-ishing. Surface finish is also important in the case of toolsand dies for moulding as well as drawing operations. Sur-face roughness and dimensional accuracy of a spark-erodedwork material depend on discharge currents, electrode ma-terials and electrode polarity[13].

2. Past work

Review of the research work reveals that much work hasbeen done on various aspects of electro-discharge machiningon low carbon steels, carbides and few die-steels with onlyone or two electrode materials. Soni and Chakraverti[13–15]performed electric discharge machining on high carbon highchromium die steel to investigate the physico-mechanicaleffect on ED machined surface, material removal rate andwear ratio, and also studied the effect of electrode materialproperties on surface roughness and dimensional accuracy.George and Venkatesh[16] investigated the optimum ma-chining conditions for EDM of 5 Cr die steel. The cementedcarbides are hard-to-machine, and pose difficulty in for-mation of rake profile, complex geometrical contours, chipbreaker etc. as inserts, which can be easily given the requiredshape by electric discharge machining. Pandey and Jillani[17] studied the electrical machining characteristics of ce-mented carbides. Raman et al.[18] noticed an improvementin machining characteristics of GT-20 grade of cementedcarbides by electric discharge machining using copper andcopper–tungsten electrodes.

Arthur et al.[19] concluded that EDM allows tool steelsto be heat treated to full hardness before EDM, avoiding theproblem of dimensional variations, which are common afterpost heat treatment. Jeswani[20] made an analytical study ofphysico-mechanical characteristics of spark machined sur-faces limited to metal removal rate (MRR), electrode wearand surface integrity.

Diameteral overcut also depends upon finishing androughing spark gap and crater size. Due to the presence ofside sparks, overcuts are found to occur in the work ma-terial. Side spark gap is half of the diameteral differenceof electrode and eroded hole in the work material. Sparkgap must be considered when selecting an electrode size toachieve a particular hole diameter. Frontal spark gap deter-mines the ultimate depth of the blind hole. The variation

274 S. Singh et al. / Journal of Materials Processing Technology 149 (2004) 272–277

between discharges in terms of their electrical characteris-tics and strike location within the gap is indeed influencedby several factors[21–23]. It is established that only sparkpulses are responsible for metal erosion. Short-circuit; opencircuits and arcing pulses are collectively termed as ineffec-tive pulses[5]. It has been experimentally investigated thatduring EDM there is an appreciable amount of diffusionof metals from the tool electrode to the work material andvice versa[24].

From the literature survey, it has been observed that noextensive work has been done with different tool electrodematerials on the work material En-31 steel (used for coldforming rolls, Knurling tools, press tools, lathe centres, etc.).There exists a great need for investigating the effect of vari-ous electrode materials and pulsed discharge currents on ma-terial removal rate, diameteral overcut, electrode wear andsurface roughness in electric-discharge machining of En-31tool steel.

3. Experimental method and procedure

The electric discharge machine, model SPARKMAN S-10(die-sinking type) with servo-head (constant gap) and posi-tive polarity for electrode (reverse polarity) was used to con-duct the experiments. Commercial grade EDM oil (sp. gr. =0.763, F. point= 94◦C) was used as dielectric fluid. Ex-periments were conducted with positive polarity of elec-trode. The pulsed discharge current was applied in varioussteps in positive mode with four different electrode materi-als. The chemical composition of the work material is givenin Table 1. Table 2presents the experimental conditions. Thework specimen was En-31 tool steel, which is widely usedin tool and die industry.

Table 1Chemical composition (wt.%) of En-31 tool steel

Elements Composition (wt.%)

C 1.05Si 0.3Mn 0.6Cr 1.0V –Fe Balance

Table 2Experimental conditions

Sparking voltage 40 VDischarge current in steps 6, 7.5, 9, 10.5 and 12 AServo system Electro hydraulicElectrode polarity PositiveDielectric used Commercial grade EDM oilDielectric flushing Side flushing with pressureWork material polarity NegativeWorkpiece hardness Hardened and tempered to 55 HRc

Design scheme of experimental parameters for EDM.

The electrodes were machined in cylindrical shape ona lathe machine (HMT, LB17/1000). The diameter of allelectrodes was 16.0 mm. The work materials were machinedto a size 150 mm× 150 mm× 20 mm, on shaper machine,and were hardened and tempered to 55 HRc (measured onRockwell hardness tester C-scale). All surfaces were groundfinished.

Two electrodes each of copper, copper–tungsten, brassand aluminium were taken. The diameter of electrode wasmeasured with a micrometer (0–25 mm, LC 0.01 mm) andits initial mass was measured with a single pan electri-cal balance (make: Dhona, max 200 g). The work materialwas mounted on the T-slot table and positioned at the de-sired place and clamped. The electrode was clamped on theV-block, and its alignment was checked with the help ofthe try square. A depth of cut of 10 mm was set for the ma-chining of all work materials. Finally the required powerswitches were switched ‘ON’ for operating the desired dis-charge current values.

After the machining operation, the electrode was taken outand weighed again on the single pan electrical balance. AnOptical microscope was used to measure the diameter of theeroded hole. A surface roughness tester (LC 0.001�m, range100�m) was used, giving theRa values in�m. The sameexperiment was repeated with different electrode materials.

4. Results and discussions

It was observed during the experiment that the size of thecrater increases with an increase in current, which ultimatelyaffects the surface finish and diameteral overcut. The calcu-lations of the MRR and electrode wear were based on themeasurement of machined volume and percentage weightloss, respectively.

4.1. Discharge current against material removal rate

Fig. 1 shows the effect of pulsed discharge current onMRR for the En-31 work material. The copper and alu-minium electrodes achieve the best MRR with the increasein discharge current, followed by copper–tungsten electrode.Brass does not indicate significant increase in MRR with theincrease in discharge current. Copper gives the best MRR onEn-31 work material. The increase in MRR with the increasein discharge current is due to the fact that the spark dis-charge energy is increased to facilitate the action of meltingand vaporisation, and advancing the large impulsive force inthe spark gap, there by increasing the MRR. Experimentalinvestigations have shown that in addition to current, MRRis also dependent upon the electrode material, work materialand dielectric flushing. The MRR is also controlled by thefrequency of the sparks. It is observed that low dischargecurrents and higher frequencies correspond to low stock re-moval. Effective machining rate with brass electrode couldnot be achieved and the mirror-shape of the tool electrode

S. Singh et al. / Journal of Materials Processing Technology 149 (2004) 272–277 275

6 7 8 9 10 11 12

0

10

20

30

40

50

60

En-31Copper electrodeCopper tungsten electrodeBrass electrode

Aluminium electrode

MR

R (m

m3 /m

in.)

Discharge Current (Amp.)

Fig. 1. Variation of material removal rate with discharge current.

on the work material was found to be coated with a thinlayer of the tool material. No plausible reason is availableand further investigation is needed.

4.2. Discharge current against diameteral overcut

Fig. 2shows the effect of discharge current on diameteralovercut for En-31 work material. Copper electrode showsthe most consistent overcut with the increase in current. Alu-minium is also the best electrode material that shows lowdiameteral overcut. Copper–tungsten and brass gave poor di-mensional accuracy by resulting in higher diameteral over-cut. The diameteral overcut is low due to the fact that atlow current with reverse polarity, erosion is less. As sparkenergy is low at low current, the crater formed on the workmaterial is small in depth and hence results in good dimen-sional accuracy[20]. Overcut increases with the increase of

6 7 8 9 10 11 12

0.00

0.05

0.10

0.15

0.20

0.25

0.30

En-31

Copper electrodeCopper tungsten electrodeBrass electrodeAluminium electrode

Dia

met

eral

ove

rcut

(mm

)

Discharge current(Amp)

Fig. 2. Variation of diameteral overcut with discharge current.

discharge current but up to a certain limit. Thus, overcut de-pends upon the gap voltage and chip size, which vary withthe amperage used. The Cu–W composite electrode resultsin high overcut due to its high spark dispersing effects.

4.3. Discharge current against electrode wear

Fig. 3 shows that the copper and copper–tungsten elec-trodes have minimal wear. Brass and aluminium show aconsiderable increase in the electrode wear with the in-crease in the discharge current. The EDMing has been donewith reverse polarity (non-conventional polarity), where theelectrons (negative ions) strike the tool electrode surfaceliberating greater energy at this surface, and an electrodematerial with higher melting point wears less. Electrodewear is mainly due to high-density electron impingement(electrical), thermal effect, mechanical vibrations (shocks)

276 S. Singh et al. / Journal of Materials Processing Technology 149 (2004) 272–277

6 7 8 9 10 11 12-5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

En-31

Copper electrodeCopper tungsten electrodeBrass electrodeAluminium electrode

Ele

ctro

de w

ear

(%)

Discharge current (Amp.)

Fig. 3. Variation of electrode wear with discharge current.

generated by metal particles from the work material andimperfections in the microstructure of electrode material.Copper and copper–tungsten show minimum electrode wearat all values of current, for the tested work material.

4.4. Discharge current against surface roughness

Fig. 4 shows the effect of discharge current on surfaceroughness for the En-31 material. By comparing the fourplots, it is observed that copper–tungsten gives low valuesof surface roughness at high discharge currents on En-31.It is also seen that copper and aluminium electrode resultsin poor machined surface at high currents, which is due to

6 7 8 9 10 11 12

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

En-31 Copper electrode)Copper-tungsten electrodeBrass electrodeAluminium electrode

Surf

ace

roug

hnes

s,R

a (µ

m)

Discharge current (Amp.)

Fig. 4. Variation of surface roughness with discharge current.

the fact that higher MRR of Cu and Al metal electrodesis accompanied by larger and deeper craters, resulting ina greater surface roughness. Brass also gives good perfor-mance with only small rate of increase in surface roughnesswith increase in current, since at low discharge currents,spark energy is low, leading to formation of small craterson the ED machined surface and thereby improving sur-face finish. Hence smaller craters form, results in good sur-face finish [22]. The ED machined surface is distinguishedby the presence of the recast white layer, micro-structuralchanges, surface roughness, residual stresses, micro-cracks,micro-hardness and deposition of carbon content. Pulse cur-rent and pulse ON duration can be utilised to significantly

S. Singh et al. / Journal of Materials Processing Technology 149 (2004) 272–277 277

improve the thickness of the white layer [25]. The aboveeffects have not been studied.

5. Conclusions

After analysing the results of the experiments on En-31tool steel with different electrode materials, the followingconclusion are arrived at:

• For the En-31 work material, copper and aluminium elec-trodes offer higher MRR.

• Diameteral overcut produced on En-31 is comparativelylow when using copper and aluminium electrodes, whichmay be preferred for En-31 when low diameteral overcut(higher dimensional accuracy) is the requirement.

• Copper and copper–tungsten electrodes offer compara-tively low electrode wear for the tested work material.Aluminium electrode also shows good results while brasswears the most, of all the tested electrodes.

• Of the four tested electrode materials, Cu and Al elec-trodes produce comparatively high surface roughnessfor the tested work material at high values of currents.Copper–tungsten electrode offers comparatively low val-ues of surface roughness at high discharge currents givinggood surface finish for tested work material.

• Copper is comparatively a better electrode materials asit gives better surface finish, low diameteral overcut,high MRR and less electrode wear for En-31 work ma-terial, and aluminium is next to copper in performance,and may be preferred where surface finish is not therequirement.

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