Non-conventional Machining Processes as Viable Alternatives for

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Journal of Scientifi c & Industrial Research Vol. 62 , July 2003, pp 678-682 Non-conventional Machining Processes as Viable Alternatives for Production With Specific Reference to Electrical Discharge Machining H S Payal" a nd B L Sethi * "Department of M echani ca l & Production Engin eering, Beant Co ll ege of Engineering & T ec hn ology, Gurdaspur 143 52 1 * Departmelll of Mechani ca l & Industrial Engineering, Thapar Institute of Engineeri ng & Technology, Patiala 147004 Rece ived: 23 August 2002; accepted: 28 November 2002 The so -ca ll ed non-conventi ona l mac hining techniques can no longer be ca ll ed 'non tr ad iti onal', si nce th ey have wide range of app li ca tions. El ec tri ca l Di sc harge M ac hining (EDM ) is one such te chnique, which so far was limit ed to tool room wo rk fo r di e and mould making. The inll ow of hi gh techn ology and products has produced phenomenal chan ges in th e technologi ca l environmen t. Industri es, which have to ent er th e critical phase of competition in th e global market for their prod ucts. require th e advancement in non-conventional machining process to mee t the challenges. This paper desc ri bes progress with respect to present situ ati on and future trends in non-conventional machining. particularly , about th e Electrical Discharge Machining to review extensively what has bee n done in th e past. It is bas ical ly a cr iti ca l assessme nt of the presenl sta te-of-art of th e wide and compl ex field of non-con ventional mac hining, ba5 ed on the inf o rm at ion obtain ed from th e re levant lit erature, industr y. and discussion at seminar and co nferen ces. Thi s is al so illu str ated by several spec i Il c ex.a mp l es . Some rece nt deve lopments anu new trends are hi gh li ghted. Some of th ese tec hniqu es ca n be compet iti ve with conventional manufacturing meth ods. In so me of th e cases they arc th e only efficient so lution for realizing th e specillc industrial pruducts. Keywords : Electrical di sc harge machinin g. Non-conventional machining proce sses Introduction Rapid tec hn ological developme nt s in th e fi e ld of new materia ls and a ll oys with ever- in creas in g strength. hardness, toughnes s, heat res istance, and wear res istance, have Imposed many probl e ms dur in g the machini ng of th ese materials by con ve nti o nal means. As a res ult of research and development (R&D) for the last five to six decades or so, several new techn iques of ma chining have emerged whi ch can be class ifi ed, as given in Table I. These new tec hniq ues of mac hinin g kn o wn as non-conventional machining processes, are diverse in nature and differ from each other by th eir characteris ti c features, operati o n. a nd fi elds of app li ca ti on. The co mp etiti ve pressure for progress means lhat in ena in eerin g. we mu st always st ri ve to make quicker, and be tt er th an before ' . This is an account of th e results of one such attempt. Manufacturing in th e future will require mu ch more " AUlhor for correspondence: E-mail: p:lya Uls <f!)yn hoo. CllIl1 powerful strategies fo r cont rol of processes in a hi ghl y automated manufacturing enviro nme nt. Effective utili za ti on of th e newest manufacturing equipment depends ma inl y on th e applications of T ab le 1- of non-conventional mac hining process es Energy mode Mechan i ca l Chemica l Electrochemi ca l Thermal Process Energy source US M Ultra son ic Vibration A./M WJM AWJM IJM AFM MAF C HM ECM EDM EBM I BM LBM PAM Pneull1a ti c: Hydraulic Hydrau l ic Hydraulic Hyuraulic M ag netic field Cor ro si ve agent Hi gh currenl Hi gh voltage I on i zed mat erial loniz eu mat erial Amplified light Ioni zed rnd lerial

Transcript of Non-conventional Machining Processes as Viable Alternatives for

Journal of Scientifi c & Industrial Research

Vol. 62, July 2003, pp 678-682

Non-conventional Machining Processes as Viable Alternatives for Production With Specific Reference to Electrical Discharge Machining

H S Payal" and B L Sethi *

"Department of M echanical & Production Engineering, Beant Co llege of Engineering & Technology, Gurdaspur 143 52 1

* Departmelll of Mechanica l & Industrial Engineering, Thapar Institute of Engineeri ng & Techno logy, Patiala 147004

Received: 23 August 2002; accepted: 28 November 2002

The so-call ed non-conventi onal machining techniques can no longer be called 'non trad itional ' , si nce they have wide range of appli ca tions. Electri ca l Di scharge M achining (EDM ) i s one such technique, which so far was limited to tool room work fo r die and mould making. The inllow of high technology and products has produced phenomenal changes in the technological environmen t. Industries, which have to enter the critical phase of competition in the global market for their products. require the advancement in non-conventional machining process to meet the challenges. This paper descri bes progress with respect to present situati on and future trends in non-conventional machining. particularly, about the Electrical Discharge Machining to review extensively what has been done in the past. It is bas ical ly a critica l assessment of the presen l state-of-art of the wide and complex field of non-conventional machining, ba5ed on the information obtained from the re levant literature, industry. and discussion at seminar and conferences. Thi s is also illustrated by several speci Ilc ex.amples . Some recent developments anu new trends are high lighted. Some of these techniques can be competiti ve with conventi onal

manufacturing methods. In some of the cases they arc the only effici ent solution for realizing the specillc industrial pruducts.

Keywords: Electrica l discharge machining. Non-conventional machining processes

Introduction

Rapid technologica l deve lopments in the fi e ld of new materia ls and all oys wit h ever- increas ing st rength. hardness , toughness, heat res istance, and wear res istance, have Imposed many problems during the machini ng of these material s by conventi onal means. As a result of resea rch and development (R&D) for the last five to six decades or so, several new techn iques of machining have emerged which can be class ifi ed, as given in Table I. These new tec hniq ues of machining known as non-conventional machining processes, are diverse in nature and differ from eac h other by their character isti c features, operati on. and fi elds of app licati on.

The competiti ve pressure for progress means lhat in enaineerin g. we must always st ri ve to make thin~s quicker, che~pe r, and better than before ' . This is an account of the results of one such attempt. Manufacturing in the future will require much more

" AUlhor for co rrespondence: E-mail: p:lyaUls <f!)ynhoo.CllIl1

powerful strategies fo r control of processes in a hi ghl y automated manufacturing environment. Effective utili zati on of the newest manufacturing equipment depends mainl y on the appli cations of

Tab le 1- Clas~ilicati o n of non-conventional machining processes

Energy mode

Mechan ica l

Chemica l

Electrochemical

Thermal

Process Energy source

US M Ultrason ic Vibration

A./M

WJM

AWJM

IJM

AFM

MAF

CHM

ECM

EDM

EBM

I BM

LBM

PAM

Pneull1atic:

Hydraulic

Hydrau l ic

Hydraul ic

Hyurauli c

M agnetic field

Corrosi ve agent

High currenl

High volt age

Ion ized material

l onizeu material

Amplified light

Ioni zed rnd lerial

PA YAL & SETHI: NON-CONVENTIONAL MACHINING PROCESSES 679

information technology (IT). Existing methods for control of manufacturing operations, mainly based on the craftsmanship of the technic ian, are becoming obsolete and have to be repl aced by science base methods .

Conventional methods of manufacturing process depend primarily upon the interac tion of mechanical energy with the work piece, while the non­conventional machining processes are based on the directed energy source concept2

, such that, performing these operations without intimate contac t between energy source and work piece obtaining the advantages of e liminating cutting tools , such as abrasi ve wheels, dri ll s. Such sources inc lude high power LASER beam, e lectron beam, e lectro di scharge machining, e lec trochemica l machining, etc. These non-conve nti onal machining techniques may come into the picture as poss ible alternative machining methods for many reasons, the malll are: machinability, work piece shape, and comp lex ity, surface integrity and prec is ion require ment. These processes are so called as they use new forms of energy and tools . As of now, different types of advanced machining processes have been developed , including the basics processes, hybrid process, and different versions of both .

Stiff market competition and continuously grow ing de mand for improved product performance has lead to the deve lopment of an ever-growing variety and quality of material s, like carbides, ceramics , composites, semiconductors variety of glass, and diamond. These material s possess properties like high strength and stiffness at e levated temperatures, extreme hardness, high brittl eness, high strength to weight ratio, hi gh corrosion and oxidation resistance, and chemical inertness. A lthough these properties result in superi or product performance, the ir prec ise shaping andlor machining can be difficult. Conventi onal machining or shaping methods result in very high mac hining cost, and degradation s of strength and some useful properties. Therefore, there is a need to develop processes capab le of giving adequate material removal rate (MRR) as well as minimum damage to the material properties' . In additi on to the above requirements, comp lex ity of work surface shape and s ize, surface integrity and miniaturi zation (i .e. micro-machining) requirements have also played a vital role in the deve lopment of different advanced (i .e. non-con ventional or non­traditional) machining processes.

An Overview of EDM

EDM is among the earliest non-traditional manufacturing process; anyone who has ever-seen what happens when a bolt of lightening strikes the ground will have an idea of the process of EDM. The origin of EDM goes back to 1770, when Joseph Priestly discovered the eros ive effect of electrical discharges. But for a long period this area was not much di scussed. In 1943, B Lazarenko and N Lazarenko did have an idea of expl oiting the destructive effect of an e lectrical di scharge and deve loping a controlled process for machining material s that are conductors of electricity. With that idea the EDM process was born. At first ,EDM was only used in an emergency, and then mostl y for busting broken taps and drill s from expensive workpieces4 During the 1960 the CLRP and ISME conferences were held for the first time in Czechslovakia which proved to be a dri ving force in the progress of the EDM process. The significant contribution of co-operative research in the field of non-conventional machining process and in EDM fie ld, particularly goes to CLRP, In 1964, Optiz proposed a program for co-operati ve research on EDM, involving fo llowing main steps':

( i) Which parameters influence the process and to define these paramete rs.

(ii) To draw up the general researc h program.

(iii) To carry out standard test programs in order to obtain comparative results . A uniform standard procedure would be necessary to carry out the next step.

(iv) To finalize research program in which the participants would take a part.

In 1966, more detailed directions were drafted. New tests had to be carried out unde r more specified conditions , such as the pul se ene rgy levels, pulse shape (rectangle), and pul se duration. Experiments with relaxation generators were discontinued . During 1967 many additional directions and proposals for improving measuring techniques became necessary. In 1968 the results of the participating institutes were examined. The results are found to be in agreement , particularly in view of results derived earlier in the co-operative work. During the experiments, it proved necessary to standardize (in addition to the above­mentioned parameters) the pulse durati on time, polarity of the electrodes, type of flu shing, and the

680 J SCI INO RES VOL 62 J ULY 2003

geo metri ca l confi guration of the test e lectrodes. In order to determine the pul se current and the pul se effi c iency, spec ia ll y des igned measuring devices have been deve loped and used by the parti c ipants.

In 197 1, experimenta l work used of d ie e lectri c flui d and its pressure: series of experiments performed at current level of 5-10-20-50A and pul se duration were fo und to vary between 5J.ls and 2000J.ls . The fo ll owing researche rs5 carri ed out the malll experimenta l work afte r 1971 .

Koning

Peke lharing

Heuvelman

Kaczmarek

Uni versity o f Aachen

U ni versity o f De lft

Uni versity o f E ind hoven

U ni vers ity o f Krakow

Uni ve rsity of Louvain

Centre for Techn ology

Uni vers ity of Trondhe im

Snoeys

Ten Horn

Rasch

The progress in E DM process got acce lerated immed iate ly afte r 197 1 and the commerc ia l appl icat ion of the process has made it market dri ven. Table 2 g ives an overview of EDM progress. Bas ica ll y, EDM is a therma l process , in whi ch materi a l is removed by me lting and vapori zing small areas at the surface area of the work piece. EDM process occurs between work pi ece and too l e lec trode of conduc ting mate ri a ls subme rged in a liquid ca lled di elec tri c, which ac ts as an insul ator. A suitable gap know n as spark gap is mainta ined between the work and too l e lectrode. A mong non-traditi ona l machining processes, EDM is cons idered to be one o f the mos t important process for machining intricate and complex shapes in var ious e lec tri ca ll y conducti ve mate ri a ls inc luding hi gh strength temperature-

Tahle 2 - An overv iew or EOM

Origin or EOM 1770

'2 Eros ive ellcct or EOM 1<)'1.1 ., Con tro ll ed prol'ess ror lll ,lCil in ing 1<)44

4 First mach ine patcnted 1<)50

5 Stan ur EOM - USA. Japan and 1<)50

Switzerland

6 Advan ce stage pat ent-USA 1<)5 2

7 Slow speed EOM 1<)5 2- 1 <)() R

X ONC,CNC EOM 1<) 70- 1<)75

<) Si mp le: orbi tal and <llbpti ve control 1<) 75 - 19))0

10 10 and morc all lO Illat ion I<) XI-llJ90

II Hi ghe r MRR and production EOM IlJ lJ I-Onwards

res istant (HSTR) a ll oys . In recent years, EDM has shi fted , more than ever before, from its island of " non-traditi ona l M achining " towards the broad spec trum of fac tory automati on.

Introduc tion of vari ous automati c devices and system of EDM machine has enhanced its popul a rity and utility, viz. Auto matic tool changer (ATe) automati c w ire feeder (AF), automati c swarf removal (AS R), auto matic work changer (A We) , and auto matic pos iti on compensator. With the adopti on of such fun cti ons in EDM, fl ex ible manu fac turing ce ll s a re be ing des igned6

. The unattended nature of ED M also makes it a cost-effec ti ve process for a three shift pe r day operati on without adding manpower7. The process a lso he lps to reduce in ventory and shorten de li veries , which contribute to improved cash fl ow and reduced operati onal expenses.

Growth of EDM

EDM has rapidl y earned its p lace a long with milling and grinding equipment as a proact ive, mainstream techno logy, E DM is bes t known fo r its ability to machine complex shapes in ve ry ha rd meta ls. The most common use of EDM is machining dies, tools, and mo lds made of hardened stee l, tungsten carbide, hi gh-speed steel and other work p iece mate ri a ls that are di fficult to machi ne by " traditional" methods. The process has a lso solved severa l probl ems re lated to the machi ning of "exoti c" Haste ll oy, N itra lloy, Waspa loy and N imon ic, w hi ch are used on a large scale in aerospace ind ustry.

Because of technica l ad vances in e lec trode wear. accurac ies and surface speed , EDM has rep laced many o f the traditi ona l processes in some app li cati on . Another factor contributing, to the gro wing use of E DM is the ex pans ion of the wo rk e nve lope, pa rti cularl y w hen it comes to he ights and tape rs. S tandard wire EDM can cut parts 400 111 m ta ll w ith a

stra ightness of ± 0.0 12 per side. W ith the reduct ion in e lec trode wear and inc reased sophi sticati on of EDM contro ls in rams, new EDM processe.' uses simp le­shaped e lec trodes to 3-D mill complex.

When To Use EDM

T he range of app lications for which EDM offers the ri ght so luti on has broadened greatl y. re lat ive to the conventional machining processes, like mill ing, turning, drill ing, and broaching. Because EDM is a no­contact and no-force process, it is well su ited for mak ing fragi Ie parts that cann ot take the stress or

PA Y AL & SETHI: NON-CONVENTIONAL MACHINING PROCESSES 68 1

machining. Grinding and cutting tool s find difficulty with thin wall s. Collets, Jet engine blade, cooling ribs of moulds, turbine blades, car engine, missile parts, carbide dies, toxic ' materials, spool valves and key ways for cy linder are now conveniently made by EDM. Foll owing are the sampl ing opportunities of EDM.

For Sharp Inside Corners - Unti l the square end mill is perfected , EDM wi ll conti nue to remain a superior process for internal corners.

For Th e Most Complex Geomefl y - In particu lar, EDM makes sense when the complex surface is diffi cult for a milling cutter to reach.

Where Deep CUffing Is Required - More specifi call y avo id milling where tool LID (tool length to diameter rat io) is particul arl y hi gh; milling is avo ided in such cases .

III V llaff ended Cuttillg - EDM is much easier process to automate because it is more predictable than milling .In a process that uses robots for load ing electrodes and work pi eces , a complete EDM process including electrode production can run round the clock effecti vely with minimal human attenti on.

For High-experTise Parts In general, programming time is shorter for milling an EDM electrode than for milling the required form in metal , and thi s difference becomes hi gher fo r sophi sticated milling operati ons. Due to hi gher programming time in milling operati ons, EDM remains the first choice.

When EDM Fill ish Is Specified - The surface CJ ualit y that ED M can prese ntl y deli ver is better th an ever. Advances re lated to add iti ves. e.g. not onl y improve the fin ish as a graphite electrode can achi eve; they also res ult in a shall ower heat affected zone (HAZ).

Process Replw;cll len l - Both wire and sinker EDM ca n replace grinding and ji g grinding fo r dies, powder metal d ies . punc hes, core pin s and dowel holes as the), all ow un at tended machining and less expensive des ign. As a replacement for 2-axis mi ll in g, wire EDM can slab of large slugs in stead of creating piles of" chi ps. For short run stamping (under 5000 pieces) the need to make a d ie set is eliminated and EDM provides a low cos t tooling so luti on as a substitute for low volume broaching of sp ines, gear teeth, etc. Appl ications that require co mp lex and extensive fi xt urin g with conventi onal machining are readil y handl ed by EDM because no cOl1lac t and no mac hinin g force are needed. Thus, onl y simple fixtur ing is required.

Specific Industrial Examples

The evolution of EDM from a basic arc­disintegrator to a fully automated machining center clearly indicates the importance of EDM in modern manufacturing industry. It has been reported that ae rospace or aerospace related industries own 40 per cent EDMs, general manufacturing about 16 per cent and spec ialty jobs 15 per cent. Machine tool s are gett ing smarter with time. The machines themse lves are now handling many of the tasks and fun cti ons that were earli er being done by humans. The automat ion in EDM fulfils some of the spec ific app li cat ion in support of the process given subsequentl y.

High Aspect Holes And Industrial Application­There are different methods used to produce micro holes including based on mi cro-drilling, EBM , PCM, ECMM, LBM and mi cro-electro-discharge machin ­ing. Each has its merits and .demerits, but for certain app lications, such as ink jet nozzle manufacture. where it is essential to produce a smooth internal bore inside the hole to encourage laminar f low of the ink, the micro EDM is bes t fabrication method.

Micro-EDM And Th e Two Stage Process - The two-stage process is relati ve ly recent develop ment in whi ch there are two separate mi cro EDM processes using one NC-machine to fabri cate the e lectrode and a second machine to generate the holes .

Th e Fabricatioll of Microelect rode - Wi re for EDM can be manufactured of va ri ous sizes fro m di fferent materi als including tungsten ca rbide coated wire, etc.

Quality and Productivity Aspect of EOl\l

The rapid industri al expansion demands a better quality dec ision in the shortes t possible time. In EDM , quality assurance is the max imu m ga in using the hybrid generator, better electrode mate ri a l ~ and hi gh CJuality di e lec tri c r1uids. Ex perts fix EDM parameters in software . Complex parts and multip le operati ons are permanentl y programmed and are not susceptible to stati on-to-stati on changes by e lec trodes . Too li ng can be stored with plug-in modules an d interlocked fo r batch producti on of va ri ous parts on the same machine. set up time is reduced considerab ly because EDM parameters and tabl e moves are not to be checked each time a new operati on goes in to the mac hine, Hence the machine producti vity is enhanced considerabl y.

682 J SCI INO RES VOL 62 JULY 2003

Present Trend of Research in EDM

There has been a continuou s effort towards enhanc ing the capabilities of the EDM process so as to make it more versatil e and competitive. This has also resulted in the deve lopment of many EDM assisted processes, such as wire EDM, electro­discharge grinding (EDG), and micro-spark erosion. New deve lopments, such as multiple-electrode set ups and multi-lead power supplies have made it poss ible to achieve higher cutting rates thereby making the process more competitive. On the other hand, introduction of fully automatic numerica ll y controlled machine with adaptive control system for in-process optimi zation of the process have resulted in highly effic ient metal remo'val systems. Researches have also been conducted towards the development of power suppl y syste ms for EDM, tool materia ls and die lectric fluids. Some reports have been publi shed on basic studies on discharge mechanism, optimization and numerica l control of the process. Introduction of programmable waveform generators and development of new e lec trode material s have made it poss ible to min imi ze the undesirabl e wear of the tool e lectrode down to acceptable limits. New die lec trics, other than hydrocarbon oil s have also been tried to Improve upon the effi ciency and better environment.

EDM in India - Past and Future

EDM came to India almost simultaneously with its commerci al introduction in the World Market. Initia lly the machines were imported blindly without any consideration of important factors like operator training, maintenance and spares, e lectrode material s and manufacturing. This created a situat ion where as late as in 1972-73 out of about 100 or so machines were in the country- and out of these hardly 10 to 15 percent were being really used to optimum capacity . The rest were almost not used or used only for simple operations. Thi s generated a reverse user reaction and EDM was cons idered as a white e lephant on the shop floor. In the past 15 to 20 y indigenous development and machine manufacturing have started in India. This has also he lped in inc reas ing the general awareness of thi s technology, with stress on operator training, prompt maintenance, etc . Now this techn ology is making good strides towards acceptance in user industries. It is expected that thi s new

technology will change the manufacturin cr teachin cr o 0'

particularly in tool making and wi ll ensure better turnout of fini shed products . Indigenous EDM machine production has already captured the market and in the decade or so all type of indigenous CNC­EDM will be available just as conventional machine tools. By combining two or more than two bas ic (non­conventional and/or conventional) processes, some hybrid processes have also been deve loped, so that the potential s and capabilities of each ind ividual process can be exploited simultaneously.

Conclusions

The results of such studies are useful in makin cr o the non-conventional machining processes to be effectively utili zed in the present days fo r production . The capabilities of EDM process to handle speed, surface fini sh, machine control technology as we ll as electrode development to meet every poss ible cutting condition, are making production of EDM not only poss ible, but a lso reliable as machi ning choice. It is be lieved that the production EDM will open up an opportunity for growing demand of precis ion products . Finally, it l S concluded that the price/performance trend of non-conventional machining has made it important and economicall y viable alternative to conventional mac hining process for a broader range of applicat ions.

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2 Schachra A & Lenz E, LBM and EOM- A co mpariso n on the cracks behaviour, A IlI/ C fR?, 25 ( 1976) 121· 1. 23 .

3 Jai n K Neelesh & Jain V K, Modeling of material removal in mechanical type advanced machining rocesses: A state-of­art review, fnt J Mach Tool Mall/ifact, 14 (200 I) 1573- 1635.

4 Guirau E Bud , EDM hand book (Henser Gardner Publi ca tion. Ci ncinnati ) 1997.

5 Heuvelman C J & Ten Horn B L, Revi ew of cooperat ive work on EOM in STCE of CIRP, AI/II CfR?, Key-Note­Paper ( 1974).

6 Jain V K, Non-conventional machining, Pmc First SERC School Adv Manu/act Techno f (Indian Institute of Technology, Kanpur) ( 1996) 88-9 1.

7 Payal H S, Electrical di scharge machining as viable and competiti ve opt ion for batch producti on, Proc Na t SVIIl/i

Manu/act Eng 2 r" Century (Indian In sti tute of Technology, Kanpur) (200 1) 125- 127