Dr.Deepak Lawrence.KDept. of Mechanical EngineeringNIT Calicut, India

ECM and related Processes

ME6324:Modern Machining Processes

Commercial ECM machines

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Electro Chemical Machining (ECM)

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ELECTROCHEMICAL MACHININGELECTROCHEMICAL MACHINING (ECM) is thecontrolled removal of metal by anodicdissolution in an electrolytic cell in which theworkpiece is the anode and the tool is thecathode.The electrolyte is pumped through the cuttinggap between the tool and the workpiece, whiledirect current is passed through the cell at alow voltage, to dissolve metal from theworkpiece.

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ECM

The cathode (electrode) does not touch the anode(workpiece). The product is not cut, but dissolved

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ECM

The electro-chemical machining process is based on the principle ofelectrolysis.An electrode connected to a D.C. source acts as cathode (the tool).The workpiece represents the other electrode and is poled as anode.In a watery electrolyte solution the cathode and workpiece exchange acharge that machines the workpiece without touching at the selectedpoint, generating contours, annular grooves, flutes or cavities all with thehighest precision.The material being removed separates from the electrolyte solution as metalhydroxide.

regardless of whether the material is soft or hard.The components are exposed to neither thermal nor mechanical stresses. 6

ECMECM is a subtractive method that works on the principle of anodicmetal dissolution (ion exchange).Each part to be machined requires a cathode (-) for selective materialremoval on the workpiece (+).The lack of contact between the tool (-) and the workpiece (+) isimportant.An electrolyte solution (NaCl or NaNO3) handles charge transfer in theworking gap.The resulting electron current releases metal ions from the workpiece.The shape of the tool cathode determines the shape of the materialremoval.ECM is an imaging method.The cutting speed is the DC current applied to the part. Amount of

time)The removed material comes out as a hydroxide and has to bereplaced by an adequate filter system to maintain constant gapconditions.

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ECM-Tool replication

(a) Initial condition, (b) final condition

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Schematic of the ECM system

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ECM-System DescriptionThe electric current varies from 50 to 20 000 Awith a current density of 0.2 to 3 A/mm and 30Vdc applied across a gap of 0.025 to 1.3 mmbetween the tool (cathode) and the workpiece(anode).The electrolyte flows through this cutting gap ata velocity of 30 to 60 m/s forced by a pressureof 70 to 2800 kPaTypical temperature of the electrolyte variesfrom 24 to 65°CThe tool can be fed into the workpiece at therate of 0.25 to 20 mm/min

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ECM-System DescriptionElectrolytes are usually aqueous solutions ofinorganic salts such as sodium chloride,potassium chloride, sodium nitrate, or sodiumchlorate. They may also contain proprietaryadditives.Other electrolytes, such as strong acids orsodium hydroxide solutions, can be used forspecial ECM applications or for select metals.The tool can be made of brass, bronze, copper,stainless steel, Monel, titanium, a sinteredcopper-tungstenalloy, or aluminum.The workpiece must be electrically conductive

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ECM-System DescriptionAlthough electrochemical machining is sometimesapplicable to small-lot production, the process isbest suited to larger-lot production applicationsbecause of high tooling and setup costs and highcapital equipment costs.A 15 000 A machine tool for removing metal atabout 25 x 10 3 mm3/ min may cost 2.5 to 4.5crore (not including the cost of peripheralequipment and facilities).The most frequent application of the ECM processis in the production of gas turbine engine parts andfor other aerospace applications.

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ECM-System Description

The main objective of the tooling is to handle the electricalresistance. Electrolyte has to be available in proper conditionsand quantity to handle charge transfer, hydroxide removal, andthermal management in the gap.

Areas not to be affected by ECM need to be protected againstpresence of electrolyte or electrical field. In the gap, electricalresistance has to be very low to get the most machining done bythe DC power.

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ECM-System DescriptionCathode design is important to maintain the gap and ensurethe focus on the targeted area. If this is not done, materialmay be removed from areas not to be machined!

Accuracy of the gap is key to preventing short circuit. Shortcircuit detection is built into the DC source. Cathode areasthat are not part of the process should be coated by anisolator.

Cathodes have no process-related wear because they do nottouch the anode. Wear of the cathode typically is related tomechanical damage of the isolated part on the cathode.

Cathode materials should be electrically conductive. In mostcases brass or stainless steel can be used.

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Basics of electro-chemical machining

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ECM advantages

Low-level tool wear (cathode), an ideal precondition for batch productionSurface finishes of up to Ra 0.05 µm (material is removed by atom by atom)No thermal and mechanical effects, therefore, no changes in the materialpropertiesHardness, toughness and magnetic qualities of the material remainunchangedPossibility to machine thin-walled contoursA high degree of repeat accuracy in the machining of the surface structureSimple but highly efficient production process; no need for subsequentdeburring or polishingRough-machining, finish-machining and polishing in a single operationPossibility to machine superalloysPossibility to simultaneously machine macro and micro structures

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ECM advantagesTargeted material removal at precisely defined locations

No mechanical or thermal load on the workpieces

Roughing and finishing in one pass

Suitable for deburring hard-to-reach locations and metals thatare hardened or tough to machine

No secondary burrs material will be dissolved and not cut!

Highest Productivity - fast process times (usually 5-20seconds) in combination with the ability to machine multipleparts per cycle ensure attractive cost per part

Superior process stability and good process control ensureaccuracy, constant quality, and highest repeatability

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ElectrolyteThe electrolyte has three main functions inelectrochemical machining.It carries the current between the tool and the workpiece,It removes the products of the reaction from thecutting regionIt removes the heat produced in the operation.Electrolytes must have high electrical conductivity,low toxicity and corrosivity, chemical andelectrochemical stability, and should dissolve workpiece at a consistent valence.

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Sludging electrolytesSludging electrolytes are solutions of typical salts, suchas sodium chloride.These salts are used primarily because they providesubstantial conductivity to the solutioning waterThe salts are generally not depleted in the ECM process.Instead, the water is depleted, yielding hydrogen gas (H2)and the important hydroxide ions.The hydroxide ions combine with the metal ions that arebeing removed by ECM, thus forming insoluble reactionproducts, or sludge.Sodium chloride solution is the most widely usedelectrolyte. It can be kept at constant strength by addingwater to keep the concentration constant, and itselectrical conductivity is stable over a wide pH range of 4to 13.

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Sludging electrolytesAs an electrolyte, sodium chloride also has somedisadvantages.It is fairly corrosive and produces large amounts ofsludge.Some metals, such as tungsten and pure titanium,cannot be machined in a sodium chloride electrolyte.Rough finishes are produced on aluminum alloys thatcontain silicon when this electrolyte is used.Sodium nitrate, another salt, is used singly or incombination with sodium chloride in some applicationsSodium nitrate gives smoother finishes on aluminum orcopper alloys and is less corrosive than sodium chloride,but it is more expensive

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SludgeSludge, consisting of insoluble hydroxides or hydrated oxidesof the work metal, is produced in conventional saltelectrolytes at the rate of about 6 to 8 kg of sludge perkilogram of metal .The electrolyte preferably should contain no more than 2 wt%sludge.An electrolyte with up to 2 wt% sludge is nearly as efficient asa clean solution. There is little difference in the conductivity,rate of penetration, surface finish, or accuracy of machining.A dirty solution may deposit more precipitate on the sides ofthe workpiece, depending mainly on the flow rate of theelectrolyte. There is some danger that dried precipitate maybecome lodged in the cutting gap or flow passages, causing ashort circuit or a flow disturbance.Filtering, centrifuging, and settling are generally done toremove sludge. It is usually more practical to clarify even thecheapest electrolyte (sodium chloride) than to discard andreplace it.

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Non-sludging ElectrolytesStrong alkali solutions (for example, NaOH) are used in theelectrochemical machining of heavy metals (such as tungstenand molybdenum) and their alloys.The salt in these electrolytes also provides the substantialconductivity required.However, the salt is depleted because the sodium ions of thesalt join with the metals being removed by the ECM process.Therefore, new compounds, such as sodium tungstate(Na2WO4.2H2) and sodium molybdate (Na2MoO42H2), formduring the process, and makeup of both the alkali salts andthe water are required for process control.This type of ECM operation is referred to as a depletion modebecause the alkali salt is depleted during the process.The new compounds in this process are quite soluble in waterand heavy precipitate volumes do not occur.

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Non-sludging ElectrolytesStrong acid electrolytes (for example,hydrochloric, nitric, and sulfuric) are also usedin a depletion mode.The metals being removed by electrochemicalmachining also form new compounds (such asnickel chloride, nickel nitrate, or nickel sulfate)with the electrolyte. Hydrogen gas is alsoproduced.These new compounds in acid electrolytes mustbe reduced by conditioning procedures or byperiodic replacement with unused acid.

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The concentration level of electrolyteThe concentration level of the electrolyte solution for electrochemicalmachining is usually a compromise.A concentrated solution has the advantages of lower voltage andpower requirements because of better conductivity.Faster rate of penetration and greater precision are also possiblebecause the conductivity of a concentrated solution varies less withthe changes in temperature and concentration encountered in theECM process.On the other hand, diluted, low concentration solutions cost less,dissolve more readily, and give a smoother surface on work.Low-concentration solutions also make crystallization less likely. Anover concentrated solution may become saturated and allow theformation of crystals that can damage pumps, valves, instruments,and pipes.However, a very weak solution causes local or intermittent passivity ofthe work, which makes machining difficult.A sodium chloride solution of 0.12 to 0.30 kg/ L of water performswell as an electrolyte.

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The conductivity of the electrolyteThe conductivity of the electrolyte dependsprimarily on its concentration and temperature.It must be controlled because it directly affectspower requirements and rate of penetration. Forhigher conductivity, the penetration rates arefaster.The conductivity of an electrolyte changessubstantially with temperature.For example, solutions of sodium chloride are

100% more conductive at 70°C than at 24 °C.

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The conductivity of the electrolyteAny change in electrolyte conductivity will affectECM geometry because the dimensions of thecutting gap and the side gap depend on theconductivity. This is why electrolyte temperature isclosely controlled.Selection of the operating temperature usuallyinvolves a compromise.Maintaining a lower temperature on a high-amperage operation requires additional coolingrequirements. A low-amperage operation mayrequire heating to maintain a higher temperature.To some extent, the conductivity of the electrolytelimits the current that can be used.

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The flow rate of the electrolyteThe flow rate of the electrolyte is important because the electrolyte must adequatelyremove the heat and the products of the chemical reaction.

Flow can be related to the amount of current used; the larger the ratio of flow tocurrent, the better the removal of heat and reaction products. High electrolyte flowrates often improve the uniformity of metal removal with a given electrolyte withoutreducing the metal removal rate.

However, the cost of pumping increases as the flow increases, and excessive flowrates can cause local erosion on the work piece or tool.

A flow rate of 0.95 L/min per 100 A of current is a guiding minimum for machiningsteel with an electrolyte of sodium chloride.

Tool design also influences the rate, pattern, and uniformity of electrolyte flow.

To produce a smooth, uniform surface on the work piece, the tool design must providea uniform flow rate over the entire machining area.

However, a uniform flow rate at all points is difficult to achieve given the typical flowvelocities (30 to 60 m/s) in the gap

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Electrolytes for the electrochemical machining

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Electrolytes and Machining Rates for ECM of Various WorkMaterials

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Tools (Cathodes)The metal used for a cathode or associated toolingshould have the necessary stiffness and machinability inaddition to electrical and thermal conductivity andchemical resistance to the electrolyte.Copper, brass, bronze, stainless steel, and titanium arethe materials most frequently used for ECM tools.Bronze or brass is usually the optimum choice, exceptwhere greater stiffness is needed. Titanium is especiallyuseful when machining with an acid electrolyte thatanodizes it (sulfuric acid, for example).The current can then be reversed periodically to removeplated deposits without excessive electrochemicalmachining of the cathode.

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Materials for ECM tools (cathodes)Copper alloys110, electrolytic tough pitch copper145, free-cutting copper (1/2% Te)187, free-cutting copper (1% Pb)220, commercial bronze, 90%260, brass360, free-cutting brass464, brass639, aluminum-silicon bronze706, copper nickel, 10%Copper-manganese alloy316 stainless steelTitanium (99%)Tungsten 80%, copper 20% (sintered

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Tool DesignThe tool or cathode is usually shaped like amirror image of the machined area of thecompleted part.Tool dimensions must be slightly different fromthe nominal mirror dimensions of the completedpart to allow for ECM overcut, which can rangefrom 0.025 to 1.3 mm, depending on electrolyteflow and the required dimensional accuracy

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Tools for electrochemicol mochining

The most common tool design is the open-flow type (Fig. a), in which electrolyteenters the gap through a channel in the centre of the tool and exits around theoutside of the tool to the atmosphere. Taper-free cavities can be machined with theinsulated construction shown.Tools for external machining may be of the cross-flow type (Fig. b), in whichelectrolyte enters the gap at one side of the work piece and exits at the opposite side.This may necessitate special fixtures to confine the electrolyte flow to the spacebetween the work piece and the tool. With reverse-flow tools, the electrolyte ispumped through the gap and out through the tool by using special fixtures to confinethe liquid.

(a) Hole-sinking tool of the open-flowtype, with insulated sidewall. Holes ofuniform curvature can be cut withcurved tool

(b) Dual external-cuffing tool, cross-flowtype. Special fixtures are required toconfine electrolyte.

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Tools for electro chemical machining

(a) Hole-sinking tool of the open-flow type, with insulated sidewall. Holes of uniform curvature can becut with curved tool. (b) Dual external-cuffing tool, cross-flow type. Special fixtures are required toconfine electrolyte. (c) Tool for tapering o predrilled hole. (d) Tool for sinking o stepped through hole.Electrolyte enters gap through predrilled hole. (e) Tool for enlarging interior section of o hole. A stubtool of similar design con be used to enlarge the interior of a blind hole (g) Double-wall trepanningtool; entire wall is insulated. A single-wall tool can be used, with electrolyte entering on one side andleaving on the other side, but flow rotes must be high and control is less precise. Special types areavailable for slug cut off. Oddly-shaped or tapered holes can be mode. (h) Cross-flow tool used togenerate ribs on o surface without leaving flow lines on the part. Flow is fed down one side, acrossthe face, and up the second side.

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Insulation-Tool designInsulation is important in the control of theelectrical current.The tool can be insulated in a number of ways,depending on its shapeSpraying or dipping is generally the simplestmethod of applying insulationAdequate thickness (0.05 mm to 0.5 mm) isrequiredTeflon, urethane, phenolic, epoxy, powder coating,Nylon, acetal, and fiber glass reinforced epoxy arecommonly used for insulation

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The feed rateThe feed rate, or penetration rate, is controlledin most ECM operations.The distance across the frontal gap is afunction of feed rate because, as the cathode isfed into the work piece at a higher rate, the gapcloses, causing resistance to drop.As resistance drops, amperage increases;therefore, machining rate also increases untilan equilibrium is reached.

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The feed rateAt slower feed rates, the material removal ratedecreases as the gap increases because thecathode is not keeping up with the work piecesurface.As the gap increases, the resistance rises andamperage drops.Side gap is also a function of feed rate. Frontalgaps are usually between 0.1 to 0.8 mm, andside gaps are about 0.5 to 1.3 mm

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Surface finish in ECMConsiderable variations in surface finish occurdue to the work piece characteristics andmachining conditions.Crystallographic irregularities, such as voids,dislocation and grain boundaries, differingcrystal structures and orientation, and locallydifferent alloy compositions produce anirregular distribution of current density, thusleaving the microscopic peaks and valleys thatform the surface roughness.

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Surface roughness generation in ECM

Figure shows the effect of machining feed rate on the localgap width for an alloy containing two elements X and Y.Accordingly, due to the difference in their machining ratesand their corresponding gap width, the generated maximumpeak-to-valley surface roughness Rt decreases at higher feedrates, and thus better surfaces are expected at these higherrates. 39

Surface finishThe surface roughness of a machined part varies from 0.3 to 1.9for the frontal gap area and can be as rough as 5 or more for theside gap area.

Important variables that affect surface finish are current density, feedrate, gap dimensions, electrolyte composition, viscosity, temperature,flow, and workpiece microstructure.

Microscopic surface defects, such as intergranular attack, may becaused by selective ECM attack on certain constituents in an alloy.

Such defects are usually associated with low current densities andintermetallic precipitates at grain boundaries.

Inter granular attack can shorten the fatigue life of metal and cannotbe tolerated in parts subjected to alternating high stress.Inter granular attack can usually be avoided with proper selection ofelectrolyte and ECM parameters

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AccuracyThe usual dimensional tolerances forelectrochemical machining are ± 0.13 mm forthe frontal gap and ± 0.25 mm for the side gap.It is difficult to machine internal radii smallerthan 0.8 mm external radii can be 0.5 mm orlarger.Overcut and taper, depend on the configurationof the cathode. Common results are 0.001mm/mm of taper and 0.5 mm overcut

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Removal ratesRemoval rates depend on the valence andatomic weight of the workpiece.Theoretical maximum rates range from 650 to4400 mm3/min per 1000 A.With suitable selection of electrolyte andoperating conditions, work pieces can beelectrochemically machined close to thetheoretical removal rates.As a first approximation, a value of 1600mm3/min per 1000 A can be assumed for mostmetals in preliminary planning

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Cleaning of the workpieceCleaning of the workpiece is nearly alwaysnecessary after electrochemical machining.Cleaning should be done before any residue hashardened on the work.A rinse in water may be all that is needed formetals that resist corrosion.Steel and cast iron are usually treated in alkalinecleaning solutions (with or without a water rinse) orin a water-displacing compound that may leave aprotective coating on the work piece.A mild hydrochloric acid solution is commonly usedbefore a clean water rinse.Glass bead peening or vapour blasting is also usedin numerous applications

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ECM- Process Parameters

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ECM -Applications

ECM is used for

-machining45

Electrochemical deburring (ECD).

Electrochemical deburring (ECD) is an adaptation of ECMdesigned to remove burrs or to round sharp corners on metalwork parts by anodic dissolutionThe hole in the work part has a sharp burr of the type that isproduced in a conventional through-hole drilling operation.The electrode tool is designed to focus the metal removalaction on the burr.Portions of the tool not being used for machining are

insulated. The electrolyte flows through the hole to carry awaythe burr particles 46

Applications of electrochemical machining

(a) Bottom contour of a deep hole. (b) Airfoils machined directly on a compressor disk.(c) Finishing a conical hole in a nozzle. (d) Machining a thin-wall casing withembossments. (e) Contouring a turbine blade surface. (f) Cutting slots in a valve plate.(g) Cutting spiral grooves in a friction plate. (h) Cutting multiple small cavities in Inconel718. All dimensions given in millimeters 47

ECM Applications

Electrochemical machining can be used for a variety of tasks,including some that would be difficult, impossible, or time consumingby mechanical machining.The tasks include machining hard materials (such as hardened steeland heat-resistant alloys) and oddly shaped, small, deep holes.

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ECM Applications

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Electrochemical grinding (ECG).

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Electrochemical grinding (ECG).

Electrochemical grinding (ECG) is a special form of ECM in which a rotatinggrinding wheel with a conductive bond material is used to augment theanodic dissolution of the metal work part surface.Abrasives used in ECG include aluminum oxide and diamond. The bondmaterial is either metallic (for diamond abrasives) or resin bond impregnatedwith metal particles to make it electrically conductive (for aluminum oxide).

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ELECTROCHEMICAL GRINDINGELECTROCHEMICAL GRINDING (ECG), also calledelectrolytic grinding, is similar to electrochemicalmachining (ECM), except that the cathode is anelectrically conductive abrasive grinding wheelinstead of a tool shaped like the contour to bemachinedElectrochemical grinding is used primarily tomachine difficult-to-machine alloys (such asstainless steel, Hastelloy, Inconel, Monel, andtungsten carbide), heat-treated work pieces (60 to65 HRC and harder), fragile or thermo sensitiveparts, or parts for which stress free and burr-freeresults are required.

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ELECTROCHEMICAL GRINDING

Electrochemical grinding removes metal by acombination of electrochemical and grinding actions.This electrochemical action is responsible for most (90%)of the material removal, but the grinding action of thecathode wheel removes the build up of oxide film on thesurface of the work piece.

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Inter Electrode Gap zones in ECG

ZONE 1PURE ELECTROCHEMICAL DISSOLUTIONWHEEL ROTATION HELPS IN DRAWING THE ELECTROLYTECONTAMINATION BY REACTION PRODUCTS & GASESCONDUCTIVITY CHANGESELECTROLYTE TRAPPED => GRIT/bond & W / P > ELECTROLYTIC CELL FORMATIONSMALL AMOUNT OF MATERIAL REMOVED

ZONE 2ABRASIVE GRAINS REMOVE W/P MATERIAL IN THE FORM OF CHIPSREMOVAL OF OXIDE LAYER HELPS IN EC DISSOLUTION

ZONE 3MATERIAL REMOVAL BY ELECTROCHEMICAL DISSOLUTIONWHEEL STARTS LIFTING OFF THE WORK SURFACEREMOVES SCRATCHES & BURRS < - FORMED IN ZONE 2

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ELECTROCHEMICAL GRINDINGIn electrochemical grinding ,the work piece is apositive electrode (the anode), and the wheel is anegative electrode (the cathode).The two electrodes are connected to a directcurrent source and are immersed in a conductiveionic solution (the electrolyte) containing positivelyand negatively charged ions.Under the influence of electric potential, thepositive ions migrate to the cathode, and negativeions migrate to the anode.At the cathode, the positive ions of the electrolytetake on electrons (reduction) and are discharged asneutral atoms or molecules.

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ELECTROCHEMICAL GRINDING

At the anode, the negative ions release electrons (oxidation) and aredischarged as neutral atoms or molecules.As electrolysis occurs, an oxide film is formed on the surface of the anodicworkpiece.The oxide film is an electrically insulating dielectric and, if allowed to remain,would slow or stop the process, depending on the porosity of the film.The abrasive material in the cathode wheel wipes the oxide away, exposingmetal for continued oxidation

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ELECTROCHEMICAL GRINDINGLess power is needed for electrochemical grinding thanfor electrochemical machining because the machiningarea is smaller and the abrasive in the wheel is removingthe oxide. Current ranges from 5 to 1000 A are mostcommon, with a voltage of 3 to 15 V over an electrolytegap of approximately 0.025 mmECM action (anodic dissolution) is responsible for 90% ormore of the metal removal in ECG, and the abrasiveaction of the grinding wheel removes the remaining 10 %or less, mostly in the form of oxide films that have beenformed during the electrochemical reactions at the worksurface.Because most of the machining is accomplished byelectrochemical action, the grinding wheel in ECG lastsmuch longer than a wheel in conventional grinding. Theresult is a much higher grinding ratio.

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ECG- SummaryCONVENTIONAL GRINDING

STRESSES

ECG

CONDUCTIVE-> 10 %

-> 90 %

NaCI & NaN03MATERIAL REMOVAL IS AS HIGH AS 10 TIMES OFCONVENTIONAL GRINDING ON HARD MATERIAL (> 65 HRC)

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Advantages of ECGAbsence of work hardeningElimination of grinding burnBurr-free surfacesAbsence of distortion of thin, fragile, or thermosensitive partsLess wheel truingSurface finishes of 0.125 to 1.0 µm depending onmaterial, wheel, and other variablesTolerances of ±0.025 mm under normal conditionsand ± 0.0025 mm under special conditionsLonger wheel life

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ECG-APPLICATIONSECONOMICAL IN GRINDING CARBIDE CUTTINGTOOLSEC GROUND CEMENTED CARBIDE W/P -» NODAMAGE TO MICROSTRUCTUR & NOMICROCRACKSFOR RE-PROFILING WORN LOCOMOTIVE TRACTIONMOTOR GEARSNO EFFECT ON GEAR HARDNESSBURR-FREE SHARPENING OF HYPODERMICNEEDLESSUPERALLOY, TURBINE BLADES & HONEYCOMBStructured METALS

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Disadvantages of electrochemical grindingHigher capital cost than for conventionalmachinesUse limited to electrically conductiveworkpiecesCorrosive nature of electrolytesRequired disposal and filtering of electrolytesNoncompetitive removal rates compared toconventional methods for readily machinablemetals

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ELectrolytic-In-process-Dressing (ELID) grinding

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Electrolytic-In-Process-Dressing (ELID) grinding

The ELID system consists of a grinding wheel, an electrode, apower supply and an electrolyte. The wheel is continuously dressedwhile the part is machined by means of electrochemical action.The ELID process minimises the problem of wheel loading andglazing making uninterrupted grinding and mirror surface finish ondifficult-to-machine materials possible when fine-grained wheels (#No. 6000 to 8000) of diamond are used.Primarily, a cast iron or cobalt bonding material is used with adiamond grinding wheel. With copper grinding wheels, insulatingoxide layers are difficult to produce.

Schematic of the electrolytic-in-process-dressing grinding process.

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Relationships between grain size and surface roughness

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Electrolytic-In-Process-Dressing (ELID) grinding

It uses a super diamond grid embedded metal bonded grinding wheel which iscontinuously dressed by means of an electrolytic process, to realize stablegrinding.This technique has been successful in obtaining nanometric surfaceRegarding the power supply, a rippled current that was made by combination of ahigh-frequency pulse direct current (DC) and a continuous DC is usedGenerally, a high-frequency (2 4 is standard) DC rectangular wave is preferable.

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Electrolytic-In-Process-Dressing (ELID) grinding

The wheel (tool) used is the metal bond wheel in which bonding material composedmainly of cast-iron and cobalt is sintered with abrasives.The abrasives used are mainly the so-called super-abrasives like diamond or cBN(Cubic Boron Nitride) abrasives.General abrasives such as cerium oxide, silica, alumina can also be used according togrinding purpose.The wheel serves as the positive electrode. The negative electrode is installedopposite to the grinding surface of the wheel.The clearance between these two electrodes is set at 0.1 to 0.3 mm.D.C pulse voltage is supplied between the two electrodes to electrolytically removeonly the metal bond of the wheel, allowing efficient and automatic dressing of thewheel.This dressing is continued even during the grinding work to prevent reduced wheelsharpness from wear, and wheel loading thereby realizing highly efficient mirror-surface grinding.

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ELectrolytic In-process Dressing(ELID) mechanism

According to the mechanism of this grinding method, first the wheel bond iselectrolyzed, which causes the abrasives to protrude out appropriately (1) in diagram.This process at the same time produces nonconductive oxidized iron, whichaccumulates to form a layer of coating on the wheel surface, automatically reducingthe electrolysis current. Initial dressing completes at this point (2) .When the actual grinding work is started in this state, the nonconductive oxide layer(such as Fe2O3 or Fe OH2) on the wheel surface comes in contact with the surface ofthe workpiece, and is removed by friction. As this takes places, the abrasives start togrind the workpiece, and subsequently start to gradually wear (3) This reduces theinsulation of the wheel surface, allowing the electrolytic current to flow again.As a result, the whole process starts again with the electrolysis of the wheel bondwhere the nonconductive oxide coating between the worn out abrasives has becomethin (4), allowing the abrasives to protrude again (returns to(2) state). The protrusionof the grains therefore remains constant in a general sense. 67

ELID applications

(a) Machined silicon sample using ELIDgrinding. (b) and (c) show the details of themachined surfaces with and without ELIDgrinding, respectively.

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ELID applications

Electrolytic in-process dressing (ELID) grindingtechnology is suitable for achieving mirror-qualitysurface finishing of silicon wafers used to fabricateintegrated circuits and other micro-devices,curvature glass lens used in optical instruments ,grinding of Co-Cr alloys used for artificial joints andmicro-tools used for mechanical micro cutting

Ground Co-Cr alloy for artificial jointMicro-tools

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Thank You.

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