VALUE ENGINEERING OF MICRO- MANUFACTURING USING ECM … · 111 Int. J. Mech. Eng. & Rob. Res. 2012 Hemant Jain et al., 2012 VALUE ENGINEERING OF MICRO-MANUFACTURING USING ECM AND

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Int. J. Mech. Eng. & Rob. Res. 2012 Hemant Jain et al., 2012

VALUE ENGINEERING OF MICRO-MANUFACTURING USING ECM AND ITS

APPLICATIONS

Pankaj Agrawa1, Ashish Manoria1, Hemant Jain1* and Richa Thakur1

*Corresponding Author: Hemant Jain,[email protected]

In the experiment, typical results at the microscopic level, such as fine grating and micro indentingare demonstrated. It is suggested that a strategy of combining aspect in a centralized anddistributed manner might be use to increase both productivity and flexibility in manufacturing.The current techniques for micro manufacturing mostly are silicon based. These manufacturingtechniques are not suitable for use in demanding applications like aerospace, micro factory andbio-medical industries. Micro-electrochemical machining removes materials while holding microntolerance and µECM can machine hard a navel µECM utilizing high frequency voltage pulsesand closed loop control. The work piece materials were used as “Coinage metals”. The researchstudied the effect of various parameters. The experimental data on small drilled holes agreedwith theoretical modal burns can be effectively removed by optimal µECM. A sacrificial layerhelped to improve the hole profile since. It reduced above 50% corner rounding. It electrochemicalmanufacturing localization is closely related with accuracy and the accuracy will be improvedwhen the localization is enhanced. Micro holes were electrochemical drilled in stainless steelusing nanosecond pulse power, millisecond pulse power and direct current power. Theexperimental results showed that the localization could be significantly enhanced usingnanosecond pulse power.

Keywords: Localization, Coinage metals, Accuracy, Electrochemical, Micromachining

INTRODUCTIONEngineering on the micro-scale has becomeincreasingly attractive and important due to thefurther requirements for finer, smaller and moreprecision. However, the fabrication of small

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Int. J. Mech. Eng. & Rob. Res. 2012

1 CIM, Department of Mechanical Engineering, Samrat Ashok Technological Institute, Vidisha, Madhya Pradesh 464001, India.

parts of dimensions in micrometer is still achallenge for modem manufacturing system,especially metal micromachining. Traditionalcutting machining is hardly competent formicromachining due to the difficulty of tool

Research Paper

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fabrication, cutting heat generation and toolwear. In nontraditional machining processes,micro electric discharge machining, laserbeam machining and focused ion beam andso on are superior micromachining methods,but they have their respective limitations.Electrochemical machining is an anodicdissolution process of metal work piece ionby ion (Figure 1). ECM process has been usedwidely as a practical machining method ofmetal because of its various advantages suchas no tool wear, absence of stress, high MRR.Due to the tiny size of ions, ECM possessesthe tremendous potential for micromachiningat micro to meso scale. However, ECM stilllags far behind other processes such aselectric discharge machining in the micro-machining field because of the difficulty incontrolling machined shape and largermachining gap during ECM. In this paper, theaim of the research work is to develop a set ofexperimental equipment and explore theprocess of micro-ECM.

PRINCIPLE OFELECTROCHEMICALMICROMACHININGUnder low machining current density, the goodmachining accuracy can be achieved as aresult of the passivation of NaNO

3 solution.

Since the micro tool electrode is very thin,stationary electrolyte is employed to guaranteebetter machining accuracy. When lowerconcentration of electrolyte and lowermachining voltage are employed, themachining gap is very narrow, so it is verydifficult for fresh electrolyte to flow into themachining gap, and product of electrolysis andelectrolytic heat can not be removed easily.However, the rotation of tool electrode at veryhigh speed can greatly improve thehydrodynamic conditions, and thus ensure theabundant supply of fresh electrolyte and thesuccessful removal of electrolytic product.Under lower machining voltage and lowerconcentration of electrolyte, the machining

Figure 1: Electrochemical Machining

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current is very low, thus the passive layer isformed on the work piece surface as a resultof the passivation of sodium nitrate (NaNO

3)

solution. This passive layer prevents the workfrom future dissolution. However, the rotationof tool electrode can make electrolyte in thenarrow machining gap flow rapidly in closeproximity to the tool electrode. The rapid flowof electrode thought the narrow machining gapin virtue of the rotation of tool electrode cannot only improve the removal of electrolyticproduct and the supply of fresh electrolyte butalso prevent formation of passive layer in verynarrow machining gap. When the electrodegap is kept at a very small value, the microtool electrode can give a high current density,so the machined surface of work piece in thetiny machining gap is kept activated andstands in the transpassive region in the whole

machining process, and the micro-ECM cansuccessfully. The metal of non-machinedsurface far from the tool electrode can not beremoved due to the protection of passive layer,the machined metal by micro-ECM is onlyconfined to a very small region in closeproximity to the tool electrode, and thedissolution of anodic metal can be localized.Hence the better resolution of machined shapecan be obtained.

MACHINING GAP CONTROLThe gap control between micro tool electrodeand work piece is one of most importantfactors influencing machining accuracy, sothe reasonable narrow gap must bemaintained in the process of micro-ECM.Figure 2 shows the flow chart of gap controlstrategy in this work.

Figure 2: Flow Chart of Gap Control

Start

ElectrodePositioning

Gap Forming

Set MachiningSpeed

Feeding

Feeding Forward andMachining

Short CircuitElectrode Move

Backward

Yes

No

Sampling

Machining Finish EndNo Yes

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The positions of the micro tool electrode andthe clamped Work piece are determinedthrough contact sensing function of themultifunctional machine tool, and then toolelectrode is withdrawn about 10pm to formsound machining gap. Proper machiningspeed must be set in advance through thesystem specially designed for micro-ECMaccording to the experience before machining.Then micro tool electrode is fed to startmachining and uniform machining speed isadopted during micro-ECM. The short circuitdetection system detects periodically whetheror not short circuit occurs through Hall currentsensor during micro-ECM. When toolelectrode touches work piece surface or thedistance between tool electrode and workpiece is only several microns, the machiningcurrent jump up instantly and the voltage of thesampling resistance soar. Thus the short circuitcan be detected by the short circuit detectionmodule. When short circuit occurs, the pulsedpower supply is switched off immediately andthe tool electrode is drawn back severalmicrometers rapidly to avoid short circuitdamage. Computer automatically regulatesthe feed rate and afterwards switches on thepulse supply. Then the micro tool electrode isfed to continue machining. By this machininggap control strategy, the machining gap canbe controlled within about 10 ptm.

RESEARCH METHODOLOGY

Section A

Electrochemical machining is a materialremoval process similar to electro polishing.In this process the work piece to be machinedis made the anode and the tool is made thecathode of an electrolytic cell with a salt

solution being used as an electrolyte. The toolis normally made of copper, brass, or stainlesssteel. The tool and the work piece are locatedso there is a gap between 0.1 mm to 0.6 mmbetween them the tool is designed so that it isthe exact inverse of the feature to be machined.On application of a potential differencebetween the electrodes and subsequentlywhen adequate electrical energy is availablebetween the tool and the work piece, positivemetal ions leave the work piece. Sinceelectrons are removed from the work piece,oxidation reaction occurs at the anode whichcan be represented as,

M Mn+ + ne– ...(1)

Where n is the valence of the work piecemetal. The electrolyte accepts these electronsresulting in a reduction reaction which can berepresented as,

nH2O + ne– n/2 H

2 + nOH– ...(2)

Hence the positive ions from the metalreact with the negative ions in the electrolyteforming hydroxides and thus the metal isdissolute forming a precipitate. Theelectrolyte is constantly flushed in the gapbetween the tool and the work piece toremove the unwanted machining productswhich otherwise would grow to create a shortcircuit between the electrodes. The electrolytealso carries away heat and hydrogenbubbles. The tool is advanced into the workpiece to aid in material removal.

Electrochemical machining can be used tomachine either a single hole or a series of holeswith the same characteristics. The tool isdesigned so that there is electrolyte flow botharound and along the length of the electrodeor through a hole inside the electrode so that

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the precipitates flow out. Flushing theprecipitates is crucial in hole drilling becauseotherwise the removed material would pile upand form a short circuit. Most of the material isremoved in the gap between the bottom of thetool and the work piece; however the highcurrent densities at the tip of the cathoderemoves some material at the sides of thecathode as the tool progresses into the workpiece. This enlarges the hole because furthermaterial leaves as the tool progresses into thework piece. This can be overcome by coating

the tool sides with an insulating material sothat machining occurs only at the tool base ortip. Since the hole shape depends on thestationary cathode’s shape, the holes drilledneed not be round The position of the tool andthe flow path of the electrolyte in a hole drillingoperation are shown in Figure 3. Sincematerial is removed radially in ECM, the toolmust compensate for this removal. Figure 4shows the expected cavity shape to be formedwith the given tool and the shape finallyobtained.

The amount of material removed isdetermined by Faraday’s first law which statesthat the mass of the substance removed at anelectrode is proportional to the quantity ofcurrent passed to that electrode. So,

V = Cit ...(3)

Where = volume of metal removed (mm3),C = electrochemical constant (mm3/amp-s),I = current (amps), t = time (sec).

Current and Voltage

Current density depended on the rate at whichions arrived at respective electrodes which was

proportional to the applied voltage,concentration of electrolyte, gap between theelectrodes, and tool feed rates. As the toolapproached the work, the length of theconductive current path decreased andmagnitude of current increased. This lesseningof the gap and increase in current continueduntil the current was just sufficient to remove themetal at a rate corresponding to the rate of tooladvance. The total amperage required formachining of the work piece could be calculatedby multiplying the current density and the surfacearea being machined. When the equilibriumgap approaches zero value, over voltage

Figure 3: Hole Drilling Using ECM

Electrolyte ToolInsulation

Workpiece

Figure 4: Shape of Tooland Cavity Formed After Machining

Produces

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approached applied voltage (Figure 6). Overvoltage (V) was calculated by Equation atvarious equilibrium gap for a given valency,

V = V – ZF/KA Yef ...(4)

where V is the Voltage, K is the conductivity, isthe density, Y

e is the equilibrium gap, Z is the

valency, f is the feed rate, and A is the atomicweight. Over voltage was a parameter which

Figure 5: Examples of High Accuracy of Micro Milled, Micro Holes and Micro Structure

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 6: Plot of Current Density versus Over Voltage

Machining Voltage (V)

5 6 7

160

140

120

100

80

60

40

20

0

Ma

ch

inin

g S

pe

ed

(m

/min

)

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restricted material removal rate and wassensitive to tool feed rate and equilibriummachining gap. The plot of over voltage versuscurrent density during machining analuminum sheet using brass cathode with

1.5 M sodium chloride electrolyte is given inFigures 7 and 8, The parameters for the plotWhere A = 26.97, Z = 3, V = 40 V, F = 96500,K = 0.184 Ohm-1 cm–1, T = 20 °C, andf = 0.000667 cm/s.

Figure 7: Variation of Micro-Hole Diameter with Pulse Frequency

Pulse Frequency (KHZ)

–5 0 5 10 15 20 25 30 35 40 45 50 55

360

340

320

300

280

260

240

220

200

Voltage: 6 VDiameter of Tool: 150 mFeed Rate of Tool: 150 m/minElectrolyte Component: NaNO

3

Electrolyte Concentration: 50 g/l

Mic

ro-H

ole

Dia

me

ter

(m

)

Figure 8: Variation of Micro-Hole Diameter with Applied Voltage

Applied Voltage (V)

4 5 6 7 8 9 10

380

360

340

300

280

260

240

220

200

Diameter of Tool: 150 m

Pulse Frequency: 20 KHZ

Feed Rate of Tool: 150 m/min

Electrolyte Component: NaNO3

Electrolyte Concentration: 40 g/l

Mic

ro-H

ole

Dia

me

ter

(m

)

320

118


Section B

Electrochemical micro-machining was carriedout in the electrolyte of 0.2 M1 NaNO

3 to

facilitate dissolution of metals because theremoved materials were carried away in theway of metals ions. The voltages applied inthe cathode and the anode was kept to be5 V. The work piece’s materials were a nickel

foil with 100 µm thickness.25 µm tungstenshaft prepared by the deep immersion methodwas employed as the cathode tool. To reducethe diameter of the electrochemical micromachined hole. The inter electrode gap needsto the in the order of 5 to 20 µm. In ourexperiment, the distance between the cathodetool and the anode was 5 µm (Figure 9).

Figure 9: Diagrammatic Sketch of Edges-Cuts Electrode and Deep Micro Drilledby Micro-ECM

Figure 10: (a) Feed Rate of Tool: 20 m/min (b) Feed Rate of Tool: 50 m/minMicro-Holes Machined with Different Tool Feed Rate

Figure 11: (a) Pulse Frequency: 10 KHZ; (b) Pulse Frequency: 50 KHZMicro-Holes Machined with Different Pulse Frequency

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THE PRESENT AND FUTURESTATUS OF ELECTROCHEMICALMICRO MACHININGHigh-rate anodic electrochemical dissolutionis a practical method of smoothing andshaping hard metals by employment of simpleaqueous electrolyte solutions without wear ofthe cathodic tool. ECM can offer substantialadvantages in a wide range of cavity-sinkingand shaped-hole production operations.Control of the ECM process is improving allthe time, with more sophisticated servo-systems, and better insulating coatings.However there is still a need for basicinformation on electrode phenomena at bothhigh current densities and electrolyte flow-rates. Tool design continues to be ofparamount importance in any ECM operation.The use of computer-aided design to predictcathode tool profiles will continue to advance.Recently developments in ECM practice havedwelt on the replacement of constant DC bypulsed currents (PECM). Significantimprovements in surface quality have beenclaimed. Much smaller electrode gaps may beobtained, for example, below 0.1 mm leadingto improved control of accuracy, for exampleto 0.02 to 0.10 mm, with dies, turbine blades,and precision electronic components. The keyto further advancement in PECM lies indevelopment of a low cost power supply.Successful development of technique willenable on-line monitoring of the gap size,enabling closer process control.

The model for material removal rate caninclude the effect of pulse OFF duration andflow rate to accurately predict the materialremoval rate.

The advent of new technology forcontrolling the ECM process and thedevelopment of new and improved metalalloys, which are difficult to machine byconventional means, will assure the future ofelectrochemical micro machining.

CONCLUSIONA novel ECM system was developed:

• Using high frequency pulses.

• A model was developed for materialremoval rate using pulsed current.

• The system was used to successfully formmicro holes and for profile refinement.

• Experimental data on small drilled holesagreed with theoretical data within 10%.

• Micro burrs can be effectively removed byoptimal ECM setup.

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Turbine Machined by ECM”, available athttp://www.barber-nichols.com/, accesson October 3, 2008.

2. Bhattacharyya B, Doloi B and Sridhar P S(2002), “Electrochemical Micromachining:New Possibilities for Micromachining”,Robotics and Computer IntegratedManufacturing, Vol. 18.

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4. Bhattacharyya B, Malapati M and MundaJ (2005), “Experimental Study onElectrochemical Micromachining”,Journal of Materials ProcessingTechnology, Vol. 169, No. 3, pp. 485-492.

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11. McGeough J A (2005), “Kirk-OthmerEncyclopedia of Chemical Technology”, inJ I Kroschwitz (Ed.), ElectrochemicalMachining, 5th Edition, Vol. 9, pp. 590-606,Wiley-Interscience, New York.

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Documents

VALUE ENGINEERING OF MICRO- MANUFACTURING USING ECM … · 111 Int. J. Mech. Eng. & Rob. Res. 2012 Hemant Jain et al., 2012 VALUE ENGINEERING OF MICRO-MANUFACTURING USING ECM AND