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SCIENCE OFENGINEERING

MANUFACTURE-IIASSIGNMENT - 1

SUBMITTED BYAMIT KUMAR

SISODIA2k8/me/215

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ELECTRO CHEMICAL GRINDING

Electrochemical grinding is a process that removes

electrically conductive material by grinding witha negatively chargedabrasive grinding wheel,

an electrolyte fluid, and a positively charged work

piece. Materials removed from the workpiece stay in the

electrolyte fluid. Electrochemical grinding

andelectrochemical machining are similar but a wheel

is used instead of a tool shaped like the contour of the

workpiece.

PROCESS CHARACTERISTICS:

The wheels and workpiece are electrically conductive.

Wheels used last for many grindings - typically 90% of the

metal are removed by electrolysisand 10% from the

abrasive grinding wheel.

Capable of producing smooth edges without the burrs

caused by mechanical grinding. Does not produce appreciable heat that would distort

workpiece.

Decomposes the workpiece and deposits them into the

electrolyte solution. The most common electrolytes

are sodium chloride and sodium nitrate at concentrations

of 2 lbs per gallon.

PROCESS:

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The wheels are metal disks embedded with abrasive

particles. Copper, brass, and nickel are the most commonly

used materials; aluminum oxide is typically used as an abrasive

when grinding steel. A thin layer of diamond particles will be

used when grinding carbides or steels harder than Rockwell

C65.

An electrolytic spindle with carbon brushes, acting as a

commutator, hold the wheel. The spindle receives a negative

charge from the DC power supply, which gives the workpiece a

positive charge. The electrolytic fluid is applied where the work

contacts the tool by a nozzle similar to that which supplies

coolant in conventional grinding. The fluid works with the wheel

to form electrochemical cells that oxidize the surface of the

workpiece. As the wheel carries away the oxide, fresh metal is

exposed. Removing the oxidized fluid may only require a

pressure of 20 psi or less, causing much less distortion than

mechanical grinding. The wheel is subject to little wear,

reducing the need for truing and dressing.

The main feature is the use of a grinding wheel in which aninsulating abrasive,

such as diamond particles, is set in a conducting bondingmaterial. This wheelbecomes the cathode-tool. The non-conducting particles act asa spacerbetween the wheel and workpiece, providing a constant inter-electrode gap,through which electrolyte is flushed.When a voltage of about 4 to 8 V is applied between the wheeland the

workpiece, current densities of about 120 to 240 A/cm arecreated, removingmetal mainly by ECM, although mechanical action of the non-conductingparticles accounts for an additional 5 to 10% of the total metalremoval. The

rate of machining is typically 1600 /min. The surface finish

produced byECG varies from 0.2 micrometre to 0.3 micrometre , depending

on the metal being ground.

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Accuracies achieved by ECG are usually about 0.125 mm,although someclaims have been made for accuracies an order of magnitudebetter. A

drawback of ECG is the loss of accuracy when inside comersare ground;because of the electric field effects, radii better than 0.25 to0.375 mm canseldom be achieved.A wide application of electrochemical grinding is the productionoftungsten carbide cutting tools. ECG is also useful in thegrinding of fragile

parts such as hypodermic needles and thin-wall tubes.A recent application of the technique has arisen in the offshoreindustry, forthe removal of fatigue cracks from underwater steel structures.

Seawater itself is an electrolyte, being composed mainly of

sodium chloride solution

of approximate salinity 3.5%. Although its specific conductivityis about

one-fifth of that of electrolytes normally used in ECM. it is asuitahlevehicle for ECG, and is used in the North Sea. The diamondparticlesembedded in the grinding tool are used to remove non-conducting materials,such as organic sea growth on the surface of the steel, beforethe ECG actionproperly starts. Holes about 25 mm in diameter, in steel 12 to25 mm thick.have been produced by ECG at the ends of fatigue cracks tostop furtherdevelopment of the crack and to enable the removal ofspecimens formetallurgical inspection.

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USES:

Production oftungstencarbide cutting tools.

Burr-free sharpening ofhypodermic needles

Grinding ofsuperalloyturbine blades

Form grinding of aerospace honeycomb metals

Removal offatigue cracksfrom underwater steel

structures. In this case, seawater itself acts as the

electrolyte. Diamond particles in the grinding wheel

remove any non-conducting organic matter, such

as algae, before electrochemical grinding begins.

DISADVANTAGES:Electrochemical grinding loses accuracy when grinding inside

corners, due to the effects of the electric field.

ELECTRO CHEMICAL MACHINING

Electrochemical Machining (ECM) is a non-traditional

machining (NTM) process belonging to Electrochemical

category. ECM is opposite of electrochemical or galvanic

coating or deposition process. Thus ECM can be thought

of a controlled anodic dissolution at atomic level of the

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work piece that is electrically conductive by a shaped

tool due to flow of high current at relatively low

potential difference through an electrolyte which is

quite often water based neutral salt solution.

Electrochemical machining (ECM) is a method of removing

metal by an electrochemical process. It is normally used for

mass production and is used for working extremely hard

materials or materials that are difficult to machine using

conventional methods. Its use is limited to electrically

conductive materials. ECM can cut small or odd-shaped angles,

intricate contours or cavities in hardand exotic metals, such

as titanium aluminides, Inconel, Waspaloy, and

high nickel, cobalt, and rhenium alloys. Both external and

internal geometries can be machined.

ECM is often characterized as "reverse electroplating," in that it

removes material instead of adding it. It is similar in concept

to electrical discharge machining (EDM) in that a high current ispassed between an electrode and the part, through

an electrolytic material removal process having a negatively

charged electrode (cathode), a conductive fluid (electrolyte),

and a conductive workpiece (anode); however, in ECM there is

no tool wear. The ECM cutting tool is guided along the desired

path close to the work but without touching the piece. Unlike

EDM, however, no sparks are created. High metal removal rates

are possible with ECM, with no thermal or mechanical stressesbeing transferred to the part, and mirror surface finishes can be

achieved.

In the ECM process, a cathode (tool) is advanced into an anode

(workpiece). The pressurized electrolyte is injected at a set

temperature to the area being cut. The feed rate is the same as

the rate of "liquefication" of the material. The gap between the

tool and the workpiece varies within 80-800 micrometers (.003

in. and .030 in.) As electrons cross the gap, material from theworkpiece is dissolved, as the tool forms the desired shape in

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As the potential difference is applied between the work piece(anode) and the tool (cathode), the positive ions move towardsthe tool and negative ions move towards the workpiece.

Thus the hydrogen ions will take away electrons from the

cathode (tool) and from hydrogen gas as:2H+ + 2e- = H2 at cathodeSimilarly, the iron atoms will come out of the anode (workpiece) as:Fe = Fe+ + + 2e-

Within the electrolyte iron ions would combine with chlorideions to form iron chloride and similarly sodium ions wouldcombine with hydroxyl ions to form sodium hydroxideNa+ + OH- = NaOH

In practice FeCl2 and Fe(OH)2 would form and get precipitated inthe form of sludge. In this manner it can be noted that the work

piece gets gradually machined and gets precipitated as the

sludge. Moreover there is not coating on the tool, only

hydrogen gas evolves at the tool or cathode. Fig. 2 depicts the

electro-chemical reactions schematically. As the material

removal takes place due to atomic level dissociation, the

machined surface is of excellent surface finish and stress free.

3. EquipmentThe electrochemical machining system has the followingmodules:

Power supply Electrolyte filtration and delivery system Tool feed system Working tank

The voltage is required to be applied for the electrochemicalreaction to proceed at a steady state. That voltage or potentialdifference is around 2 to 30 V. The applied potential difference,however, also overcomes the following resistances or potentialdrops. They are:

The electrode potential The activation over potential Ohmic potential drop Concentration over potential Ohmic resistance of electrolyte

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Advantages and Disadvantages

Because the tool does not contact the workpiece, its advantageover conventional machining is that there is no need to use

expensive alloys to make the tool tougher than the workpiece.

There is less tool wear in ECM, and less heat and stress are

produced in processing that could damage the part. Fewer

passes are typically needed, and the tool can be repeatedly

used.

Disadvantages are the high tooling costs of ECM, and that up to

40,000 amps of current must be applied to the workpiece. Thesaline (or Acidic) electrolyte also poses the risk of corrosion to

tool, workpiece and equipment.

ECM machines come in both vertical and horizontal types.

Depending on the work requirements, these machines are built

in many different sizes as well. The vertical machine consists of

a base, column, table, and spindle head. The spindle head has

a servo-mechanism that automatically advances the tool and

controls the gap between the cathode (tool) and the workpiece

Copper is often used as the electrode material. Brass, graphite,

and copper-tungsten are also often used because they are

easily machined, they are conductive materials, and they will

not corrode

Applications

Some of the very basic Applications of ECM are listed below:

It can be used for Die-Sinking operations.

Drilling a jet engine turbine blade.

Multiple Hole drilling.

Steam turbine blades can be machined within close limits.

ECM technique removes material by atomic level dissolution ofthe same by electrochemical action. Thus the material removal

rate or machining is not dependent on the mechanical or

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physical properties of the work material. It only depends on theatomic weight and valency of the work material and thecondition that it should be electrically conductive. Thus ECMcan machine any electrically conductive work material

irrespective of their hardness, strength or even thermalproperties.

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INTRODUCTION:

Nontraditional machining processes are extensivelyemployed to

produce geometrically complex and precision parts fromengineeringmaterials in industries as diverse as aerospace,electronics andautomotive manufacturing . There are many multiplegeometrically designed precision parts, such as deepinternal cavities,miniaturized microelectronics and fine qualitycomponents may only

be manufactured by nontraditional machiningprocesses.Chemical machining is a well known nontraditionalmachiningprocess is the controlled chemical dissolution of themachinedworkpiece material by contact with a strong acidic oralkalinechemical reagent. Special coatings called maskants

protect areasfrom which the metal is not to be removed. The processis used toproduce pockets and contours and to remove materialsfrom partshaving a high strength-to-weight ratio. Moreover, themachiningmehod is widely used to produce micro-components forvarious

industrial applications such as microelectromechanicalsystems(MEMS) and semiconductor industries.

There are several factors contributing to the popularity ofchemical machining processes as follow :a. Chemical machining process is mature and well established.b. It is simple to implement.c. There is no additional cleaning step needed.d. Cheaper machining process.

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STEPS OF MACHINING:

Chemical machining process has several steps for producingmachine parts. These are given below:

1 .Workpiece preparation: The workpiece material has to becleanedin the beginning of chemical machining process. The cleaningoperation is carried out to remove the oil, grease, dust, rust orany substance from the surface of material. A good cleaningprocess produces a good adhesion of the masking material.

There are two cleaning methods; mechanical and chemicalmethods. The most widely used cleaning process is chemicalmethod due to less damages occurred comparing to

mechanical one. Ultrasonic cleaning machine is applied withusing special cleaning solution and heating is beneficialduring the cleaning process.

2. Coating with masking material: The next step is thecoatingcleaned workpiece material with masking material. Theselected masking material should be readily strippable mask,which is chemically impregnable and adherent enough to

stand chemical abrasion during etching.

3. Scribing of the mask: This step is guided by templates toexpose the areas that receive chemical machining process.

The selection of mask depends on the size of the workpiecematerial, the number of parts to be produced, and the desireddetail geometry. Silk-screen masks are preferred for shallowcuts requiring close dimensional tolerances.

4. Etching: This step is the most important stage to producethe required component from the sheet material. This stage iscarried out by immerse type etching machine. Theworkpiece material is immersed into selected etchant and theuncovered areas were machined. This process is generallycarried out in elevated temperatures which are depended onthe etched material. Then the etched workpiece is rinsed toclean etchant from machined surface.

5. Cleaning masking material: Final step is to removemasking

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etchant can be prepared in shop. Ferric chloride (FeCl3) is themostwidely used etchant in chemical machining. It is mainly used foretching iron-based alloys as well as copper and its alloys,

aluminium,etc. Cupric chloride (CuCl2) is generally applied for copper andcopper based alloys in electronics industry because variousregeneration systems are available for the waste etchant.Alkalineetchants are introduced to the fabrication of electroniccomponentssuch as printed circuit board.

There are some other etchants can be named, but the

industrialapplication of chemical machining is generally used these threeetchants, even most of the companies use only ferric chloridedue toeconomical considerations

The application of chemical machining provides several

advantages as follow:

1. Easy weight reduction

2. No effect of workpiece materials properties such as hardness

3. Simultaneous material removal operation

4. No burr formation

5. No stress introduction to the workpiece

6. Low capital cost of equipment7. Easy and quick design changes

8. Requirement of less skilled worker

9. Low tooling costs

10. The good surface quality

11. Using decorative part production

12. Low scrap rates (3%).

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