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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:
http://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Electrochemical_machininghttp://en.wikipedia.org/wiki/Electrolysishttp://en.wikipedia.org/wiki/Sodium_chloridehttp://en.wikipedia.org/wiki/Sodium_nitratehttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Electrochemical_machininghttp://en.wikipedia.org/wiki/Electrolysishttp://en.wikipedia.org/wiki/Sodium_chloridehttp://en.wikipedia.org/wiki/Sodium_nitrate -
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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
http://en.wikipedia.org/wiki/Electrochemicalhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Hardnesshttp://en.wikipedia.org/wiki/Titanium_aluminidehttp://en.wikipedia.org/wiki/Inconelhttp://en.wikipedia.org/wiki/Waspaloyhttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Cobalthttp://en.wikipedia.org/wiki/Rheniumhttp://en.wikipedia.org/wiki/Electroplatinghttp://en.wikipedia.org/wiki/Electrical_discharge_machininghttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Anodehttp://en.wikipedia.org/wiki/Electrochemicalhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Hardnesshttp://en.wikipedia.org/wiki/Titanium_aluminidehttp://en.wikipedia.org/wiki/Inconelhttp://en.wikipedia.org/wiki/Waspaloyhttp://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Cobalthttp://en.wikipedia.org/wiki/Rheniumhttp://en.wikipedia.org/wiki/Electroplatinghttp://en.wikipedia.org/wiki/Electrical_discharge_machininghttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Anode -
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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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