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I.RajkumarAssistant professor IKALASALINGAM UNIVERSITY(Kalasalingam Academy of Research and Education)Anand Nagar, Krishnankoil 626 190DEPARTMENT OF MECHANICAL ENGINEERING
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Sheet metal forming is a process that materials undergo permanent deformation by cold forming to produce a variety of complex three dimensional shapes.SHEET METAL FORMINGkalasalingam university*[email protected]
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HIGH VELOCITY FORMINGHigh velocity forming processes began to make their mark and grow in application in 1960. Requirements such as the introduction of super tough alloys for space vehicles., supersonic aircraft and the need for shaping incredibly small, thin and brittle materials for electronic components helped in the growth of high velocity forming processes. In high velocity forming of metals, the metal is shaped in micro-seconds with pressures generated by the sudden application of large amounts of energy. kalasalingam university*[email protected]
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HIGH VELOCITY FORMINGPressures needed for forming are generated by :
Detonating explosives.Releasing compressed gases.Discharging powerful electrical sparks orElectromagnetic energy.kalasalingam university*[email protected]
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Explosive formingElectro-hydraulic forming.Magnetic formingPneumatic- mechanical high velocity forging..TYPES OF HIGH VELOCITY FORMING PROCESSESkalasalingam university*[email protected]
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EXPLOSIVE FORMINGkalasalingam university*[email protected]
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Explosive forming has evolved as one of the most dramatic of the new metalworking techniques. Explosive forming is employed in Aerospace and aircraft industries and has been successfully employed in the production of automotive-related components. Explosive Forming can be utilized to form a wide variety of metals, from Aluminium to high strength alloys
EXPLOSIVE FORMINGkalasalingam university*[email protected]
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In this process the punch is replaced by an explosive charge. The process derives its name from the fact that the energy liberated due to the detonation of an explosive is used to form the desired configuration. The charge used is very small, but is capable of exerting tremendous forces on the work piece.
In Explosive Forming chemical energy from the explosives is used to generate shock waves through a medium (mostly water), which are directed to deform the work piece at very high velocities.EXPLOSIVE FORMINGkalasalingam university*[email protected]
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Explosive forming makes use of the pressure wave generated by an explosion in a fluid to force the material being formed against the walls of the female die. The fluid has the effect of rounding off the pressure pulse generated by the detonation.
Methods of Explosive Forming
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The explosives are used in many forms- rod, sheet, granules, liquid, stick etc. Depending upon the placement of the explosive (charge), the operations fall into two categories :Stand-off operations.Contact operations.
Methods of Explosive Forming
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Standoff Method In this method, the explosive charge is located at some predetermined distance from the work piece and the energy is transmitted through an intervening medium like air, oil, or water. Peak pressure at the work piece may range from a few thousand psi to several hundred thousand psi depending on the parameters of the operation.METHODS OF EXPLOSIVE FORMINGkalasalingam university*[email protected]
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Contact Method In this method, the explosive charge is held in direct contact with the work piece while the detonation is initiated. The detonation produces interface pressures on the surface of the metal up to several million psi (35000 MPa).METHODS OF EXPLOSIVE FORMINGkalasalingam university*[email protected]
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WORKING PRINCIPLEThe system used for Standoff operation consists of following parts: - 1) An explosive charge 2) An energy transmitted medium 3) A die assembly 4) The work piece.kalasalingam university*[email protected]
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- Figure 1 shows an arrangement of Standoff Explosive forming operation. The die assembly is put together on the bottom of the tank. Work piece is placed on the die and blank holder placed above. A vacuum is then created in the die cavity. The explosive charge is placed in position over the centre of the work piece. The explosive charge is suspended over the blank at a predetermined distance. The complete assembly is immersed in a tank of water. - After the detonation of explosive, a pressure pulse of high intensity is produced. A gas bubble is also produced which expands spherically and then collapses until it vents at the surface of the water. When the pressure pulse impinges against the work piece, the metal is displaced into the die cavityWORKING PRINCIPLEkalasalingam university*[email protected]
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Explosives are substances that undergo rapid chemical reaction during which heat and large quantities of gaseous products are evolved.Explosives are divide into two classes;Low Explosives in which the ammunition burns rapidly rather than exploding, hence pressure build up is not large.High Explosive which have a high rate of reaction with a large pressure build up. EXPLOSIVES
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Various explosives used are: Solid (TNT-trinitro toluene) Liquid (Nitroglycerine) Gaseous (oxygen and acetylene mixtures). EXPLOSIVES
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Low capital investment is required to adapt the process to production. Large and expensive presses are not needed for forming. Component is generally formed in one shot only.Only one die either male or female is needed. For this reason tooling costs are greatly reduced.In general terms, the ultimate strength and yield strength are improved by high explosive forming.Large size parts unable to be handled successfully by conventional presses can be formed easily.Advantages of Explosive Forming Over Conventional formingkalasalingam university*[email protected]
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Personnel must be highly trained in the safe handling of high explosives.The technique has not been developed to the stage where a part can always be formed on the first shot.The noise developed during forming and the strict laws prohibiting the use of high explosives in populated areas, usually make it necessary to locate the facility away from populated areas. This increase transportation and handling costs. Disadvantages of Explosive Forming Over Conventional formingkalasalingam university*[email protected]
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Uses
Blanking Coining Powder compacting Cutting Embossing Drawing Expanding Flanging Sizing etc.
Applications
Aerospace components. kalasalingam university*[email protected]
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ELECTRO HYDRAULIC FORMING (EHF)kalasalingam university*[email protected]
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The ability to generate high intensity shock waves by discharging stored electrical energy across electrodes submerged in a liquid medium has been recognized for a great many years. Spark discharge in a liquid was pointed out as a potential source of power in 1944. Basic research on under water condenser discharges was the subject of several investigations in the early 1950s.The first attempts to use electro hydraulic forming ( also called capacitor discharge forming or spark forming ) process for forming metals were in the United States in 1953 to punch holes in 1.5 mm steel plates.
INTRODUCTIONkalasalingam university*[email protected]
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PRINCIPLE OF OPERATIONkalasalingam university*[email protected]
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Electro hydraulic forming involves the conversion of electrical energy to mechanical energy in a liquid medium. Discharging of an electric spark in a liquid produces shock waves and pressures which can be used for metal forming. As the energy produced is less than that produced in explosive forming, it is usually necessary to repeat the operation several times to achieve the desired results kalasalingam university*[email protected]
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METHODS TO CONVERT ELECTRICALENERGY INTO MECHANICAL ENERGY
Capacitor discharge through a gap : Voltages of 10,000 to 30,000 volts are generally used when the spark discharge method is utilized. This potential difference will jump the air gap present between two electrodes, submerged in the liquid.kalasalingam university*[email protected]
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METHODS TO CONVERT ELECTRICAL ENERGY INTO MECHANICAL ENERGY
Capacitor discharge through a wire : provides better control than the previous method because the path of the electrical discharge can be positively determined and shaped, also a more efficient energy conversion results. Moreover, with wire method, the use of lower potential difference is possible; the wire will initiate a path across a wider gap than a specified voltage will jump without the wire. A disadvantage of the wire method, however, is that a new wire is to be loaded after each shot for parts requiring multiple shots for reaching the desired results.
kalasalingam university*[email protected]
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In electrohydrauylic forming, electrical energy replaces the explosive used in explosive forming. High voltage electrical energy is discharged from a capacitor bank (a device used to store electrical energy) into a thin wire or foil suspended between two electrodes. The unit is immersed in water. As the water vaporizes, the vapour products expand converting the electrical energy to hydraulic energy. The shock wave forms (or shapes) the metal against the die. Since the energy produced for forming is less than that associated with explosives, it is usually necessary to repeat the operations several times to achieve the desired resultskalasalingam university*[email protected]
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ADVANTAGES OF EHF OVER CONVENTIONAL FORMING:Many designs requiring standard operations have some peculiarity which is not within the capability of conventional equipment, at least not without some special provision or tooling. Such works can be easily undertaking by EHF. When EHF is used, the cost of tooling will almost always be less than that for conventional equipment. Large amounts of energy can be directed into isolated areas as required in some piercing operations. Reproducibility is another main advantage. kalasalingam university*[email protected]
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DISADVANTAGES OF EHF
One of the most significant limitations of the process is the energy rating of the capacitor bank itself, and the amount of energy which can be dumped by the triggering device is another. Materials having critical impact velocities below 30 meters per second are not practical for electohydraulic forming. Neither is EHF of parts from materials having low ductility, such as the titanium alloys, likely to be successful.
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APPLICATIONS OF EHF:
A wide range of part sizes are producible by EHF, and most materials can be worked. The process is widely accepted in aerospace industries to accomplish bulging , forming, beading, drawing, blanking and piercing. kalasalingam university*[email protected]
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MAGNETIC PULSE FORMING(MPF)kalasalingam university*[email protected]
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One of the earliest developments in producing a short duration, high intensity magnetic field was reported in 1924. A special storage battery was used as the energy source.Harvey and Brower, around 1958, demonstrated the application of a magnetic system for forming metals. In 1962, first magnetic commercial forming machine was marketedUsing this technique, it is possible to apply to a metallic work piece a powerful, uniform magnetic pulse to swage and expand tubular forms, as well as to coin, shear and form flat sheets.
INTRODUCTIONkalasalingam university*[email protected]
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PRINCIPLE OF OPERATION:A basic magnetic pulse metal forming circuit consists of -An energy storage capacitor, -A switch -A coil -A power supply that provides energy to charge a capacitor
FIGURE : Schematic illustration of the magnetic-pulse-forming process. The part is formed without physical contact without physical contact with any objectkalasalingam university*[email protected]
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PRINCIPLE OF OPERATION:An insulated induction coil is either wrapped around, or placed within the work, depending on whether the work piece (metal) is required to be squeezed inward or bulged outward. The coil is shaped to produce the desired shape in the work. The power source is a capacitor bank.
Magnetic pulse forming machines generate the very high currents required to produce the magnetic fields by discharging the capacitor bank through the coil.
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As a very high momentary currents are passed through the coil an intense magnetic field is developed that causes the work to collapse, compress, shrink or expand depending upon the designs and placement of the coil.
Energy storage capacity and ability of the unit to utilize that energy determines the size of the work that can be formed. Highly conductive metals can be formed easily. Non conductive or low conductivity materials can be formed if they are wrapped or coated with a high conductivity auxiliary material.
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ADVANTAGES OF MPF OVER CONVENTIONAL FORMING:
The pressure is applied to the work piece through the medium of a magnetic field without any physical contact.Since no static forces are involved in the process, relatively high structures may be used for the support of dies.Since there is no friction between the magnetic field and the work piece, no lubricants are required and the process leaves no tool marks.Machines can be designed for repetition rates of hundreds of operations per minute.
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LIMITATIONS OF MPF :
The configuration of the work piece must be such that the area to be formed constitutes a closed loop, as in the case of rings, tubes, circular areas on flat work pieces etc., thus permitting the induced currents to flow. Slits interrupting the paths of the induced currents have a detrimental effect on MPF process.
It is not possible to apply high pressures in an arbitrarily chosen area while applying a low pressure in a closely adjacent area. kalasalingam university*[email protected]
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APPLICATIONS OF MPF :
The magnetic pulse forming process has been primarily applied in the forming of excellent conductors, such as aluminum, copper, brass and low carbon steel. It has also been used effectively to form poor conductors such as stainless steel for use with highly conductive sheaths. MPF is used widely to expand, compress or form tubular shapes and has also been used effectively to form conical, ellipsoidal and flat work pieces and assembly operations in a single step. The method is also used for piercing, shearing, embossing, cupping, sizing, banding etc. Tubular members can be joined to end fittings by magnetic swaging. kalasalingam university*[email protected]
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PNEUMATIC MECHANICAL HIGH VELOCITY FORGING kalasalingam university*[email protected]
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Just as a turret lathe can be set up to machine hundreds of identical parts, so also pneumatic mechanical, high velocity forging machine can be set up to produce hundreds of identical forging without depending on the skill of the operator. High velocity forging is a closed die hot or cold forging process which unlike conventional forging, deforms work metal at unusually high velocities. Identify, the final configurations of the forging is developed in one blow, or at most, in a few blows.
High velocity forging can be used for hot forging of the parts of the same general shapes as those produced in conventional forging hammers and presses.
COMPARASION WITH CONVENTIONAL FORGINGkalasalingam university*[email protected]
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The forces developed in high velocity forging are many times more powerful and are sufficient to shape hard to work materials.
High velocity forging, single blow, hammers require less moving weight than do conventional hammer to achieve the same impact energy per blow.
This machine also requires considerably less space than the conventional forging press. COMPARASION WITH CONVENTIONAL FORGINGkalasalingam university*[email protected]
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Pneumatic-mechanical forging machines use:the energy stored in a compressed gas e.g. nitrogen the energy released from burning fuel (petrol) oxidizer mixture to accelerate a ram to a high velocity for accomplishing a deformation stroke or hammer.
PRINCIPLE OF OPERATION:kalasalingam university*[email protected]
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In the gas operated pneumatic mechanical machines, energy in obtained by expandingA High pressure gas to drive a piston down a cylinder. In most machines, thepressurized gas is stored and made to act on the top of the ram with the help of quickrelease mechanism for rapid acceleration of the ram. The gas accelerates the ram tohigh speeds. During the process there is a very rapid build up on the work piece.Subsequent to each hammer blow the gas is recompressed. The original quantity ofgas is used time after time or no loss, resulting in very low operating cost. Repeatability is an important feature. PRINCIPLE OF OPERATION:kalasalingam university*[email protected]
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There is an inner frame and an outer frame within which the inner frame is free to move vertically. As the trigger gas steel is opened, high pressure gas from the firing chamber acts from the top face of the ram and forces the ram and the upper die downward. Reaction to the downward Acceleration of the ram raises the inner frame and lower die. The machine is made ready for the next blow by means of hydraulic jacks that elevate the ram until the trigger gas seal between the upper surface of the firing chamber and the ram top is re established. The machine uses air springs and thus the shock is not transmitted to the floor. Therefore, a special foundation is not needed, and the ,machine can be placed directly on the factory floor. PRINCIPLE OF OPERATION:kalasalingam university*[email protected]
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ADVANTAGES OF PNEUMATIC-MECHANICAL FORGING :
Metals difficult to forge by other methods can be forged successfully. Complex parts can be forged in one blow.Forging are made to size or within a minimum of machining allowance.Dimensional accuracy, surface details and often, surface finish are improved. Draft allowances can be reduced.Severe deformation results in greater gain refinement in some metals. Deep, thin sections can be forged.
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DISADVANTAGES OF PNEUMATIC-MECHANICAL FORGING :
Process generally is limited to symmetrical parts. Sharp corners and small radii can not be forged without causing undue die wear. The production rate is about the same as in hammer or hydraulic press forging, but slower than in mechanical press forging. Part size is limited to about 10 kg fore carbon steel forging, and to lesser weights for forging made of stainless steel or heat resisting alloys.
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APPLICATIONS OF PNEUMATIC-MECHANICAL FORGING :
Symmetrical or concentric forging such as gears and wheels. Parts with thin webs and high rib height to width ratios. Forging of super alloys, refractory alloys, as cast materials, low alloy steels, aluminum alloys, titanium alloys, stainless steels etc. Turbine blades and turbine wheels. Valve bodies.Rifle parts.Missile components.Housings for electronic devices.Engine housings.Rocket nozzle inserts, etc.
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ECONOMIC CHARACTERISTICSWhen making a cost comparison to aid in selecting in between one of these special processes and a conventional one, or when choosing between two different special processes, each of which appears capable of accomplishing the necessary forming, the engineer is advised therefore, to contact manufacturers of the respective equipment to obtain the most current information available. kalasalingam university*[email protected]
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ECONOMIC CHARACTERISTICSExperience indicates that the HVF processes are generally more useful in the production of smaller quantities of parts. For the production of small number of parts, the unit cost makes a HVF operation more favorable because of the generally lower initial cost of tooling, capital equipment etc. However as the number of parts is increased and the unit cost for the conventional method becomes the lower .In industries that produce extremely large numbers of parts, such as the stamping industry, are not economically feasible for most HVF techniques.
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CAPABILITY OF HVF PROCRSSESkalasalingam university*[email protected]
Characteristics Electro hydraulic FormingHigh Explosive(stand-off)Bridge wireSpark dischargeMetal working operationsTube bulging, drawing sizing, flanging, coining , blanking, stretchingTube bulging, sizing, flanging, expanding, coining , blanking, stretching, embossingDraw forming, sizing, flanging, expanding, coining ,blanking,stretching,embossing,beading,joiningSize limitations6.1542mm diameter or larger6.1542mm diameter or largerLimited only by available blank sizeShape complexityComplex surface and shapes, especially tubularComplex surface and shapes, especially tubularSmall and intricate, large and simpleCapital investmentModerateModeratelowTooling costlowlowlowLabor cost moderatemoderatemoderateProduction rate 360 parts per hr depending on part and equipment 360 parts per hr depending on part and equipment 0.5-4 parts per hr or less depending on part and facility
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CAPABILITY OF HVF PROCRSSESkalasalingam university*[email protected]
Characteristics Electro hydraulic FormingHigh Explosive(stand-off)Bridge wireSpark dischargeCycle timeLong mediummediumEnergy costslowlowhighSafety considerationsEquipment interlocks, high voltage safety practices, trained personnelEquipment interlocks, high voltage safety practices, trained personnelTrained personnelMethod of energy releaseVaporization of wiresVaporization of medium Chemical detonationPressure wave velocity20,000(6096)20,000 (6096)4,000 to 25,000(1219-7620)Pressure wave durationmicrosecondsmicrosecondsmicrosecondsEnergy range(KJ)20,000 to 175,000(27 to 237)10,000 to 110,000(13.5 to 150 )100,000 to 2,000.000(136-2712) per lb of explosive, up to 100 lb 45 kg detonatorWork-piece deformation velocity(m/s)50 to 700 (15-213)50 to 700 (15-213)60 to 400 (18 to 122)Energy transfer mediumWater or other suitable liquidWater or other suitable liquidWater, elastomers sand, molten salts
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CAPABILITY OF HVF PROCRSSESkalasalingam university*[email protected]
Characteristics High Explosive(direct contact)ElectromagneticMechanical-Pneumatic forgingMetal working operationsdraw forming, sizing, flanging, expanding, coining , blanking, stretch forming,embossing,beading, cutting, powder compacting, joining,Hardening,welding, perforating, cladding, powder compactingHot, warm, cold forming water compacting, powder forging, cropping, blanking, piercing, coiningSize limitationsPart size not limiting0.1-72(2.5-1830mm) diam. and larger in some operationsUp to 24 (610mm) diam. larger on future machinesShape complexitySimple shapesCompound surfaces, corrective forming on large complex shapesComplex shapes thin forged sectionsCapital investmentlowModerate to highmoderateTooling costNone to lowHigh if work coil is regarded as part of toolingmoderateLabor cost moderateModerate to lowmoderateProduction rate 0.5-4 parts per hr or less depending on part and facilityUp to 12,000 parts per hr for simple parts and automated transfer equipment60 to 100 parts per hr with automatic equipment, depending upon part complexity
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ECONOMIC CHARACTERISTICSkalasalingam university*[email protected]
Characteristics High Explosive(direct contact)ElectromagneticMechanical-Pneumatic forgingCycle timemediumMedium to shortlongEnergy costshighlowmoderateSafety considerationsTrained personnelEquipment interlocks, high voltage safety practices, trained personnelGuards and shields, trained personnelMethod of energy releaseChemical detonationExpanding magnetic fieldQuick-release valvePressure wave velocity4,000 to 25,000(1219-7620)Not applicableNot applicablePressure wave durationMicrosecondsMicroseconds Microseconds Energy range(KJ)0.5-8 psf high explosive 0-175,00(0-237)Up to 400,000(542)Work-piece deformation velocity(m/s)Not applicable50-1000(15-305)50-700(15-213)Energy transfer mediumDirect contact or buffer materialMagnetic field (to be operated in vacuum)High velocity ram
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REFERENCES :Production Technology by O.P.Khanna, Dhanpat Rai Publishers,1998.Materials and Processes in Manufacturing by DeGarmo, Black & Kosher, Prentice-Hall Publishers, 1997archive.metalformingmagazine.com/1997/01/7mfjan5.htm www.metalwebnews.com/howto/explosive-forming/explosive-forming.html en.wikipedia.org/wiki/Electromagnetic_forming www.fsb.hr/deformiranje/Presentation_Wentzel_TNO_PML_Rijswijk.pdfweb.mit.edu/3.082/www/team1_s03/presentation4.10.ppt www.engineershandbook.com/MfgMethods/ehforming.htm en.wikipedia.org/wiki/Electromagnetic_forming www.iap.com/metalfor.html
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