Manufacturing Technology II(ME-202)

Nontraditional Machining

Dr. Chaitanya Sharma

PhD. IIT Roorkee

Title of slide

Lesson ObjectivesIn this chapter we shall discuss the following:1. Electro Chemical Machining.2. Electro discharge machining3. Electron beam machining.4. Laser beam machinin.5. process parameters; Applications;

Advantages and limitations

Learning Activities1. Look up

Keywords2. View Slides; 3. Read Notes, 4. Listen to

lecture

Keywords:

Parts Made by Advanced Machining Processes

Figure :Examples of parts produced by advanced machining processes. (a) Samples of parts

produced from water jet cutting. (b) Turbine blade, produced by plunge EDM, in a fixture to

produce the holes by EDM.

(a)(b)

Electrical Discharge Machining

• EDM is a thermal process which makes use of spark discharges to erode

material from the workpiece. Since spark discharges occur in EDM, it is

also called as "spark machining".

• The material removal takes place in EDM through a rapid series of

electrical discharges, passing between the electrode and the workpiece

being machined.

• The fine chips of material removed from the workpiece gets flushed away

by the continuous flowing di-electric fluid.

• Repetitive discharge creates a set of successively deeper craters in the work

piece until the final shape is produced.

• The cavity formed in EDM is a replica of the tool shape used as the

erosions occur in the confined area.

• EDM process is best suited for making intricate cavities and contours in

electrically conducting hard and difficult to machine ( by conventional

methods) metals such as hardened tool-steels, carbides, titanium, inconel

and kovar.

Electrical-Discharge Machining Process

Figure (b) Examples of cavities produced by EDM process, using shaped electrodes. Two round parts (rear) are the set

of dies for extruding the aluminum piece shown in front (c) A spiral cavity produced by EDM using a slowly rotating

electrode similar to a screw thread. (d) Holes in a fuel-injection nozzle made by EDM; the material is heat-treated steel.

Figure : Schematic illustration of the electrical-discharge machining process.

Operation of EDM

• In EDM process, the workpiece and tool are submerged into a non-conducting,dielectric fluid which is separated by a small gap (for sparking).

• Dielectric fluid (hydrocarbon oil or de-ionized water) insulates the workpiece fromthe tool and creates the resistance to the flow of electricity between the electrodes.

• It also helps in cooling down the tool and workpiece, clears the inter-electrode gap(IEG), and concentrates the spark energy to a small CS area under the electrode.

• As the two electrodes come closer to one another, the electric field intensity increasesbeyond the strength of the dielectric enabling it to break and thereby allow thecurrent to flow between the two electrodes.

• As a result of this effect, intense heat gets generated near the zone, which melts andevaporates the material in the sparking zone. As the flow of current is momentarilystopped, some fresh dielectric liquid particles come in position between the inter-electrode gap which restores the insulating properties of the dielectric.

• The solid particles (debris) are carried away by the flowing dielectric. Flushing refersto the addition of new liquid dielectric to the inter-electrode volume.

• The sparks occur at spots where the tool and the workpiece surfaces are the closestand since the spots change after each spark (because of the material removal aftereach spark), the spark travels all over the surfaces. This results in uniform removal ofmaterial, hence exact shape get reproduced on the workpiece

Mechanism of Material Removal in EDM Process

The material removal process by a single spark is as follows:• An intense electric field develops in the gap between electrode and workpiece.• There are some contaminants inside the dielectric fluid which build a high

conductivity bridge between the electrode and workpiece.• When the voltage increases, the bridge and dielectric fluid between the

electrode and workpiece heat up.• The dielectric is ionized to form a spark channel.• The temperature and pressure rapidly increase and a spark is generated.• A small amount of material is evaporated on the electrode and workpiece at the

spark contact point.• Bubbles rapidly expand & explode during sparking until voltage is turned off.• Next the heating channel collapses and the dielectric fluid enters into the gap• in-order to flush away the molten metal particles.

Advantages of EDM

• Any materials that are electrically conductive can bemachined by EDM, regardless of their hardness, strength,toughness and microstructure

• Tool (electrode) and workpiece are free from cutting forces.

• Edge machining & sharp corners are possible in EDM process

• The tool making is easier (copper, brass and graphite).

• The process produces good surface finish (SF) (free fromburr), accuracy and repeatability.

• Hard die materials with complicated shapes can be easilyfinished with good SF and accuracy.

• Due to the presence of dielectric fluid, there is very littleheating of the bulk material.

Advantages of EDM

Limitations of EDM

• Low MRR, process is economical only for very hard and difficult to machine materials.

• Poor surface quality due to re-cast layers and micro-cracks, needs further polishing .

• The EDM process is not suitable for non-conductors.

• Rapid electrode wear makes the process more costly.

Disadvantages of EDM

Applications of EDM

• EDM is used for making die for wire drawing, extrusion, heading, forging etc. from hardened steel and stamping tool with intricate cavities.

• In machining of exotic materials that are used in aerospace and automatic industries.

• For making fragile parts which cannot take the stress of machining.

• Deep cavities, slots and ribs can be easily made by EDM for collets, jet engine blade slots,

• Micro-EDM process can successfully produce micro-pins, micro-nozzles and micro-cavities.

Applications of EDM

Stepped Cavities Produced by EDM Process

Figure : Stepped cavities produced with a square electrode by the EDM process. The

workpiece moves in the two principle horizontal directions (x – y), and its motion is

synchronized with the downward movement of the electrode to produce these cavities.

Also shown is a round electrode capable of producing round or elliptical cavities.

The Wire EDM Process

Figure : Schematic illustration of the wire EDM

process. As many as 50 hours of machining can

be performed with one reel of wire, which is

then discarded.

Wire EDM is a variation of EDM and is commonly known as wire-cut EDM or wire cutting.In this process, a thin metallic wire is fed on-to the workpiece, which is submerged ina tank of dielectric fluid such as deionized water.WEDM can cut plates as thick as 300mm and is used for making punches, tools and diesfrom hard metals that are difficult to machine with other methods.The wire, which is constantly fed from a spool, is held b/w upper & lower diamond guides.The guides are usually CNC-controlled and move in the x–y plane.Greater flexibility due to independent movement of upper guide helps in programming thewire-cut EDM, for cutting very intricate and delicate shapes.

The process is used in the following areas:Aerospace, Medical, Electronics andSemiconductor applicationsTool & Die making industries.For cutting the hard Extrusion DiesIn making Fixtures, Gauges & CamsCutting of Gears, Strippers, Punches & DiesManufacturing hard Electrodes.Manufacturing micro-tooling for Micro-EDM,

Micro-USM and such other micromachiningapplications.

Wire EDM

Figure : Cutting a thick plate with wire EDM.

(b) A computer-controlled wire EDM M/c.

Laser-Beam Machining (LBM)Laser (Light Amplification by the Stimulated Emission of Radiation) machining islocalized, non-contact thermal machining and is almost reaction-force free.

LBM uses a laser beam (narrow beam of intense monochromatic light) to machine orcut required shapes or profile or pattern in almost all types of materials.

The laser beam is focused onto the work-piece and can be moved relatively to it.Photon energy is absorbed by target material in the form of thermal energy orphotochemical energy.

The high amount of heat thus generated either melts, burns, or vaporizes away thematerial at the focused region.

Any material that can properly absorb the laser irradiation can be laser machined. Thespectrum of laser machinable materials includes hard and brittle materials as well assoft materials. Some of the examples include metals, ceramics, leather etc.

LBM, find applications in following areas:Heat treatment, Welding, Ablation or cuttingof plastics, glasses, ceramics, semiconductorsand metals , Material deposition, Laser-enhanced jet plating and etching ,Lithography, Surgery ,Photo-polymerizationµ-stereo-lithography

General Applications of Lasers in Manufacturing

Gas is blown into the cut to clear

away molten metals, or other

materials in the cutting zone.

In some cases, the gas jet can

be chosen to react chemically with

the workpiece to produce heat and

accelerate the cutting speed (LACE)

Advantages, Disadvantages & Applications of LBMAdvantages

Ability to cut almost all materials including fragile materials (which are easily cut by laser)

No limit to cutting paths as the laser point can move in any paths.

No direct contact b/w tool and workpiece; thus no need of the work holding system.

Flexibility exists in precision cutting of simple or complex parts.

There is no tooling cost or associated wear costs.

Electron Beam Machining (EBM)

In EBM, material is removed by means of afocused beam of high velocity electronsthat strike the workpiece.

Electrons are emitted from an electron gunand are accelerated to speeds (200,000km/s ) of about 75% of the speed of light,by voltages as high as 50 kV between theanode and cathode.

A magnetic lens focuses the electron beamonto the workpiece, and energy densities of2 the order of 108W/cm2.

The kinetic energy of the electrons is

As a result of extremely high energy density of thebeam and the short duration of beam-workpieceinteraction, thermal effects on the workpiece materialare limited to a heat affected zone that seldomexceeds 0.025 mm in depth.

High beam-power density also enables high aspect ratio holes to be drilled, often as large as 15 to 1.

Figure: Schematic illustration of the electron-beam machining process.

Electron-beam

Advantages, disadvantages & Applications of EBMAdvantages:

1. Extremely high energy density of the beam and the short duration of beam-workpiece interaction,

2. Limited thermal effects on the workpiece material,

3. Extremely limited and small HAZ ( 0.025 mm in depth).

Disadvantages:

1. Unlike LBM, this process requires a vacuum, so workpiece size is limited to the sizeof the vacuum chamber.

2. Holes are taperd if thickness is more than 0.13 mm.

3. Cumbersome and expensive process.

4. Low MRR.

5. Recast and metal spatter from the spot of beam strike need removal by abrasivecleaning

Applications:

1. EBM is mainly used for micromachining operations such as drilling, perforating,slotting,, engraving etc. on thin materials.

2. EBM is suited particularly for materials with high melting points and low thermalconductivity.

3. To drill extremely small hole of diameter 0.03 mm in turbine blades fortranspiration cooling, holes in mixer plates, combustion chamber rings.

Plasma Arc CuttingPlasma - a superheated, electrically

ionized gasPAC uses a plasma stream. operating

at temperatures in the range from18,000o - 25,000o F to cut metal

The high-velocity plasma stream isdirected at the workpiece, melting itand blowing the molten metal throughthe kerf

Plasma Arc Machining /Cutting (PAM/PAC)

PAM is a thermal cutting process whichuses plasma( ionized gas) to cut metals.

IN PAM/PAC arc formed between theelectrode and the workpiece is constrictedby a fine bore, copper nozzle whichincreases the temperature (20 000°C )andvelocity (speed of sound) of the plasmaemanating from the nozzle.

On initiation, the pilot arc is formedwithin the body of the torch between theelectrode and the nozzle. For cutting, thearc must be transferred to the workpiece

The process differs from the oxy-fuel process in that theplasma process operates by using the arc to melt the metalwhereas in the oxy-fuel process, the oxygen oxidises themetal and the heat from the exothermic reaction melts themetal.

Thus, unlike the oxy-fuel process, the plasma process can beapplied for cutting conductive metals which form refractoryoxides such as stainless steel, aluminium, cast iron andnickel based materials, copper alloys, titanium alloys .

Advantages, disadvantages & Applications of PACAdvantages :

1. PAC is a low cost alternative to oxy-fuel and laser profiling.

2. Superior quality, more versatile and enhanced accuracy.

3. Cut faster than oxy-fuel and LBM.

4. Economical due to faster production rate.

5. Wide range of materials (ferrous & nonferrous) & thickness ( up to 80 cm )

6. Easy to use

Disadvantages :

1. Faster electrode and nozzle wear .

2. Increased cost of operation due to frequent replacement of electrode & nozzle.

3. Cannot non-conductive materials such as wood or plastic.

4. Cut edges are beveled by 4-6 degree.

Applications:

• PAM/PAC is predominantly used in metal fabrication and sheet metal industries for cutting and gauging of hard materials.

• PAM s is used for cutting conductive materials such as carbon steel, stainless

• steel, aluminum

Process variants

Dual gas Water injection Water shroud

Air plasmaHigh tolerance plasma

• ECM is opposite of electrochemical or galvanic coating or depositionprocess. ECM technique removes material by atomic level dissolution ofthe same by electrochemical action.

• Thus ECM can be thought of a controlled anodic dissolution at atomic levelof the work piece that is electrically conductive by a shaped tool due to flowof high current at relatively low potential difference through an electrolytewhich is quite often water based neutral salt solution.

Fig. : schematically shows the basic principle of ECM.

Initial state of ECM Steady state of ECM

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Electrochemical Machining

Figure 27.6 Schematic illustration of the electrochemical machining process.

• In ECM, material removal takes place due to atomic dissolution of work material. Electrochemical dissolution is governed by Faraday’s laws.

• Electrical energy in combination with chemical reactions to remove material -Work material must be a conductor

• Reverse of electroplating– Part is the anode (+) and the tool is the cathode (-)

– Metal is “pulled” away from work

(positive pole) and transported to a cathode

Advantages:

1.MRR is not dependent on the mechanical or physical properties of the work material.

2.No surface damage, no burr, low tool wear,

3.High MRR for hard-to machine materials

4.Hard to soft materials made of conductive material can be machined.

5.Cutting tool can be made from soft material.

6.Low heat generated during process.

7.No cutting forces.Die sinking 3D Profiling

Electrochemical MachiningElectrochemical machining is the

controlled removal of metal by anodic dissolution in an electrolytic medium in which the workpiece is the anode and the tool is the cathode. Two electrodes are placed closely with a gap of about 0.5mm and immersed in an electrolyte which is a solution of sodium chloride. When an electrical potential of

about 20V is applied between the electrodes, the ions existing in the electrolyte migrate towards the electrodes. Positively charged ions are

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Parts Made by Electrochemical Machining

Figure 27.7 Typical parts made by electrochemical machining. (a) Turbine blade made of

nickel alloy of 360 HB. Note the shape of the electrode on the right. (b) Thin slots on a 4340-

steel roller-bearing cage. (c) Integral airfoils on a compressor disk.

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Knee Implants

Figure 27.8 (a) Two total knee replacement systems showing metal implants (top

pieces) with an ultra-high molecular-weight polyethylene insert (bottom pieces). (b)

Cross-section of the ECM process as applies to the metal implant. Source: Courtesy of

Biomet, Inc.

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Electrochemical-Grinding Process

Figure 27.9 (a) Schematic illustration of the electrochemical-grinding process. (b)

Thin slot produced on a round nickel-alloy tube by this process.

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Chemical Milling

Figure 27.2 (a) Missile skin-panel section contoured by chemical milling to improve the stiffness-to-

weight ratio of the part. (b) Weight reduction of space-launch vehicles by the chemical milling of

aluminum-alloy plates. These panels are chemically milled after the plates first have been formed

into shape by a process such as roll forming or stretch forming. The design of the chemically

machined rib patterns can be modified readily at minimal cost.

Chemical milling:

• Shallow cavities produced on plates, sheets, forgings, and extrusions

Procedure for chemical milling Steps :

1 – Residual stresses should relieved in order to prevent warping

2 – Surfaces to be thoroughly degreased and cleaned

3 - Masking material(tapes,paints,elastomers & plastics ) is applied

4 – masking is peeled off by scribe and peel technique

5 – The exposed surfaces are etched with etchants

6 – After machining the parts to be thoroughly washed to prevent further reactions with residue etchant

7 – rest of the masking material is removed and the part is cleaned and inspected

8 – additional finishing operations are performed on chemically milled parts

9 – this sequence is repeated to produce stepped cavities and various contours

Process capabilities:• Chemical milling used in the aerospace industry

• Tank capacities for reagents are as large as 3.7m x15m

• Process also used for micro electronic devices

• Surface damage may result due to preferential etching and intergranular attack

Chemical blanking:• Chemical blanking is similar to chemical milling

• Material is removed by chemical dissolution rather than by shearing

• Burr free etching of printed-circuit boards, decorative panels, thin sheet metal stampings as well as production of small and complex shapes

Photochemical blanking :

• Modification of chemical milling

• Material removed from flat thin sheet by photographic techniques

• Design is prepared at a magnification of 100x

• Photographic negative is reduced to the size of finished part

• Sheet blank is coated with photosensitive material (Emulsion)

• Negative placed over coated blank and exposed to ultra violet light which hardens the exposed area

• Blank is developed which dissolves the exposed areas

• Blank is then immersed into a bath of reagent or sprayed with the reagent which etches away the exposed areas

Steps for photochemical blanking

Chemical-Machining

Figure 27.3 (a) Schematic illustration of the chemical-machining process. Note that no forces

or machine tools are involved in this process. (b) Stages in producing a profiled cavity by

chemical machining; note the undercut.

• This process is reversal of the electro plating

• Electrolyte acts as current carrier

• High rate of electrolyte movement in tool work piece gap washes metal ions away from the work piece ( ANODE)

• This is washed just before they have a chance to plate on the tool ( cathode)

• Shaped tool made of brass , copper , bronze , or stainless steel

• Electrolyte is pumped at a high rate through the passages in the tool

• Machines having current capacities as high as 40,000 A and as low as 5A are available

Electrochemical machining

Process capabilities

• Used to machine complex cavities in high strength material

• Applications in aerospace industry,jet engines parts and nozzles

• ECM process gives a burr free surface

• No thermal damage

• Lack of tool forces prevents distortion of the part

• No tool wear

• Capable of producing complex shapes and hard materials

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Chemical-Machining

Figure 27.3 (a) Schematic illustration of the chemical-machining process. Note that no forces

or machine tools are involved in this process. (b) Stages in producing a profiled cavity by

chemical machining; note the undercut.

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Parts Made by Chemical Blanking

Figure 27.5 Various parts made by chemical blanking. Note the fine

detail. Source: Courtesy of Buckbee-Mears, St. Paul.

The End