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 machining.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 & difficult to m/c ( by conventional methods)

metals such as hardened tool-steels, carbides, titanium, inconel & 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.

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.

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

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.

Wire EDM Process

Fig : Schematic illustration of wire EDM process.

• Wire EDM is a variation of EDM and is commonly known as wire-cut EDMor wire cutting.

• In WEDM, a thin metallic wire is fed on-to the workpiece, which issubmerged in a tank of dielectric fluid such as deionized water.

• WEDM can cut plates as thick as 300mm and is used for making punches,tools & dies from hard metals that are difficult to machine by othermethod

• The wire, which is constantly fed from a spool, is held b/w upper & lowerdiamond guides.

• As many as 50 hours of machining can be performed with one reel of wire,which is then discarded.

• The guides are usually CNC-controlled andmove in the x–y plane.

• Greater flexibility due to independentmovement of upper guide helps inprogramming the wire-cut EDM, forcutting very intricate and delicate shapes.

Applications of Wire EDM

The process is used in the following areas:• Aerospace, Medical, Electronics and Semiconductor

applications• Tool & Die making industries.• For cutting the hard Extrusion Dies• In making Fixtures, Gauges & Cams• Cutting of Gears, Strippers, Punches & Dies• Manufacturing hard Electrodes.• Manufacturing micro-tooling for Micro-EDM, Micro-USM

and such other micromachining applications.

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 is localized, non-contact thermal machining and is almostreaction-force free.

• LBM uses a laser beam (narrow beam of intense monochromatic light) tomachine or cut required shapes or profile in almost all materials.

• The laser beam is focused onto the work-piece and can be moved.

• Photon energy is absorbed by target material in the form of thermalenergy or photochemical energy.

• The high amount of heat thus generated either melts, burns, or vaporizesaway the material at the focused region.

• Any material that can properly absorb thelaser irradiation can be laser machined e.g.hard and brittle materials as well as softmaterials such as metals, ceramics, leatheretc.

Applications of LBM

• LASER, find applications in following areas:

• Heat treatment, Welding, Machining, Material deposition, Lithography, Surgery ,Photo-polymerization µ-stereo-lithography, Laser-enhanced jet plating and etching ,

• LBM is used for Ablation or cutting of plastics, glasses, ceramics, semiconductors and metals.

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 of LBM

Advantages

• Ability to cut almost all materials including fragile materials.

• 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.

• Laser produces high quality cuts without extra finishingrequirements.

Disadvantages of LBM

• Disadvantages

• Laser processes involve high capital investments and highoperating costs.

• Laser holes are tapered to some extent (approximately 1% of the drill depth)

• It cannot drill blind holes to precise depths.

• Heat of lasers may change mechanical properties of metallic materials & alloys in HAZ

• Long processing time in larger holes due to trepanning action.

• Reflected laser lights can lead to safety hazards.

• Assist or cover gases are required for safety purposes.

Applications of LBM

• Applications

• Electronic and automotive industries.

• LBM is used for making very accurate sized holes as small as 5 microns in metals, ceramics and composites without wrapage.

• It is widely used for fine & accurate drilling & cutting of metallic & non-metallic materials.

• LBM can cut wood, plastics, leather, glass, ceramics and fiber optics.

Electron Beam Machining (EBM)

• In EBM, material is removed by means of a focused beam of high velocityelectrons that strike the workpiece.

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

• A magnetic lens focuses the electron beam onto the workpiece, and highenergy densities of the order of 108W/cm2 are produced.

• K.E. of the electrons is converted into heat that is sufficient to causerapid melting and vaporization of the workpiece material .

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

Electron-beam

• To eliminate scattering of the beamof electrons, the work is done in ahigh-vacuum chamber.

• Electron-beam machining can processa wide range of materials.

Advantages of EBM

Advantages:

1. Extremely high energy density of the beam and the shortduration 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 workpiecesize is limited to the size of the vacuum chamber.

2. Holes are tapered 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 needremoval by abrasive cleaning

Applications of EBM

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 andlow thermal conductivity.

3. To drill extremely small hole of diameter 0.03 mm in turbine bladesfor transpiration cooling, holes in mixer plates, combustion chamberrings.

Principle of ECM

• ECM is opposite of electrochemical or galvanic coating ordeposition process.

• ECM technique removes material by atomic level dissolution ofthe same by electrochemical action.

• Thus ECM can be thought of a controlled anodic dissolutionat atomic level of the work piece that is electricallyconductive by a shaped tool due to flow of high current atrelatively 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

Electrochemical Machining

Figure 27.6 Schematic illustration of the electrochemical machining process.

Electrochemical Machining

• ECM is controlled removal of metal by anodic dissolution in anelectrolytic medium in which workpiece is anode and tool is cathode.

• Two electrodes are placed closely with a gap of about 0.5mm andimmersed in an electrolyte which is a solution of sodium chloride.

• When an electrical potential of about 20V is applied betweenelectrodes, ions existing in electrolyte migrate towards electrodes.

• Positively charged ions are attracted towards the cathode and thenegatively charged ions are attracted towards the anode.

• This initiates the flow of current in the electrolyte.

• The rate at which the ions arrive at their respective electrodedetermines density of current and varies in direct proportion toapplied voltage, number of ions present & gap between electrodes.

• The electrolysis process that takes place at the cathode liberateshydroxyl ions (negatively charged) and free hydrogen.

• The hydroxyl ions combine with the metal ions of anode to forminsoluble metal hydroxides & material is thus removed from the anode.

• This process continues and tool reproduces its shape in the workpiece

• In ECM, material removal occurs due to atomic dissolution of work (conductive) material.

• Electrochemical dissolution is governed by Faraday’s laws.

• Electrical energy in combination with chemical reactions to remove material.

• Simply ECM is 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 tool (negative pole) in an electrolyte bath.

• Electrolyte flows rapidly between the two poles to carry off deplated material, so it does not plate onto tool.

• Electrode materials: Cu, brass, or stainless steel

• Tool has inverse shape of part : Tool size & shape must allow for gap.

• No surface damage, no burr, low tool wear, high MRR for hard-to machine materials

Processes Variant:

Processes Variant

Processes Variant:

– Electrochemical machining (ECM)

– Electrochemical deburring (ECD)

– Electrochemical grinding (ECG

Advantage of ECM

Advantages:

1. MRR is not dependent on mechanical or physical properties of WP.

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 m/ced.

5. Cutting tool can be made from soft material.

6. Low heat generated during process.

7. No cutting forces.

8. Excellent surface finish.

Disadvantages:

1. Since ECM depends on atomic weight & valency of work materialtherefore work material should be electrically conductive.

2. Low machining accuracy due to its wider machining gap.

Applications of ECM.

Application:

1. ECM can machine any electrically conductive workmaterial irrespective of their hardness, strength oreven thermal properties.

2. Moreover as ECM leads to atomic level dissolution, thesurface finish is excellent with almost stress freemachined surface and without any thermal damage sofragile and thin section can be machined.

3. ECM is used for : Die sinking, Profiling and contouring,Trepanning , Grinding , Drilling , Micro-machining etc.

Die sinking 3D Profiling

ECM 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

Parts Made by Electrochemical Machining

Fig: 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.

Case Study: Knee Implants

Fig: (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.

Electrochemical-Grinding Process

Fig: (a) Schematic illustration of the electrochemical-grinding process.

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

• In ECG, Electrode is in the form of rotating grinding wheel.

The End

Plasma Arc Machining /Cutting (PAM/PAC)

• PAM is a thermal cutting process which uses plasma (a superheated,electrically ionized gas) to cut metals.

• In PAM arc formed between the tungsten electrode and the workpieceis constricted by a fine bore, copper nozzle.

• Nozzle increases the temperature (≈ 20 000°C) and velocity (≈ speed ofsound) of the plasma emanating from the nozzle.

• On initiation, the pilot arc is formed within the body of the torchbetween the electrode and the nozzle.

• For cutting, the arc must be transferred to theworkpiece and the plasma gas flow is increased sothat the deeply penetrating plasma jet cutsthrough the material & molten material isremoved by efflux plasma.

• Electrode (tungsten) has a negative polarity andworkpiece a positive polarity so that majority ofarc energy (≈ two thirds) is used for cutting.

• It can be applied for cutting conductive metals whichform refractory oxides e.g. SS, Al, CI, Ni, Cu & Tialloys .

Process variants

Dual gas Water injection Water shroud

Air plasmaHigh tolerance plasma

Plasma Arc Cutting Vs Oxy Fuel Cutting

• PAC differs from oxy-fuel process. Plasma process operatesby using the arc to melt the metal whereas in the oxy-fuelprocess, oxygen oxidises the metal and heat from theexothermic reaction melts the metal.

• Thus, unlike the oxy-fuel process, plasma process can beapplied for cutting conductive metals like stainless steel,aluminium, cast iron and nickel based materials, copper alloys,titanium alloys .

•

Advantages of PAC

Advantages :

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 of PAC

Disadvantages :

1.Faster electrode and nozzle wear .

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

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

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

Applications of PAC

Applications:

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

• Unlike the oxy-fuel process, the plasma process can beapplied for cutting conductive metals which formrefractory oxides such as stainless steel, aluminium,cast iron and nickel based materials, copper alloys,titanium alloys .

Chemical Milling

Fig: (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.

• 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

Chemical Milling

Process capabilities:

• Chemical milling used in the aerospace industry

• Tank capacities for reagents are as large as 3.7mx15m

• Process also used for micro electronic devices

• Surface damage may result due to preferential etchingand intergranular attack

Chemical blanking:

• Chemical blanking is similar to chemical milling

• Material is removed by chemical dissolution ratherthan by shearing

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

Chemical Blanking

Parts Made by Chemical Blanking

Fig: Various parts made by chemical blanking. Note the fine detail. Source: Courtesy of Buckbee-

Mears, St. Paul.

• Photochemical blanking Modification of chemical milling

• Material removed from flat thin sheet by photographictechniques

• 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 violetlight which hardens the exposed area

• Blank is developed which dissolves the exposed areas

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

• The masking material is removed and the part is washedthoroughly to remove all chemical residues

Photochemical Blanking

Chemical-Machining

Fig (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.