Ion beam machining

Ion beam machining is generally a surface finishing process in which the material removal takes place by sputtering of ions.
It is also called the etching process. This is a different process from electric discharge, electron beam, laser beam and plasma arc machining. 

Working Principle :

This process is very simple. It consists of bombarding the work with accelerated ions which collide with the surface atoms of the work. Each bombarding ions, as a result of collisions, dislodges surface layer.
It consists of an electron gun discharging free electrons into a chamber filled with argon gas. The gas is ionized by electrons. The top of the chamber is known as ion-beam generating apparatus. At the other end, the workpiece is fixed to a table which can be oscillated and rotated so that different points on the work surface can be subjected to an ion beam.

Accuracy :

• Etching rates vary up to 2000 Å per min.
• Accuracy of the etching process is considerably high mainly due to the small amount of material removal.
• Tolerances in the vicinity of + 50  Å  to - 50  Å  are possible.

Applications of IBM :

• It is applied mostly in micro-machining of electronic components.
• Typical materials that can be etched included glass, alumina, quartz, crystal, silica, agates, porcelains, numerous metals, cermets and oxides.
• It is also be used to deposit materials such as platinum, tungsten and silicon oxide insulators on another material substrate.

Advantages of IBM :

• IBM is almost universal.
• No chemical reagents or etching are required.
• Etching rates are easily controlled.
• There is no undercutting as with another chemical etching process.

Disadvantages of IBM :

• IBM is relatively expensive.
• Etching rates are slow.
• No heat is generated so there is little possibility of some thermal or radiation damage.

Plasma arc machining

In this article, we have to discuss plasma arc machining how it works and its application and main advantages and also disadvantages of this process.

PAM : 

When a flowing gas is heated sufficiently high temperature to become partially ionized, it is known as 'plasma'. This virtually a mixture of free electrons, positively charged ions and neutral atoms.
PAM is a material removal process in which the material is removed by ionized gas of high temperature (11,000 to 30,000 C) which applied by a high-velocity jet on the workpiece.

Working principle :

The principle of plasma arc machining is in a plasma torch, known as the gun or plasma-Tron, a volume of gas such as H₂, N₂, O₂, etc. is passed through a small chamber in which a high-frequency spark (arc) is maintained between the tungsten electrode (cathode) and the copper nozzle (anode), both of which are water-cooled. In certain torches, an insert gas-flow surface rounding the main flame is provided to shield the gas from the atmosphere. The high-velocity electrons generated by arc collide with the gas molecules and produce dissociation of diatomic molecules of the gas resulting in ionization of the atoms and causing large amounts of thermal energy to be liberated. The plasma forming gas is forced through a nozzle duct of the torch in such a manner as to stabilize the arc. Much of the heating of the gas takes place in the constricted region of the nozzle duct resulting into relatively high exit gas velocity and very high core temperature up to 30,000 C. the plasma jet steams effectively blows the material and the high-velocity gas stream effectively blows the molten metal away.

Accuracy:

• This is a roughing operation to an accuracy of about 15 mm with a corresponding surface finish.
• Accuracy on the width of slots and diameter of holes is ordinary from +0.8 mm to -0.9 mm on 6 to 30 mm thick plates, and +3 mm to -3 mm on 100 to 150 mm thick plates.

Applications of PAM:

• This is chiefly used to cut stainless steel and aluminium alloys. 
• Profile cutting of metals, particularly of these metals and alloys, has been the most prominent commercial application of PAM. 
• PAM has been used successfully in turning and milling of materials which are hard and difficult to machine.

Advantages :

• The main advantages of this process are that it is almost equally effective on any metal, regardless of its hardness of refractory nature.
• There being no contact between the tool and work piece, only a simply supported work piece structure is enough.

Disadvantages :

• The main disadvantages of this process are the metallurgical change of the surface.
• Safety precautions are necessary for the operator and those in near by areas. 
• This adds additional cost. 

Electron beam machining

Electron beam machining is processed by a high-velocity focused stream of electrons which heats, melts and vaporizes the work material at the point of bombardment thus metal will be removed. 

The production of free electrons is obtained from thermo-electronic cathodes wherein metal are heated to the temperature at which the electrons have sufficient speed for escape and make the space around the cathode. Thus the electrons accelerate and are carried by an electric field and by controlled magnetic fields are using for focusing and concentration of that electric field. The kinetic energy of a beam of free electrons is transformed into heat energy thus the interaction of the electrons with the workpiece material. Therefore EBM is also called thermo-electric process.

Operation principle of electron beam machining :

A beam of electrons is emitted from the electron gun which is basically a triode consisting of

• A cathode which is a hot tungsten filament ( 2500 degree C ) emitting high negative potential electrons.
• A grid cup, negatively based with respect to the filament.
• An anode which is heats at ground potential, and through which the high-velocity electrons pass.

A gun is supplied with electric current from a high voltage D.C source. The flow of electrons is controlled by the negative bias applied to the grid cup. The electrons passing through the anode are accelerated to two-third of the velocity of light by applying 50 to 150 kV at the anode and this speed id maintained till they strike the workpiece.
A magnetic deflection coil is used to make the electron beam circular and its a cross-sectional diameter is 0.01 to 0.02 mm and deflect it anywhere.
A microscope with a magnification of 40 on the workpiece enables the operator to accurately locate the beam impact and observe the machining operation.
As the beam impacts on the workpiece surface the kinetic energy of high-velocity electrons is converted into the thermal energy and it vaporized the material at the spot of its impact.

The application of the above process is also found in electron-beam drilling in which an organic or synthetic backing material is sandwiched on the other side of the component.

Accuracy :
Tolerance is about 10% of slot width or hole diameter.
Taper about 4 degrees included angle 
Depth to diameter ratio can reach 20:1 with multiple pulses.
Heat affected zone of up to 0.03 mm deep has been observed.

What is electric discharge machining

Electric discharge machining also is known as spark erosion.
It is the process of material removal based on the principle of metals by an interrupted electric spark discharge between the electrode tool and workpiece.

Working principle :

The main components of the EDM process are : 

• Electric power supply
• The dielectric medium 
• The workpiece and tool
• A servomotor

The basic process of EDM is really quite simple. The workpiece and the tool are electrically connected to D.C electric power supply. The workpiece is connected to the positive terminal of the electric source so that becomes the anode. The tool is a cathode. A gap is known as the 'spark-gap' in the range of 0.005 to 0.05 mm is maintained between the work piece and the tool.

In the spark gap produced spark is visible evidence of the flow of electricity. This electric spark produces intense heat with temperatures reaching 8000 to 12000 degree Celsius. The spark is controlled by very carefully and localized so that it affects the surface of the material only. 
The EDM process can be used in two different ways: 
1. A preshaped or formed electrode used as tool usually made from graphite is shaped to the form of the cavity it is to reproduce. The formed electrode is fed vertically down and the reverse shape of the electrode is eroded (burned) into the solid workpiece. 
2. A continuous-travelling vertical-wire electrode used as a tool and its diameter of a small needle or less is controlled by the computer. It follows a programmed path to erode the workpiece or cut a narrow slot to produce the required shape. 


The electrode ( Tools ) :

The shape of the tool will be basically the same as that of the product desired except that an allowance is made for side clearance and overcut.
The electrode materials generally used can be classified as metallic materials, non-metallic materials and a combination of metallic and non-metallic materials. Usually, copper, yellow brass, zinc and graphite are used for tools. Some low wearing tools are also used like silver-tungsten, copper-tungsten and metallized graphite. For commercial applications copper is best suited for fine machining. Aluminium is used for die-sinking while cast iron for rough machining.
One of the advantages of EDM is due to the fact that a tool made of a material softer than the workpiece material and which is a good conductor of electricity can be used to machine material of any hardness.
The wear of the tool in the EDM process due to electron bombardment is inevitable. 
The tool wears rates to determine the machining accuracy, tool movement, and tool consumption. 
The tool wear is the function of the rate of metal removal, the material of the workpiece, current setting, machining area, gap between the tool and workpiece and the polarity of the tool.

Wear ratio = Volume of work material removed / Volume of electrode consumed

Wear ratio = Depth of cut / Decrease in usable of the electrode

The wear ratio for carbon electrodes is up to 100:1.

For copper 2:1 

For brass 1:1

For copper tungsten 8:1

Purpose of Dielectric fluids :

• It used as a coolant for the workpiece and the tool. 
• It works as an insulating material during the charging of the condenser, as a result, perfect condition for efficacious spark discharge and its conduction when ionized is obtained. 
• The eroded materials are carried away by this medium. 
• It is a coolant in quenching the spark and prevents the arcing. 

Requirements of Dielectric fluids :

• Remain electrically nonconducting until the required break-down voltage has been reached.
• Breakdown electrically in the shortest possible time once the breakdown voltage has been reached.
• Have a good degree of fluidity.
• Be cheap and easily available.
• Be capable of carrying away the swarf particles.
• Inflammable. 
• It should be a hydrocarbon compound. 
• It should not produce toxic gases or vapours during the operation. 

Accuracy :

Tolerance value ( +- 0.05 ) mm could be easily achieved by EDM normal production.

By close control of the several variables tolerance ( +-0.003 ), mm could also be achieved.

Taper value id about to 0.005 to 0.05 mm per 100 mm depth.

An overcut of 5 to 100 micron is produced.

In no wear machining using graphite electrode a surface finish within 3.2 microns can be achieved.

Application of EDM :

The EDM provides economic advantages for making stamping tools, wire drawing and extrusion dies, header dies, intricate mould cavities.

It extremely used in aerospace industries, refractory metals, hard carbides and hardenable steels.

Some of its applications are following below :

• Drilling of micro-holes.
• Thread cutting.
• Helical profile milling.
• Wire-cutting EDM.
• Rotary forming.
• Curved hole drilling.
• Vacuum tubes.

Electrochemical Grinding

Electro-chemical grinding is also called electrolyte grinding.

Electrolyte grinding is a modification of both the grinding and electrochemical machining.
In this process, machining is affected both by the grinding action and by the electro-chemical process. Hence, it may also be called mechanically assisted electrochemical machining.

In ECG the metal bonded grinding wheel impregnated with a non-conductive abrasive is made the cathode and the workpiece the anode as in ECM.
The electrolyte which is usually sodium nitrate, sodium chloride, sodium nitrite, potassium nitrite with a concentration of 0.150 to 0.300 kg/litre of water, is passed through a nozzle in the machining zone in order to complete the electrical bridge between anode and cathode.
The work and wheel do not make contact with each other because they are kept apart by the insulating abrasive particles which protrude from the face of the grinding wheel.
The electrolyte is carried past the work surface at high speed by the rotary action of the grinding wheel. metal is removed from the workpiece by the simultaneous electrolytic and abrasive action.

It can be seen that the process is similar to conventional grinding in that an abrasive grinding wheel is used, and the work is fed against the rotating wheel. In fact, 10% of the work is removed by abrasive cutting and 90% by electrolytic action.

The grinding wheel used are conventional shape and structure metal bond, diamond grit wheels are used for grinding tungsten carbide tips. carbon bond wheels are used upon the hard alloy steels such as the stainless steels. 

The machine is similar in design to surface grinder or tool and cutter grinder and equipment includes a tank, filter and pump for the supply of electrolyte and a power unit for delivering a heavy D.C current. The current applied is in the range of 50 to 3000 A at 4 to 10 V.

Accuracy :

• Tolerance is about +0.02 to -0.02 are held on the rather complex grinding operation.
• For closer tolerances, the proportion of material removed by abrasive should be increase.
• Surface finish is held in a range of 0.2 to 0.4 micron on carbide and 0.4 to 0.8 micron on steel.
• A sharp corner is difficult to obtain and minimum radius of 0.2 mm.

Applications :

• Any material which is electrically conductive may be ground by the electrolytic process.
• Mainly used for resharpening and reconditioning of carbide tools and other materials that are hard to grind.
• Grind and cut thin sections
• Grind difficult materials without distortion or burr.

For more to know about: Advantages and disadvantages of Electo-chemical Grinding

Chemical Machining

Chemical machining is stock removal process for the production of desired shapes and dimensions through selective or overall removal of material by a controlled chemical attack with acids or alkalis.

The metal is gradually transformed into metallic salt by chemical reaction and is ultimately removed in this form.

The process can be suitably applied to different types of operation such as milling on a milling machine, blanking, and engraving. The chemical machining process can be classified as :

• Chemical milling
• Chemical blanking 
• Chemical engraving

Chemical milling :

Chemical milling sometimes called chilling or contour or etching is used mainly to produce shapes by selective or overall removal of metal parts from the relatively large surface area.

The main purpose is to achieve shallow but complex profiles, reduction in weight by removing unwanted material from the surface as in the skin of an aircraft.

Chemical milling complete in four steps :

1. Cleaning
2. Masking
3. Etching
4. De-masking

Application of CHM :

• Chemical machining has been applied successfully in a great number of usages where the depth of material removal is critical to a few microns and tolerance are closed.
• The surface finish obtained in the process is in the range of 0.5 to 2 microns.
• One of the major application of CHM is in the manufacture if burr-free, intricate stampings.

You know to check it out: Advantages and disadvantages of chemical machining 

Ultrasonic Machining

Working Principle :
 
In ultrasonic machining, a tool vibrating longitudinally at 20 to 30 kHz with amplitude between 0.01 to 0.06 mm is pressed on the work surface with a light force.
As tool vibrates with a specific frequency, an abrasive slurry, usually a mixture of abrasive grains and water of definite proportion is made to flow under pressure through the tool-workpiece interface. This causes micro-indentation fracture on the material. 

Small abraded particles are removed along the surface which is perpendicular to the direction of the tool vibration. When the material has removed a cavity of the same profile of the tool face is formed. The abrasive particles gradually erode as the machining process continues. As a result, fresh abrasive particles are needed to be supplied in the machining zone. Abrasive particles associated with the liquid is fed to the m/c zone and it ensures the removal of the worn-out grains and material.

The commonly used abrasives are :

aluminium oxide ( alumina ), boron carbide, silicon carabid and diamond dust. 

The abrasive slurry is circulated to the work-tool interface by pumping. A refrigerated cooling system is used to cool the abrasive slurry to a temperature of 5 to 6 degree C. A good method is to keep the slurry in a bath in the cutting zone. The liquid to produce abrasive slurry should have the following characteristics :

• Good wetting characteristics
• Low viscosity
• High thermal conductivity
• Anti-corrosive property
• Low cost

The size of abrasive varies in between 200 and 2000 grit. Course grades are good fro roughing whereas finer grades (1000 grit) are used for finishing.

Cutting rate of USM depending upon the following factors :

• The grain size of abrasive
• Abrasive materials
• Concentration of slurry
• Amplitude of vibration
• Frequency

Accuracy :

• The maximum speed of penetration in soft and brittle materials such as soft ceramics are of the order of 20 mm/min but fro hard and brittle materials penetration rate is lower. 
• Dimensional accuracy upto (+0.005 to -0.005 mm).
• Minimum corner radius of 0.10 mm is possible in finish machining. 

Applications :

• In performing machining operations like drilling grinding, profiling and milling operation on all materials both conduction and non-conducting also.
• In machining of glass, ceramics, tungsten and other hard carbides.
• In making tungsten carbide and diamond wire drawing dies and dies for forging and extrusion processes.
• For USM is used in drill a hole in teeth of any shape without creating any pain.
• Circular, as well as non-circular holes, can be done with straight or curved axes.
• Fabrication of silicon nitride turbine blades.

 Limitations of USM :

• The machining rate is extremely low when compared to the conventional machining process.
• The power consumption is also very high when compared to other similar processes.
• This method is limited for the machining of small workpieces only.
• Machining of deep holes is difficult, as the slurry movement is restricted.
• The tool wear is relatively high.

Recent Development in USM :

Recently a new development in USM has taken place in which a tool impregnated with diamond dust is used and no slurry is used. The tool oscillates at ultrasonic frequencies as well as rotated. If it is not possible to rotate the tool the workpiece may be rotated.

This innovation has removed some of the drawbacks of the conventional process of drilling deep holes. for instance, the hole dimensions can be kept within +0.125 to -0.125 mm. Holes up to 75 mm, in depth have been drilled in ceramics without any fall in the rate of machining as is experienced in conventional processes.

You can also know more :
Advantages and disadvantages of ultrasonic machining

Abrasive Jet Machining

The principle of Abrasive jet machining involves the use of a high-speed stream of abrasive particles carried by a high-pressure gas or air on the work surface through a nozzle. The metal removal occurs due to erosion caused by the abrasive particles impacting the work surface at high speed.

Working Principle :

The filtered gas supplied under a pressure of 2 or 8 kgf/cm2 to the mixing chamber containing the abrasive powder and vibrating at 50 Hz and is then passed into a connecting hose.  

This abrasive and gas mixture emerges from a small nozzle mounted on a fixture at a high velocity ranging from 150 to 300 m/min. The abrasive powder feed rate is controlled by the amplitude of vibration of the mixing chamber. A pressure regulator controls the gas flow and pressure.

The carrier gas should be cheap, nontoxic and easily available. Air and nitrogen are two of the most widely used gas in AJM.

The abrasives generally employed are aluminium, oxide, silicon carbide, glass powder or specially prepared sodium bicarbonate.

The average particle sizes vary from 10 to 50 microns. Larger sizes are used for rapid removal rate while smaller sizes are used for good surface finish and precision work.

Since nozzles are subjected to a great degree of abrasion wear, they are made of hard materials such as tungsten carbide or synthetic sapphire to reduce the wear rate. 

Water jet machining is another variation where a high-pressure jet of water is directed on a surface of removal material.

The metal removal rate depends upon :

• Diameter of nozzle
• Composition of the abrasive-gas mixture 
• Jet pressure 
• The hardness of abrasive particles
• Particle size
• Velocity of Jet 
• The distance of workpiece from jet

Accuracy :
With close control of the various parameters a dimensional tolerance of 
(+0.05 mm to -0.05 mm) can be obtained. on normal production work accuracy of
(+0.01 mm to -0.01 mm) is easily held.

Applications :

• Cutting slots
• Thin sections
• Contouring
• Drilling 
• For producing shallow crevices
• Deburring 
• Producing intricate shapes in hard and brittle materials
• Cleaning and polishing of plastics, nylon and Teflon components
• Frosting of the interior surface of the glass tubes 

Advantages :

• AJM is suitable for materials of any hardness and brittleness like ceramics, germanium, glass.
• Ability to cut fragile and heat-sensitive materials without damage as no heat is generated due to the passing of gas or air.
• Low capital cost
• Holes of intricate shapes could be produced efficiently.
• It can be utilized for cutting, drilling, polishing, deburring, cleaning of the materials.

Disadvantages :

• The material removal rate is slow 
• The machining accuracy is poor and the nozzle wear rate is high.
• Additional cleaning of the work surface may occur because the abrasive particles may remain embedded in the work surface
• Abrasive particles cannot be reused.
• A dust collecting chamber is a basic requirement to prevent atmospheric pollution to cause health hazards.