The unconventional machining is also called as non-traditional machining.
The unconventional machining use for the machine the hard and brittle materials such as carbides, stainless steel, Hastalloy, nitralloy, waspalloy, and any other that cannot be machined by a conventional process using the conventional machines such as lathe, milling, shaper, planer etc.
These materials are widely used in the field of nuclear industry, space research, missile technology and in other industries which requires high strength to weight ratio, heat resisting quality, hardness and toughness.
By using the conventional machining the time taken for machining is more and the less surface finishes, as well as no accuracy.
Therefore by using unconventional machining the time taken is less and the surface finish and accuracy are excellent.
The unconventional machining it uses some form of energy for metal removal there is no direct contact between the tool and workpiece. his process is also uneconomical, time-consuming and sometimes impossible to machine complex shapes.
- Unconventional Machining
- Advantages of the unconventional machining process
- Disadvantages of the unconventional machining process
- Classification of the unconventional machining process.
- Abrasive Jet Machining
- Ultrasonic machining
- Electro-Chemical Machining
- Electrical Discharge Machining
- Electron beam machining
- Laser Beam Machining
1.1 Advantages of the unconventional machining process
- It has a high degree of accuracy.
- It provides a great surface finish.
- The complex shapes can be easily machined.
- It has higher tool life.
- The metal removal rate is high.
1.2 Disadvantages of the unconventional machining process
- This process has the higher cost.
- It requires high operator skills.
- It is complicated in setup.
2. Classification of the unconventional machining process
the following are the classification of unconventional machining processes,
- Mechanical energy based unconventional machining process
- Abrasive jet machining (AJM)
- Ultrasonic MAchining (USM)
- Chemical energy based unconventional machining
- Chemical machining (CHM)
- The electrochemical-based unconventional machining process
- Electro-chemical machining (ECM)
- Electrochemical grinding (ECG)
- Thermo-electric energy based unconventional machining
- Ion-Beam machining (IBM)
- Plasma ARC machining (PAM)
- Electron-Beam machining (EBM)
- Laser-Beam machining (LBM)
2.1 Abrasive Jet Machining (AJM)
A high-velocity jet of dry air, nitrogen, or carbon dioxide containing abrasive particles (typically∼0.025mm) is aimed at the workpiece surface under controlled conditions.
As particle impact the work surface, they cause small cracks, and the gas stream carries both the abrasive particles and the fractured (wear) particles away.
The gas supply pressure is of the order of 850kPa and jet velocity can be as high as 300 m/s and is controlled by a valve.
It consists of a mixing chamber in which abrasive particle such as aluminium oxide, silicon carbide, diamond powder, glass particles are used.
Air or gas may be nitrogen or carbon dioxide is used to mix with the abrasive particles.
From the mixing chamber, the mixture is supplied to the nozzle which is the high strength of a material i.e., tungsten carbide.
From the nozzle, the abrasive particles with velocity 150 to 300m/min are made impact the workpiece due to the high-velocity metal removed from the workpiece with no contact with the tool.
2.1.1 Design consideration for AJM
- Material removal is done with cutting speeds between 25 – 125 mm/min.
- Dimensional tolerances are in the range of ± 2 to ± 5 µm.
- Surface finish with Ra values varies from 0.3 to 2.3 µm.
2.1.2 Advantages of AJM
- It has the ability to cut hard materials such as composites, ceramics, and glass.
- Good for materials that cannot withstand high temperature.
- The complex shape can be produced in the hard and brittle material.
- Ability to cut the heat sensitive materials.
- Low initial cost.
2.1.3 Disadvantages of AJM
- IT is an expensive process.
- Flaring can become large.
- This process not suitable for mass production because of high maintenance requirement.
- the metal removal rate is slow.
- The nozzle wear rate is more.
- Aditional cleaning is necessary.
2.1.4 Application of AJM
- It is used for deburring, etching, and cleaning of brittle metals, alloys, and non-metallic materials.
- Polishing of plastic, Nyon can be done easily.
- complicated shapes can be produced in hard and brittle materials.
- Drilling can be done easily.
- The fragile material can be easily machined.
2.2 Ultrasonic Machining (USM)
In ultrasonic machining, ultrasonic waves are produced by means of magnetostrictive effects which is converted into mechanical vibration.
In this machining, the material removed from the workpiece by microchipping and erosion with fine abrasive grains in the slurry.
The tip of the tool vibrates at a frequency of 20kHz and low amplitude (0.0125mm – 0.075mm). The tool has the same shape as the cavity to be machined.
It consists of an electromechanical transducer which is connected to an AC supply. The velocity transformer which holds the tool firmly.
An abrasive gun is used to supply an abrasive slurry, which is a mixture of abrasive grains and the water in between tool and workpiece interface under a pressure.
When the AC source is supplied with high frequency, the transducer starts vibrating in longitudinally direction by magnetostriction, which is transmitted to the tool through a mechanical focusing device called velocity transformer.
As the tool vibrates is pressed on to the work surface with light force and the abrasive slurry to flow through between tool-workpiece surface.
The impact force rising out of the vibrations of the tool end and the flow of abrasive slurry causing thousands of microscopic grains to remove them from work material by abrasion.
2.2.1 Advantages of USM
- Workpiece after machining is free any stress.
- Extremely hard and brittle materials can be easily machined.
- Very good accuracy and surface finish can be obtained.
- The operational cost is low.
- The process is environmentally friendly as it is noiseless without any chemical reactions and heating.
- This process is economical.
- Better efficiency can be obtained.
- It is suitable for both conducting and non-conducting materials.
2.2.2 Disadvantages of USM
- In this, the metal removal rate is low and cannot be used for large machining cavities.
- Initial cost and cost of the tool is very high, frequency tool replacement is required as tool wear takes place in this operation.
- Not for soft and ductile material due to their ductility.
- Power consumption is quite high.
- The slurry may have to be replaced frequently.
- The tool life is low.
2.2.3 Applications of USM
- The ultrasonic machining is suited for materials that are hard and brittle, such as ceramics, carbides, precious stones, titanium and hardened steels.
- It is used for tool, punch and dies making.
- It is widely used for several machining operations like turning, grinding, trepanning and milling.
- USM can make the hole of round and other shapes.
- The USM is used for threading.
- For machining the glasses and ceramics.
2.3 Electro-Chemical Machining (ECM)
In this machining, an electrolyte acts as a current carrier and high rate of electrolyte movement in the tool and workpiece gas washes the metal ions away from the workpiece before they have to change to plate onto the tool.
It is the reverse of electroplating. Modification of this process are used for turning, slotting, trepanning, and profiling operation in which the electrode becomes the cutting tool.
The tool is made up of brass, copper, bronze, or stainless steel. which is used to perform the work on the workpiece.
The electrolyte is a highly conductive inorganic salt solution, such as sodium chloride mixed with water or sodium nitrate. It is pumped at a high rate through the passage in the tool.
A DC power supply in the range of 5 – 25 V maintains densities, which for most of the application are 1.5 – 8A/mm² of the active machined surface.
machines having current capacities as high as 40000A and as 5A are available.
The entry rate of the tool is proportional to the current density since the metal removal rate is the only function of ion exchange rate, it is not affected by the strength, hardness, or toughness of the workpiece.
2.3.1 Design consideration for ECM
- It is suitable for producing sharp square corners or flat bottoms.
- Controlling the flow of electrolyte is difficult, so irregular cavities may be produced to required shape with dimensional accuracy.
2.3.2 Advantages of ECM
- Machining of hard and brittle material is possible with good quality of surface finish and accuracy.
- Complex shapes can be easily machined.
- There is almost negligible tool wear, so the cost of tool making is an only one-time investment for mass production.
- There is no use of force, no direct contact between tool and workpiece.
- No use of heat, so mechanical and thermal remaining stresses are absent in the workpiece.
- Very close tolerances can be obtained.
2.3.3 Disadvantages of ECM
- All non-conducting materials cannot be machined.
- The tool and workpiece should be chemically still with the electrolyte solution.
- Designing and making tool is difficult but its life is long recommended only for mass production.
- The accurate feed rate of the tool is required.
2.3.4 Applications of ECM
- This process used to produce the complex cavities in high strength materials, particularly in aerospace industries for mass production.
- It is also used for machine forging-die cavities (die sinking) and to produce small holes.
- It can be used as a deburring process.
- There is no tool wear and it can be used to the machine.
- Internal finishing of surgical needles and also for their sharpening.
- Drilling of small and deeper holes with a very good quality of internal surface finish.
- It is used for making inclined and blind holes and finishing of conventionally machined surfaces.
2.4 Electrical Discharge Machining (EDM)
The electrical discharge machining is also called as electro-discharge or spark erosion machining based on erosion of metal by spark discharges.
The basic EDM system consists of the shaped tool (electrode) and the workpiece, connected to a DC supply and placed in a dielectric (electrically non-conducting) fluid.
When the potential difference between the tool and workpiece is high, spark discharges through the fluid, removing a very small amount of metal from the work surface.
In this process the voltage between 50V and 980V and currents from 0.1A to 500A. the workpiece is fixed in the tank containing the dielectric fluid.
The gap between the tool and workpiece is important, thus the downward feed of the tool is controlled by the feed mechanism, which automatically maintains the constant gap.
The spark gap normally varies from 0.005mm to 0.50mm.
The most common electric fluid are minerals oil such as kerosene and distilled & deionised water is used in special applications. The present trend is the use of clear low viscosity fluids.
The EDM process can be used on any materials that are an electrical conductor.
The melting point and heat of melting are important physical properties that determine the volume of metal removed per discharge. As they increase, the rate of metal removal decreases.
Because of the shortcoming of mechanical energy, the hardness, strength, and toughness of the workpiece do not impact the removal rate.
The removal rate and surface roughness increase with increasing current density and decreasing frequency of sparks.
Electrode for EDM is made of graphite, brass, copper, or copper-tungsten alloy are used.
The tool is shaped by forming, casting, powder metallurgy, or machining techniques.
The electrode as small as 0.1mm in diameter has been used and the depth to hole diameter can range up to 400:1.
Tool wear point is important for dimensional accuracy and shape produced. lower the melting point higher the tool wear. so normally graphite with highest wear resistance is used as electrodes.
In this machining, the tool wear can be minimised by reversing the polarity and using copper tools, a process is known as no wear EDM.
2.4.1 Design Consideration for EDM
- Parts should be designed so that the required electrode can be shaped properly and economically.
- Deep slots and narrow opening should be neglected.
- Surface finish specified should not be too fine.
- To achieve high production rate, the bulk material removal should be done by conventional processes.
2.4.2 Advantages of EDM
- Costlier for machining very hard material.
- Maintain the high degree of dimensional accuracy, so it is recommended for tool and die making.
- Complex geometries can be produced.
- Highly critical sections and weak materials can also be processed without any risk of their deformation because in this process applies direct pressure on the workpiece.
- Fine holes can be drilled easily and accurately.
- The adequate form of the high value of MRR can be achieved as compared to other non-conventional machining processes.
2.4.3 Disadvantages of EDM
- This process cannot be applied to the large-sized workpiece, as size for the workpiece is helpless by the size of setup.
- Electrically non-conducting materials cannot be processed by EDM.
- Due to the use of very high temperature at the machining zone, there are chances of deformation of the workpiece in case of these sections.
- EDM process is not capable to produce sharp corners.
- MRR achieved in EDM process is enough lower than the MRR in case of the conventional machining process, so it cannot be taken as an alternative to conventional machining processes at all.
- Redressing of the tool is required for deep holes.
2.4.4 Application of EDM
- This process is highly costlier of very hard material as tool wear is independent of the hardness of workpiece material.
- It is very useful in tool manufacturing.
- It is also used for broach making, making holes with straight or curved axes, and for making complicated cavities which cannot be produced by conventional machining operations.
- EDM is widely used for die making as complex cavities are to be made in the die making.
- Used in wire cutting and rotary form cutting.
- Curved hole drilling can be produced.
2.5 Electron beam machining (EBM)
The electron beam machining arrangement is made as shown in fig. The cathode is made of tungsten or tantalum.
Cathode filaments are heated to a temperature of around 2500°C, which leads to the thermo-ionic emission of electrons, which is further increased by maintaining a very low vacuum within the chamber.
Just after the cathode, there is a grid. A high negative bias is applied to this grid so that the electrons generated by this cathode do not diverge and approach the next element.
The anode which is in the form of a beam attracts the electron beam and gradually gets accelerated.
As they leave the anode section, the electron may achieve lens velocity as high as half the velocity of light.
After the anode, the electron beam passes through a series of the magnetic lens.
The magnetic lens shapes the beam and tries to reduce the divergence. The magnetic lens improves the quality of the electron beam.
Then the electron beam passes through the final sections of deflection coils.
The deflection coil can modify the electron beam by the small amount to improve the shape of the machined holes.
The workpiece is placed inside the vacuum chamber as shown in the figure.
2.5.1 Design Considerations for EBM
- EBM can provide holes of diameter in the range of 100µm to 2mm with a depth up to 15mm, i.e., with length/diameter ratio of around 10.
- Burr formation does not occur in EBM.
- The heat affected zone is rather narrow due to shorter pulse duration in EBM.
- The number of holes drilled per second depends on the diameter of the hole. power density and depth of the hole, as well as material type.
- EBM does not apply any cutting force on the workpiece.
2.5.2 Advantages of EBM
- EBM provides very high drilling rates when small holes with large aspect ratio are to be drilled.
- It can machine almost any material irrespective of their mechanical properties.
- Work holding and fixturing cost is very less because of the absence of mechanical cutting force. So, fragile and brittle materials can also be processed.
- Heat affected zone in EBM is less due to shorter pulse.
- EBM can provide holes of any shape by combining beam deflection using electromagnetic coils with high accuracy.
- It is a fast process.
- Utilizing the CNC table for the machining.
2.5.3 Disadvantages of EBM
- The high cost of the equipment and necessary regular maintenance applicable to any equipment using the vacuum system.
- Valuable amount of non-productive pump down period for attaining the desired vacuum.
- Only small cuts are possible.
- The hole shape is affected by the depth of the workpiece.
- It requires the highly skilled operator.
2.5.4 Applications of EBM
- Used to drill the fine orifices, less than 0.002mm, in reaction turbine blades etc.
- To produce metering holes injector nozzles in diesel engine rockets etc.
- Used to scribe thin films.
- To remove small broken taps from holes.
- The gas orifices drilled for wiredrawing dies.
2.6 Laser Beam Machining (LBM)
The laser beam has wide industrial applications including some of the machining processes.
A laser is an optical transducer which converts the electrical energy into coherent light.
Laser stands for “light amplification by stimulated emission of radiation”.
The laser being coherent or consistent in nature a specific property to generate high power density.
The laser is man-made ruby crystal, containing chromium or Aluminium oxide.
LBM uses the light energy of a laser beam to remove material by vaporization and ablation.
In this process, the coherent or consistent light beam is focused optically for a particular period of time.
The beam is pulsed so that the released energy results in an impulse against the work surface that does melting and evaporation.
In this process, the metal removing is same as that of EDM process but the method of generation of heat is different. The application of heat is very focused in case of LBM as compared to EDM.
The LDM setup consists of a laser tube, a pair of reflectors, one at each end of the tube, a flash tube or lamp, an amplification source, a power supply unit and a cooling system.
This whole setup is fitted inside an enclosure, which carries good quality reflecting surface inside.
The flash lamp goes to laser tube, that excites the atoms of the inside media, which absorb the radiation of incoming light energy. This enables the light to travel to and fro between two reflecting mirrors.
The reflecting mirror does not reflect the total light back and a part of it goes out in the form of a coherent stream of monochromatic light. This amplified stream of light is focused on the workpiece with the help of a converging lens.
A good workpiece material is the one which has light energy absorption power, poor reflectivity, poor thermal conductivity, low specific heat, and low melting point.
A cooling mechanism circulates coolant in the laser tube assembly to avoid its overheating during continuous operations.
2.6.1 Design Considerations for LBM
- The reflectivity of the workpiece surface an important consideration in laser-beam machining because they reflect less.
- Design with sharp corners should be avoided since they can be difficult to produce.
- Any adverse effects on properties of machined materials caused by high temperature and heat affected zone should be the investigation.
2.6.2 Advantages of LBM
- Materials which cannot be machined by conventional methods are machined by LBM.
- There is no tool, so no tool wear.
- Application of heat is focused, so rest of the workpiece is latest affected by the heat.
- Precise holes and cavities are obtained.
- Metals and non-metals can be machined.
- Micro-drilling is possible.
- Rubber and plastics can be machined.
- No tool wear.
2.6.3 Disadvantages of LBM
- Initial cost and operating cost is high.
- Recommended for some specific operations only, as production rate is very slow.
- Cannot be used for high light reflecting materials.
- A highly skilled operator is required.
- The efficiency is low.
- Used for thin materials only.
- Materials removal rate is slow.
2.6.4 Applications of LBM
- LBM is used to perform different machining operations like drilling, slitting, slotting, scribing operations.
- It is used for drilling holes of the small diameter of the order of 0.025mm.
- Making complex parts in this and hard materials like integrated circuits and printed circuit boards.
- Machining of mechanical components of the watch.
- Smaller machining of very hard materials parts.
- LBM used in surgery.
- For making small holes.
- Unconventional machining by Mechanical engineering CET
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