Non-Conventional Machining
Non-conventional manufacturing processes is defined as a group of processes that remove excess material by various techniques involving mechanical, thermal, electrical or chemical energy or combinations of these energies but do not use a sharp cutting tools as it needs to be used for traditional manufacturing processes
classification of NTM processes is carried out depending on the nature of energy used for material removal.
1. Mechanical Processes
2. Electrochemical Processes
3. Electro-Thermal Processes
4. Chemical Processes
1-EDM Die Sinking
It is also known as spark erosion machining or spark machining. Material of workpiece removed due to erosion caused by electric spark. Working principle is described below.
Working Principle of Electric Discharge Machining
Electric discharge machining process is carried out in presence of dielectric fluid which creates path for discharge. When potential difference is created across the two surfaces of die electric fluid, it gets ionized. An electric spark/discharge is generated across the two terminals. The potential difference is developed by a pulsating direct current power supply connected across the two terminals. One of the terminals is positive terminal given to workpiece and tool is made negative terminal. Two third of the total heat generated is generated at positive terminal so workpiece is generally given positive polarity. The discharge develops at the location where two terminals are very close. So tool helps in focusing the discharge or intensity of generated heat at the point of metal removal.
Applications of focused heat raise the temperature of workpiece locally at a point, this way two metals is melted and evaporated.
Electric Discharge Machining Process Details
The working principle and process of EDM is explained with the help of line diagram in
The process details and components are explained below
Base and Container.
A container of non-conducting, transparent material is used for carrying out EDM. The container is filled with dielectric solution. A base to keep workpiece is installed at the bottom of container. The base is made of conducting material and given positive polarity.
Tool
Tool is given negative polarity. It is made of electrically conducting material line brass, copper or tungeten. The tool material selected should be easy to machine, high wear resistant. Tool is made slightly under size for inside machining and over sized for cut side machining. Tool is designed and manufactured according to the geometry to be machined.
Dielectric Solution
Dielectric solution is a liquid which should be electrically conductive. This solution provides two main functions, firstly it drive away the chips and prevents their sticking to workpiece and tool. It enhance the intensity of discharge after getting ionized and so accelerates metal removal rate.
Power Supply
A DC power supply is used, 50 V to 450 V is applied. Due to ionization of dielectric solution an electrical breakdown occurs. The electric discharge so caused directly impinges on the surface of workpiece. It takes only a few micro seconds to complete the cycle and remove the material. The circuit cam be adjusted for auto off after pre-decided time interval.
Tool Feed Mechanism
In case of EDM, feeding the tool means controlling gap between workpiece and the tool. This gap is maintained and controlled with the help of servo mechanism. To maintain a constant gap throughout the operation tool is moved towards the machining zone very slowly. The movement speed is towards the machining zone very slowly. The movement speed is maintained by the help of gear and rack and pinion arrangement. The servo system senses the change in gap due to metal removal and immediately corrects it by moving the tool accordingly. The spark gap normally varies from 0.005 mm to 0.50 mm.
Workpiece and Machined Geometry
The important point for workpiece is that any material which is electrical conductor can be machined through this process, whatever be the hardness of the same. The geometry which is to be machined into the workpiece decides the shape and size of the tool.
Application of Electric Discharge Machining
This process is highly economical for machining of very hard material as tool wear is independent of 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. However, it is capable to do all operations that can be done by conventional machining.
Advantages of EDM
(a) This process is very much economical for machining very hard material.
(b) Maintains high degree of dimensional accuracy so it is recommended for tool and die making.
(c) Complicated geometries can be produced which are very difficult otherwise.
(d) Highly delicate sections and weak materials can also be processed without nay risk of their distortion, because in this process tool never applies direct pressure on the workpiece.
(e) Fine holes can be drilled easily and accurately.
(f) Appreciably high value of MRRR can be achieved as compared to other non-conventional machining processes.
Disadvantages and Limitations of EDM Process
There are some limitations of EDM process as listed below :
(a) This process cannot be applied on very large sized workpieces as size of workpiece is constrained by the size of set up.
(b) Electrically non-conducting materials cannot be processed by EDM.
(c) Due to the application of very high temperature at the machining zone, there are chances of distortion of workpiece in case of this sections.
(d) EDM process is not capable to produce sharp corners.
(e) MRR achieved in EDM process is considerably lower than the MRR in case of conventional machining process so it cannot be taken as an alternative to conventional machining processes at all.
.
2-EDM Wire Cut
Introduction
This is a special type of electric discharge machining that uses a small diameter wire as a cutting tool on the work. Working a principle of wire cut electric discharge machining is same as that of electric discharge machining.
Process Details of WCEDM
Process details of WCEDM are almost similar to EDM with slight difference. The details of the process are indicated in the line diagram shown in Figure. Its major difference of process details with EDM process details are described below.
Tool Details
The tool used in WCEDM process is a small diameter wire as the electrode to cut narrow kerf in the work piece. During the process of cutting the wire is continuously advanced between a supply spoil and wire collector. This continuous feeding of wire makes the machined geometry insensitive to distortion of tool due to its erosion. Material of wire can be brass, copper, tungsten or any other suitable material to make EDM tool. Normally, wire diameter ranges from 0.076 to 0.30 mm depending upon the width of kerf.
Tool Feed Mechanism
Two type of movements are generally given to the total (wire). One is continuous feed from wire supply spoal to wire collector. Other is movement of the whole wire feeding system, and wire along the kerf to be cut into the workpiece. Both movements are accomplished with ultra accuracy and pre-determined speed with the help of numerical control mechanism.
Dielectric Fluid and Spray Mechanism
Like EDM process dielectric fluid is continuously sprayed to the machining zone. This fluid is applied by nozzles directed at the tool work interface or workpiece is submerged in the dielectric fluid container.
Rest of the process details in case of WCEDM process are same as that in case of EDM process.
Application of WCEDM
WCEDM is similar to hand saw operation in applications with good precision. It is used to make narrow kerf with sharp corners. It does not impose any force to workpiece so used for very delicated and thin work pieces. It is considered ideal for making components for stamping dies. It is also used to make intricate shapes in punch, dies and other tools.
Advantages of WCEDM
Advantages are listed below :
(a) Accuracy and precision of dimensions are of very good quality.
(b) No force is experienced by the workpiece.
(c) Hardness and toughness of workpiece do not create problems in machining operation.
Disadvantages and Limitations of WCEDM
The major disadvantages of this process are that only electrically conducting materials can machined. This process is costly so recommended for use specifically at limited operations.
4-Trepanning Cutting
Trepanning is a manufacturing process used to produce holes or circular grooves by using one or more cutters and revolving them around a center.
Trepanning machining is used to produce the following features and parts:
5-Electric-Discharge Texturing
A fast and reliable process of providing surface textures on cold mill rolls to outstanding accuracy and consistency, with lower operating costs and improved environmental conditions offered by other texturing processes such as shot / grit blasting.
Advanced electronic control provides independent selection of surface roughness (Ra) and Peak Count (PC) values over a wide range of roll textures, to suit a vast range of applications. Waviness (Wa) values are maintained well within the limits specified by major automotive companies.
With texture ranges from Ra 0.5 to 12 µM, regardless of roll hardness, the most stringent requirements for coated and uncoated strip qualities can be satisfied Machines of various sizes and specifications operate continuously to provide textured rolls for Tandem, Varying roll shapes, such as parallel, CVC and cambered can be easily textured. Integrated automatic roll loaders and roll shop management software provides texturing capabilities of +1000 rolls/month.
Features
Benefits
Electrode Material
Properties of EDM Electrode
Brass
Categories of Electrodes
1-Metallic Electrodes
Copper
Tungsten
Steel
Brass
Aluminum
Advantages
Disadvantages
2-Non Metallic Electrodes
Graphite
Limitations
3-Combination Electrodes
Copper Graphite Electrode
4-Coated Electrodes
Two types
6-Ultrasonic Machining
Definition:
Ultrasonic Machining is a non-traditional process, in which abrasives contained in a
slurry are driven against the work by a tool oscillating at low amplitude (25-100
microns) and high frequency (15-30 kHz).
Introductions
Ultrasonic machining (USM) is one of the non-traditional machining process. Working principle of this process resembles with conventional and metal cutting as in this process abrasives contained in a slurry are driven at high velocity against the workpiece by a tool vibrating at low amplitude and high frequency. Amplitude is kept of the order of 0.07 mm and frequency is maintained at approximately 20,000 Hz. The workpiece material is removed in the form of extremely small chips. Normally very hard particle dust is included in the slurry like, Al2O2, silicon carbide, boron carbide or diamond dust.
Working principle of USM is same as that of conventional machining that is material of workpiece is removed by continuous abrasive action of hard particles vibrating in the slurry. Abrasive slurry acts as a multipoint cutting tool and does the similar action as done by a cutting edge.
Process Details
USM process is indicated in line diagram shown in Figure 5.3. Details of the process are discussed below
Abrasive Slurry
Abrasive slurry consists of dust of very hard particles. It is filled into the machining zone. Abrasive slurry can be recycled with the help of pump.
Workpiece
Workpiece of hard and brittle material can be machined by USM. Workpiece is clamped on the fixture I the setup.
Cutting Tool
Tool of USM does not do the cutting directly but it vibrates with small amplitude and high frequency. So it is suitable to name the tool as vibrating tool rather than cutting tool. The tool is made of relatively soft material and used to vibrate abrasive slurry to cut the workpiece material. The tool is attached to the arbor (tool holder) by brazing or mechanical means. Sometimes hollow tools are also used which feed the slurry focusing machining zone.
Ultrasonic Oscillator
This operation uses high frequency electric current which passes to an ultrasonic oscillator and ultrasonic transducer. The function of the transducer is to convert electric energy into mechanical energy developing vibrations into the tool.
Feed Mechanism
Tool is fed to the machining zone of workpiece. The tool is shaped as same to the cavity of be produced into the workpiece. The tool is fed to the machining area. The feed rate is maintained equal to the rate of enlargement of the cavity to be produced.
Tool holder. OR Horn.
The tool holder holds and connects the tool to the transducer. It virtually transmits
the energy and in some cases, amplifies the amplitude of vibration. Material of tool
should have good acoustic properties, high resistance to fatigue cracking.
Commonly used tool holders are Monel, titanium, stainless steel. Tool holders
are more expensive, demand higher operating cost.
Tool holder can be classified as :
Amplifying Tool Holder Non-Amplifying Tool Holder
Tool
Tools are made of relatively ductile materials like Brass, Stainless steel or Mild steel
so that Tool wear rate (TWR) can be minimized. The value of ratio of TWR and MRR
depends on kind of abrasive, work material and tool materials.
Applications
Advantages
1. It can be used machine hard, brittle, fragile and non conductive material
2. No heat is generated in work, therefore no significant changes in physical
structure of work material
3. Non-metal (because of the poor electrical conductivity) that cannot be machined
by EDM and ECM can very well be machined by USM.
4. It is burr less and distortion less processes.
5. It can be adopted in conjunction with other new technologies like EDM,ECG,ECM.
Disadvantages
The Ultrasonic Cleaning Process
When ultrasonic energy is introduced into a cleaning solution, cavitation, the foundation of ultrasonic cleaning occurs. Ultrasonic energy causes alternating patterns of low and high pressure phases. During the low pressure phases, minute bubbles – or vacuum cavities form. During the subsequent high pressure phases, cavitation takes place, where the bubbles implode violently. Cavitation provides an intense scrubbing action that leads to unsurpassed cleaning speed when compared with simple soaking or immersion with agitation. Additionally, the bubbles are small enough to penetrate even microscopic crevices.
The Basic Ultrasonic Cleaning System
A generator, a transducer and a tank make up our ultrasonic cleaning system. The generator supplies electrical power to the transducer, which converts to mechanical energy in the form of pressure waves. Ultrasonic energy enters the cleaning solution in the tank, generating the cavitation that precision-cleans its contents.
The Ultrasonic Deburring
It is also a cleaning process. It is close to ultrasonic machining, deburring is removal of burr. Cleaning of burr is carried out, only work surface is cleaned not a whole part. It is cleaned with slurry.
3-Electrical discharge grinding (EDG),
The detail of EDG process has been illustrated in Fig. In this process, a rotating eclectically conductive metallic wheel is used which is known as grinding wheel. The grinding wheel used in this process, having no any abrasive particles and rotates its horizontal axis. Due to the similarities of process with conventional grinding and material is removed due to the electrical discharge, it is known as electrical discharge grinding (EDG). In this process, the spark is generated between rotating wheel and workpiece. The rotating wheel and workpiece both are separated by dielectric fluid and during machining both (workpiece and wheel) are continuously deeped into dielectric fluid. The dielectric fluids are mainly Kerosene oil, Paraffin oil, Transformer oil or de-ionized water. The main purpose of dielectric is to make a conductive channel during ionization when suitable breakdown voltage is applied. The servo control mechanism utilized to maintain the constant gap between workpiece and wheel in range of 0.013-0.075 mm. A pulse generator is used for maintaining the DC pulse power supply in ranges of voltage, current and frequency are 30-400V, 30-100A and 2-500 kHz respectively When pulse power supply is applied, the spark takes place into gap due to the ionization and striking of ions and electrons at their respective electrodes. Due to spark, high temperature generated between ranges of 8000°C to 12000°C or as so high upto 200000c by each spark resulting material is melted from both the electrodes. Simultaneously DC pulse power supply switch is deactivated resulting the breakdown of spark occurs and fresh dielectric fluid entering into gap. Due to the high flushing efficiency, the molten materials flush away in form of micro debris from gap and formed the crater on work surface
Fig. 1 Detail of EDG process
7-PLASMA ARC MACHINING (PAM)
It is also one of the thermal machining processes. Here the method of heat generation is different than EDM and LBM.
Working Principle of PAM
In this process gases are heated and charged to plasma state. Plasma state is the superheated and electrically ionized gases at approximately 5000oC. These gases are directed on the workpiece in the form of high velocity stream. Working principle and process details are shown in Figure 5.7.
Process Details of PAM
Details of PAM are described below.
Plasma Gun
Gases are used to create plasma like, nitrogen, argon, hydrogen or mixture of these gases. The plasma gun consists of a tungsten electrode fitted in the chamber. The electrode is given negative polarity and nozzle of the gun is given positive polarity. Supply of gases is maintained into the gun A strong arc is established between the two terminals anode and cathode. There is a collision between molecules of gas and electrons of the established arc As a result of this collision gas molecules get ionized and heat is evolved. This hot and ionized gas called plasma is directed to the workpiece with high velocity. The established arc is controlled by the supply rate of gases
Power supply (DC) is used to develop two terminals in the plasma gun. A tungsten electrode is inserted to the gun and made cathode and nozzle of the gun is made anode. Heavy potential difference is applied across the electrodes to develop plasma state of gases.
Cooling Mechanism
As we know that hot gases continuously comes out of nozzle so there are chances of its over heating. A water jacket is used to surround the nozzle to avoid its overheating.
Tooling
There is no direct visible tool used in PAM. Focused spray of ho0t, plasma state gases works as a cutting tool.
Workpiece
Workpiece of different materials can be processed by PAM process. These materials are aluminium, magnesium, stainless steels and carbon and alloy steels. All those material which can be processed by LBM can also be processed by PAM process.
Applications of PAM
The chief application of this process is profile cutting as controlling movement of spray focus point is easy in case of PAM process. This is also recommended for smaller machining of difficult to machining materials.
Advantages of PAM Process
Advantages of PAM are given below :
(a) It gives faster production rate.
(b) Very hard and brittle metals can be machined.
(c) Small cavities can be machined with good dimensional accuracy.
Disadvantages of PAM Process
(a) Its initial cost is very high.
(b) The process requires over safety precautions which further enhance the initial cost of the setup.
(c) Some of the workpiece materials are very much prone to metallurgical changes on excessive heating so this fact imposes limitations to this process.
(d) It is uneconomical for bigger cavities to be machined.
Plasma v/s Oxyfuel |
In the end, every operation has to weigh the benefits of each technology to choose which is best. If a user only cuts very thick mild steel, or frequently needs to heat metal for shaping or bending it, oxyfuel is clearly a good choice. If more versatility is required, calling for metals other than mild steel being cut, or if much of a user’s cutting requirements are 35 mm or below, then plasma is a technology that has some compelling advantages.
Compared to oxyfuel, plasma offers many benefits that are worth considering; faster cut speeds, better cut quality, less rework, higher productivity, more material flexibility, increased efficiency and lower total costs of operation which all will lead to greater profitability. Looking at a complete picture helps to determine the true cost of cutting metal and in choosing which technology is the best for each application.
8-Laser Beam Machining
Introduction
Laser beam have wide industrial applications including some of the machining processes. A laser is an optical transducer that converts electrical energy into a highly coherent light beak. One must know the full name of laser, it stands for “light amplification of stimulated emission of radiation”. Laser being coherent in nature has a specific property, if it is focused by conventional optical lenses can generate high power density.
Working Principle of LBM
LBM uses the light energy of a laser beam to remove material by vaporization and ablation. The working principle and the process details (setup) are indicated in Figure 5.6. In this process the energy of coherent light beam is focused optically for predecided longer 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. Here the way of metal removing is same as that of EDM process but method of generation of heat is different. The application of heat is very finely focused in case of LBM as compared to EDM
Figure : Working Principle and Process Details of LBM
Laser Tube and Lamp Assembly
This is the main part of LBM setup. It 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 a enclosure, which carries good quality reflecting surfaces inside. In this setup 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 partial reflecting mirror does not reflect the total light back and apart of it goes out in the form of a coherent stream of monochromatic light. This highly amplified stream of light is focused on the workpiece with the help of converging lense. The converging lense is also the part of this assembly.
Workpiece
The range of workpiece material that can be machined by LBM includes high hardness and strength materials like ceramics, glass to softer materials like plastics, rubber wood, etc. A good workpiece material high light energy absorption power, poor reflectivity, poor thermal conductivity, low specific heat, low melting point and low latent heat.
Cooling Mechanism
A cooling mechanism circulates coolant in the laser tube assembly to avoid its over heating in long continuous operation.
Tool Feed Mechanism
There is no tool used in the LBM process. Focusing laser beam at a pre-decided point in the workpiece serve the purpose of tool. As the requirement of being focused shifts during the operation, its focus point can also be shifted gradually and accordingly by moving the converging lense in a controlled manner. This movement of the converging lense is the tool feed mechanism in LBM process.
Lasing Medium
Many materials can be used as the heart of the laser. Depending on the lasing medium lasers are classified as solid state and gas laser. Solid-state lasers are commonly of the following type
• Ruby which is a chromium – alumina alloy having a wavelength of 0.7 μm
• Nd-glass lasers having a wavelength of 1.64 μm
• Nd-YAG laser having a wavelength of 1.06 μm
These solid-state lasers are generally used in material processing.
The generally used gas lasers are
• Helium – Neon
• Argon
• CO2 etc.
Lasers can be operated in continuous mode or pulsed mode. Typically CO2 gas laser is operated in continuous mode and Nd – YAG laser is operated in pulsed mode.
Applications.
LBM is used to perform different machining operations like drilling, slitting, slotting, scribing operations. It is used for drilling holes of small diameter of the order of 0.025 mm. It is used for very thin stocks. Other applications are listed below
(a) Making complex profiles in thin and hard materials like integrated circuits and printed circuit boards (PCBS).
(b) Machining of mechanical components of watches.
(c) Smaller machining of very hard material parts.
Advantages of LBM
(a) Materials which cannot be machined by conventional methods are machined by LBM.
(b) There is no tool so no tool wear.
(c) Application of heat is very much focused so rest of the workpiece is least affected by the heat.
(d) Drills very find and precise holes and cavities.
Disadvantages of LBM
Major disadvantages of LBM process are given below :
(a) High capital investment is involved. Operating cost is also high.
(b) Recommended for some specific operations only as production rate is very slow.
(c) Cannot be used comfortably for high heat conductivity materials light reflecting materials.
(d) Skilled operators are required.
Limitations
9-Abrasive Water Jet Cutting
Process Description
Abrasive water jet cutting utilizes a high velocity coherent stream of water and abrasive that can be used to cut almost all materials. Water at 40,000 to 55,000 psi accelerates through a sapphire, ruby or diamond orifice. The stream passes through a mixing region where the vacuum, induced by the stream, sucks in abrasive. Momentum of the water stream accelerates and entrains abrasive as it passes through the nozzle. The stream exits the nozzle as a three phase mixture of air, water and abrasive particles with a cutting diameter of 0.020” to 0.060”. The high velocity abrasive particles impact on the kerf face and do the actual cutting. Kerf material is removed as microchips, with no negligible affects on the material.
The cutting stream carries 0.5 to 1.5 pounds per minute of abrasive. The quantity of abrasive is dependent on the cutting stream size, which is selected based on the material to be cut. Garnet is by far the most commonly used abrasive. It is environmentally clean, contains no, free silica, and combines good cutting ability with reasonable wear on the consumables.
The main attributes of the cut are: no heat, narrow kerf, good edge finish, and high accuracy. Successful, cost effective, abrasive waterjet applications take advantage of these characteristics.
Benefits of Abrasive Jet Cutting
Abrasive waterjet offers many advantages not found in other cutting techniques.
a. No heat affected zone (HAZ)
b. Low contact force of cutting stream.
c. No distortion and warping
d. Burr-free
e. Can cut any material and thickness
f. Near net shape cutting eliminates secondary operations.
g. Can achieve high accuracy’s of up to +/- 0.001.
h. Material thickness of 0.002 to 12” can be cut.
i. Small kerf width allows for tight nesting and optimal material usage.
j. Flexibility
Typical Applications
Abrasive water jet has the ability to cut almost all materials and thickness’. Most uses are for cutting of specialty materials such as stainless steel and aluminum. Its flexibility makes it useful for all applications, but of course some uses are better than others. The following is a list of applications where waterjet is the best approach:
• Shape cutting of ¼” and thicker aluminum
• Net size cutting of ½” and thicker stainless steel
• Blank cutting of parts for final machining
• Screen Cutting.
• Precision cuts in ½” and thicker mild steel
• Hardened materials
• Intricate shapes in delicate materials.
• Tube cutting
Cut Quality
Cut quality describes the kerf edge and taper. The feed rate controls the amount of jet lag. Cutting speed and edge quality are directly related. At high feed rates the jet has increased curvature as it passes through the cut. Reduced cutting speeds can result in a good edge finish of 125 microinch, having a ground appearance and minimal taper. High feed rates for cuts give striations through the full cut depth.
Most common cut Material
Materials that are reflective, conductive, heat resisting, or heat sensitive are ideal material for abrasive water jet cutting. As the material thickness increases AWJ becomes the preferred cutting technique, especially where accuracy must be maintained.
Heat sensitive and heat resisting materials such as stainless steel, alloy steel, titanium, inconel and hastelloy can be cut with no material effects
10- ELECTRO CHEMICAL MACHINING
Introduction:
Electrochemical machining (ECM) process uses electrical energy in combination with chemical energy to remove the material of workpiece. This works on the principle of reverse of electroplating.
Working Principle of ECM
Electrochemical machining removes material of electrically conductor workpiece. The workpiece is made anode of the setup and material is removed by anodic dissolution. Tool is made cathode and kept in close proximity to the workpiece and current is passed through the circuit. Both electrodes are immersed into the electrolyte solution. The working principle and process details are shown in the Figure 5.5. This works on the basis of Faraday’s law of electrolysis. The cavity machined is the mirror image of the tool. MRR in this process can easily be calculated according to Faraday’s law.
Process Details
Workpiece
Workpiece is made anode, electrolyte is pumped between workpiece and the tool. Material of workpiece is removed by anodic dissolution. Only electrically conducting materials can be processed by ECM.
Tool
A specially designed and shaped tool is used for ECM, which forms cathode in the ECM setup. The tool is usually made of copper, brass, stainless steel, and it is a mirror image of the desired machined cavity. Proper allowances are given in the tool size to get the dimensional accuracy of the machined surface.
Power Supply
DC power source should be used to supply the current. Tool is connected with the negative terminal and workpiece with the positive terminal of the power source. Power supply supplies low voltage (3 to 4 volts) and high current to the circuit.
Electrolyte
Water is used as base of electrolyte in ECM. Normally water soluble NaCl and NaNO3 are used as electrolyte. Electrolyte facilitates are carrier of dissolved workpiece material. It is recycled by a pump after filtration.
Tool Feed Mechanism
Servo motor is used to feed the tool to the machining zone. It is necessary to maintain a constant gap between the workpiece and tool so tool feed rate is kept accordingly while machining.
In addition to the above whole process is carried out in a tank filled with electrolyte. The tank is made of transparent plastic which should be non-reactive to the electrolyte. Connecting wires are required to connect electrodes to the power supply.
Chemistry of Process.
During ECM, there will be reactions occurring at the electrodes i.e. at the anode or work piece and at the cathode or the tool along with within the electrolyte.
Let us take an example of machining of low carbon steel which is primarily a ferrous alloy mainly containing iron. For electrochemical machining of steel, generally a neutral salt solution of sodium chloride (NaCl) is taken as the electrolyte. The electrolyte and water undergoes ionic dissociation as shown
below as potential difference is applied
NaCl ↔ Na+ + Cl-
H2O ↔ H+ + (OH)-
As the potential difference is applied between the work piece (anode) and the tool (cathode), the positive ions move towards the tool and negative ions move towards the work piece. Thus the hydrogen ions will take away electrons from the cathode (tool) and from hydrogen gas as:
2H+ + 2e- = H2 ↑ at cathode
Similarly, the iron atoms will come out of the anode (work piece) as:
Fe = Fe+ + + 2e-
Within the electrolyte iron ions would combine with chloride ions to form iron chloride and similarly sodium ions would combine with hydroxyl ions to form sodium hydroxide
Na+ + OH- = NaOH
Fig. depicts the electro chemical reactions schematically. As the material removal takes place due to atomic level dissociation, the machined surface is of excellent surface finish and stress free
Cathode Reaction
Na+ + e- = Na
Na+H20 = Na(OH)+H+
2H++2e- =H2 ↑
It shows that there is no deposition on tool but only gas is formed, whereas,
in cathode in machining an iron.
Anode Reaction
Iron (Fe) ↔ Fe++ + 2e-
Fe++ +2cl- ↔ Fecl2
Fe++ +2(OH)- ↔ Fe(OH)
Fecl2 +2(OH)- ↔ Fe(OH)2 +2cl-
It shows that metal (work piece) i.e. Fe goes into solution and hence machined to produce reaction products as iron chloride and iron-hydroxide as a precipate. Interesting part is that the removal is an atom by atom, resulting in higher surface finish with stress and crack free surface, and independent of the hardness of work material.Smaller the interlectrode gap(IEG) the gap,
greater will be the current flow because resistance decreases and higher will be rate of metal removal from the anode.
That voltage or potential difference is around 2 to 30 V. The applied potential difference, however, also overcomes the following resistances or potential drops. They are:
• The electrode potential
• The activation over potential
• Ohmic potential drop
• Concentration over potential
• Ohmic resistance of electrolyte.
Applications
1. ECM can be used to make disc for turbine rotor blades made up of HSTR alloys
2. ECM can be used for slotting very thin walled collets
3. ECM can be used for copying of internal and external surfaces, cutting of curvilinear slots, machining of intricate patterns, production of long curved profiles, machining of gears and chain sprockets, production of integrally bladed nozzle for use in diesel locomotives, production of satellite rings and connecting rods, machining of thin large diameter
diaphragms.
4. ECM principle has be employed for performing a number of machining operations namely, turning, treplaning, broaching, grinding, fine hole drilling, die sinking, piercing,deburring, plunge cutting etc.
5. ECM can also be used to generate internal profile of internal cams.
Advantages
ECM offers impressive and long lasting advantages.
1. ECM can machine highly complicated and curved surfaces in a single pass.
2. A single tool can be used to machine a large number of pieces without any loss in its shape and size. Theoretically tool life is high
3. Machinability of the work material is independent of its physical and mechanical properties. The process is capable of machining metals and alloys irrespective of their strength and hardness.
4. Machined surfaces are stress and burr free having good surface finish
5. It yields low scrap, almost automatic operation, low overall machining time, and reduced inventory expenses.
6. There is no thermal damage and burr free surface can be produced.
Disadvantages
1. High capital cost of equipment
2. Design and tooling system is complex
4. Spark damage may become sometimes problematic
5. Non conductive material cannot be machined.
6. Blind holes cannot be machined in solid block in one stage
7. Corrosion and rust of ECM machine can be hazard
8. Space and floor area requirement are also higher than for conventional machining methods..
11- Electron Beam Machining
Process
Electron beam is generated in an electron beam gun. Electron beam gun provides high velocity electrons over a very small spot size. Electron Beam Machining is required to be carried out in vacuum. Otherwise the electrons would interact with the air molecules, thus they would loose their energy and cutting ability. Thus the workpiece to be machined is located under the electron beam and is kept under vacuum. The high-energy focused electron beam is made to impinge on the workpiece with a spot size of 10 – 100 μm. The kinetic energy of the high velocity electrons is converted to heat energy as the electrons strike the work material. Due to high power density instant melting and vaporization starts and “melt – vaporisation” front gradually progresses, as shown in Fig. Finally the molten material, if any at the top of the front, is expelled from the cutting zone by the high vapour pressure at the lower part.
Localized heating by focused
electron beam Gradual formation of hole
Penetration till the auxiliary support Removal due to high vapour pressure
Fig. 9.6.2 Mechanism of Material Removal in Electron Beam Machining
Equipment
Fig shows the schematic representation of an electron beam gun, which is the heart of any electron beam machining facility. The basic functions of any electron beam gun are to generate free electrons at the cathode, accelerate them to a sufficiently high velocity and to focus them over a small spot size. Further, the beam needs to be maneuvered if required by the gun.
The cathode as can be seen in Fig. 9.6.3 is generally made of tungsten or tantalum. Such cathode filaments are heated, often inductively, to a temperature of around 25000C. Such heating leads to thermo-ionic emission of electrons, which is further enhanced by maintaining very low vacuum within the chamber of the electron beam gun. Moreover, this cathode cartridge is highly negatively biased This cathode is often in the form of a cartridge so that it can be changed very quickly to reduce down time in case of failure.
Just after the cathode, there is an annular bias 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 annular anode, in the form of a beam.
The nature of biasing just after the cathode controls the flow of electrons and the biased grid is used as a switch to operate the electron beam gun in pulsed mode.
After the anode, the electron beam passes through a series of magnetic lenses and apertures. The magnetic lenses shape the beam and try to reduce the divergence. Apertures on the other hand allow only the convergent electrons to pass and capture the divergent low energy electrons from the fringes. This way, the aperture and the magnetic lenses improve the quality of the electron beam.
Then the electron beam passes through the final section of the electromagnetic lens and deflection coil. The electromagnetic lens focuses the electron beam to a desired spot. The deflection coil can manoeuvre the electron beam, though by small amount, to improve shape of the machined holes.
Generally in between the electron beam gun and the workpiece, which is also under vacuum, there would be a series of slotted rotating discs. Such discs allow the electron beam to pass and machine materials but helpfully prevent metal fumes and vapour generated during machining to reach the gun.
Electron beam guns are also provided with illumination facility and a telescope for alignment of the beam with the workpiece.
Workpiece is mounted on a CNC table so that holes of any shape can be machined using the CNC control and beam deflection in-built in the gun.
One of the major requirements of EBM operation of electron beam gun is maintenance of desired vacuum. Level of vacuum within the gun is in the order of 10-4 to 10-6 Torr. {1 Torr = 1mm of Hg}
Electron Beam Process – Parameters
The process parameters, which directly affect the machining characteristics in Electron Beam Machining, are:
• The accelerating voltage
• The beam current
• Pulse duration
• Energy per pulse
• Power per pulse
• Lens current
• Spot size
• Power density
As has already been mentioned in EBM the gun is operated in pulse mode. This is achieved by appropriately biasing the biased grid located just after the cathode. Switching pulses are given to the bias grid so as to achieve pulse duration of as low as 50 μs to as long as 15 ms.
Beam current is directly related to the number of electrons emitted by the cathode or available in the beam. Beam current once again can be as low as 200 μamp to 1 amp. Increasing the beam current directly increases the energy per pulse. Similarly increase in pulse duration also enhances energy per pulse. High-energy pulses (in excess of 100 J/pulse) can machine larger holes on thicker plates A higher energy density, i.e., for a lower spot size, the material removal would be faster though the size of the hole would be smaller.
Process Capability
EBM can provide holes of diameter in the range of 100 μm to 2 mm with a depth upto 15 mm, i.e., with a l/d ratio of around 10. Generally burr formation does not occur in EBM
A wide range of materials such as steel, stainless steel, Ti and Ni super-alloys, aluminum as well as plastics, ceramics, leathers can be machined successfully using electron beam. As the mechanism of material removal is thermal in nature as for example in electro-discharge machining, there would be thermal damages associated with EBM. However, the heat-affected zone is rather narrow due to shorter pulse duration in EBM. Some of the materials like Al and Ti alloys are more readily machined compared to steel. EBM does not apply any cutting force on the workpieces. Thus very simple work holding is required. This enables machining of fragile and brittle materials by EBM
Advantages and Limitations
EBM provides very high drilling rates when small holes with large aspect ratio are to be drilled. Moreover it can machine almost any material irrespective of their mechanical properties. As it applies no mechanical cutting force, work holding and fixturing cost is very less. Further for the same reason fragile and brittle materials can also be processed. The heat affected zone in EBM is rather less due to shorter pulses. EBM can provide holes of any shape by combining beam deflection using electromagnetic coils and the CNC table with high accuracy.
However, EBM has its own share of limitations. The primary limitations are the high capital cost of the equipment and necessary regular maintenance applicable for any equipment using vacuum system. Moreover in EBM there is significant amount of non-productive pump down period for attaining desired vacuum. Though heat affected zone is rather less in EBM but recast layer formation cannot be avoided
12-Roll Forming
Pressure during rolling
Typical pressure variation along the contact length in flat rolling.The peak pressure is located at the neutral point. The area beneath the curve, represents roll force.
Friction in rolling: It depends on lubrication, work material and also on the temperature. In cold rolling the value of coefficient of friction is around 0.1 and in warm working it is around 0.2. In hot rolling it is around 0.4. In hot rolling sticking friction condition is also seen and then friction coefficient is observed up to 0.7. In sticking the hot wok surfaceadheres to roll and thus the central part of the strip undergoes with severe deformation.
Roll configurations in rolling mills
Various Roll Configurations (a) Two-high (b) Three-high
(c) Four-high (d) Cluster mill (e) Tandem mill
13-High Energy Rate Forming (HERF) Processes
Introduction:
The forming processes are affected by the rates of strain used.
Effects of strain rates during forming:
Principle / important features of HERF processes:
Advantages of HERF Processes
metals.
is economically justifiable.
eliminating intermediate forming steps and pre forming dies.
production.
Limitations:
Applications:
14 Explosive Forming
Introduction:
A punch in conventional forming is replaced by an explosive charge.
Explosives used can be:
Factors to be considered while selecting an HERF process:
Types of explosive forming:
1) Unconfined type or Standoff technique
2) Confined type or Contact technique
1) Unconfined type (or Standoff technique)
Principle:
The work is firmly supported on the die and the die cavity is evacuated. A definite
quantity of explosive is placed suitably in water medium at a definite stand off distance from the work. On detonation of the explosive charge, a pressure pulse (or a shock wave) of very high
intensity is produced.
Fig. Unconfined Type Explosive Forming
A gas bubble is also produced which expands spherically and then collapses. When the
pressure pulse impinges against the work (plate or sheet0, the metal is deformed into the die
with a high velocity of around 120 m/s (430km/h). The vacuum is necessary in the die to prevent adiabatic heating of the work which may lead to oxidation or melting.
Role of water:
Process Variables
Advantages;
Shock wave is efficiently transmitted through water and energy is transmitted effectively on the work
Limitations:
Applications:
2) Confined System ( or Contact Technique)
Principle:
The pressure pulse or shock wave produced is in direct contact with the work piece
(usually tubular) and hence the energy is directly applied on the work without any water
medium.
The tube collapses into the die cavity and is formed. It is used for bulging and flaring
operations.
Fig. Confined (Contact) type Explosive Forming
Advantages:
Disadvantages:
Applications;
Bulging and flaring of tubes.
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