ME0028 - NON TRADITIONAL MACHINING TECHNIQUES Unit 1 - INTRODUCTION Introduction to Non Traditional machining methods – Need for Non Traditional machining - Sources of metal removal – classification classification on the basis of energy sources – Parameters Parameters influencing selection of process.
Unit 2 - MECHANICAL ENERGY TECHNIQUES Abrasive Jet Machining (AJM): Operating principles – Equipment – Parameters influencing metal removal –Benefits – Applications – Advantages and Limitations.
Water Jet Machining (WJM): Operating principles – Equipment – Parameters influencing metal removal –Benefits – Applications – Advantages and Limitations.
Ultra Sonic Machining (USM): Operating principles – Equipment and sub systems – Parameters influencing metal removal – Benefits and Applications – Advantages and Limitations
Unit 3 - ELECTRICAL ENERGY TECHNIQUES Electro Chemical Machining (ECM): Operating principles – Equipment and sub systems – Parameters influencing metal removal – Benefits and Applications – Advantages and Limitations – current developments in ECM.
Electro Chemical Grinding (ECG): Operating principles – Equipment and sub systems – Parameters influencing metal removal – Benefits – Applications – Advantages and Limitations
Unit 4 - THERMO ELECTRICAL ENERGY TECHNIQUES Electrical Discharge Machining (EDM) and Wire Cut Electrical Discharge Machining (WCEDM) : Operating principles – Equipment and sub systems – Parameters influencing metal removal – Benefits – Applications – Advantages and Limitations.
Electrical Discharge Grinding (EDG): Operating principles – Equipment and sub systems – Parameters influencing metal removal – Benefits – Applications – Advantages and Limitations
Unit 5 - THERMAL ENERGY TECHNIQUES Operating principles – Equipment and sub systems – Parameters influencing metal removal – Benefits – Applications – Advantages and Limitations of Electron Beam Machining (EBM), Plasma
ARC Machining (PAM) and Laser BEAM Machining (LBM).
Unit 1 - INTRODUCTION Manufacturing processes can be broadly divided into two groups and they are primary manufacturing processes and secondary manufacturing processes. The former ones provide basic shape and size to the material as per designer‘s requirement. Casting, forming, powder metallurgy are such processes to name a few. Secondary manufacturing processes provide the final shape and size with tighter control on dimension , surface characteristics etc. Material removal processes are mainly the secondary manufacturing processes. Material removal processes once again can be divided into mainly two groups and they are ―Conventional Machining Processes ‖ and ―Non-Traditional Manufacturing Processes ‖. Examples of conventional machining processes are turning, boring, milling, shaping, broaching, slotting, grinding etc. Similarly, Abrasive Jet Machining (AJM), Ultrasonic Machining (USM), Water Jet and Abrasive Water Jet Machining (WJM and AWJM), Electro-discharge Machining (EDM) are some of the Non Traditional Machining (NTM) Processes.
To classify Non Traditional Machining Processes (NTM), one needs to understand and analyse the differences and similar characteristics between conventional machining processes and NTM processes. CHARACTERISTICS OF CONVENTIONAL MACHINING PROCESSES:
Conventional Machining Processes mostly remove material in the form of chips b y applying forces on the work material with a wedge shaped cutting tool that is harder than the work material under machining condition. Such forces induce plastic deformation within the work piece leading to shear deformation along the shear plane and chip formation. Fig shown below depicts such chip formation by shear deformation in conventional machining.
Thus the major characteristics of conventional machining are: • Generally macroscopic chip formation by shear deformation • Material removal takes place due to application of cutting forces – energy – energy domain can be classified as mechanical • Cutting Cutting tool is harder than work piece at room temperature as well as under machining conditions
Unit 1 - INTRODUCTION Manufacturing processes can be broadly divided into two groups and they are primary manufacturing processes and secondary manufacturing processes. The former ones provide basic shape and size to the material as per designer‘s requirement. Casting, forming, powder metallurgy are such processes to name a few. Secondary manufacturing processes provide the final shape and size with tighter control on dimension , surface characteristics etc. Material removal processes are mainly the secondary manufacturing processes. Material removal processes once again can be divided into mainly two groups and they are ―Conventional Machining Processes ‖ and ―Non-Traditional Manufacturing Processes ‖. Examples of conventional machining processes are turning, boring, milling, shaping, broaching, slotting, grinding etc. Similarly, Abrasive Jet Machining (AJM), Ultrasonic Machining (USM), Water Jet and Abrasive Water Jet Machining (WJM and AWJM), Electro-discharge Machining (EDM) are some of the Non Traditional Machining (NTM) Processes.
To classify Non Traditional Machining Processes (NTM), one needs to understand and analyse the differences and similar characteristics between conventional machining processes and NTM processes. CHARACTERISTICS OF CONVENTIONAL MACHINING PROCESSES:
Conventional Machining Processes mostly remove material in the form of chips b y applying forces on the work material with a wedge shaped cutting tool that is harder than the work material under machining condition. Such forces induce plastic deformation within the work piece leading to shear deformation along the shear plane and chip formation. Fig shown below depicts such chip formation by shear deformation in conventional machining.
Thus the major characteristics of conventional machining are: • Generally macroscopic chip formation by shear deformation • Material removal takes place due to application of cutting forces – energy – energy domain can be classified as mechanical • Cutting Cutting tool is harder than work piece at room temperature as well as under machining conditions
CHARACTERISTICS OF NON TRADITIONAL MACHINING PROCESSES:
Non Traditional Machining (NTM) Processes on the other other hand are characterised as follows: • Material removal removal may occur with chip formation or even no chip formation may take place. For example in AJM, chips are of microscopic size and in case of Electrochemical machining material removal occurs due to electrochemical dissolution at atomic level • In NTM, there may not be a physical tool present. For example in laser jet machining, machining is carried out by laser beam. However in Electrochemical Machining there is a physical tool that is very much required for machining • In NTM, the tool need not be harder than the work piece material. For example, in EDM, copper is used as the tool material to machine hardened steels. • Mostly NTM processes do not necessarily use mechanical energy to provide material removal. They use different energy domains to provide machining. For example, in USM, AJM, WJM mechanical energy is used to machine material, whereas in ECM electrochemical dissolution constitutes material removal.
Material removal volume in brittle material is considered to be Hemispherical with diameter equal to chordal length of the indentation. Material removal in Ductile material is considered to be equal to the indentation volume due to particulate impact.
CLASSIFICATION OF NON TRADITIONAL MACHINING PROCESSES:
Thus classification of NTM processes is carried out depending on the nature of energy used for material removal. The broad classification is given as follows: • Mechanical Processes ⎯ Abrasive Jet Machining (AJM) ⎯ Abrasive Flow Machining (AFM) ⎯ Ultrasonic Machining (USM) ⎯ Ultrasonic Assisted Machining (UAM) ⎯ Rotary Ultrasonic assisted Machining (RUM) ⎯ Orbital Grinding (OG) ⎯ Water Jet Machining (WJM) ⎯ Abrasive Water Jet Machining (AWJM) • Electrochemical Processes ⎯ Electrochemical Machining (ECM) ⎯ Electro Chemical Grinding (ECG) ⎯ Electro Chemical Discharge Grinding (ECDG) ⎯ Electro Chemical Deburring (ECD) ⎯ Electro Chemical Honing (ECH) ⎯ Electro Stream Drilling (ESD) ⎯ Electro Jet Drilling (EJD) ⎯ Shaped Tube Electrolytic Machining (STEM) • Electro-Thermal Processes ⎯ Electro-discharge machining (EDM) ⎯ Laser Beam Machining (LBM) ⎯ Electron Beam Machining (EBM) ⎯ Electro Discharge Wire Cutting (EDWC) or Wire Cut Elecro discharge Machining (WCEDM) ⎯ Electro Discharge Grinding (EDG) ⎯ Plasma Arc Machining (PAM) ⎯ Ion Beam Machining (IBM) ⎯ Thermal Deburring (TD) • Chemical Processes ⎯ Chemical Milling (CHM) ⎯ Chemical Blanking (CHB) ⎯ Chemical Engraving (CHE) ⎯ Electro Polishing (ELP) ⎯ Photochemical Milling (PCM) ⎯ Thermo Chemical Machining (TCM)
NEED FOR NON TRADITIONAL MACHINING:
Conventional machining sufficed the requirement of the industries over the decades. But new exotic work materials as well as innovative geometric design of products and components were putting lot of pressure on capabilities of conventional machining processes to manufacture the components with desired tolerances economically. This led to the development and establishment of NTM processes in the industry as efficient and economic alternatives to conventional ones. With development in the NTM processes, currently there are often the first choice and not an alternative to conventional processes for certain technical requirements. The following examples are provided where NTM processes are preferred over the conventional machining process: • • • • •
Intricate shaped blind hole – e.g. square hole of 15 mmx15 mm with a depth of 30 mm Difficult to machine material – e.g. same example as above in Inconel, Ti-alloys or carbides. Low Stress Grinding – Electrochemical Grinding is preferred as compared to conventional grinding Deep hole with small hole diameter – e.g. φ 1.5 mm hole with l/d = 20 Machining of composites.
CASE STUDIES:
The industries always face problems in manufacturing of components because of several reasons. This may be because of the complexity of the job profile or may be due to surface requirements with higher accuracy and surface finish or due to the strength of the materials. To elaborate such difficulties, let us discuss some case studies. CASE I This is a case of a square blind hole in any material with higher surface finish of about 10 micron (rms). This can not be obtained in foundry (range of surface finish 1250-2500 micron) or in forging (750-1250 micron) practices. This has to be finished in a machine shop. If it is a case of single piece or a mass production — slotting or broaching of blind hole without a recess can not be done. For the above reason, first a through hole drilling and then plugging to required length, increase the production time unnecessarily.
CASE II Consider the same square hole machining with certain accuracy requirements. In this case, the preliminary processing will be either foundry or forging depending on the properties of the material, but can not give the desired accuracy even if we can make the hole. To achieve accuracy we must go for machining using an end milling cutter, but then, there will be a restriction on the corner radius depending on the size of the cutter. Again, if the corner is milled by corner milling attachment then there is a restriction on the length to diameter ratio of diameter means length across flat to flat). This trouble is seen due to fact that the shape of the job and its accuracy pose problems in manufacturing.
CASE III Now let us consider the job of same material as above, where shape and accuracy is not a problem as it is circular with open tolerances. In this case preliminary processing like casting and forging are not possible, so final operation is to be done by drilling. As the L/D ratio is more than 30, think of the difficulty in manufacturing as The drill may break The hole may be inclined too. This results to the conclusion that the size of the job some times creates a problem as well. CASE IV Now consider a contoured hole generation in a very hard material like WC or satellite for making a die block. Any conventional machining excepting diamond contour grinding is not possible. The contour grinding will provide the lay line orientations along the feed direction resulting in inadequate lubrication between the die and blank because of the directional property of the lay lines, again, the profil e errors of the die will be more
Due to the cross sensitivity factors of the 3D motions of the cutter. This increases production time (grinding is a low material removal process) since the other cutters (milling) do not have strength to machine these types of hard materials. So we can conclude that the increased strength of work material and inadequacy or complicacy of the control system (3D profiling) make it difficult to manufacture components to desired expectations.
CASE V Now let us consider a case, how improper technology deteriorates the reliability of a given component. Take an example of automobile piston and cylinder assembly. The basic requirement between the piston and cylinder is to provide a single degree of freedom to the movement of the piston (along the axial direction) with a restriction to rotational freedom to the piston.
To ease manufacture we choose cylindrical boring and maintain close clearance between the small end bearing of the connecting rod and gudgeon pin. These results — first, its vibration within the limits of two clearances (small end and big end) at higher speed of the piston causing lateral vibrations (rotation and transverse as well) of the pistons, thereby restrict the permissible speed; and second, the smaller contact area between the cylinder and piston perpendicular to the gudgeon pin axis results in high stress value at the contacting surface — increases the wear of the both and decreases the life of the engine; thirdly, the conventional boring operation will lead to a higher roughness value along the axial direction of the boring, resulting in high friction condition between the two due to the interlocking between the two crests (roughness) of piston and cylinder. This also prevents inadequate lubrication between piston and cylinder because the lubricant can not rise to the top from bottom due to the interlocking phenomenon. Any way, lapping may improve the surface finish but can not completely eliminate the phenomena.
Now, by some means, if we can impact orientation along the peripheral directions then rotational vibration may decrease with minimization of friction between the two mating parts (since the lay-lines are oriented axially) and the lubrication may be improved as lubricants now find way through capillary path towards the top. However, this can be further improved if we consider an elliptical piston and cylinder where elliptical hole generation is the only problem.
From these examples if it quite clear that the convention methods (e.g. turning, milling, grinding, lapping) are to be improved by unconventional machining methods that can Sustain productivity with increasing strength of the work material, Maintain productivity with desired shape, accuracy, and surface integrity requirements, and Improve the capability of automation system and decreasing their sophistication (decreasing the investment cost) requirements i.e. converting 3D control to 1D control of the tool movements.
Practice test:
1) 2) 3) 4) 5) 6) 7)
Brief about material removal volume in brittle materials. Brief about material removal volume in ductile materials. Classify Electro Thermal process? Classify Mechanical process? Classify Chemical process? Classify Electro Chemical process? Differentiate Primary and Secondary manufacturing.
Unit 2 - MECHANICAL ENERGY TECHNIQUES ABRASIVE JET MACHINING (AJM):
In Abrasive Jet Machining (AJM), abrasive particles are made to impinge on the work material at a high velocity. The jet of abrasive particles is carried by carrier gas or air. The high velocity stream of abrasive is generated by converting the pressure energy of the carrier gas or air to its kinetic energy and hence high velocity jet. The nozzle directs the abrasive jet in a controlled manner onto the work material, so that the distance between the nozzle and the work piece and the impingement angle can be set desirably. The high velocity abrasive particles remove the material by micro-cutting action as well as brittle fracture of the work material. Fig. 9.1.3 schematically shows the material removal process.
AJM is different from standard shot or sand blasting, as in AJM, finer abrasive grits are used and the parameters can be controlled more effectively providing better control over product quality. In AJM, generally, the abrasive particles of around 50 μm grit size would impinge on the work material at velocity of 200 m/s from a nozzle of I.D. of 0.5 mm with a stand off distance of around 2 mm. The kinetic energy of the abrasive particles would be sufficient to provide material removal due to brittle fracture of the work piece or even micro cutting by the abrasives. Equipment
In AJM, air is compressed in an air compressor and compressed air at a pressure of around 5 bar is used as the carrier gas as shown in Fig. 9.1.4. Fig. 9.1.4 also shows the other major parts of the AJM system.
Gases like CO2, N2 can also be used as carrier gas which may directly be issued from a gas cylinder. Generally oxygen is not used as a carrier gas. The carrier gas is first passed through a pressure regulator to obtain the desired working pressure. The gas is then passed through an air dryer to remove any residual water vapour. To remove any oil vapour or particulate contaminant the same is passed through a series of filters. Then the carrier gas enters a closed chamber known as the mixing chamber.
The abrasive particles enter the chamber from a hopper through a metallic sieve. The sieve is constantly vibrated by an electromagnetic shaker. The mass flow rate of abrasive (15 gm/min) entering the chamber depends on the amplitude of vibration of the sieve and its frequency. The abrasive particles are then carried by the carrier gas to the machining chamber via an electro-magnetic on-off valve. The machining enclosure is essential to contain the abrasive and machined particles in a safe and eco-friendly manner. The machining is carried out as high velocity (200 m/s) abrasive particles are issued from the nozzle onto a work piece traversing under the jet. Process Parameters:
• Abrasive
⎯ Material – Al2O3 / SiC / glass beads ⎯ Shape – irregular / spherical ⎯ Size – 10 ~ 50 μm ⎯ Mass flow rate – 2 ~ 20 gm/min • Carrier gas
⎯ Composition – Air, CO2, N2 ⎯ Density – Air ~ 1.3 kg/m
3
⎯ Velocity – 500 ~ 700 m/s ⎯ Pressure – 2 ~ 10 bar ⎯ Flow rate – 5 ~ 30 lpm
• Abrasive Jet ⎯ Velocity – 100 ~ 300 m/s ⎯ Mixing ratio – mass flow ratio of abrasive to gas –
⎯ Stand-off distance – 0.5 ~ 5 mm 0 0 ⎯ Impingement Angle – 60 ~ 90 • Nozzle
⎯ Material – WC / sapphire ⎯ Diameter – (Internal) 0.2 ~ 0.8 mm ⎯ Life – 10 ~ 300 hours The important machining characteristics in AJM are 3
• The material removal rate (MRR) mm /min or gm/min • The machining accuracy • The life of the nozzle THE EFFECT OF PROCESS PARAMETERS ON MRR IN AJM:
MODELLING OF MATERIAL REMOVAL:
As mentioned earlier, material removal in AJM takes place due to brittle fracture of the work material due to impact of high velocity abrasi ve particles. Modelling has been done with the following assumptions:
(i) Abrasives are spherical in shape and rigid. The particles are characterised by the mean grit diameter (ii) The kinetic energy of the abrasives are fully utilised in removing material (iii) Brittle materials are considered to fail due to brittle fracture and the fracture volume is considered to be hemispherical with diameter equal to chordal length of the indentation (iv) For ductile material, removal volume is assumed to be equal to the indentation volume due to particulate impact. Fig. 9.1.6 schematically shows the interaction of the abrasive particle and the work material in AJM.
Applications
• For drilling holes of intricate shapes in hard and brittle materials • For machining fragile, brittle and heat sensitive materials • AJM can be used for drilling, cutting, deburring, cleaning and etching. • Micro-machining of brittle materials Limitations
• MRR is rather low (around ~ 15 mm3/min for machining glass) • Abrasive particles tend to get embedded particularly if the work material is ductile • Tapering occurs due to flaring of the jet • Environmental load is rather high.
WATER JET MACHINING (WJM) :
It utilizes a high velocity stream of water as cutting agent. The mechanism of metal removal is erosion. Water is pumped at a sufficiently high pressure, 100-1000 MPa using intensifier technology. An intensifier works on the simple principle of pressure amplification using hydraulic cylinders of different cross-sections. From the intensifier /pump the water goes to an accumulator. The accumulator helps in eliminating pulsation and also acts as an energy reservoir such that cutting action may not be continuous. From the accumulator the water is lead to the nozzle through a high pressure thick stainless steel tubes. The material of the nozzle may be sintered diamond sapphire or tungsten carbide. When water at such pressure is issued through a suitable nozzle/orifice (generally of 0.075- 0.4 mm dia), the potential energy of water is converted into kinetic energy, yielding a high velocity jet (300-1000 m/min) and inducing high stress in the work material. The induced stress exceeds the ultimate shear stress of the material, rupture takes place. Such high velocity water jet can machine thin sheets/foils of aluminium, leather, textile, frozen food, plastics, wood, ceramics etc . HYDRAULIC SETUP FOR WJM:
Intensifier, shown in Fig. 5 is driven by a hydraulic power pack. The heart of the hydraulic power pack is a positive displacement hydraulic pump. The power packs in modern commercial systems are often controlled by microcomputers to achieve programmed rise of pressure etc. The hydraulic power pack delivers the hydraulic fluid to the intensifier at a pressure of ph. The ratio of cross-section of the two cylinders in the intensifier is say . Thus, pressure amplification would take place at the small cylinder as follows.
Thus, if the hydraulic pressure is set as 100 bar and area ratio is 40, pw = 100 x 40 = 4000 bar. By using direction control valve, the intensifier is driven by the hydraulic unit. The water may be directly supplied to the small cylinder of the intensifier or it may be supplied through a booster pump, which typically raises the water pressure to 11 bar before supplying it to the intensifier. Sometimes water is softened or long chain polymers are added in ―additive unit‖. Thus, as the intensifier works, it delivers high pressure water. As the larger piston changes direction within the intensifier, there would be a drop in the delivery pressure. To counter such drops, a thick cylinder is added to the delivery unit to accommodate water at high pressure. This is called an ―accumulator‖ which acts like a ―fly wheel‖ of an engine and minimises fluctuation of water pressure. High-pressure water is then fed through the flexible stainless steel pipes to the cutting head. It is worth mentioning here that such pipes are to carry water around 4000 bar (400 MPa) with flexibility incorporated in them with joints but without any leakage. Water jets do not create hazardous materials or vapours. It is truly a versatile, productive, cold cutting process.
The most important benefit of the water jet cutter is its ability to cut material without interfering with the materials inherent structure as there is no "heat affected zone" (HAZ). This allows metals to be cut without harming their intrinsic properties. WJM can be achieved using two approaches as enumerated below:
WJM - Pure WJM - with stabilizer In pure WJM, commercially pure water (tap water) is used for machining purpose. However as the high velocity water jet is discharged from the orifice, the jet tends to entrain atmospheric air and flares out decreasing its cutting ability. Hence, quite often stabilisers (long chain polymers) that hinder the fragmentation of water jet are added to the water. Effect of Stand of distance (SOD) with MRR:
MRR increases with the increase of SOD up to a certain limit after which it remains unchanged for a certain tip distances and then falls gradually. A large SOD produces divergence of jet which affects the accuracy and quality.
Small MRR at low SOD is due to reduction in nozzle pressure with decreasing distance. Drop in MRR at large SOD is due to a reduction in the jet velocity with increasing distance. OPERATING PARAMETERS: Fluid:
Type : Water or Water with additives Additives : Glycerin, Polyethylene Oxide or long chain polymers. Pressure : 1 to 10 kbar (100 to 1000 MPa) Jet Velocity : 300 to 1000 m/min. Flow rate : upto 8 Lit/min. Jet force on work piece : 0.5 to 15 kg (5 to 15 N). Nozzle:
Material : Hardened Steel, WC, Synthetic Sapphire. Diameter : 0.075 to 0.4 mm. SOD : 2.5 to 50 mm. Angle: normal to 30˚ positive rake.
Advantage:
1) Energy transfer media (water) is cheap, non toxic and easy to dispose. 2) Work area remains clean and dust free. 3) Process is environmentally safe and friendly manufacturing. 4) Low maintenance and operating cost is low as it has no moving parts. 5) Intricate contour can be cut. 6) There is no thermal damage. 7) Cut can be started anywhere without the need for predrilled holes 8) Minimum burr produced. 9) Extremely fast set-up and programming 10) Very little fixturing for most parts 11) Machine virtually any 2D shape on any material 12) Very low side forces during the machining Disadvantage:
1) Initial cost is high. 2) Hard material cannot be cut. 3) Lack of suitable pumping devices. 4) Noisy operation.
ULTRA SONIC MACHINING (USM): Ultrasonic machining is a non-traditional machining process. USM is grouped under the mechanical group NTM processes. Fig. 9.2.1 briefly depicts the USM process.
In ultrasonic machining, a tool of desired shape vibrates at an ultrasonic frequency (19 ~ 25 kHz) with an amplitude of around 15 – 50 μm over the work piece. Generally the tool is pressed downward with a feed force, F. Between the tool and work piece, the machining zone is flooded with hard abrasive particles generally in the form of water based slurry. As the tool vibrates over the work piece, the abrasive particles act as the indenters and indent both the work material and the tool. The abrasive particles, as they indent, the work material, would remove the same, particularly if the work material is brittle, due to crack initiation, propagation and brittle fracture of the material. Hence, USM is mainly used for machining brittle materials {which are poor conductors of electricity and thus cannot be processed by Electrochemical and Electro-discharge machining (ECM and EDM)}. Machine:
The basic mechanical structure of an USM is very similar to a drill press. However, it has additional features to carry out USM of brittle work material. The work piece is mounted on a vice, which can be located at the desired position under the tool using a 2 axis table. The table can further be lowered or raised to accommodate work of different thickness. The typical elements of an USM are (Fig. 9.2.7)
Slurry delivery and return system Feed mechanism to provide a downward feed force on the tool during machining
The transducer, which generates the ultrasonic vibration The horn or concentrator, which mechanically amplifies the vibration to the required amplitude of 15 – 50 μm and accommodates the tool at its tip. The ultrasonic vibrations are produced by the transducer. The transducer is driven by suitable signal generator followed by power amplifier. The transducer for USM works on the following principle
Piezoelectric effect Magnetostrictive effect Electrostrictive effect
Magnetostrictive transducers are most popular and robust amongst all. Fig. 9.2.8 shows a typical magnetostrictive transducer along with horn. The horn or concentrator is a wave-guide, which amplifies and concentrates the vibration to the tool from the transducer. Mechanisms of Material Removal in USM and its modelling:
As has been mentioned earlier, USM is generally used for machining brittle work material. Material removal primarily occurs due to the indentation of the hard abrasive grits on the brittle work material. As the tool vibrates, it leads to indentation of the abrasive grits. During indentation, due to Hertzian contact stresses, cracks would develop just below the contact site, then as indentation progresses the cracks would propagate due to increase in stress and ultimately lead to brittle fracture of the work material under each individual interaction site between the abrasive grits and the workpiece. The tool material should be
such that indentation by the abrasive grits does not lead to brittle failure. Thus the tools are made of tough, strong and ductile materials like steel, stainless steel and other ductile metallic alloys. Other than this brittle failure of the work material due to indentation some material removal may occur due to free flowing impact of the abrasives against the work material and related solid-solid impact erosion, but it is estimated to be rather insignificant. Thus, in the current model, material removal would be assumed to take place only due to impact of abrasives between tool and workpiece, followed by indentation and brittle fracture of the workpiece. The model does consider the deformation of the tool. In the current model, all the abrasives are considered to be identical in shape and size. An abrasive particle is considered to be spherical but with local spherical bulges as shown in Fig. 9.2.2. The abrasive particles are characterised by the average grit diameter, dg. It is further assumed that the local spherical bulges have a uniform diameter, db and which is related to the grit diameter by db = μdg2. Thus an abrasive is characterised by μ and dg.
Fig.9.2.3 During indentation by the abrasive grit onto the workpiece and the tool, the local spherical bulges contact the surfaces and the indentation process is characterised by d b rather than by d g. Fig. 9.2.3 shows the interaction between the abrasive grit and the work piece and tool. MRR
dg - Diameter of grit. f - Frequency of the tool vibration. C - Concentration of abrasive grit. A - total surface area of the tool facing the workpiece. F - Tool feed force. a0 - Amplitude of vibration. σ w - Flow strength of work material. λ - σ w / σ t μ = db / dg2
EFFECT OF OPERATING PARAMETERS ON MRR IN USM:
Assumption of cook’s model:
This model is based on the brittle fracture of the work materials and under following assumptions.
The abrasive grits are spherical in nature. Material removal is based on hemispherical fracture mechanism due to the indentation.
Tool and abrasive are rigid.
PROCESS PARAMETERS AND THEIR EFFECTS:
During discussion and analysis as presented in the previous section, the process parameters which govern the ultrasonic machining process have been identified and the same are listed below along with material parameters. • Amplitude of vibration (ao) – 15 – 50 μm • Frequency of vibration (f) – 19 – 25 kHz • Feed force (F) – related to tool dimensions • Feed pressure (p) • Abrasive size – 15 μm – 150 μm • Abrasive material
– Al2O3 - SiC - B4C - Boronsilicarbide - Diamond
• Flow strength of work material • Flow strength of the tool material • Contact area of the tool – A • Volume concentration of abrasive in water slurry – C Applications
Used for machining hard and brittle metallic alloys, semiconductors, glass, ceramics, carbides etc.
Used for machining round, square, irregular shaped holes and surface impressions. Machining, wire drawing, punching or small bla nking dies. Limitations
Low MRR Rather high tool wear Low depth of hole Practice Test:
1. 2. 3. 4. 5. 6. 7.
What are the limitations of USM? What are the materials that can be machined using WJM? Write the application of WJM. Write the operating condition of fluid for WJM. Write the operating condition of nozzle for WJM. Write the assumptions of Cook‘s model in USM. Brief about the effect of SOD with MRR in WJM.
Unit 3 - ELECTRICAL ENERGY TECHNIQUES Electro Chemical Machining (ECM):
Electrochemical Machining (ECM) is a non-traditional machining (NTM) process belonging to Electrochemical category. ECM is opposite of electrochemical or galvanic coating or deposition process. Thus ECM can be thought of a controlled anodic dissolution at atomic level of the work piece that is electrically conductive by a shaped tool due to flow of high current at relatively low potential difference through an electrolyte which is quite often water based neutral salt solution.
Principle of operation of ECM:
During ECM, there will be reactions occurring at the electrodes i.e. at the anode or workpiece and at the cathode or the tool along with within the electr olyte. 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
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 workpiece. Thus the hydrogen ions will take away electrons from the cathode (tool) and from hydrogen gas as:
Similarly, the iron atoms will come out of the anode (work piece) as:
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
In practice FeCl2 and Fe(OH)2 would form and get precipitated in the form of sludge. In this manner it can be noted that the work piece gets gradually machined and gets precipitated as the sludge. Moreover there is not coating on the tool, only hydrogen gas evolves at the tool or cathode. Fig. 2 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.
ELEMENT OF ECM PROCESS:
(i) Cathode tool (of a shape which is almost the mirror image of the cavity to be machined into the work piece). (ii) Anode work piece. (iii)Source of D.C power. (iv)Electrolyte, a conductive liquid.
Advantages of Nacl or KCl electrolyte are
(i) Inexpensive. (ii) Non toxic. (iii)No fire hazard. (iv)Can be used for variety of materials. (v) Current efficiency is high. (vi)At high concentration conductivity is easy to control. Equipment
The electrochemical machining system has the following modules:
Power supply Electrolyte filtration and delivery system Tool feed system Working tank
Fig. 4 schematically shows an electrochemical drilling unit.
2-30V dc of the order of 50 to 40,000 amps is supplied across the anode and the cathode. The electrolyte flows in the gap between the tool and the work piece at a velocity of 30 to 60 m/s. The temperature of the electrolyte is maintained at about 40°C. In operation, the tool (cathode) is moved towards the workpiece (anode) and the electrolyte by electrolysis process dissociates into ions carrying positive and negative' electrical charges. The cathode attracts the positively charged ions (cations) from the electrolyte, while the negatively changed ions (anions) move towards the anode, thereby completing the electric circuit. The anions cause the anode to dissociate and dissolve into the electrolyte. These dissolved metal ions are continuously carried away by the flowing electrolyte. The metal removal thus takes place, and gets the shape of the tool. The commonly used electrolytes are sodium chloride (NaCl), sodium nitrate (NaN03) potassium chloride (KCl), sodium hydroxide (NaOH), sodium fluoride (Na2F) and sulfuric acid (H2S04).
Functions of Electrolytes:
The electrolyte is the working medium and part of tooling in an ECM process. It performs three important functions: (i) It carries the current between the tool and the work piece . (ii) It carries away the products of machining from the work – tool gap. (iii)It removes the heat produced in operation from the working surfaces.
Characteristics of ECM:
Tool and work material – electrically conductive. Atomic level dissolution. Surface finish excellent. Almost stress free machined surface. No thermal damage.
MODELLING OF MATERIAL REMOVAL RATE :
Material removal rate (MRR) is an important characteristic to evaluate efficiency of a non-traditional machining process. In ECM, material removal takes place due to atomic dissolution of work material. Electrochemical dissolution is governed by Faraday’s laws. The first law states that “ the amount of electrochemical dissolution or deposition is proportional to amount of charge passed through the electrochemical cell ” , which may be expressed as:
where W = mass of material dissolved or deposited
The second law states that “ the amount of material deposited or dissolved further
depends on Electrochemical E quivalence (ECE) of the material that is again the ratio of the atomic weigh and valency” . Thus
Derived in Class
PROCESS PARAMETERS Power Supply Type Voltage Current
direct current 2 to 35 V 50 to 40,000 A
Current density Electrolyte Material
2
0.1 A/mm to 5 A/mm
2
NaCl and NaNO o
Temperature Flow rate Pressure Dilution
o
3
20 C – 50 C 20 lpm per 100 A current 0.5 to 20 bar 100 g/l to 500 g/l
Working gap Overcut Feed rate Electrode material Surface roughness, R
a
0.1 mm to 2 mm 0.2 mm to 3 mm 0.5 mm/min to 15 mm/min Copper, brass, bronze 0.2 to 1.5 μm
The important process variables that affect the ECM operation are as follows: 1) Voltage: The voltage across the cutting gap between the tool and work influences the current and hence the material removal rate. This is the primary controlling factor in most ECM operations. However for a given constant voltage, current also depends on the electrical resistance between the gap, which is further a factor of 'the conductivity of the electrolyte and the gap size. 2) Feed Rate: This .is the rate of penetration in ECM process. For a given voltage, both the frontal gap and side gap (see Fig. 2-8) are inversely proportional to the feed rate. The distance across the frontal gap is a function of the feed rate. As the gap reduces, the resistance drops, increasing the amperage and thus increasing the machining rate. Similarly, side gap also affects the feed rate. Frontal gaps are usually in the range of 0.1 to 0.8 mm, while the side gaps are of the order of 0.5 to 1.3 mm. 3. Current density: The feed rate varies directly with the current. Higher machining rates requires higher current densities .with higher voltages which in turn increases the power consumption 4) Electrolyte flow rate: Electrolyte flow rate is important in controlling the machining rate, and temperature control of the ECM process. Flow rate also has an influence on the level of turbulence and hence on the surface finish and taper on the work material. The flow should carry away the sludge formed along with it.
Advantages 1. 2. 3. 4. 5. 6. 7.
Very hard materials can be machined. Complex shapes can be produced. Tool wear is negligible. High surface finish can be obtained. Very thin metals sheets can be machined. Residual stresses induced are almost nil, and no distortion. There is no cutting forces therefore clamping is not required except for controlled motion of the work piece. 8. There is no heat affected zone. 9. Very accurate. 10. Relatively fast and a time saving process. 11. During drilling, deep holes can be made or several holes at once. 12. ECM deburring can debur difficult to access areas of parts.
Limitations: 1. 2. 3. 4. 5. 6.
More expensive than conventional machining. Need more area for installation. Electrolytes may destroy the equipment. Not environmentally friendly (sludge and other waste) High energy consumption. Material has to be electrically conductive.
APPLICATIONS:
ECM is used for machining hard metals and alloys, for making dies and tools, machining of complex contours, machining of tungsten carbide and high strength heat resisting alloys. The important applications are1. Machining hard heat resistant alloys. 2. Machining of thorough holes of any cross section. 3. Machining of shaped cavities (like forging dies). 4. Machining of complex external shapes (like turbine blades). 5. Wire cutting of heavy slugs of metals. 6. Machining of blind holes (regular and irregular shapes). 7. Die sinking 8. Profiling and contouring 9. Trepanning 10. Grinding 11. Drilling 12. Micro-machining
ELECTRO CHEMICAL GRINDING (ECG):
The work is machined by the combined action of electrochemical effect and conventional grinding operation. The majority of the metal removal from electrolytic action. The tool electrode is a rotating metal diamond or aluminium oxide wheel and it acts as cathode. The work acts as anode and hence current flow between work and wheel. A constant gap of about 0.25mm is maintained between work and wheel and through this gap an essentially neutral electrolyte is circulated. The Electrolyte is carried past the work surfaces at high speed by the rotary action of the grinding wheel. The grinding wheel runs at speeds o 900 - 1800 rpm. In this process 90% of the metal is removed by electrolytic actions and only about 10% by abrasive grinding. Electrolyte:
aqueous solution o sodium silicate, sodium nitrate, borax.
Grinding Wheel : Abrasives :
Aluminium oxide, dimond.
Size:
60-80 mesh grid.
Bonding agents: copper, brass and nickel.
The abrasives continuously remove the machining products from the working area. In the machining system shown in Fig. 2, the wheel is a rotating cathodic tool with abrasive particles (60 – 320 grit number) on its periphery. Electrolyte flow, usually NaNO3, is provided for ECD. The wheel rotates at a surface speed of 20 to 35 m/s, while current ratings are from 50 to 300 A. Material removal rate
When a gap voltage of 4 to 40 V is applied between the cathodic grinding wheel and the anodic workpiece, a current density of about 120 to 240 A/cm2 is created. The current density depends on the material being machined, the gap width, and the applied voltage. Material is mainly removed by ECD, while the MA( mechanical abrasion) of the abrasive grits accounts for an additional 5 to 10 percent of t he total material removal. ECG is a hybrid machining process that combines mechanical abrasion and ECD. The machining rate, therefore, increases many times; surface layer properties are improved, while tool wear and energy consumption are reduced. While Faraday‘s laws govern the ECD phase, the action of the abrasive grains depends on conditions existing in the gap, such as the electric field, transport of electrolyte, and hydrodynamic effects on boundary layers near the anode. The contribution of either of these two machining phases in the material removal process and in surface layer formation depends on the process parameters. Figure 3 shows the basic components of the ECG process. The contribution of each machining phase to the material removal from the work piece has resulted in a considerable increase in the total removal rate QECG, in relation to the sum of the removal rate of the electrochemical process and the grinding processes QECD and QMA, when keeping the same values of respective parameters as during the ECG process. As can be seen in Fig 4, the introduction of MA, by a rotary conductive abrasive wheel, enhances the ECD process. The work of the abrasive grains performs the mechanical depolarization by abrading the possible insoluble films from the anodic workpiece surface. Such films are especially formed in case of alloys of many metals and cemented carbides. A specific purpose of the abrasive grains is, therefore, to depassivate mechanically the workpiece surface. In the machining zone there is an area of simultaneous ECD and MA of the workpiece surface, where the gap width is less than the height of the grain part projecting over the binder. Another area of pure electrochemical removal where the abrasive grains do not touch the workpiece surface exists at the entry and exit sides of the wheel. The important functions of abrasive particles are:-
It acts as insulators to maintain a small gap between wheel and work piece. They remove electrolysis products from the work area. To cut chips if the wheel should continue the work particularly in the event of power failure.
Advantage of ECG over conventional grinding:
Disadvantages 1. Higher capital cost than conventional machines
2. Process limited to electrically conductive materials 3. Corrosive nature of electrolyte 4. Requires disposal and filtering of electrolyte
ELECTRO CHEMICAL HONING PROCESS: Process
A process similar to electrochemical grinding involving the use of honing stones rather than a grinding wheel. In this process, the stock removal capabilities of ECM are combine with the accuracy capabilities of honing. The basic principles of the process regarding voltage, current, electrolyte and materials processed are the same as those described under ECM. The process, as shown in Fig. 3.11, consists in rotating and reciprocating the tool inside the cylindrical components. The electrolyte is fed under pressure
through holes in the tool so that at every point there is uniform flow and velocity. The gap between the tool and the work piece is usually adjusted by the use of an expanding tool. At
The start of the operation, the gap is approximately 1 mm and increases during the cycle. Bonded abrasive honing stones are forced out with equal pressure in all directions from slots in the tool. These stones are non-conductive and assist in electrochemical action. They also abrade the residue left by the electrochemical action. A clean surface thus generated is ready for further chemical attack. If the work piece is not round but tapered or wavy, the stones cut away all high areas and remove t he geometric error. As in conventional honing, the abrasive should be selfdressing. If the stones glaze, the stock removal rate is reduced. If they dress too rapidly, the operating cost becomes high. The design of the tool and the adjusting head mechanism is important for proper abrasive application.
Accuracy and Surface Finish
Size tolerance of 0.01 mm on the diameter can be obtained and roundness can be maintained at less than 0.005 mm. Surface roughness of the order of 0.1-0.5 micron CLA is obtainable by this method but if it is required to have a specified roughness, the stones are put to work for a few seconds after the power is switched off. The surface finish obtained in this manner is dependent upon the size of abrasive grains, speed of rotation and reciprocation, and duration of the ‗run out‘ period.
Advantages
The advantages of electrochemical honing are similar to those claimed for electrolytic grinding: increased metal removal rate particularly on hard materials, burr-free action, less
pressure required between stones and work, reduced noise and distortion when honing thinwalled tubes, cooler action leading to increased accuracy with less material damage.
Applications
The process is easily adaptable to cylindrical parts for trueing the inside surfaces. The size of the cylinder that can be processed by this method is limited only by the current and electrolyte that can be supplied and distributed. Any surface roughness compatible with the material being cut is duplicated over a number of component parts.
Practice Test: 1. Define Faraday‘s laws. 2. What are the elements of ECM process? 3. Write the disadvantages of ECG. 4. Write the characteristics of ECM. 5. What are the advantages of Nacl or KCl electrolyte in ECM? 6. What are the functions of electrolyte used in ECM process? 7. What are the functions of abrasive particles in ECG?
Unit – 4. THERMO ELECTRICAL ENERGY TECHNIQUES
Electrical Discharge Machining (EDM):
It is also known as spark erosion or spark machining. It is a process of metal removal based on the principle of erosion of metals by an interrupted electric spark discharge between the electrode tool (cathode) and the work (anode). In EDM process electric energy is used to cut the material to final shape and size. Efforts are made to utilize all the energy by applying it at the exact spot where the operation to be carried out. No complicated fixtures are needed for holding the job and even very thin jobs can be machined to the desired dimensions and shape. All the operating is carried out in a single setup. This process may be applied to machine steels, super alloys, refractories etc. When a difference of potential is applied between two conductors immersed in a dielectric fluid, the fluid will ionize if the potential difference reaches a high enough valve and spark will occur. If the potential difference is maintained then the spark will develop into an arc, otherwise spark extinguishes. The control erosion of metal is achieved by the rapidly recurring spark discharge produced between two electrodes one tool and other work end spark impinging against the surface of the work piece which must be an electrically conducting body. A suitable gap known as spark gap is maintained between tool and the work by a servo motor which feeds the tool downward towards the work piece.
The MRR depends on spark gap maintained. If both the electrodes are made of same material it has been found that the greatest erosion takes place upon the positive electrode. To remove maximum metal and have minimum wear of tool, the tool is made the cathode and work piece the anode. The two electrodes are separated by a dielectric fluid medium such as paraffin or transformer oil which is pumped through the tool or work piece at a pressure of 2 kgf/cm2. The current may vary from 0.5 to 400amp at 40 to 300 V DC. Pulse duration 2 to 2000. The moment the spark occurs; sufficient pressure is developed between the work and tool. The repetitive sparks releases their energy in the form of local heat as a result of which local temp of around 10000C is reached at the spot hit by electrons and at such high pressure and temp, the metal is melted and some of vaporized and some particles are carried away by dielectric fluid circulated around it, forming a crater around the work piece.
Spark Generator:
Supplies adequate voltage to initiate and maintain the disc harge current intensity and discharge duration and controlling the recurring rhythm of the discharge. Electrode Material
Electrode material should be such that it would not undergo much tool wear when it is impinged by positive ions. Thus the localized temperature rise has to be less by tailoring or properly choosing its properties or even when temperature increases, there would be less melting. Further, the tool should be easily workable as intricate shaped geometric features are machined in EDM. Thus the basic characteristics of electrode materials are: 1. High electrical conductivity – electrons are cold emitted more easily and there is less bulk electrical heating 2. High thermal conductivity – for the same heat load, the local temperature rise would be less due to faster heat conducted to the bulk of the tool and thus less tool wear 3. Higher density – for the same heat load and same tool wear by weight there would be less volume removal or tool wear and thus less dimensional loss or inaccuracy 4. High melting point – high melting point leads to less tool wear due to less tool material melting for the same heat load 5. Easy manufacturability 6. Cost – cheap The followings are the different electrode materials which are used commonly in the industry: • Graphite • Electrolytic oxygen free copper • Tellurium copper – 99% Cu + 0.5% tellurium • Brass Dielectric Fluid
In an EDM operation, a dielectric fluid in an essential working medium. It is flushed through the spark gap or supplied to the gap through a hole in the tool or from external jets. In some cases the fluid is also supplied through openings in the work piece wherever feasible. The flow of this working medium is designed to take in specific directions to favour the machining operations. The different flow patterns are discussed in the subsequent sections. Functions of Dielectric Fluids
The functions of a dielectric fluid in EDM are as follows: 1. To serve as a spark conductor in the spark gap between the tool and work material. 2. To act as a coolant to quench the spark and to c ool the tool and work piece. 3. To carry away the condensed metal particles and to maintain the gap for continuous and smooth operation.
Requirements of a Dielectric Fluid
The essential requirements in the selection of a dielectric fluid are as follows: 1. It should have a stable and sufficient dielectric strength to act as insulation between the tool and the work piece. 2. It should have a low viscosity (for smooth flow) and high wettability. 3. It should be chemically inert, so that the tool, work piece, container, etc., are not attacked. 4. It should be able to deionise immediately after the spark discharge. 5. It should have a high flash point. 6. It should not emit toxic vapours and should not have unpleasant odours. 7. It should not alter its basic properties under operating conditions of temperature variations, contamination by metal particles and products of decomposition. 8. It should be economical for use. Advantages:
1. It can be applied to all electrically conducting metals & alloys irrespective of their melting point, hardness, toughness and brittleness. 2. Any complicated shape that can be made by fabricating of tool can be reproduced on the work piece. 3. The machining faster than conventional machining. 4. It can be employed for extremely hard material. 5. No residual stresses. Disadvantages:
1. Excessive tool wear. 2. High specific power consumption. 3. Machining heats the work piece considerably. Applications:
Manufacture of process tools, extrusion dies, forging dies and moulds. – drill the holes in hardened points like nozzle.
WIRECUT EDM:
This process is similar to contour cutting with a band saw. A slow moving wire travels along a prescribed path, cutting the work piece with
discharge sparks. Material removal is affected as a result of spark erosion as the wire electrode is fed through the work piece. In most of the cases, horizontal movement of the worktable, controlled by CNC on modern machines, determines the path of cut. However, some machines move the wire horizontally to define the path of cut, leaving the part stationary. On both types of machining configurations, the wire electrode moves vertically over sapphire or diamond wire guides, one above and one below the work piece. The electrode wire is used only once, then discarded because the wire loses its form after one pass through the work piece. A steady stream of deionized water or other fluid is used to cool the work piece and electrode wire and to flush the cut area. Viewed from above, the electrode wire cuts a slot or ―kerf‖. The width of the kerf is the wire diameter plus EDM overcut as illustrated in figure. Strater or threading holes are required. In steel or other material a drilled hole suffices for carbide; the hole may have to be produced by EDM or micro EDM. Wire should have sufficient tensile strength and fracture toughness. Wire is made of brass, copper or tungsten. (About 0.25mm in diameter).
THEORIES OF MATERIAL REMOVAL CONCEPTS: a) High pressure theory. b) Static field theory. c) High temperature theory. a) High pressure theory:
Due to sudden stoppage of electro dynamic waves, high impulsive pressure responsible for the erosion of electrodes is released on the electrode surface.
Pressure of electrical discharge reported might be as high as 1000 kgf/mm2, but expected plastic deformation was not found on the surface.
From energy distribution of discharge spectrum, the pressure in the ar c column remains between 10 and 100 kgf/cm2.
Actually the pressure is less than that reported because the acting area of discharge pressure is thought to be wider than the crater area.
It is obtained that duration during which pressure acts is longer than discharge periods.
To conclude, in smaller energy discharges, the discharge pressure alone would not be sufficient to erode the material, but in conjunction with some other factor, such as heat, the pressure might blow out molten metal from the electrode surface.
b) Static field theory:
Two charged electrodes experience an electrostati c force according to Coulomb‘s law. Accordingly, the force between the electrodes produce stress on the electrodes which, when the gap is very small, may cross the ultimate stress limit of the electrode material resulting in a tensile rupture.
The force involved in tensile rupture erosion arises because the extremely high current densities beneath the surface of the anode produce a strong electric field gradient, which acts on the positive ions of the crystal lattice.
When this force reaches the tensile strength of the material, a tensile fracture occurs removing one or several particles.
However, this field theory is found to be incorrect because forces into metal lattice due to colliding electrons belong to the force created by the static field.
c) High temperature theory:
According to this theory, due to the bombardment of high energetic electrons on the electrode surface, the spot attains high temperature about 10,000˚C especially with materials of low thermal conductivity.
At this high temperature, material at that spot instantaneously melts and vaporises leaving a crater on the surface.
This high temperature is not generated by electron bombardments alone. The Joule – heating by high density current is also considered to contribute. Gradually high temperature theory became promising. Many experiments were carried out for exploring this theory.
It was established that the energy given to anode per unit area Wa and whole discharge energy Wo bear the following relation.
Where d = the gap between electrodes, ƛ = mean free path of electron, b = l/ ƛ , l = cathode fall area, m = mass of electron, mg = mass of gas molecule, Fa, Fc = anode and cathode work functions, Vc = cathode fall.
FLUSHING TECHNIQUES:
The circulation of dielectric fluid between the electrode and the workpiece is called flushing. The effective flushing removes waste products from the gap whereas the bad flushing results in low MRR and poor surface finish. The good flushing system is one that shoots the dielectric to the place where the sparking occurs. It is observed that flushing in blind cavities is difficult. So, flushing does not perform good in blind cavities.
Pressure (or Injection) Flushing
Pressurized clean dielectric is pumped into the EDM gap via either a predrilled hole in the tool (Fig.) or the workpiece (Fig). The liquid flows upwards in the peripheral gap sweeping the erosion products into the open work tank. Occasionally, debris builds up in the gap on either tool or workpiece surface and so-called ―evacuation-discharges‖ result.
The gap gradually enlarges because of this kind of discharge; with more of these discharges taking place in the ‗down-stream‘ part of the gap, because of the higher debris content of the dielectric fluid having passed further through the gap, a tapered hole is always, produced even though the tool used is parallel sided.
Vacuum (or Suction) Flushing
Incontrast to pressure flushing, vacuum flushing sucks used dielectric fluid laden with erosion products either through the electrode (directly or indirectly through the electrode holder) Fig) or through the workpiece (Fig). Clean dielectric fluid from the work tank flows into the peripheral gap to replace the used dielectric sucked out. Since this peripheral gap is flushed with clean (or at worst only slightly contaminated) fluid, very few evacuation discharges occur down the sodes of the tool, though some taper may occur at the bottom of the cavity. Obviously, vacuum flushing produces an almost constant side gap.
Side Flushing
There are some applications in which the drilling of flushing holes through tool or workpiece are not possible (for example when EDMing coining dies or deep narrow slots or processes in plastics molding dies). In these cases the gap has to be flushed using carefully
adjusted nozzles forcing sets of fluid to flow evently around the periphery of the tool electrode (Fig.8.40a) The nozzle surrounds between a sixth to a third of the tool periphery. The direction of the jet of dielectric fluid has to be carefully adjusted to coincide with the angle of the gap so that the flow is directed parallel to the surfaces which define the gap (Fig.8.41a) If the direction of flushing is not parallel with the electrode surface (Fig.8.41a) and (b), then turbulence results and only a small proportion of the dielectric fluid actually enters the gap, so that the actual flushing will be inadequate. Another disadvantageous result of the fluid jet being applied perpendicular to the side of a tool is that deflection and/or vibration of the tool may occur (Fig. 8.41b). An important working rule is to avoid directing the jets of fluid symmetrically to opposite sides of a tool since the flows will tend to cancel each other at the bottom of the cavity with the consequence that erosion products are not flushed awa y (Fig. 8.41c)
Fig. 8.42 shows a special application of side flushing in which clean dielectric fluid is assisted into the EDM gap by rotation of the electrode and the viscosity of the fluid. Many geometrical variations of this technique are possible and commonly used, particularly in wire cut EDM where the longitudinal feed of the wire introduces fresh dielectric fluid into the EDM gap.
Reciprocating Electrode Flushing
When EDMing deep cavities, the tool can be moved periodically up and down, thus, introducing fresh dielectric fluid into the gap and expelling contaminated dielectric fluid from the gap respectively. At each cycle (Fig. 8.43) fresh liquid is mixed with contaminated fluid by the upward movement of the electrode and the mixture is partially expelled from the
cavity by the downward movement. So at each strode the contamination of the dielectric fluid by the EDM debris is reduced.
Side flushing and reciprocating electrode flushing may be used together so that erosion debris leaves from the opposite side of the gap from the inlet jet. Evaluation discharge cause tapering similar to that experienced in pressure flushing, though the flushing is less effective and the side tapering rather larger with this method than with pressure flushing.
ELECTRICAL DISCHARGE GRINDING (EDG):
Practice Test:
1. Define breakdown in EDM. 2. What is the dielectric used in EDM? 3. What are the different types of pulsing unit in EDM? 4. What is the main basic mechanism of dielectric breakdown in the three states of matter? 5. Write the application of EDM. 6. What are the factors that determine the efficiency of EDM? 7. What are the basic units constituting an EDM equipment?
UNIT – 5 THERMAL ENERGY TECHNIQUES
CONSTRUCTION AND WORKING OF LASER: The Lasing Process
Lasing process describes the basic operation of laser, i.e. generation of coherent (both temporal and spatial) beam of light by ―light amplification‖ using ―stimulated emission‖. In the model of atom, negatively charged electrons rotate around the positively charged nucleus in some specified orbital paths. The geometry and radii of such orbital paths depend on a variety of parameters like number of electrons, presence of neighbouring atoms and their electron structure, presence of electromagnetic field etc. Each of the orbital electrons is associated with unique energy levels. At absolute zero temperature an atom is considered to be at ground level, when all the electrons occupy their respective lowest potential energy. The electrons at ground state can be excited to higher state of energy by absorbing energy form external sources like increase in electronic vibration at elevated temperature, through chemical reaction as well as via absorbing energy of the photon. Fig. 9.6.7 depicts schematically the absorption of a photon by an electron. The electron moves from a lower energy level to a higher energy level. On reaching the higher energy level, the electron reaches an unstable energy band. And it comes back to its ground state within a very small time by releasing a photon. This is called spontaneous emission. Schematically the same is shown in Fig. 9.6.7 and Fig. 9.6.8. The spontaneously emitted photon would have the same frequency as that of the ―exciting‖ photon. Sometimes such change of energy state puts the electrons in a meta-stable energy band. Instead of coming back to its ground state immediately (within tens of ns) it stays at the elevated energy state for micro to milliseconds. In a material, if more number of electrons can be somehow pumped to the higher meta-stable energy state as compared to number of atoms at ground state, and then it is called “population inversion”. Such electrons, at higher energy meta-stable state, can return to the ground state in the form of an avalanche provided stimulated by a photon of suitable frequency or energy. This is called stimulated emission. Fig. 9.6.8 shows one such higher state electron in meta-stable orbit. If it is stimulated by a photon of suitable energy then the electron will come down to the lower energy state and in turn one original photon, another emitted photon by stimulation having some temporal and spatial phase would be available. In this way coherent laser beam can be produced.
F ig. 9.6 9.6.7 .7 Energy bands in materials
Fig. 9.6.9 schematically shows working of a laser. There is a gas in a cylindrical glass vessel. This gas is called the lasing medium. One end of the glass is blocked with a 100% reflective mirror and the other end is having a partially reflective mirror. Population inversion can be carried out by exciting the gas atoms or molecules by pumping it with flash lamps. Then stimulated emission would initiate lasing action. Stimulated emission of photons could be in all directions. Most of the stimulated photons, not along the longitudinal direction would be lost and generate waste heat. The photons in the longitudinal direction would form coherent, highly directional, intense laser beam.
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 – alumina alloy having a wavelength of 0.7 μm • NdNd-glass lasers having a wavelength of 1.64 μm • NdNd-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 – Neon • Argon • CO etc. 2
Lasers can be operated in continuous mode or pulsed mode. Typically CO gas laser is 2
operated in continuous mode and Nd – Nd – YAG YAG laser is operated in pulsed mode.
1. SOLID STATE LASER :
Fig. 9.6.10 shows a typical Nd-YAG laser. Nd-YAG laser is pumped using flash tube. Flash tubes can be helical, as shown in Fig. 9.6.10, or they can be flat. Typically the lasing material is at the focal plane of the flash tube. Though helical flash tubes provide better pumping, they are difficult to maintain.
Fig. 9.6.11 shows the electrical circuit for operation of a solid-state laser. The flash tube is operated in pulsed mode by charging and discharging of the capacitor. Thus the pulse on time is decided by the resistance on the flash tube side and pulse off time is decided by the charging resistance. There is also a high voltage switching supply for initiation of pulses.
2. GAS LASER:
Fig. 9.6.12 shows a CO2 laser. Gas lasers can be axial flow, as shown in Fig. 9.6.12, transverse flow and folded axial flow as shown in Fig. 9.6.13. The power of a CO2 laser is typically around 100 Watt per metre of tube length. Thus to make a high power laser, a rather long tube is required which is quite inconvenient. For optimal use of floor space, high powered CO2 lasers are made of folded design.
In a CO2 laser, a mixture of CO2, N2 and He continuously circulate through the gas tube. Such continuous recirculation of gas is done to minimize consumption of gases. CO2 acts as the main lasing medium whereas Nitrogen helps in sustaining the gas plasma. Helium on the other hand helps in cooling the gases. As shown in Fig. 9.6.12 high voltage is applied at the two ends leading to discharge and formation of gas plasma. Energy of this discharge leads to population inversion and lasing action. At the two ends of the laser we have one 100% reflector and one partial reflector. The 100% reflector redirects the photons inside the gas tube and partial reflector allows a part of the laser beam to be issued so that the same can be used for material processing. Typically the laser tube is cooled externally as well. As had been indicated earlier CO2 lasers l asers are folded t o achieve high power. Fig. 9.6.13 shows a similar folded axial flow laser. In folded laser there would be a few 100% reflective turning mirrors for manoeuvring the laser beam from gas supply as well as high voltage supply as shown in Fig. 9.6.13.
LASER BEAM MACHINING (LBM): Modes of Laser Beam Operation
There are two modes of operations using laser beams: a) Continuous wave b) Pulsed wave In the continuous wave mode, the laser beam produces heat as a constant stream of steady power. The beam maintains its level of energy throughout the cycle of operation, hence it is most suitable for seam welding. In the pulsed wave mode, the laser beam produces heat in the form of short but intense burst of light energy. This energy dissipation in short
duration is sufficient to overcome the absorption threshold of most materials. This mode of beam is suitable for both welding and machining. The advantage of short pulse is that there is hardly any heat affected zone (HAZ). However, in this mode the depth of penetration of the beam is limited.
Material removing mechanism
As presented in Fig. 3, the unreflected light is absorbed, thus heating the surface of the specimen. On sufficient heat the workpiece starts to melt and evaporates. The physics of laser machining is very complex due mainly to scattering and reflection losses at the machined surface. Additionally, heat diffusion into the bulk material causes phase change, melting, and/or vaporization. Depending on the power density and time of beam interaction, the mechanism progresses from one of heat absorption and conduction to one of melting and then vaporization. High intensity laser beams are not recommended since they form a plasma plume at or near the surface of the material with a consequent reduction in the process efficiency due to absorption and scattering losses. Machining by laser occurs when the power density of the beam is greater than what is lost by conduction, convection, and radiation, and moreover, the radiation must penetrate and be absorbed into the materia l.
Metal Removal Rate
The material removal rate in LBM is low and are of the order of 4000 mm3/hour. The approximate energy required to remove a given amount of material can be determined using the specific heat and latent heats of fusion and evaporisation of the work material.
Advantages
• In laser machining there is no physical tool. Thus no machining force or wear of the tool takes place. • Large aspect ratio in laser drilling can be achieved along with acceptable accuracy or dimension, form or location • Micro-holes can be drilled in difficult – to – machine materials • Though laser processing is a thermal processing but heat affected zone specially in pulse laser processing is not very significant due to shorter pulse duration.
Limitations
• High initial capital cost • High maintenance cost • Not very efficient process • Presence of Heat Affected Zone – specially in gas assist CO2 laser cutting • Thermal process – not suitable for heat sensitive materials like aluminium glass fibre laminate as shown in Fig.9.6.14
APPLICATIONS OF LASER BEAM MACHINING
The industrial applications of laser beams are welding, cutting, drilling, engraving and heat treatment.
Cutting:
Practically any material can be cut by a laser beam. Any intricate shape/contour can be machined at high speeds using lasers without any special jigs and, fixtures. Hence, lasers are most - suitable for precision works, like in toolings.
Drilling:
Another important application of laser is the drilling operations. Small holes in thin plates can be produced using lasers, which is difficult by conventional drilling. For a thicker work piece, a sequence of controlled laser pulses is projected to' gradually increase the hole depth. Most industrial drilling operations using lasers are used for hole drilling in fuel filters, carburetor nozzles, hypodermic needles, jet engine blade cooling holes, etc.
Marking & Engraving:
Lasers are suitable for marking & engraving to produce a controlled surface pattern on a work piece. Bar codes, company logos, part number, etc., can be easily produced. Such markings can be done on metals, glass and papers also.
ELECTRON BEAM MEACHINING (EBM)
Similar to LBM except laser beam is replaced by high velocity electrons. When electron beam strikes the work piece surface, heat is produced and metal is vaporized. Surface finish achieved is better than LBM. Used for very accurate cutting of a wide variety of metals.
Principle of operation •
In electron beam machining the kinetic energy of fast moving electrons, st riking the surface of the work piece, is converted into heat energy.
•
Such a beam can be easily concentrated to heat a small selected zone and to obtain a very high temperature there.
•
The operation is done in a vacuum chamber in order to avoid dissipation of the kinetic energy of the stream by collisions between the electrons and molecules of atmospheric gases.
Working Arrangement: •
The system consists of a suitable cathode (Electron emitter) and an anode in a vacuum chamber.
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When a high potential difference (30-175kV dc) is applied between the heated cathode (tungsten element) and the anode, a stream of rapidly moving electrons leave the cathode and is accelerated towards the anode.
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The kinetic energy imparted to the electrons is transferred to the work piece in the form of heat energy as the electrons bombard its s urface.
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An increase in the accelerating potential difference results in a greater heat input at the surface of the work.
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With an accelerating voltage of 15kV, the beam speed may be around 224000 km/sec.
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The electrons attain a speed close to 75% of the speed of light.
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Magnetic lenses focus the electron beam on the work surface.
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The energy density on a 0.025 mm diameter spot is as high as 106W/mm.
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The most important feature of electron-beam machining is that an extremely deep and narrow penetration can be obtained.
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Since, the rate of energy input is very high, the base metal is melted instantaneously and vaporised at the point where the beam impinges.
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This creates a hole in the base metal, which is deepened by the electron bea m.
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The diameter of the hole is very small, and the metal vapours filled in it help to stabilise the beam.
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As the base metal assembly is moved , the hole advances through the metal while the molten metal behind solidifies.
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Using this process an extremely deep and narrow penetration c an be obtained.
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The heat generated is about 2500°C, which is sufficient to melt and vaporize the metal and cause drilling operation.
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Since a very narrow hole is produced this, it is suitable for drilling very small holes.
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The depth of penetration depends on the beam diameter, power density, and the accelerating voltage.
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Moreover the depth of eroded material per pulse depends on the densit y of the work piece material as well as on the beam diameter.
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Practically, the number of pulses that produce a given hole depth is usually found to decrease with an increase in the acceleration voltage.
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For a fixed set of process conditions, the number of pulses required incre ases hyperbolically as the depth of the hole increases.
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In practical terms, this conclusion means that when a certain depth has been reached, any further EBM to deepen the hole would require a very large increase in the number of pulses.
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The machining time, in EBM, required to drill a hole depends on the number of pulses required to erode a certain depth and pulse frequency.
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For slotting by EBM, the machining time is affected by slot length, beam diameter, pulse duration, and number of pulses required to remove a specified depth.
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The EBM rate is usually evaluated in terms of the number of pulses required to evaporate a particular amount of material.
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The use of electron counters, which register the number of pulses, enables ready adjustment of the machining time to produce a required depth of cut.
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Work piece material properties such as boiling point and thermal conductivity play a significant role in determining how readily they can be machined.
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Other thermal properties such as electrical conductivity are considered as additional factors.
APPLICATIONS
1. Drilling •
Drilling applications with an electron beam machine fit ted with a system for numerically controlling the beam power, focus and pulse duration, and mechanical motion.
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Cylindrical, conical, and barrel-shaped holes of various diameters can be drilled
with consistent accuracy at rates of several thousand holes per second . •
Holes at an inclination angle of about 15° were also possible.
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The largest diameter and depth of holes that can be accurately drilled by EBM are, respectively, 1.5 mm and 10 mm and that the aspect depth-to-diameter ratio is normally in the range of 1:1 to 1:15 .
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For deeper holes, in the range of 2.5 to 7.5 mm.
2. Perforation of thin sheets
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For perforation by EBM to be economically acceptable, 104 to 105 holes per second have to be produced.
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Thus single pulses lasting only a few microseconds are needed.
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In some applications the sheet or foil is stretched on a rotating drum, which is simultaneously shifted in the direction of its axis.
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Rows of perforations following a helical line are thereby produced.
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Manipulators capable of linear and rotating movements in four axes are used for EBM perforation of jet engine components.
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Foil made of a synthetic material has been perforated with 620 holes per square millimeter for filter application at a rate of one hole every 10 μs.
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EBM perforation can be applied to the production of filters and masks of color television tubes.
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Other applications for perforation lie in sieve manufacture, for sound insulation and in glass fiber production.
3. Slotting •
Rectangular slots of 0.2 by 6.35 mm in 1.57mm thick stainless steel plate are produced in 5 min using 140 kV, 120 μA, a pulse width of 80 μs, and a frequency of 50 Hz.
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The rate of slotting depends on the work piece thickness .
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In this regard 0.05-mm-thick stainless steel was cut at a rate of 100 m/min, while 0.18-mm-thick stainless steel was cut at 50 m/min using similar machining
conditions.
The RICHARDSON-DUSHMAN equation for thermionic emission capability of the cathode in EBM. 2
J = AT e
– (EW/KT)
Where J = Current density (amp/cm2) of the emitted current. W = work function of the material of the cathode (volts) T = absolute temperature of cathode (K-Kelvin) E = electronic charge (coulomb) K = Boltzmann constant (1.3 X 10
-23
J/K)
A = constant (120 amp/cm 2 (degree)2)
Accelerating voltage : 50 to 200 kV Beam current : 100 to 1000 µA Power : 0.5 to 50 kW.
PLASMA ARC MACHINING (PAM):
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When the temperature of a gas is raised to about 2000°C, the gas molecules become dissociated into separate atoms. At higher temperatures, 30,000°C, these atoms become ionized. The gas in this stage is termed plasma.
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Machining by plasma was adopted in the early 1950s as an alternative method for oxy-gas flame cutting of stainless steel, aluminum, and other nonferrous metals.
MACHINING SYSTEM •
In plasma machining a continuous arc is generated between a hot tungsten cathode and the water-cooled copper anode .
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A gas is introduced around the cathode and flows through the anode.
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The temperature, in the narrow orifice around the cathode, reaches 28,000°C, which is enough to produce a high-temperature plasma arc.
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Under these conditions, the metal being machined is very rapidly melted and vaporized.
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The stream of ionized gases flushes awa y the machining debris as a fine spray creating flow lines on the machined surface.
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The removal rates by this method are substantially higher than those of conventional single-point turning operation.
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Plasma arc As shown in Fig. 1, the arc is struck from the rear electrode of the plasma torch to the conductive workpiece causing temperatures as high as 33,300°C.
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The double arcing effect between the nozzle and the workpiece damages the electrode and the workpiece.
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High heat transfer rates are found to occur during plasma arc due to the transfer of all the anode heat to the workpiece.
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Owing to the greater efficiency of plasma arc systems, they are often used for machining metals.
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Plasma arc does not depend on a chemical reaction between the gas and the work metal.
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Because the temperature is high, the process is suitable for any electrically conductive material including those that are resistant to oxy-fuel gas cutting.
MATERIAL REMOVAL RATE •
During PAM absorbing the heat energy from the plasma jet directed to the workpiece activates metal removal.
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The plasma torch blows the molten and evaporated metal away as a fine spray or vapor.
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The resulting cutting rates and hence the machinability depend on the workpiece being machined as well as the type of the cutting and shielding gases that determine the maximum temperature transfer rates.
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The power consumption factor needed in plasma beam rough turning of some alloys.
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A low factor indicates either low energy required or high removal rates. The machining speed is found to decrease with increasing the thi ckness of the metal or the cut width in case of beveling.
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As the power is increased, the efficient removal of melted metal is found to need a corresponding rise in the gas flow rate.
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During plasma machining of 12-mm-thick steel plate using 220 kW the machining speed is 2500 mm/min, which is 5 times greater than that for oxy-gas cutting.
SURFACE FINISH AND ACCURACY : •
Plasma arc cutting can yield better surface finishes then oxy-acetylene gas cutting.
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However, the cut edges are round, with a corner radius of about 4 mm. There are also problems of taper (about 2-5°).
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The kerf width is around is 2.5 to 8 mm.
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Accuracy of the width of slots and diameter of holes is in the range of ±0.8 mm or 5 to 30 mm plates and it is as high as ±3.0 mm on 100 to 150 mm thick plates.
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The depth of heat affected zone is very high and depends on the work material, its thickness and cutting speeds.
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About 4 mm of heat affected zone is common on 25 mm thick plates, which can be reduced considerably by increasing the cutting speed.
APPLICATIONS :
1. PAM is an attractive turning method for difficult-to-machine materials by conventional methods. In this regard, cutting speeds of 2 m/min and a feed rate of 5 mm per revolution produced a surface finish of 0.5 mm Rt. The depth of cut can be controlled through the machining power or surface speed (Fig. 5).
2. Computer numerical controlled PAM is used for profile cutting of metals that are difficult to tackle by oxyacetylene gas technique such as stainless steel and aluminum. A large number of parts can also be produced from one large sheet thus eliminating shearing operations. 3. PBM can cut 1.5-mm-deep, 12.5-mm-wide grooves in stainless steel at 80 mm3/min, using 50 kW as the cutting power. Such a high machining rate is 10 times the rate of grinding and chipping methods. Lower machining rates are obtainable when these grooves are cut in nonconductive materials. The groove dimension however depends on the traverse speed, arc power, and the angle and height of the plas ma arc. 4. The process is recommended for parts that have subsequent welding operations. 5.
A plasma arc can cut tubes of wall thickness of up to 50 mm. In this case no deburring is required before tube welding.
6. Underwater NC plasma cutting can achieve machining accurac y of ±0.2 mm in 9 m at low cutting speeds.
Advantages:
1. Requires no complicated chemical analysis or maintenance 2. Uses no harmful chlorinated fluorocarbons, solvents, or acid cleaning chemicals 3. Operates cleanly, often eliminating the need for vapor degreasing, solvent wiping, ultrasonic cleaning, and grit blasting 4. Requires no worker exposure to harmful chemicals 5. Needs less energy to operate 6. Carbon steels thinner the 50 mm can be cut at accelerated speeds. 7. Due to the high speed of cutting the deformation of sheet metals is reduced while the width of the cut is minimum and the quality is high. 8. It can also be used for underwater cutting.
Disadvantages :
1. The large power supplies needed (220 kW) are required to cut through 12-mm-thick mild steel plate at 2.5 m/min. 2. The process also produces heat that could spoil the workpiece and produce toxic fumes.
Primary gas used in PAM is Argon – nitrogen (4:1), Hydrogen. Shielding gas used in PAM is Nitrogen, oxygen, air, CO2 and water.
PLASMA ARC TORCHES (PLASMATRON) Indirect Arc Plasma Torches
The arc established between the electrode and the torch is forced through the nozzle by the working gas fed into the chamber. The anode spot of the arc moves on the inside wall of the nozzle passing, whereas the column if firmly stabilized along the axis of the electrode and torch. In passing through the nozzle, part of the working gas becomes heated, ionized, and emerges from the torch as the plasma jet. The outer layer of the gas flowing around the arc column remains relatively cool and forms an electric and thermal insulation between the plasma jet and nozzle bore, thus protecting the nozzle from erosion. In addition, the external coat of gas intensively cools the arc column due to which the cross-section of the column
decreases and the current density diameter of the column the constricting action of the arc intrinsic magnetic field also increases:
Where P is the inward pressure of the magnetic field, I the arc current (Amp), D the conductive diameter of column and C the speed of light (=3 x 10
10
cm/s).
Thus the thermal constriction in the plasmatron (thermal pinching effect) induces an increased magnetic constriction (magnetic pinch effect). The current density reaches 100A/mm and the temperature reaches several thousand degrees. Emerging from the torch the plasma jet widens slightly. This leads to the appearance of an axial pressure gradient of the arc magnetic field, accelerating the plasma jet to supersonic speeds.
Direct Arc Plasmatrons Torches
In direct arc plasma torches the work piece is the anode, whereas the nozzle is electrically neutral, being used for the construction and stabilization of the arc only. In contrast to the indirect-arc plasma torches, the plasma jet of the direct-arc type is brought into coincidence with the arc column and for that reason has a higher temperature and thermal power.
