Non Conventional Machining

Thermal Metal Removal Processes

Contents

Introduction Overview Basic description

EDM EBM IBM PAM LBM

What is thermal process ?

Thermal energy usually applied to small portion of work surface, causing that portion to be removed by fusion and/or vaporization.

Thermal Energy Processes - Overview

Very high local temperatures Material is removed by fusion or

vaporization Physical and metallurgical damage to

the new work surface In some cases, resulting finish is so

poor that subsequent processing is required

Thermal Energy Processes

•Electric discharge machining (EDM)

•Electron beam machining(EBM)

•Laser beam machining(LBM)

•Plasma arc machining(PAM)

•Ion beam machining (IBM)

Electrical Discharge Machining (EDM)

Required electrically conductive materials. Work piece – anode and tool cathode. Independent of

material hardness. Removal of material through melting or vaporisation

caused by a high frequency spark discharge. EDM machined surface may be deleterious to fatigue

properties due to the recast layer. good selection of the proper electrode material for the

work piece Produce deep holes, slots cavities in hard materials without drifting or can do irregular contour.

Electric Discharge Processes Metal removal by a series of discrete electrical

discharges (sparks) causing localized temperatures high enough to melt or vaporize the metal

Can be used only on electrically conducting work materials

Two main processes: 1. Electric discharge machining 2. Wire electric discharge machining Since spark discharges occur in EDM, it is also

called as "spark machining".

EDM Operation

One of the most widely used non traditional processes Shape of finished work surface produced by a formed

electrode tool Sparks occur across a small gap between tool and

work Requires dielectric fluid, which creates a path for

Each discharge as fluid becomes ionized in the gap

Work Materials in EDM

Only electrically conducting work materials

Hardness and strength of the work material are not factors in EDM

Material removal rate is related to melting point of work material

Advantages of EDM

The major advantages of the process are: Any materials that are electrically conductive can be machined

by EDM. Materials, regardless of their hardness, strength, toughness and

microstructure can be easily machined / cut by EDM process The tool (electrode) and workpiece are free from cutting forces Edge machining and sharp corners are possible in EDM process The tool making is easier as it can be made from softer and

easily formable materials like copper, brass and graphite.

The process produces good surface finish, accuracy and repeatability. Hardened work-pieces can also be machined since the deformation caused by it does not affect the final dimensions. EDM is a burr free process. Hard die materials with complicated shapes can be easily finished with good surface finish and accuracy through EDM process. Due to the presence of dielectric fluid, there is very little heating of the bulk material.

Limitations of EDM

Material removal rates are low, making the process economical only for very hard and difficult to machine materials.

Re-cast layers and micro cracks are inherent features of the EDM process, thereby making the surface quality poor.

The EDM process is not suitable for non-conductors. Rapid electrode wear makes the process more costly. The surfaces produced by EDM generally have a matt type

appearance, requiring further polishing to attain a glossy finish

EDM Applications

Tooling for many mechanical processes: Molds for plastic injection moulding, extrusion dies,

wire drawing dies, forging and heading dies, and sheet metal stamping dies

Production parts: delicate parts not rigid enough to withstand conventional cutting forces, hole drilling

where hole axis is at an acute angle to surface, and Machining of hard and exotic metals

Applications of EDM

The process is widely used for machining of exotic materials that are used in aerospace and automatic industries.

EDM being a non-contact type of machining process, it is very well suited for making fragile parts which cannot take the stress of machining. The parts that fit such profiles include washing machine agitators; electronic components, printer parts and difficult to machine features such as the honeycomb shapes.

Deep cavities, slots and ribs can be easily made by EDM as the cutting forces are less and longer electrodes can be used to make such collets, jet engine blade slots, mould cooling slots etc.

Micro-EDM process can successfully produce micro-pins, micro-nozzles and micro-cavities.

Electron Beam Machining (EBM)

Electron beam machining (EBM) has been used in industry since the 1960s, initially in nuclear and aerospace welding applications. Uses high velocity stream of electrons focused on workpiece surface to remove material by melting and vaporization Drilling small holes, cutting, engraving, and heat treatment are a set of modern applications used in semiconductor manufacturing as well as micro- machining areas

Components of EBM

EBM Operation

1.EB gun accelerates a continuous stream of electrons to about 75% of light speed

2.Beam is focused through electromagnetic lens, reducing diameter to as small as 0.025 mm (0.001 in)

3.On impinging work surface, kinetic energy of electrons is converted to thermal energy of extremely

4. high density which melts or vaporizes material in a

very localized area

Parameters affecting EBM performance

EBM Applications:

1.Works on any known material2. Ideal for micromachining Drilling small diameter holes - down to 0.05 mm(0.002 in)3. Cutting slots only about 0.025 mm (0.001 in.)4. Wide Drilling holes with very high depth-to-diameter ratios greater than 100:15. EBM perforation can be applied to the production of filters and masks of color television tubes. Other applications for perforation lie in sieve manufacture, for sound insulation and in glass fiber production.

Advantages :

1. Drilling is possible at high rates (up to 4000 holes per second).2. No difficulty is encountered with acute angles.3. Drilling parameters can easily be changed during machining.4. No limitation is imposed by workpiece hardness, ductility, and surface reflectivity.5. No mechanical distortion occurs to the workpiece since there is no contact6. The process is capable of achieving high accuracy and repeatability of 0.1 mm for position of holes and 5 percent for the hole diameter. 7. The process produces the best surface finish compared to other processes. 8. The cost is relatively small compared to other processes used to pro- duce very small holes.

Disadvantages:

1. High capital equipment cost2. Long production time due to the time needed to generate a vacuum3. The presence of a thin recast layer4. Need for auxiliary backing material

Ion Beam machining (IBM)

Definition =ion beam machining is an important nonconventional manufacturing technology used in Micro/Nanofabrication, using a stream of accelerated ions to remove the atoms on the surface of the object.Ion beam machining (IBM) takes place in a vacuum chamber using charged ions fired from an ion source toward the work piece by means of an accelerating voltage. The mechanism of material removal in IBM differs from that of EBM. It is closely related to the ejection of atoms, from the surface, by other ionized atoms (ions) that bombard the work material. The process is, therefore, called ion etching, ion milling, or ion polishing.

Working and Principle :

2.The machining system has an ion source that produces a sufficiently intense beam, with an acceptable spread in its energy for the removal of atoms from the workpiece surface by impingement of ions. 3. A heated tungsten filament acts as the cathode, from which electrons are accelerated by means of high voltage (1 kV) toward the anode.

1.The beam removes atoms from the workpiece by transferring energy and momentum to atoms on the surface of the object. When an atom strikes a cluster of atoms on the workpiece, it dislodges between 0.1 and 10 atoms from the workpiece material.

6.A magnetic field is produced between the cathode and anode that makes the electrons spiral.7.The path length of the electrons is, there- fore, increased through the argon gas, which, in turn, increases the ionization process.8. The produced ions are then extracted from the plasma toward the workpiece, which is mounted on a water-cooled table having a tilting angle of 0° to 80°.9. Machining variables such as acceleration voltage, flux, and angle of incidence are independently controlled.

4. During the passage of these electrons from the cathode towards the anode, they interact with argon atoms in the plasma source, to produce argon ions.5.Reaction takes place is Ar + e− →Ar+ + 2e

IBM system components

Applications of IBM :

1.IBM is used in smoothing of laser mirrors as well as reducing

the thickness of thin films without affecting their surface finish.

2. Using two opposing beams, a thin circular region on a rotating sample can produce samples for transmission electron microscopy.3. Polishing and shaping of optical surfaces by direct sputtering of pre- forms in glass, silica, and diamond is performed using patterning masks.4. The process can produce closely packed textured cones in different materials including copper, nickel, stainless steel, silver.5.Atomically clean surfaces can be produced by IBM that are used in the adhesion of gold films to silicon and aluminum oxide substrate.

Advantages of IBM :

1.the ibm is used as a micro- and nano-machining tool, to modify or machine materials at the micro- and nanoscale. 2.Ibm tools are designed to etch or machine surfaces, an ideal FIB might machine away one atom layer without any disruption of the atoms in the next layer, or any residual disruptions above the surface. 3.The ibm is also commonly used to prepare samples for the transmission electron microscope.4. ibm is also used for maskless implantation 5.Other techniques, such as ion millingor electropolishing can be used to prepare such thin samples.

Disadvantages of IBM :

1. High capital equipment cost2. Long production time due to the time needed to generate a vacuum3. The presence of a thin recast layer4. Need for auxiliary backing material

Laser Beam Machining (LBM)

technology that uses a laser beam (narrow beam of intense monochromatic light)

to cut required shapes or profile or pattern in almost all types of materials.

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

The process can be used to make precise holes in thin sheets and materials.

Principle of Laser

The word laser is an acronym for Light Amplification by the Stimulated Emission ofRadiation. When an atom absorbs a quantum of energy from a light source, the orbitalelectron of an atom jumps to a higher energy level. The electron later drops to its originalorbit and emits the absorbed energy. If the electron, which is already at high energy level,absorbs the second quantum of energy, it emits two quanta of energy and after emittingthe energy it returns back to its original orbit. The energy that is radiated has the samewave length as the simulating energy.

Mechanism of Material Removal / Cutting :

Place the workpiece on the table. As there is absence of cutting forces, fewer work holding devices are needed.

The focal point of the laser is intentionally focused onto the surface of the workpiece for providing the heat in a concentric manner.

Due to the striking of laser beam, heat is generated at the work-piece surface and as a result, the material vaporizes instantly, producing kerf in the material.

The movement of machine-axis is through the computer control which helps to achieve the required profiles on the workpiece. Heat Affected Zone (HAZ) is minimal in laser as compared to flame cutting.

To clear the molten metal that has yet not vaporized or clogged on the surface of the workpiece, the assist gas, (inert gas or exothermic gas is used for this propose) under pressure is passed on-to the workpiece. The use of different assist gases with different work materials

Advantages:

The ability to cut almost all materials No limit to cutting paths as the laser point can move in any paths. No cutting lubricants are required As there is an absence of direct contact between the tool and

workpiece; thus no forces are induced and as a result it is not necessary to provide the

work holding system to hold the workpiece. The fragile materials are easy to cut on a laser without any support. Flexibility exists in precision cutting of simple or complex parts. There is no tooling cost or associated wear costs due to it. Laser produces high quality cuts without extra finishing requirements

Disadvantages:

Laser processes involve high capital investments and high operating costs.

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

It cannot drill blind holes to precise depths. Hence there is limitation on its thickness.

Heat affected through the lasers may change the mechanical properties of the metallic materials and alloys

The processing time in larger holes is slower due to trepanning action (process) involved in it.

Reflected laser lights can lead to safety hazards. Assist or cover gases are required for safety purposes.

Applications:

One of the problems associated with the conventional approach in cutting of tough materials such as titanium alloy is that, at high cutting speeds the life of the cutting tool is very short. As the titanium alloys are used extensively in the aerospace industry, there is a tremendous interest and curiosity for developing this technique especially for enabling higher cutting rates.

Laser machining is used for making very accurate sized holes as small as 5 microns in metals, ceramics and composites without warpages.

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

Electronic and automotive industries also find extensive applications for laser beam machining.

LBM Applications

Drilling, slitting, slotting, scribing, and marking operations

Drilling small diameter holes - down to 0.025 mm(0.001 in) Generally used on thin stock Work materials: metals with high hardness andstrength, soft metals, ceramics, glass and glassepoxy, plastics, rubber, cloth, and wood

Plasma Arc Cutting (PAC)

Uses a plasma stream operating at very high temperatures to cut metal by melting

It is an arc cutting process wherein the severing of the metal is obtained by melting a localized area with a constricted arc and removing the molten material with a high velocity jet of hot, ionized gas issuing from the orifice.

GASES:

The gases that are used in plasma-arc cutting:

1. Nitrogen

2. Nitrogen + hydrogen

3. Nitrogen + argon

4. Compressed air

1.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 5000C⁰ These gases are directed on the work piece in the form of high velocity stream. Working principle and process details are shown in Figure 5.

Operation of PAC

•Plasma = a superheated, electrically ionized gas

•PAC temperatures: 10,000C to 14,000C (18,000F to 25,000F)

•Plasma arc generated between electrode in torch and anode workpiece

•The plasma flows through water-cooled nozzle that constricts and directs stream to desired location

Advantages

(i) It cuts carbon steel up to 10 times faster than oxy-fuel cutting, with equal quality more economically. (ii) It leaves a narrower kerf. (iii) Plasma cutting being primarily a melting process can cut any metal. (iv)Arc plasma torches give the highest temperature available from many practicable sources. The energy seems to be unlimited in this method.

Disadvantages

(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.

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.

Applications of PAC

•Most applications of PAC involve cutting of flat metal sheets and plates

•Hole piercing and cutting along a defined path

•Can be operated by hand-held torch or automated by CNC

•Can cut any electrically conductive metal •Most frequently cut metals: carbon steel,

stainless steel, aluminium

APPLICATION OF PLASMA ARC

1. Plasma cutting is used to cut particularly those nonferrous and stainless metals that cannot be cut by the usual rapid oxidation induced by ordinary flame torches.

2. Plasma cutting can be used for stack cutting, plate bevelling, and shape cutting and piercing.

3. With some modifications, plasma arc cutting can be used under water.4. Plasma arc cutting finds applications in many industries such as

shipyard, chemical, nuclear and pressure vessel.5. It is used for removing gates and risers in foundry. 6. It cuts hot extrusions to desired length. 7. It is used to cut any desired pipe contour. 8. It is also employed for gouging applications. 9. It finds use in the manufacture of automotive and railroad components.

VARIOUS TYPE OF PLASMA ARC CUTTING

Air Plasma Arc Cutting Dual-flow Plasma Arc Cutting Underwater Plasma Arc Cutting

Comparison :