NON CONVENTIONAL MACHINING METHODS

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 non conventional machining processes in the

industry as efficient and economic alternatives to conventional ones. With development

in the non conventional machining processes, currently there are often the first choice and

not an alternative to conventional processes for certain technical requirements. It is based

on unconventional machining techniques using Laser beam, Electron beam, Electric arc

etc. Conventional machining involves the direct contact of tool and work -piece, whereas

unconventional machining does not require the direct contact of tool and work piece.

Conventional machining has many disadvantages like tool wear which are not present in

Non-conventional machining.

Non conventional machining methods utilises :-

Electrical energy

Thermal energy

Chemical energy [1]

The classification of the machining processes is based upon the type of energy used, the

mechanism of metal removal in the process, the source of the intermediate energy

required for material removal and the medium for transfer of those energies.[2]

TABLE-1

TYPES OF

ENERGY

BASIC

MECHANISM

OF METAL

REMOVAL

TRANSFER

MEDIA

ENERGY SOURCE PROCESSES

Mechanical Erosion shear High velocity

of particles,

physical

contact

Pneumatic/hydraulic

pressure

Abrasive Jet

Machining,

Ultrasonic

Machining

Chemical Chemical

ablation

Reactive

environment

Corrosive agent Chemical

Machining

Electro

Chemical

Ion

displacement

Electrolyte High current Electro

Chemical

Machining,

Electro

Chemical

Grinding

Thermo-

Electric

Fusion

vaporization

Hot gases

Electron

radiation

Ionized material

high voltage

Amplified light

IBM, PAM,

EDM, EBM,

LBM

IBM-Ion beam machining, PAM-Plasma Arc Machining, EDM-Electrical Discharge

Electro Chemical Machining, EBM-Electron Beam machining, LBM-Laser Beam

Machining.

Process Selection—In order to make use of the non traditional machining processes

efficiently, it is necessary that the exact nature of the machining problem must be known.

The points which should be looked into before the selection of these processes are;-

Physical parameters

Properties of the work material and the shape to be machined

Process capability or machining characteristics

Economic considerations

The applications of the non traditional machining processes are also influenced by the

workpiece shape end size to be produced, viz. holes, through holes, surfacing, through

cutting and special applications.

The process capability or machining characteristics can be analyzed with respect to:-

o Metal removal rate obtained

o Tolerance maintained

o Surface finish obtained

o Depth of surface damage

o Power required for machining

The economics of the various processes are analyzed by considering:-

Capital cost

Tooling cost

Consumed power cost

Metal removal rate efficiency

Wear of tooling[3]

ABRASIVE JET MACHINING

Abrasive jet machining (AJM), also known as abrasive micro-blasting, pencil

blasting and micro-abrasive blasting,[4] is an abrasive blasting machining process that

uses abrasives propelled by a high velocity gas to erode material from the workpiece.[5]

The filtered gas, supplied under a pressure of 2 to 8 kgf/cm2 to the mixing chamber

containing the abrasive powder and vibrating at 50 Hz entrains the abrasive particles and

is then passed into connecting hose. This abrasive and gas mixture emerge from a small

nozzle mounted on a fixture at a high velocity ranging from 150-300m/min. To control

the size and shape of the cut either the workpiece or the nozzle is moved by cams,

pantographs or other suitable mechanisms. Air and nitrogen are the most widely used gas

in AJM.[6]

EQUIPMENT

AJM machines are usually self-contained bench-top units. First it compresses the gas and

then mixes it with the abrasive in a mixing chamber. The gas passes through a

convergent-divergent nozzle before entering the mixing chamber, and then exits through

a convergent nozzle. The nozzle can be hand held or mounted in a fixture for automatic

operation.[5]

Nozzles must be highly resistant to abrasion and are typically made of tungsten carbide or

synthetic sapphire. For average material removal, tungsten carbide nozzles have a useful

life of 12 to 30 hours, and sapphire nozzles last about 300 hours. The distance of the

nozzle from the workpiece affects the size of the machined area and the rate of material

removal.[7]

The abrasives generally employed are Aluminium Oxide , Silicon Carbide, Glass powder

or specially prepared Sodium bicarbonate. The average particle sizes vary from 10 to 50

microns. Larger sizes are used for rapid removal rate while smaller sizes are used for

good surface finish and precision work.

The metal removal rate depends upon

the diameter of nozzle

composition of abrasive-gas mixture

jet pressure

hardness of abrasive particles and that of work material

particle size

velocity of jet

distance of work piece from the jet[8]

ADVANTAGES AND DISADVANTAGES

The main advantages are

its flexibility

low heat production

ability to machine hard and brittle materials.

Its flexibility owes from its ability to use hoses to transport the gas and abrasive to any

part of the workpiece.

One of the main disadvantages is its slow material removal rate; for this reason it is

usually used as a finishing process. Another disadvantage is that the process produces a

tapered cut.

APPLICATION

Common uses include cutting heat-sensitive, brittle, thin, or hard materials. Specifically it

is used to cut intricate shapes or form specific edge shapes.[9]

ULTRA SONIC MACHINING

Ultrasonic machining, also known as ultrasonic impact grinding, is a machining

operation in which an abrasive slurry freely flows between the workpiece and a vibrating

tool. It differs from most other machining operations because very little heat is produced.

The tool never contacts the workpiece and as a result the grinding pressure is rarely more

than 2 pounds, which makes this operation perfect for machining extremely hard and

brittle materials, such as glass, sapphire, ruby, diamond, and ceramics.

Abrasives contained in a slurry are driven at high velocity against the work by a tool

vibrating at low amplitude (.003in) and high frequency (20-100khz).The tool oscillates in

a direction perpendicular to the workpiece surface and is fed slowly into the workpiece so

that the shape of the tool is formed in the part

The action of the abrasives impinging against the work surface performs the cutting

Tool materials - soft steel, stainless steel

Abrasive materials - boron nitride, boron carbide, aluminum oxide, silicon carbide and

diamond

The vibration amplitude should be set approximately equal to the grit size, and the gap

size should be maintained at about two times the grit size

The ratio of work material to tool material removed during the cutting process ranges

from ~100:1 for cutting glass down to ~1:1 for cutting tool steel

Workpiece materials: hard and brittle such as ceramics, glass and carbides; successfully

used on certain metals such as stainless steel and titanium

Shapes obtained by USM include non round holes, holes along a curved axis and coining

operation, in which an image pattern on the tool is imparted to a flat work surface. [10]

The liquid to produce abrasive slurry should have the following characteristics:-

Good welding characteristic

Low viscosity

High thermal conductivity

Anti corrosive property

Approximately having equal density with abrasive

Low cost[11]

The surface finish of ultrasonic machining depends upon the hardness of the

workpiece/tool and the average diameter of the abrasive grain used. Up close, this

process simply utilizes the plastic deformation of metal for the tool and the brittleness of

the workpiece.

Machine time-- Machine time depends upon the frequency at which the tool is vibrating,

the grain size and hardness (which must be equal or greater than the hardness of the

workpiece), and the viscosity of the slurry fluid. Common grain materials used are silicon

carbide and boron carbide, because of their hardness. The less viscous the slurry fluid, the

faster it can carry away used abrasive.[10]

APPLICATIONS

Introducing round holes

Performing machining operations like drilling, grinding, profiling etc.

In machining glass, ceramic, tungsten

Cutting threads

CHEMICAL MACHINING

Chemical machining is the stock removal process for the production of the desired shapes

and dimensions through selective or overall removal of material by controlled chemical

attack with acids or alkalis.

Steps of chemical machining

Chemical machining process has several steps for producing machine parts. These are

given below:

1 Workpiece preparation

2. Coating with masking material

3. Scribing of the mask

4. Etching

5. Cleaning masking material

Maskants

Masking material which is called maskant is used to protect workpiece surface from

chemical etchant. Polymer or rubber based materials are generally used for masking

procedure. The selected maskant material should have following properties [12].

1. Tough enough to withstand handling

2. Well adhering to the workpiece surface

3. Easy scribing

4. Inert to the chemical reagent used

5. Able to withstand the heat used during chemical machining

6. Easy and inexpensive removal after chemical machining etching

Workpiece material                                       Masking material

Aluminium and alloys                 Polymer, Butyl rubber, neoprene

Iron based alloys                         Polymer, Polyvinyl chloride,Polyetilien butyl rubber

Nickel                                                 Neoprene

Magnesium                                  Polymer

Copper and alloys                     Polymer

Titanium                                     Polymer

Silicon                                        Polymer

ADVANTAGES

Easy weight reduction

No effect of workpiece materials properties such as hardness

Simultaneous material removal operation

No burr formation

No stress introduction to the workpiece

Low capital cost of equipment

Easy and quick design changes

Requirement of less skilled worker

Low tooling costs

The good surface quality

Using decorative part production

DISADVANTAGES

Difficult to get sharp corner

Difficult to chemically machine thick material (limit is

depended on workpiece material, but the thickness should be

around maximum 10 mm)

Scribing accuracy is very limited, causes less dimensional

accuracy

Etchants are very dangerous for workers

Etchant disposals are very expensive[12][13]

ELECTRO CHEMICAL MATCHING

Electrochemical machining (ECM) is a method of removing metal by an

electrochemical process. It is normally used for mass production and is used for working

extremely hard materials or materials that are difficult to machine using conventional

methods. ECM is often characterized as "reverse electroplating," in that it removes

material instead of adding it.It is similar in concept to electrical discharge machining

(EDM) in that a high current is passed between an electrode and the part, through an

electrolytic material removal process having a negatively charged electrode (cathode), a

conductive fluid (electrolyte), and a conductive workpiece (anode); however, in ECM

there is no tool wear. The ECM cutting tool is guided along the desired path close to the

work but without touching the piece. Unlike EDM, however, no sparks are created. High

metal removal rates are possible with ECM, with no thermal or mechanical stresses being

transferred to the part, and mirror surface finishes can be achieved.

In the ECM process, a cathode (tool) is advanced into an anode (workpiece). The

pressurized electrolyte is injected at a set temperature to the area being cut. The feed rate

is the same as the rate of liquefaction of the material. The area between the tool and the

workpiece varies within 80-800 micrometers (.003 in. and .030 in.) As electrons cross the

gap, material on the workpiece is dissolved, as the tool forms the desired shape. The

electrolytic fluid carries away the metal hydroxide formed in the process.[14] [15]

ADVANTAGES AND DISADVANTAGES

Because the tool does not contact the workpiece, its advantage over conventional

machining is that

there is no need to use expensive alloys to make the tool tougher than the

workpiece.

There is less tool wear in ECM, and

less heat and stress are produced in processing that could damage the part. Fewer

passes are typically needed, and the tool can be repeatedly used.

Disadvantages are the high tooling costs of ECM, and that up to 40,000 amps of current

must be applied to the workpiece. The saline electrolyte also poses the risk of corrosion

to tool, workpiece and equipment.

APPLICATIONS

Some of the very basic Applications of ECM are listed below:

It can be used for Die-Sinking operations.

Drilling a jet engine turbine blade.

Multiple Hole drilling.

Steam turbine blades can be machined within close limits.[15]

ELECTRO CHEMICAL GRINDING

Electro chemical grinding is one of the latest methods of grinding. This method is

introduced in early 1970’s. In this process a grinding wheel in which an insulating

abrasive is set in a conducting bonding material is employed. The D.C power required

here is 5-15V. So for obtaining a voltage between 5 to 15V a step down transformer

is used. The metal bonded abrasive wheel (or) the grinding wheel acts as a cathode.

The work piece acts as anode. Very small distance is maintained between the anode

and cathode.

An electrolyte is allowed to pass through the gap between the electrodes. The

insulating abrasive particles get spread over the surface of the wheel. The height of

the abrasive particles over the wheel in the gap between the electrodes indicates

effective gap between the electrodes. The electrolysis can take place effectively in

between this gap only. The current densities used are 2A/cm2 to 3A/cm2.

electrolyte that can be employed here should satisfy the following properties. 

1) High electrical conductivity

2) Low viscosity and high specific heat chemical stability.

3) Resistance to formation of passive film on work surface.

4) Non-corrosive and non-toxic in nature

5) Readily available and inexpensive

The electrolyte should perform many functions like

1) These should complete the electrical circuit between tool and work piece.

2) Electrolyte must allow all the desirable machining processes to occur.

3) It should function as a coolant by carrying away the heat generated during the

chemical reactions.

4) Electrolyte should be effective in carrying away the products obtained by reactions

in machining zone.

Removal of metal in ECG

Most of metal removal is done by electrochemical action. But some of the metal is

also removed by the contact of abrasive particles to work piece. The abrasive particles

have two main functions in electro chemical grinding.

1) To find the effective gap between the anode and cathode

2) To remove any passive layer formed over the work piece.

ADVANTAGES 

1) Metal removal rate is very high.

2) Though the machine requires very high investment increased metal removal rate

and less abrasive consumption acts as more than compensate for extra capital cost. On

large scale production the cost per piece gets highly reduced.

3) Less risk of thermal damage as the heat generated is very low.

4) No presence of burrs on the finished surface.

5) High surface finish and no grinding scratches are present on the finish surface.

6) Pressure over the wheel due to work gets minimized.

APPLICATION

1) This process is extensively used for grinding carbide tools. Electro chemical

grinding provides a savage of 75% in wheel cost and 50% in labor cost

2) Electro chemical grinding is also used for grinding fragile (or) very hard and tough

materials.

DISADVANTAGES

Electrochemical grinding loses accuracy when grinding inside corners, due to the

effects of the electric field. [16]

ION BEAM MACHINING PROCESS

Focused ion beam machining is carried out on any type of work material. In ion beam

machining process the stream of ions of a particular inert gas like Argon or Neon is

accelerated in a medium of vacuum. The high energy of this impact is sent to the work

material. This powerful beam removes atom from the work material by transferring the

energy and the momentum. Clusters of atom are removed due to this process from the

work material.

Ion beam machining uses the principle called as sputtering. Sputtering is a technique

where the stream of ions is made to bombard the work surface. The energy in form of

kinetic energy is transferred to the surface.

The apparatus is a filament that acts like a cathode. A suitable anode is placed near the

cathode. Poles of magnet are mounted and the argon gas is made to pass inside the

vacuum chamber.

High vacuum conditions are necessary for this process. Extraction grids are used to

remove argon from the ion source.

ION BEAM SOURCE

Most widespread are instruments using Liquid-metal ion sources (LMIS),

especially gallium ion sources. Ion sources based on elemental gold and iridium are also

available. In a Gallium LMIS, gallium metal is placed in contact with a tungsten needle

and heated. Gallium wets the tungsten, and a huge electric field (greater than 108 volts per

centimeter) causes ionization and field emission of the gallium atoms.

Source ions are then accelerated to an energy of 5-50 keV (kiloelectronvolts), and

focused onto the sample by electrostatic lenses. LMIs produce high current density ion

beams with very small energy spread.[17]

ADVANTAGES AND DISADVANTAGES

ADVANTAGES

The process is almost universal

No chemical reagents or etching

Etching rates are easily controlled

DISADVANTAGE

Expensive method

Etching rates are slow

There is a possibility of some thermal or radiation damage

APPLICATIONS

Applied in micro machining components

For etching typical materials like glass, alumina, quartz etc.[18]

PLASMA ARC MACHINING

Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas to

melt and displace material in its path. Called PAM, this is a method of cutting metal with

a plasma-arc, or tungsten inert-gas-arc, torch. The torch produces a high velocity jet of

high-temperature ionized gas called plasma that cuts by melting and removing material

from the workpiece. Temperatures in the plasma zone range from 20,000° to 50,000° F

(11,000° to 28,000° C).

It is used as an alternative to oxyfuel-gas cutting, employing an electric arc at very

high temperatures to melt and vaporize the metal. [19]

Plasma cutting is a process that is used to cut steel and other metals of different

thicknesses (or sometimes other materials) using a plasma torch. In this process, an inert

gas (in some units, compressed air) is blown at high speed out of a nozzle; at the same

time an electrical arc is formed through that gas from the nozzle to the surface being cut,

turning some of that gas to plasma. The plasma is sufficiently hot to melt the metal being

cut and moves sufficiently fast to blow molten metal away from the cut. Plasma is an

effective means of cutting thin and thick materials alike. Hand-held torches can usually

cut up to 2 in (48 mm) thick steel plate, and stronger computer-controlled torches can cut

steel up to 6  inches (150 mm) thick. Since plasma cutters produce a very hot and very

localized "cone" to cut with, they are extremely useful for cutting sheet metal in curved

or angled shapes

STARTING METHODS

Plasma cutters use a number of methods to start the arc. In some units, the arc is created

by putting the torch in contact with the work piece. Some cutters use a high voltage, high

frequency circuit to start the arc. This method has a number of disadvantages, including

risk of electrocution, difficulty of repair, spark gap maintenance, and the large amount of

radio frequency emissions. Plasma cutters working near sensitive electronics, such as

CNC hardware or computers, start the pilot arc by other means. The nozzle and electrode

are in contact. The nozzle is the cathode, and the electrode is the anode. When the plasma

gas begins to flow, the nozzle is blown forward. A third, less common method is

capacitive discharge into the primary circuit via a silicon controlled rectifier.

COSTS

Plasma torches were once quite expensive. For this reason they were usually only found

in professional welding shops and very well-stocked private garages and shops. However,

modern plasma torches are becoming cheaper, and now are within the price range of

many hobbyists. Older units may be very heavy, but still portable, while some newer

ones with inverter technology weigh only a little, yet equal or exceed the capacities of

older ones. [20]

ADVANTAGES

Extremely effective on any metal

No contact between tool and workpiece

DISADVANTAGES

Metallurgical change on the surface

Safety precautions are necessary and this adds to the cost[21]

ELECTRICAL DISCHARGE MACHINING

Electric discharge machining (EDM), sometimes colloquially also referred to as spark

machining, spark eroding, burning, die sinking or wire erosion, is a manufacturing

process whereby a desired shape is obtained using electrical discharges (sparks). Material

is removed from the workpiece by a series of rapidly recurring current discharges

between two electrodes, separated by a dielectric liquid and subject to an electric voltage.

One of the electrodes is called the tool-electrode, or simply the ‘tool’ or ‘electrode’, while

the other is called the workpiece-electrode, or ‘workpiece’.

When the distance between the two electrodes is reduced, the intensity of the electric

field in the volume between the electrodes becomes greater than the strength of the

dielectric (at least in some point(s)), which breaks, allowing current to flow between the

two electrodes. This phenomenon is the same as the breakdown of a capacitor

(condenser) (see also breakdown voltage). As a result, material is removed from both the

electrodes. Once the current flow stops (or it is stopped - depending on the type of

generator), new liquid dielectric is usually conveyed into the inter-electrode volume

enabling the solid particles (debris) to be carried away and the insulating proprieties of

the dielectric to be restored. Adding new liquid dielectric in the inter-electrode volume is

commonly referred to as flushing. Also, after a current flow, a difference of potential

between the two electrodes is restored to what it was before the breakdown, so that a new

liquid dielectric breakdown can occur.

Electrical discharge machining is a machining method primarily used for hard metals or

those that would be very difficult to machine with traditional techniques. EDM typically

works with materials that are electrically conductive, although methods for machining

insulating ceramics with EDM have also been proposed. EDM can cut intricate contours

or cavities in pre-hardened steel without the need for heat treatment to soften and re-

harden them. This method can be used with any other metal or metal alloy such as

titanium, hastelloy, kovar, and inconel. Also, applications of this process to shape

polycrystalline diamond tools have been reported.

ADVANTAGES

Complex shapes that would otherwise be difficult to produce with conventional

cutting tools

Extremely hard material to very close tolerances

Very small work pieces where conventional cutting tools may damage the part

from excess cutting tool pressure.

There is no direct contact between tool and work piece. Therefore delicate

sections and weak materials can be machined without any distortion.

A good surface finish can be obtained.

Very fine holes can be easily drilled.

DISADVANTAGE

The slow rate of material removal.

The additional time and cost used for creating electrodes for ram/sinker EDM.

Reproducing sharp corners on the workpiece is difficult due to electrode wear.

Specific power consumption is very high.

Power consumption is high.

"Overcut" is formed.

Excessive tool wear occurs during machining.

Electrically non-conductive materials can be machined only with specific set-up of the

process.[22]

LASER BEAM MACHINING

The word laser is an acronym for Light Amplification by the Stimulated Emission of

Radiation.

The three most important attributes of laser light are:

It is coherent i.e. all photons that make up the beam are in phase with each other.

 

It is collimated, because photons that diverge from the parallel are lost through the

chamber walls a very parallel beam is issued.

 

It is monochromatic, literally one colour, that is of one wavelength. Different media

used to stimulate the photons generate different wavelengths, but each type of laser has

a specific wavelength (e.g. CO2 is 10.6 mM). The purity of the medium used is of

paramount importance.

PROCEDURE

  The workpiece rests on a sacrificial table (minimal point contact, when heavily pitted by

laser overshoot is simply thrown away, hence the name). Workholding is minimal due

to absence of cutting forces and when used is mainly for location.

·        The focal point of the laser is focused onto the surface of the workpiece. The

follower takes into account any variation in height of the workpiece.

·        The material vapourises instantly, producing a kerf in the material.

·        The machine axes move to generate the correct profile. The speed of cutting is such

that the Heat Affected Zone (HAZ) is minimal - compared to flame cutting.

·        A gas assist jet clears the molten metal that has not vapourised (as in oxy-fuel

cutting). Note: the gas assist gas may be one of two types, inert and exothermic. Inert

gasses commonly used are Nitrogen and Argon. Exothermic gasses, Air or pure

Oxygen. Inert gasses help keep oxidisation to a minimum, cool the cutting zone and

prevent flammable materials burning. Exothermic gasses cause a reaction that

improves cutting performance.

·        Welding is broadly similar except for the omission of the gas assist jet. In this case

the column of molten metal needs to remain in place until after the beam has passed,

to allow solidification.

·        A significant advantage of laser welding is that filler rods need not be used and

two dissimilar metals can be welded. The two pieces to be welded are butted together (to

a close tolerance) the laser beam passes along the intersection, melting both sides and

‘stirring’ the metals together. Very accurate welds with good structural integrity (due in

part to the small HAZ) can be made with a laser beam. EBM provides very high drilling

rates when small holes with large aspect ratio are to be drilled. Moreover it can machine

almost any material irrespective of their mechanical properties. As it applies no

mechanical cutting force, work holding and fixturing cost is very less. Further for the

same reason fragile and brittle materials can also be processed. The heat affected zone in

EBM is rather less due to shorter pulses. EBM can provide holes of any shape by

combining beam deflection using electromagnetic coils and the CNC table

with high accuracy.

Though heat affected zone is rather less in EBM but recast layer

vacuum. However this can be reduced to some extent using vacuum load

significant amount of non-productive pump down period for attaining desired

applicable for any equipment using vacuum system. Moreover in EBM there is

high capital cost of the equipment and necessary regular maintenance

However, EBM has its own share of limitations. The primary limitations are the

high initial capital cost

high maintenance cost

not very efficient process

not suitable for heat sensitive material[23][24]

ELECTRON BEAM MACHINING

A high velocity stream of electrons is focused on the workpiece surface to remove

material by melting and vaporization. The production of free electrons is obtained from

thermo electronic cathodes wherein metal are heated to the temperature at which the

electrons acquire sufficient speed for escaping the space around the cathode.

Electron beam gun accelerates a stream of electrons to ~3/4 c and focused through an

electromagnetic lens. Kinetic energy of beam converted to thermal energy of extremely

high density, melting or vaporizing material in a very localized area

EBM must be carried out in a vacuum

ADVANTAGES

can drill holes or cut slots

can cut any known material, metal or non metal

no cutting tool wear

distortion free machining

DISADVANTAGES

high equipment cost

employment of high skill labour

only small cuts can be made

APPLICATIONS

To drill fine gas orifices

To produce metering holes in injector nozzles

To scribe thin films

To remove small broken taps from holes