Unconventional Machining Processes

Introduction

Manufacturing Technology I

Metal casting (Sand casting and other casting processes)

Materials Joining (Arc welding, TIG, EBW, PAW, etc.)

Bulk deformation (Metal Forming – Forging, Rolling, Extrusion)

Sheet metal processes (Shearing, bending, drawing, etc.)

Manufacturing of plastic materials (Injection molding, etc).

Manufacturing Technology II (Material removal process)

Metal Cutting or Mechanical Abrasion

Centre lathe and special purpose lathes

Shaper, Planer, Slotter, Milling, Drilling, Broaching, Gear cutting, etc.

Grinding, Honing, Lapping, etc.

CNC and DNC

Introduction – Contd.

3

Machining – produces finished products with high degree of accuracy.

Conventional machining

Utilizes cutting tools (harder than workpiece material).

Needs a contact between the tool and workpiece.

Needs a relative motion between the tool and workpiece.

Absence of any of these elements – makes the process a unconventional or nontraditional one.

Big boon to modern manufacturing industries.

The need for higher productivity, accuracy and surface quality – led to combination of two or more machining actions, called hybrid machining processes.

History of Machining

4

In ancient days – hand tools (stones, bones or stick).

Later – hand tools of elementary metals (bronze or iron)

Till 17th Century – tools were either hand operated or driven mechanically by very elementary methods.

Wagons, ships, furniture, etc. – were produced.

Introduction of water, steam and electricity – power driven machine tools

Caused a big revolution in 18th and 19th centuries.

1953 – Numerical control machine tools – enhanced the product productivity and accuracy.

5

Traditional or Conventional Machining

6

Metal Cutting Processes

7

Abrasive Machining

8

Cylindrical grinding

Flat surface grinding

Abrasive Machining

9

Centreless grinding

Need for Unconventional Machining

10

• Greatly improved thermal, mechanical and chemical properties of modern materials –

Not able to machine thru conventional methods. (Why???)

• Ceramics & Composites – high cost of machining and damage caused during machining

– big hurdles to use these materials.

• In addition to advanced materials, more complex shapes, low rigidity structures and

micro-machined components with tight tolerances and fine surface finish are often

needed.

• To meet these demands, new processes are developed.

• Play a considerable role in aircraft, automobile, tool, die and mold making industries.

Need for Unconventional Machining

11

• Very high hardness and strength of the material. (above 400 HB.)

• The work piece is too flexible or slender to support the cutting or grinding forces.

• The shape of the part is complex, such as internal and external profiles, or small

diameter holes.

• Surface finish or tolerance better than those obtainable conventional process.

• Temperature rise or residual stress in the work piece are undesirable.

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Unconventional Machining Processes - Classification

Electrical

Mechanical Based Processes

13

1. Working principles

2. Equipment used

3. Process parameters

4. MRR

5. Variation in techniques used

6. Applications

AJM

WJM

AWJM

USM

14

Electrical Based Processes

1. Working principle

2. Equipment used

3. Process parameters

4. Surface finish & MRR

5. Electrode/Tool

6. Power & Control circuits

7. Tool wear

8. Dielectric

9. Flushing

10. Applications

Electrical

EDM

WEDM

15

Chemical & Electrochemical Based Processes

1. Working principles

2. Etchants & Maskants

3. Techniques of applying maskants

4. Process parameters

5. Surface finish & MRR

6. Electrical circuits in case of ECM

7. Applications

CHM

ECM

ECG

ECH

16

Thermal Based Processes

1. Working principles

2. Equipment used

3. Types

4. Beam control techniques

5. Applications

LBM

PAM

EBM

Mechanical based Unconventional Processes

USM – thru mechanical abrasion

in a medium (solid abrasive

particles suspended in the

fluid)

WJM – Cutting by a jet of fluid

AWJM – Abrasives in fluid jet.

IJM – Ice particles in fluid jet.

Abrasives or ice – Enhances

cutting action.

17

Thermal based Unconventional Processes

Thru – melting & vaporizing

Many secondary phenomena –

surface cracking, heat

affected zone and striations.

Heat Source:

Plasma – EDM and PBM.

Photons – LBM

Electrons – EBM

Ions – IBM

Machining medium: different

for different processes.

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Chemical & Electrochemical based

Unconventional Processes

CHM – uses Chemical dissolution

action in an etchant.

ECM – uses Electrochemical

dissolution action in an

electrolytic cell.

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USM

• Mechanical Removing Techniques

– Ultrasonic Machining (USM)

Ultrasonic Machining

• In ultrasonic machining (USM), also called ultrasonic grinding, high-frequency

vibrations delivered to a tool tip, embedded in an abrasive slurry, by a booster or

sonotrode, create accurate cavities of virtually any shape; that are, negatives of the tool.

• Since this method is non-thermal, non-electrical, and non-chemical, it produces

virtually stress-free shapes even in hard and brittle work-pieces. Ultrasonic drilling

is most effective for hard and brittle materials; soft materials absorb too much

sound energy and make the process less efficient.

6/27/2015

Ultrasonic Machining

• Almost any hard and brittle material, including aluminum oxides, silicon, silicon carbide, silicon nitride, glass, quartz, sapphire, ferrite, fiber optics, etc., can be ultrasonically machined.

• The tool does not exert any pressure on the work-piece (drilling without drills), and is often made from a softer material than the work-piece, say from brass, cold-rolled steel, or stainless steel and wears only slightly.

• The roots of ultrasonic technology can be traced back to research on the piezoelectric effect conducted by Pierre Curie around 1880. He found that asymmetrical crystals such as quartz and Rochelle salt (potassium sodium titrate) generate an electric charge when mechanical pressure is applied. Conversely, mechanical vibrations are obtained by applying electrical oscillations to the same crystals. Ultrasonic waves are sound waves of frequency higher than 20,000 Hz.

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Ultrasonic Machining

• The tool, typically vibrating at a low amplitude of 0.025 mm at a frequency of 20 to 100 kHz, is gradually fed into the work-piece to form a cavity corresponding to the tool shape.

• The vibration transmits a high velocity force to fine abrasive grains between the tool and the surface of the work-piece. In the process material is removed by micro-chipping or erosion with the abrasive particles.

• The grains are in a water slurry which also serves to remove debris from the cutting area. The high-frequency power supply for the magneto-strictive or piezoelectric transducer stack that drives the tool is typically rated between 0.1 and 40 kW.

6/27/2015

Channels and holes ultrasonically machined in a polycrystalline silicon wafer.

Ultrasonic Machining

• The abrasive particles (SiC, Al2O3 or BC d= 8~ 500 µm) are suspended in water or oil.

• The particle size and the vibration amplitude are ususally made about the same.

• The particle size determines the roughness or surface finish and the speed of the cut.

• Material removal rates are quite low, usually less than 50 mm3/min.

6/27/2015

Coin with grooving carried out with USM

Ultrasonic Machining • The mechanical properties and fracture

behavior of the work-piece materials also play a large role in both roughness and cutting speed. For a given grit size of the abrasive, the resulting surface roughness depends on the ratio of the hardness (H) to the modulus of elasticity (E). As this ratio increases, the surface roughness increases.

• Higher H/E ratios also lead to higher removal rates: 4 mm3/min for carbide and 11 mm3/min for glass.

6/27/2015

ultrasonic machining can be used to form intricate,

finely detailed graphite electrodes.

Ultrasonic Machining

• Machines cost up to $20,000, and production rates of about 2500

parts per machine per day are typical.

• If the machined part is a complex element (e.g., a fluidic element) of

a size > 1 cm2 and the best material to be used is an inert, hard

ceramic, this machining method might well be the most appropriate

6/27/2015

900 watt Sonic-mill, Ultrasonic Mill

Ultrasonic Machining

Advantages and disadvantages of ultrasonic machining.

6/27/2015

Advantages Disadvantages

Machining of any material regardless of conductivity Low material removal rate

Precision machining of brittle hard materials Tool wears fast

Does not produce e lectric, thermal or chemical defe cts at

the surfa ce

Machining area and depth are quite restricted

Can drill circular or non-circular holes in very hard

materials

Less stress because of its non-thermal nature

Abrasive Water-Jet Cutting

• A stream of fine grain abrasives mixed with air or suitable carrier gas, at high pressure, is directed by means of a nozzle on the work surface to be machined.

• The material removal is due to erosive action of a high pressure jet.

• AJM differ from the conventional sand blasting process in

the way that the abrasive is much finer and effective control over the process parameters and cutting. Used mainly to cut hard and brittle materials, which are thin and sensitive to heat.

Abrasive Jet Machining Setup

Typical AJM Parameters

• Abrasive

– Aluminum oxide for Al and Brass.

– SiC for Stainless steel and Ceramic\

– Bicarbonate of soda for Teflon

– Glass bed for polishing.

• Size

– 10-15 Micron

• Quantity

– 5-15 liter/min for fine work

– 10-30 liter/min for usual cuts.

– 50-100 liter/min for rough cuts.

Typical AJM Parameters

• Medium

– Dry air, CO2, N2

– Quantity: 30 liter/min

– Velocity: 150-300 m/min

– Pressure: 200-1300 KPa

• Nozzle

– Material: Tungsten carbide or saffire

– Stand of distance: 2.54-75 mm

– Diameter: 0.13-1.2 mm

– Operating Angle: 60° to vertical

Typical AJM Parameters

• Factors affecting MRR:

– Types of abrasive and abrasive grain size

– Flow rate

– Stand off distance

– Nozzle Pressure

• Advantages of AJM • Low capital cost.

• Less vibration.

• Good for difficult to reach area.

• No heat is genera6ted in work piece.

• Ability to cut intricate holes of any hardness and brittleness in the material.

• Ability to cut fragile, brittle hard and heat sensitive material without damage

• Disadvantages of AJM:

• Low metal removal rate.

• Due to stay cutting accuracy is affected.

• Parivles is imbedding in work piece.

• Abrasive powder cannot be reused.

• Applications of AJM:

• For abrading and frosting glass, it is more economical than acid

etching and grinding.

• For doing hard suffuses safe removal of smears and ceramics oxides on metals.

• Resistive coating etc from ports to delicate to withstand normal scrapping.

• Delicate cleaning such as removal of smudges from antique documents.

• Machining semiconductors such as germanium etc.

Water Jet Machining

• The water jet machining involves directing a high pressure (150-1000 MPa) high velocity (540-1400 m/s) water jet(faster than the speed of

sound) to the surface to be machined. The fluid flow rate is typically from 0.5 to 2.5 l/min

• The kinetic energy of water jet after striking the work surface is reduced to zero.

• The bulk of kinetic energy of jet is converted into pressure energy.

• If the local pressure caused by the water jet exceeds the strength of the surface being machined, the material from the surface gets eroded and a cavity is thus formed.

• The water jet energy in this process is concentrated over a very small area, giving rise to high energy density(1010 w/mm2) High

Water Jet Machining Setup

Continue…

• Water is the most common fluid used, but additives such as alcohols, oil products and glycerol are added when they can be dissolved in water to improve the fluid characteristics.

• Typical work materials involve soft metals, paper, cloth, wood, leather, rubber, plastics, and frozen food.

• If the work material is brittle it will fracture, if it is ductile, it will cut well:

• The orifice is often made of sapphire and its diameter rangesfrom 1.2 mm to 0.5 mm:

Water Jet Equipments

• It is consists of three main units

(i) A pump along with intensifier.

(ii)Cutting head comprising of nozzle and work table movement.

(iii) filter unit for debries,pout impurities.

• Advantages

- no heat produced

- cut can be started anywhere without the need for predrilled holes

- burr produced is minimum

- environmentally safe and friendly manufacturing.

Application – used for cutting composites, plastics, fabrics, rubber, wood products

etc. Also used in food processing industry.

Abrasive Water jet machining

• The rate of cutting in water jet machining, particularly while cutting ductile material, is quite low. Cutting rate can be achieved by mixing

abrasive powder in the water to be used for machining.

• In Abrasive Water Jet Cutting, a narrow, focused, water jet is mixed with abrasive particles.

• This jet is sprayed with very high pressures resulting in high velocities that cut through all materials.

• The presence of abrasive particles in the water jet reduces cutting forces and enables cutting of thick and hard materials (steel plates over

80-mm thick can be cut).

• The velocity of the stream is up to 90 m/s, about 2.5 times the speed of sound.

Continue..

• Abrasive Water Jet Cutting process was developed in 1960s to cut

materials that cannot stand high temperatures for stress distortion or metallurgical reasons such as wood and composites, and traditionally difficult-to-cut materials, e.g. ceramics, glass, stones, titanium alloys

Electrochemical

Machining and

Micromachining

The fundamentals of

electrochemical surface treatment

• Electrochemical surface treatment is based on anodic metal dissolution.

• Metal dissolution by a) active dissolution, b)transpassive dissolution

• Dimensional resolution is mainly determined by current and potential distribution around a cathodic matrix

• Forced convection removes bubbles (by H2 and O2 evolution) and oxidic and hydroxidic debris e.g. Fe(OH)3 and other oxidation-solvolysis products

Schematic current voltage curve with

active and transpassive metal dissolution

Electrochemical shaping of metals

Active dissol. and

mass transfer Transpassive dissol.

With fast sweep

Transpassive dissolution

with mechanical scraping

Some examples of electrochemical machining of hard metals: Primary Current density distribution

Current density distributions

• Primary: neglects charge transfer kinetics

and influence of mass transfer.Decisive:

only distributions of pure Ohmic resistances

• Secondary: Adding charge transfer

resistances to purely Ohmic resistances

• Tertiary: Mainly determined by mass

transfer conditions

Primary current density distribution between two

parallel plates and at the electrode edge

Addition of electrolyte resistances Rp and Rv add

to charge transfer resistance to give primary and

secondary current distributions

Even current density distribution

At rotating disc electrode under mass

transport limited condition ( limiting

current density) is a typical tertiary

c.d. distribution

Electropolishing

Mass transfer controlled transport of

dissolution products through a thin,

statistically fluctuating layer of debris

generates the polishing effect

Current densities amount from hundred

to several hundred mA cm-2

Electropolishing electrolytes

• Composition given in lecture manuscript

• Almost all contain phosphoric acid

• Almost all – exception electropolishing W –

are strongly acidic

• Some contain organic cosolvents

• Are obtained and optimized by trial and

error

Electrochemical machining electrolytes

• Are neutral (neither basic nor acidic) with the exception of basic electrolyte for molybdenum

• Most of them contain sodium nitrates or perchlorate

• Current densities amount to several A cm-2

• Copious exchange of electrolyte must be secured to remove Joule`s heat and all debris

Electrochemical micromachining

During double layer charging: primary current distribution

The surface of the workpeace which is farther

away charges more slowly than next to the tool

l x ρ x Cspec = ح

with l = length of current line

ρ = specific resistance of electrolyte (approximately 10 Ω cm)

ζ is charging time; as potential changes exponentially with time:

Φo – Φ = (Φo – Φ )t=(1-exp(-t/ ح)

Improve the resolution of anodic

dissolution from millimetres to

micrometres

Applying pulses in nanoseconds

instead of direct currents

Example from L.Cagnon, V. Kirchner, M. Kock, R. Schuster, G. Ertl, Th. Gmelin and H. Kueck, Z. Phys. Chem. 217, (2003), 299 - 313

Example from M. Kock, V.

Kirchner and R. Schuster,

Electrochim. Acta 48, (2003) 3213 - 3219

The LIGA – Process for building

Micro-structures by x-ray-assisted masking

and cathodic metal deposition

• Resolution is determined by precision

of masks and their copy on photo-

resist – hence x-ray copying

Summary

• With maximal cutting rates corresponding to several A cm-2 electrochemical machining is too slow to be generally applicable instead of mechanical machining

• But ultrahard alloys can only be treated by electrochemical machining which usually gives also a good polishing finish

• Applying nanosecond pulses increases the dimensional resolution, so that also micrometer structures can be produced – it is still an open question how to utilize these possibilities in commercial processes.

Electric Discharge Machining (EDM)

EDM Operation

One of the most widely used non-traditional processes

Shape of a finished work surface produced by a shape of electrode

tool

Can be used only on electrically conducting work materials

Requires dielectric fluid, which creates a path for each discharge as

fluid becomes ionized in the gap.

Metal is melted/vaporized by the series of electrical discharges

Can be very precise and produces a very good surface finish

Work Materials in EDM

Work materials must be electrically conducting

Hardness and strength of work material are not

factors in EDM

Material removal rate depends on melting point of

work material

Module 4a 4

Principle of the process

Structure and configuration

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

Special form of EDM uses small diameter wire as electrode to cut a

narrow kerf in work

Wire EDM

Operation of Wire EDM

Work is fed slowly past wire along desired cutting path.

CNC used for motion control.

While cutting, wire is continuously advanced between supply

spool and take-up spool to maintain a constant diameter.

Dielectric fluid is required.

Applied using nozzles directed at tool-work interface or

submerging work part

Wire EDM Applications

Ideal for stamping die components

Since kerf is so narrow, it is often possible to fabricate

punch and die in a single cut

Other tools and parts with intricate outline shapes,

such as lathe form tools, extrusion dies, and flat

templates

Dental part cut from

nitinol material by

wire EDM

Laser-Beam Machining

Uses a concentrated beam of light to vaporize

part of the workpiece

Usually produces a rough surface with a heat-

affected zone

Can cut holes as small as .005 mm with

depth/diameter ratios of 50:1

Uses the light energy from a laser to remove material by vaporization and ablation

Laser Beam Machining (LBM)

The punch

is a light

beam

Laser-Beam Machining

Laser-Beam Machining

Laser-Beam Machining

Example of a part cut by laser-beam machining

Splatter marks appear where the laser first cuts into the material

Laser-Beam Machining

Design Considerations:

- Non-reflective workpiece surfaces are

preferable

- Sharp corners are difficult to produce; deep

cuts produce tapers

- Consider the effects of high temperature on

the workpiece material

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 and strength,

soft metals, ceramics, glass and glass epoxy, plastics,

rubber, cloth, and wood

Electron Beam Machining

Vaporizes material using electrons accelerated

to 50-80% the speed of light

Produces finer surface finish and narrower cut

width than other thermal cutting processes

Requires a vacuum; generates hazardous X rays

Electron Beam Machining

Electron Beam Machining

An electron beam in a very low-pressure atmosphere of helium

Plasma Arc Cutting

PAC

Objectives

• Define plasma arc cutting (PAC).

• Explain how a PAC cutter operates.

• Identify the parts of a PAC cutter.

• Explain advantages and disadvantages of the PAC system.

• Identify materials that can be cut with the PAC.

• Explain safety associated with using the plasma arc cutter.

History

• The plasma-arc process had its origin almost 50 years ago, during the height of World War II.

• Plasma cutting was accidentally discovered by an inventor who was trying to develop a better welding process.

• In an effort to improve the joining of aircraft materials, a method of welding was developed that used a protective barrier of inert gas around an electric arc to protect the weld from oxidation.

History

• It was discovered that by restricting the opening through which the inert gas passed, the heat produced by the process was greatly increased.

• At the same time, the smaller opening caused the flow of gas to speed up dramatically, ultimately blowing out a channel in the work.

• The plasma-arc cutting process started seeing commercial use in the first few years of the sixties.

• It was an extremely expensive process to undertake, and most cutting was performed by large burning services.

4th State of Matter

Plasma

• Plasma has two meanings.

– The fluid portion of blood.

– A state of matter that is found in the region of an electrical

discharge (arc).

• Plasma created by an arc is an ionized gas that has both

electrons and positive ions whose charges are nearly equal

to each other.

• Plasma is present in any electrical discharge.

Plasma

• Plasma consists of charged particles that conduct the electrons across the gap.

– Both the glow of a neon tube and the bright fluorescent light bulb are examples of low-temperature plasmas.

• Plasma results when a gas is heated to a high enough temperature to convert into positive and negative ions, neutral atoms, and negative electrons.

– The temperature of an unrestricted arc is about 11,000°F

– The temperature created when the arc is concentrated to from a plasma is about 23,000°F.

Machines

• Most, if not all, of the light portable plasma cutters are 110

volt machines.

– Suited primarily for cutting sheetmetal and other light work.

• The next level up are the 220 volt machines with 50 to 80

amp output current.

– These are portable from the standpoint that one person can put it

on a truck and take it to the job.

How PAC works

• Plasma cutters work by sending an electric arc through a gas that is passing through a constricted opening. – The gas can be shop air, nitrogen, argon, oxygen. etc.

• This elevates the temperature of the gas to the point that it enters a 4th state of matter. – Scientists call this additional state plasma. As the metal being cut is part of the

circuit, the electrical conductivity of the plasma causes the arc to transfer to the work.

• The restricted opening (nozzle) the gas passes through causes it to squeeze by at a high speed. This high speed gas cuts through the molten metal.

• The gas is also directed around the perimeter of the cutting area to shield the cut.

How a Plasma Cutter works

• A complete plasma cutter consists of a

– power supply,

– a ground clamp,

– and a hand torch.

• The main function of the power supply is to convert the AC line voltage into a user-adjustable regulated (continuous) DC current.

• The hand torch contains a trigger for controlling the cutting, and a nozzle through which the compressed air blows. An electrode is also mounted inside the hand torch, behind the nozzle.

PAC System

Operation

• Initially, the electrode is in contact with (touches) the nozzle.

• When the trigger is squeezed, DC current flows through this contact.

• Next, compressed air starts trying to force its way through the joint and out the nozzle.

Operation

• Air moves the electrode back and establishes a fixed gap between it and the tip. – The power supply automatically increases the voltage in order to maintain

a constant current through the joint - a current that is now going through the air gap and turning the air into plasma.

• Finally, the regulated DC current is switched so that it no longer flows through the nozzle but instead flows between the electrode and the work piece. This current and airflow continues until cutting is halted.

Starting the Arc

• In many of today's better plasma cutters, a pilot arc between the electrode and nozzle is used to ionize the gas and initially generate the plasma prior to the arc transfer.

• Other methods that have been used are touching the torch tip to the work to create a spark, and the use of a high-frequency starting circuit.

PAC versus Oxy-Fuel

• In general, fabricators consider oxy-fuel to be superior to

plasma for cutting steel when thicknesses exceed about 1/2

inch.

• This is because of the slight bevel (4 to 6 degrees) in the cut

face that plasma produces. It is not noticeable in thinner

materials, but becomes more so as thicknesses increase.

Also, at thicknesses above 1/2 inch, plasma has no cutting

speed advantage over oxy-fuel.

PAC versus Oxy-Fuel

• If you are planning to cut non-ferrous metals such as

stainless or aluminum, which cannot be cut by oxy-fuel,

consider a 50 to 80 amp, 220 volt plasma cutter.

• Plasma cutting is by far the simplest and most economical

way to cut a variety of metal shapes accurately.

• Plasma cutters can cut much finer, faster, and more

automatically than oxy-acetylene torches.

PAC Cutting Examples

Plasma Torch

• A device depending on its design, which

allows the creation and control of the plasma

for welding and cutting processes.

• Plasma torch supplies electrical energy to a

gas to change it into the high energy state of a

plasma

Torch Body

• Made of a special plastic that is resistant to

high temperatures, ultraviolet light, and

impact.

• Provides a good grip area and protects the

cable and hose connections to the head.

• Torch body is available in a variety of lengths

and sizes.

Torch head

• Torch head is attached to the torch body where the cables and hoses attach to the electrode tip, nozzle tip, and nozzle.

• Torch and head may be connected at any angle such as 90°, 75°, 180° (straight), or it can be flexible.

• Because of the heat in the head produced by the arc, some provisions for cooling the head and its internal parts must be made. – Cooling for low power torches may be either by air or water.

– High power torches must be liquid cooled.

• It is possible to replace just the torch head on most torches if it becomes worn or damaged.

Power switch

• Manual power switch used to start and stop

the power source, gas, and cooling water.

• Thumb switch on the torch body most often

used.

• Foot control can be used.

Common Torch Parts

• Parts of the torch

– Electrode

– Tip nozzle insulator

– Nozzle tip

– Nozzle guide

• Nozzles and the metal parts are usually made out of copper, and they may be plated.

• The plating of copper parts will help stay spatter-free longer.

PAC

Electrode tip

• Electrode tip is often made of copper electrode with a tungsten tip attached.

• Use of copper/tungsten tip has improved the quality of work they can produce.

• By using copper, the heat generated at the tip can be conducted away faster.

• Keeping the tip as cool as possible lengthens the life of the tip and allows for better quality cuts for a longer time.

• Old torches you must grind the tungsten electrode

Nozzle Insulator

• Located between the electrode tip and the nozzle tip

• Provides the critical gap spacing and electrical separation of the parts.

• The spacing between the electrode tip and nozzle tip called the electrode setback is critical to the proper operation.

Nozzle tip

• Has a small, cone-shaped, constricting orifice in the center.

• The electrode setback space, between the electrode tip and nozzle tip is where the electric current forms the plasma.

• The preset close-fitting parts provide the restriction of the gas in the presence of the electric current so the plasma can be generated.

• The diameter of the constricting orifice and electrode setback are major factors in the operation of the torch.

• As the diameter of the orifice changes, the plasma jet action will be affected.

• When the setback distance is changed, the arc voltage and current flow will change.

Nozzle

• Sometimes call the cup.

• Made of ceramic or any other high-

temperature resistant substance.

• Helps prevent the internal electrical parts

from accidental shorting and provides control

of the shielding gas or water injection if they

are used.

Water shroud

• Water shroud nozzle may be attached to some

torches.

• Water surrounding nozzle tip is used to

control the potential hazards of light, fumes,

noise, or other pollutants produced.

Power Requirements

• Requires a drooping arc voltage or constant current, direct

current, high-voltage, power supply.

• Drooping arc voltage allows for a rapid start of the plasma

arc at the high open circuit voltage and more controlled

plasma arc aas the voltage rapidly droops.

– Ranges from 50-200 volts closed circuit

– Ranges from 150-400 volts open circuit.

Amperages

• High voltage

• Amperage range from 10-200 amps.

• Some automated machines may have 1,000

ampere capacities.

• Higher the amperage capacity the faster and

thicker they will cut.

Cutting speeds

• High cutting speeds are possible

– Up to 300 inches per minute

– 25 feet a minute

– ¼ mile an hour

Metals to be cut

• Any material that is conductive can be cut using the PAC process.

• In a few applications nonconductive materials can be coated with conductive material so that they can be cut.

• Most popular materials cut – Car o steel up to 1”

– Stai less steel up to 4”

– Alu i u up to 6”

• Other metals commonly cut – Copper

– Nickel-alloys

– High-strength, low alloy steels

– Clad materials

– Expanded metal

Starting Methods

• Two methods are used to establish a current

path through the gas

– High frequency alternating current

– Momentary shorting

High frequency alternating current

• Most common

• Uses a high frequency alternating current carried through the conductor, the electrode and back from the nozzle tip.

• High frequency current will ionize the gas and allow it to carry the initial current to establish a pilot arc.

• After the pilot arc has been started, the high frequency starting circuit can be stopped.

• When the torch is brought close enough to the work, the primary arc will follow the pilot arc across the gap, and the main plasma is started.

• Once the main plasma is started, the pilot arc power can be shut off.

Momentary shorting

• Requires the electrode tip and nozzle tip to be

momentarily shorted together.

• This is accomplished by automatically moving

them together and immediately separating

them again.

Safety

• Electrical shock – Because the open circuit voltage is much higher for this process than for any other,

extra caution must be taken.

– The chance that a fatal shock could be received from this equipment is much higher than from any other welding equipment.

• Moisture – Often water is used with PAC torches to cool the torch, improve the cutting

characteristics, or as a part of a water table.

– A y ti e ater is used it’s ery i porta t that there e o leaks or splashes. – The chance of electrical shock is greatly increased if there is no moisture on the

floors, cables, or equipment.

Safety

• Noise – Because the plasma stream is passing through the nozzle orifice at a high speed, a loud sound

is produced.

– The sound level increases as the power level increases.

– High le els of sou d a ha e a u ulati e effe t o o e’s heari g. • Light

– PAC produces light radiation in all three spectrums. • Large quantity of visible light, if the eyes are unprotected, will cause night blindness.

• Most dangerous of the lights is ultraviolet. This light can cause burns to the skin and eyes.

• Infrared can be felt as heat, and it is not as much a hazard.

• Fumes – PAC produces a large quantity of fumes that are potentially hazardous.

– A specific means for removing them from the work space should be in place.

Safety

• Gases

– Some of the plasma gas mixtures include hydrogen.

– Hydrogen is a flammable gas.

– Make sure that the system is leak-proof.

• Sparks

– Danger of accidental fire is present.

– Use a fire watch person if excessive sparks are present.