Table of Contents

TITLE....................................................................................................................................................2

1.1.0 Gas cutting (Oxy-Cutting)................................................................................................2

1.2.0 OBJECTIVES....................................................................................................................2

1.3.0 INTRODUCTION..............................................................................................................2

1.4.0 THEORY.......................................................................................................................3

1.5.0 APPARATUS................................................................................................................6

1.6.0 PROCEDURE................................................................................................................7

1.7.0 RESULTS......................................................................................................................8

1.8.0 DISCUSSION................................................................................................................9

1.9.0 CONCLUSION............................................................................................................11

2.1.0 Plasma cutting.................................................................................................................12

2.2.0 OBJECTIVE................................................................................................................12

2.3.0 INTRODUCTION AND THEORY.............................................................................12

2.4.0 APPARATUS..............................................................................................................18

2.5.0 EXPERIMENTAL SETUP AND PROCEDURE........................................................18

2.6.0 DATA ANALYSIS AND DISCUSSION....................................................................19

2.7.0 CONCLUSION............................................................................................................27

3.1.0 AIR ARC GOUGING......................................................................................................28

3.2.0 OBJECTIVE................................................................................................................28

3.3.0 INTRODUCTION AND THEORY.............................................................................28

3.4.0 APPARATUS..............................................................................................................31

3.5.0 EXPERIMENTAL SETUP AND PROCEDURE........................................................32

3.6.0 RESULT AND ANALYSIS........................................................................................33

3.7.0 DISCUSSION OF RESULTS......................................................................................35

3.8.0 CONCLUSION............................................................................................................37

3.10.0 REFERENCE......................................................................................................................38

1

TITLEStudy the science and system operation of thermal cutting using Gas cutting, Plasma cutting

and Arc gouging process

1.1.0 Gas cutting (Oxy-Cutting)

1.2.0 OBJECTIVES

To study the basic operation of gas cutting using Oxy-Cutting process

To analyze the Oxy-Cutting process on the carbon steel, stainless steel and aluminium

plate

To find out the oxygen ignition temperature for carbon steel & reactions involved

1.3.0 INTRODUCTION

Gas cutting or also known as Oxy-fuel cutting is a chemical reaction between pure

oxygen and steel to form iron oxide. It uses a cutting torch to cut the materials as well as

heating metal to its kindling temperature. A high pressure stream of pure oxygen is directed

on the metal after it has been preheated to a temperature above its kindling point. The oxygen

causes a chemical reaction known as oxidation to take place rapidly. When oxidation occurs

rapidly, it is called combustion or burning. But when it occurs slowly, it is known as rusting.

Kindling temperature or also known as ignition temperature is when the melting temperature

for the oxide of the metal is lower than the melting temperature of base metal. For example,

the kindling temperature of iron is approximately 1600 F. The preheat flames are used to

raise the surface of the steel to approximately 1800 F. As the steel is oxidized and blown

away to form a cavity, the preheated flames and oxygen stream are moved at constant speed

to form a continuous cut. Oxy-fuel gas cutting is commonly performed with oxyacetylene

cutting which is the combination of oxygen and acetylene gas. Table 1 shows the types of

fuel gas used for Oxy-fuel gas cutting.

2

Fuel Gas Temperature (Celsius)

Acetylene 3087

MAPP 2927

Natural gas 2538

Propane 2526

Propylene 2867

Hydrogen 2660

Table 1: Fuel Gases Used for Flame Cutting

3

1.4.0 THEORY

In Oxy-fuel gas cutting, the burning away of the metal is a chemical reaction between

iron (Fe) and oxygen (O). The reaction between them forms an iron oxide (Fe3O4). Heat is

produced by the metal as it burns. This heat also helps to carry the cut along. The gas cutting

was commonly performed by Oxyacetylene cutting, which is the combination of oxygen and

acetylene gas. Oxygen is a colorless, tasteless, odorless gas and is slightly heavier than air. It

is non-flammable but supports the combustion with the other elements and it is one of more

common elements in its free state. Acetylene is a flammable fuel gas composed of carbon and

hydrogen having the chemical formula C2H2. When it burns with oxygen, acetylene

produces a hot flame, with a temperature between 5700 F and 6300 F. It is a colorless gas and

having a disagreeable odour that is readily detected even when the gas is highly diluted with

air.

There are three types of gas flames commonly used for all oxygen gas process. They

are carburizing, neutral and oxidizing. Carburizing flames always shows discreet colours, the

inner core is bluish white, the intermediate core is white, the outer envelope flame is light

blue and the feather at the top of the inner core is greenish. The highly carburizing flame is

longer with yellow or white feathers on the inner core. The slightly carburizing flame has a

shorter feather on the inner core and becomes whiter. The temperature for carburizing flame

is about 5400 F.

The neutral flame is the most common preheat flame used for gas cutting. The neutral

flame occurs when the carburizing flame is increased with the oxygen. The feather will

disappear from the inner core and all that will be left is the dark blue inner flame and the

lighter blue outer one. The temperature for this flame is about 5600 F.

Oxidizing flame occurs when a little more oxygen is added to the preheat flame. It

will quickly become shorter and the flame will start to neck down at the base, next to the

flame parts. The inner flame core changes from dark blue to light blue. Oxidizing flames are

much easier to look at because they are less radiant than neutral flame. Figure 1 shows the

type of flame used for Oxy-acetylene gas cutting.

4

Figure 1: Type of flame used in Oxy-acetylene gas cutting.

5

1.5.0 APPARATUS

- Oxy-acetylene gas cutting equipment.

- Carbon steel, aluminium metal and stainless steel workpiece of material.

6

1.6.0 PROCEDURE

1. The equipment of the preheating flame was adjusted to neutral flame.

2. The torch was holded perpendicular to the carbon steel with the inner cores of the

neutral flames about 1/6 inches above the end of the size to be cut.

3. Hold the torch in this position until the heating spot is bright red.

4. The cutting oxygen valve was opened slowly but steadily by pressing down the

cutting valve lever.

5. A shower of sparks will fall from the opposite side of the work. This shows that the

flame has started to pierce the metal. The cutting torch was moved forward along the

line just as fast enough for the flame to continue to penetrate the work completely.

6. The torch was moved slowly along the cutting work. Watch the cut to know the

progress of the cut by adjusting the torch movement necessary.

7. Make sure to move the torch at the correct speed, not too fast and not too slow.

8. Step 1 until 7 was repeated by using oxidizing flame and carburizing flame

respectively for carbon steel material.

9. Step 1 until 8 was repeated on Aluminium metal plate and Stainless Steel material

respectively.

10. Analyze the result obtained.

7

1.7.0 RESULTS

Carbon Steel plate Stainless Steel plate

Aluminium plate

8

1.8.0 DISCUSSION

When we use gas cutting to cut carbon steel, it can cut easily without any problem.

The reaction between oxygen and iron that forms iron oxide has a lower temperature than the

melting temperature of the base metal (carbon steel). The iron oxide temperature is

approximately 1500 C while the melting temperature of the carbon steel during the reaction is

approximately 1540 C. Thus, the kindling temperature for carbon steel is possible for gas

cutting operation. The high pressure stream of pure oxygen will blast the iron oxide, blowing

the molten spot and produce cuts.

When we use gas cutting to Aluminium metal plate, we find out that the Aluminium

cannot be cut. This is because during the reaction, the oxygen will react with Aluminium to

form Aluminium oxide, that generate the temperature approximately 1700 C. The kindling

temperature for Aluminium is not possible to produce cut because the temperature for

Aluminium oxide during the reaction is higher than the base metal(Aluminium), where the

temperature of the base metal during the reaction is 660 C.

For stainless steel material, we find out that it cannot be cut using gas cutting process.

This is because stainless steel contains high amount of chromium element that reacts with

oxygen during the burning reaction. The oxygen and chromium will forms chromium oxide,

which produce the temperature approximately 1990 C. The temperature for the base

metal(stainless steel) during the burning reaction is approximately 1868 C. This shows that

the chromium oxide temperature during the reaction is higher than the base metal. Thus, the

kindling temperature for stainless steel makes it possible to cut the material using gas cutting

process.

To produce good quality cuts, there are some factors that need to be considered during

gas cutting process. The first factor is the preheat flame. The characteristics for proper heat

flames on the cut is the top edge quite square must be less than 1/16 inches melt over. The

face of cut contains an easily removable thin layer of slag which covers a clean surface with

well define drag lines from top to bottom. Besides, there should be a little to no easily

removable slag on the bottom edge of the cuts.

The second factor is the pure oxygen stream. The oxygen stream should be high

pressure to provide adequate quantities of oxygen to react sufficiently with a narrow bond of

9

steel and to blow slag clear of the cut. The stream also must be columnar in shape and extend

visibly for at least six inches in a nozzle test. The third factor is oxygen bore design. The dirty

or damaged oxygen bore will result in a non-uniform cutting stream. This will cause defects

such as undercutting (belly) in the cut face. To correct the defects make sure the speed,

nozzle size and oxygen pressure are suitable. If no improvement, change the nozzle to known

good quality or reduce travel speed accordingly.

Besides, the cutting speed is the factor for good quality cuts. The quality increases as

the speed is decreased. This is because higher levels push the ability of the cutting bore or

stream to deliver the full amount of high purity oxygen with the perfect stream geometry to

the kerf. As the speed increases, the drag lines lean further to the rear. This results in some

rounding of the bottom edge in shape cutting. At higher speeds, undercuts or bellies will

appear along the cut face destroying the desired flatness. At extreme speeds, the drag will be

so severe that the bottom corner of the cut will not be completed at the finish of the cut. But,

slow speeds can cause problem as well. The availability of too much oxygen can cause

instability of operation when not enough cutting action is created at the leading edge.

The travel speed also can be considered for good quality cuts. If the travel speed is too

fast, the amount of the metal volume being oxidized and the heat being generated is

insufficient to maintain the cutting operation through the plate thickness. But if the speed is

too slow, large amount of heat are generated. However, there is now not enough oxygen to

handle the greater volume of metal which must be oxidized for the cut to be successfully

completed.

10

1.9.0 CONCLUSION

At the end of the experiment, we are finally understands about the operation of the gas

cutting process. The gas cutting commonly performs by Oxy-acetylene gas cutting but other

types of fuels can be used for the cutting process depends on some characteristics of the

materials to be cut. There are three types of preheat flames which are carburizing, neutral and

oxidizing flames that has different characteristics for cutting purposes. Besides, not all types

of metal can be cut for Oxy-acetylene gas. For carbon steel types, only low and medium iron

content of carbon steel can be cut using the Oxy-acetylene gas cutting process. We also had

understands the term kindling temperature or ignition temperature which means the oxides of

the metal during the burning reaction must be lower than the temperature of the base metal so

that gas cutting is possible during the process. The reaction of oxygen with the metal than

form metal oxide is the results whether it is possible or not to cut the metal. The example of

the metal that is impossible to be cut using the gas cutting process is Aluminium and

Stainless Steel where the temperature of the aluminium oxide and the chromium oxide is

higher than the base metal. Furthermore, there are several factors also need to be considered

to produce good quality cuts.

11

2.1.0 Plasma cutting

2.2.0 OBJECTIVE

1. To study the basic operation of plasma cutting process

2. To understand the properties of plasma cutting process and its benefits

2.3.0 INTRODUCTION AND THEORY

Plasma cutters come varying in shapes and sizes. Regardless of its size, all plasma cutters

functioning on the same principle and are constructed around roughly the same design.

Figure1: Plasma cutter when cutting the metal (Dual gas plasma cutting)

Plasma cutters work by channeling a pressurized gas, such as nitrogen, argon, or oxygen,

though a small channel. In the center of this channel, it consists of negatively charged

electrode and when we apply power to the negative electrode, touch the tip of the nozzle to

the metal and the connection creates a circuit. It can clearly see in the figure 1 above that it is

using dual gas, that is one gas for plasma and the other gas is for shielding gas. This shield

gas is used to shield the cut area from atmosphere and producing a good surface cutting.

12

Figure 2: conventional plasma cutting using single gas

From experiment we did, the plasma cutter we used is conventional type that is use a single

gas (usually air or nitrogen) that both produce plasma for cutting the metal. Most of this

system is rated below 100 Amps. A powerful spark was generating between the electrode

and the metal. As the inert gas passes through the channel, the spark heats the gas until it

reaches the fourth state of matter. Fourth state of matter which is plasma, when we keep

rising the temperature of the gas, the electron and the nucleus constituent of an atom cannot

stay together and the atom are stripped of the electron and the plasma is formed.

13

Figure 3: State of matter and properties of matter from low temperature to high temperature

As we can see on figure 3 above, electron move freely among the ions and their

negatively charges cancel out the effect of positively charged ions. It can say that the plasma

contain no charged or roughly zero charge and when the charges move independently they

generate electrical currents with magnetic fields, as a result, they are affected by each other’s

fields

Once the energy of heat releases the electrons from the atom, the electrons begin to

move around quickly. The electrons are negatively charged, and they leave behind their

positively charged nuclei. These positively charged nuclei are known as ions. When the fast-

moving electrons collide with other electrons and ions, they release vast amounts of energy.

This energy is what gives plasma its unique status and unbelievable cutting power.

14

Figure 4: Plasma cutter before cut the metal

This reaction creates a stream of directed plasma, approximately 30,000 F (16,649 C)

and moving at 20,000 feet per second (6,096 m/sec) that reduces metal to molten slag.The

plasma itself conducts an electrical current and the cycle of creating the arc is continuous as

long as power is supplied to the electrode and the plasma stays in contact with the metal that

to be cut. In order to ensure this contact, protect the cut from oxidation and regulate the

unpredictable nature of plasma, the cutter nozzle has a second set of channels. These channels

release a constant flow of shielding gas around the cutting area. The pressure of this gas flow

effectively controls the radius of the plasma beam.

A plasma cutting can be operated in the two modes, which is transferred and non-

transferred mode. In transferred mode, the electric current flows between the plasma torch

electrode (cathode) and the work piece (anode). While in the non-transferred mode, the

electric current was flows between the electrode and the torch nozzle. In other words, in non-

transferred arc torches, the electrodes are inside the body of the torch itself (creating the arc

inside the torch) whereas in transferred one of electrode is outside (and is usually the

conductive material to be cut), allowing the arc to form outside of the torch over a larger

distance. Both modes of operation are illustrated in Figure 5.

15

Figure 5: Schematic diagram for transferred and non-transferred arc plasma cutting

The difference between this two type of arc is the work piece is cut by the heat of

constricted arc between an electrode and base metal (transferred arc) while when the heat of

constricted arc between the electrode and the constricting nozzle (non-transferred arc). Base

on study, the benefit of transferred arc is it can prevent heat loss and it can used in a twin-

torch setup, whereas one torch is cathode and the other anode, which has the earlier benefit of

a regular transferred single-torch system, but allows their use non-conductive materials, as

there is no need for it to form the other electrode. The transferred mode was widely used in

plasma cutting because useable heat input to the work piece is more efficiently applied when

the arc is in electrical contact with the work piece.

While the non-transferred arc, where the material is not in contact with the electrical

circuit. It means that electrically non-conducting material may also be cut by this process and

was suitable for low cutting performance value for example plastic.

16

Figure 6: Weldability of plasma cutting

Non-transferred arc plasma cutting was limited to cutting non conductivity of work

piece, it is not suitable for metal cutting. Both modes was suitable for cutting the work piece

and mainly transferred arc is the best mode to cut all type of work piece especially in metal

cutting as shown in figure 6 above.

17

2.4.0 APPARATUS

Figure 7: Setup of plasma cutting

From the figure 7 above, a 3 main components of plasma cutter is 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 electrode, and a nozzle through which the

compressed air blows. An electrode is also mounted inside the hand torch, behind the nozzle.

2.5.0 EXPERIMENTAL SETUP AND PROCEDURE

1. Initially, the electrode was in contact with (touches) the nozzle.

2. When the trigger is pressed, DC current flows through this contact.

3. Next, compressed air started forcing its way through the joint and out the nozzle.

4. 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.)

5. 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 was stop.

18

2.6.0 DATA ANALYSIS AND DISCUSSION

Before starting any plasma cutting, first of all we must set the type of gas of cutting

and the most common and cheapest gas for Plasma Cutting is compressed Air. It works best

with most metal plates up to 25mm thick and air usually contains oxygen; consequently,

when used for cutting, it leaves oxidized cut surfaces. Compressed air also was used for

plasma gouging on carbon steel as well as cutting of aluminium.

Other gas cutting such as oxygen can be used on carbon steel up to 60mm and

produces a fine and better cut quality surface; stainless steel and aluminium also can be cut

too using oxygen gas, but it might give low quality of cut surfaces.

19

Plasma

Gas / Shield

Mild Steel Stainless Aluminum

Air / Air Good cut

quality/speed.

Economical

Good cut

quality/speed

Economical

Good cut

quality/speed

Economical

Oxygen

(O2) / Air

Excellent cut

quality/speed. Very

little dross

Not recommended Not recommended

Nitrogen

(N2) / CO2

Fair cut quality, some

dross. Excellent parts

life

Good cut quality

Excellent parts life

Excellent cut quality.

Excellent parts life

Nitrogen

(N2) / Air

Fair cut quality, some

dross. Excellent parts

life

Good cut quality

Excellent parts life

Good cut quality

Excellent parts life

Nitrogen

(N2) / H20

Fair cut quality, some

dross. Excellent parts

life

Excellent cut

quality. Excellent

parts life

Excellent cut quality.

Excellent parts life

Argon

Hydrogen /

N2

Not recommended Excellent on thick

>1/2"

Excellent on thick

>1/2"

Table 1: Type of gas cutting and shielding gas of plasma cutting for mild steel, aluminium

and stainless steel

20

When we want to cut metal sheets of up to 75mm in plasma cutting, nitrogen gas is

usually used because it can provides high quality of non-oxidized cuts.

Stainless steel and aluminium are usually cut by using mixtures of Argon and Hydrogen,

especially when the material is thicker than 75mm and the result give a higher quality of

cutting surface. Type of gas cutting and shielding gas for plasma cutting for mild steel,

stainless steel and aluminium can be clearly shows in table 1 above. Composition of gas

should be taken into consideration so that it can meet the process, material and application

requirement.

All of this requirement must be consider cause it can affect the investment cost and

the today is that more simple and inexpensive machinery was used, making plasma cutting a

realistic alternative to other cutting methods. The selection of gas or gases for plasma arc

cutting is based on such factors as the required quality of the cut, the thickness of metal to be

cut and the gas cost. For cutting thin metal a single gas flow is often used to provide both the

plasma and the arc shielding, but for cutting thicker metal, dual gas flows are used. The

single gas flow may be air, nitrogen, nitrogen/hydrogen, oxygen or argon. The dual gas flows

may be nitrogen, nitrogen/hydrogen, oxygen, argon or argon/hydrogen mixtures.

21

There are a few practices of plasma cutting that can increase your efficiency, improve

the cut quality and prolong the life of plasma cutting equipment. Standoff, travel speed,

cutting nozzle and pressure of gas must be consider before doing any plasma cut to obtain a

better quality of cutting surface as well as its performance.

Figure 8: Maintain a 1/16 to 1/8 in. standoff to increase the longevity of consumables,

produce a cleaner cut and maximize your machine’s cutting capacity.

Figure 9: Using a non-cutting hand as a brace to help and maintain a standoff as well as

provide a cleaner cut.

22

From figure 8 and 9, maintain the optimal 1/16 to 1/8 in. distance between the tip and

the work piece and use your non-cutting hand as supportive equipment for other hand. This

provides freely movement in all directions while help to maintain a constant standoff and

steady of our cutting hand. When piercing, this distance should be increased. The golden rule

should be followed: Pierce high, cut low. If we do not, cutting/piercing quality and

consumables life will suffer. The height of the torch during piercing is particularly important.

If we pierce too low molten metal splashes upwards and spatters the front of the nozzle and

shield damaging the parts; damaged parts result in low cut quality.

Arc "snuffing" occurs when the torch pierces touching the metal, or when it keeps in

contact with the sheet surface while cutting. If the arc is "snuffed", the electrode, nozzle, gas

swirler and, sometimes, the torch, are destroyed. Piercing at a height of 1.5-2X the

recommended cut height protects the torch and parts from damage.

Figure 10: Maintaining the proper travel speed, the sparks will exit the work piece at a 15ᵒ to

20ᵒ angle.

23

The spark should exit the material at a 15ᵒ to 20̊ᵒ angle opposite the direction of travel.

If the spark was going straight down, it means the cutting was moving too slowly but if it

sprays back, it means the cutting was moving too fast. Cutting too fast or too slow will cause

cutting quality problems and impaired consumables longevity. If the speed was too slow the

cut pieces will develop "low speed dross" a large bubbly accumulation of dross along the

bottom edge. Slow speeds also may cause a widening of the kerf and excessive amounts of

top spatter. If the speed was too fast the arc will lag backward in the kerf causing a bevelled

edge, a narrow kerf and a small hard bead of dross along the bottom edge of the cut piece.

High speed dross is difficult to remove. The correct cutting speed will produce minimal

dross--the result will be a clean edge that needs little rework before the next step in the

manufacturing process.

Figure 11: Typical quality problem that related to plasma cutting and ways to solve it

24

As we can see in figure 11, dross which is a burr formation on the bottom of the kerf

and spatter on the top of the kerf and burr refers to resolidified metal and metal oxide that

adheres to the bottom of the plasma- cut surface. Spatter can also form on the top edge of the

plasma cut-surface. Burr formation depends on a number of process variables, such as the

cutting speed, burner distance, current intensity, voltage, the plasma gas and the plasma

technology. It is also affected by variables such as the material itself, its thickness, surface

condition and temperature changes in the material during cutting. Burrs can also form if the

cutting speed is too high or too low. Generally there is a middle range between these two

extremes in which a burr-free cut can be achieved. The plasma cutting technique and gas used

are important factors in avoiding the formation of burrs.

Figure 11: Quality parameter of plasma cutting

The kerf width in plasma cutting is about one and a half to two times greater than the

diameter of the nozzle outlet. Kerf width is influenced by the cutting speed. If the cutting

speed is reduced, the kerf is wider.

Figure 12: Angle deviation of plasma cutting

25

u

During plasma cutting, the cut surface generally runs at a slight angle due to the

temperature gradient in the plasma arc. The greatest energy input occurs at the top of the kerf,

causing more material to melt there than at the bottom. The more the arc is constricted, the

smaller the resulting cut angle. The cut angle is also influenced by the distance of the burner

and the cutting speed. In conventional plasma cutting, the cutting angle on both sides is

normally 4 to 8°. When using plasma technology with greater constriction, the cut angle can

be reduced to less than 1 degree so that cutting parts possess common cutting edges.

Use the right nozzles as specified by the manufacturer for different amperage ratings.

The best cut quality and parts life is usually achieved when the amperage is set just under the

maximum of the nozzle's rating. If the amperage is too low, the cut won’t be square; if it is

too high, nozzle life will be shortened.

Keep plasma gas clean and dry. Use correct air pressure and flow. The plasma gas

pressure or flow setting should not exceed the factory recommended settings. Excess pressure

in the plasma chamber makes it harder for the High Frequency spark to jump the gap. The

pilot arc is effectively blown out before it is fully established. High gas pressure accounts for

the majority of hard starting problems.

Keep the torch and consumables clean. Torches, if properly cared for, can last for

months or even years. Torch threads must be kept clean and seating areas should be check for

contamination or mechanical damage. Any dirt, metal dust or excess O-ring lubricant should

be cleaned out of the torch. To clean the torch, use a cotton swab and electrical contact

cleaner or hydrogen peroxide.When cutting periodically clean oxides from the electrode and

nozzle. This build up disrupts gas flow and shortens life of consumables besides causing

difficulties in arc starting.

Check air filters and hose cables regularly for leaks and rectify. Drain the air filters

and clean the elements thoroughly every week. The flow and pressure of gas and coolant for

water cooled torches should be check every day. If the flow is insufficient, consumables will

not be cooled properly and parts life will be reduced. Inadequate flow of cooling water due to

worn pumps clogged filters; low coolant level etc. is a common cause of parts and torch

failure.

26

2.7.0 CONCLUSION

As we can conclude, a plasma system was highly versatile and a productive cutting

tools which it process ability to perform various processes as well as its applications to

operate in various locations. It also can work on various metal types, forms, and thicknesses

that give it distinct advantages over competitive cutting technologies.

27

3.1.0 AIR ARC GOUGING

3.2.0 OBJECTIVE

1. To understand the system operation in air arc gouging

2. To study the difference between gouging and cutting

3. To identify when the air arc gouging is applied

4. To understand properties of air arc gouging when different type of current was used

3.3.0 INTRODUCTION AND THEORY

Figure 1: Air arc gouging diagram

28

Air arc cutting is an arc cutting process in which metals to be cut are melted by the

heat of a carbon arc and the molten metal is removed by a blast of air. The circuit diagram for

air arc cutting or gouging is shown by figure 1 above, the equipment consists of a special

holder that uses carbon or graphite electrodes and the air jet is external to the consumable

carbon-graphite electrode which strikes the molten metal immediately behind the arc. A push

button or a hand valve on the electrode holder controls the air jet.

Figure 2: Process diagram for air arc gouging cutting

As shown in figure 2 above, the arc cutting electrode holder is designed for the air arc

process and the holder includes a small circular grip head which contains the air jets to direct

compressed air along the electrode. It also has a groove for gripping the electrode and this

head can be rotated to allow different angles of electrode with respect to the holder.

A heavy electrical lead and an air supply hose are connected to the holder through a

terminal block. The system also contains a valve that included in the holder for turning the

compressed air on and off. Holders are available in several sizes depending on the duty cycle

of the work performed, the welding current and size of carbon electrode used. For extra

heavy duty work, water-cooled holders are used.

The electrode holder operates at air pressures that varying between 60 and 100 psi.

During this operation, bare carbon or graphite electrodes become smaller due to oxidation

caused by heat buildup. Copper coating these electrodes reduces the heat buildup and prolong

their use.

The copper-coated electrode provides better electrical conductivity between it and the

holder as well as it better for maintaining the original diameter during operation. Copper-

29

coated electrodes can be divided into two types, which is dc and ac type. The composition

ratio of the carbon and graphite is slightly different for these two types of current. The dc

type is more common while the ac type contains special elements to stabilize the arc.

Electrodes range in diameter from 5/32 to 1 in. (4.0 to 25.4 mm). Normally electrode was in

12 inches. (300 mm) long; however, 6 in. (150 mm) electrodes are available

30

3.4.0 APPARATUS

Figure 3: Circuit block diagram for Air arc gouging

The apparatus or equipment that we can clearly see at figure 3 above is

1. Power source

2. Compressed air

3. Electrode lead

4. Work piece lead

5. Hand held electrode holder (Torch holder)

6. Carbon-graphite electrode

7. Mild steel (Work piece)

31

3.5.0 EXPERIMENTAL SETUP AND PROCEDURE

The operating procedures for air arc cutting and gouging are basically the same. The

procedures are as follows:

a) The machine was adjusted to the correct current for electrode diameter.

b) The air compressor was started and the regulator was adjusted to the correct air pressure.

The lowest air pressure was used as possible to blow away the molten metal.

c) The electrode was inserted into the holder and the carbon electrode 6 inches was

extended beyond the holder. Ensure that the electrode point is properly shaped.

d) The arc was stroked and then the air-jet valve was opened. The air-jet disc can swivel,

and the V-groove in the disc automatically aligns the air jets along the electrode. The

electrode is adjusted relative to the holder.

e) Control the arc and the speed of travel according to the shape and the condition of the cut

desired.

f) Always cut away from the operator as molten metal sprays some distance from the

cutting action. This process can cut or gouge metal in the flat, horizontal, vertical, or

overhead positions.

32

3.6.0 RESULT AND ANALYSIS

Electrode diameter DC with electrode positive

(DCRP)

DC with electrode negative

(DCSP)

in. mm Min Amps Max Amps Min Amps Max Amps

1/8 3.2 60 90 - -

5/32 4.0 90 150 - -

3/16 4.8 200 250 150 180

¼ 6.4 300 400 200 250

5/16 7.9 350 450 - -

3/8 9.5 450 600 300 400

½ 12.7 800 1000 - -

5/8 15.9 1000 1250 - -

¾ 19.1 1250 1600 - -

1 25.4 1600 2200 - -

Table 1: Suggested current range for commonly used electrode types and sizes

Figure 4: Air arc gouging (DCRP)

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Figure 5: Air arc gouging (DCSP)-arrow

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3.7.0 DISCUSSION OF RESULTS

Base on experiment that have been done, usually air arc gouging was used a direct

current of electrode negative (DCSP) when cutting cast irons. For normal use, the electrode is

operated with the electrode positive which is reverse polarity and also alternating current

(AC). The theory about polarity explain that Direct Current Electrode Negative (DCEN)

form a narrow weld bead and deeper penetration. This is because the arrangement of

electrode for DCEN is in negative pole while the workpiece is positive pole. The electron

flows from negative pole to the positive pole.

Since the electron flow from electrode to the workpiece, approximately 70 percent of

the heat of arc is concentrated at the work, and approximately 30 percent at the electrode then

produce a deep penetration and narrow weld bead shape. DCEN results in faster melt-off of

the electrode and faster deposition rate. The Direct Current Electrode Positive (DCEP) is the

arrangement of direct current when the electrode is in positive pole while the workpiece is

negative pole.

The electron flows from the workpiece to the electrode and approximately 70 percent

of heat at the arc is concentrated at the electrode while approximately 30 percent of heat is

concentrated at the workpice which produce a wide and shallow weld bead shape since the

heat is less concentrated at the work. Comparing using electrode positive, the using of

electrode negative will reduce rapidly heating on the electrode. Thus for gouging, the suitable

type of polarity use is DCSP because it mainly use to remove the molten metal in base metal

and by using this polarity it can easily remove and repairing as well as to cut the base metal.

As we can see the clearly different between air arc gouging and others cut was

production of fume. In air arc gouging, an electric arc at the end of a consumable carbon rod

melts the metal and a continuous blast of compressed air violently blows the molten metal

away. The molten metal react strongly with air and the force of air blast tends to vaporize

much of the molten metal into fine droplets, creating a high level of fume consisting of metal

vapor, carbon dust, and metallic by products.

Cutting also uses an electric arc to melt the metal being gouged but the plasma gas

itself pushes the molten metal out of the groove and it is done less violently compare with air

arc gouging, less molten metal vaporizes, reducing the metallic vapor and reaction with the

surrounding atmosphere. When air is used as the plasma gas, some reaction occurs, but the

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volume of air is still lower than comparing with air arc gouging. If inert gas is used, the

molten metal in the gouge is protected from the surrounding atmosphere and has little chance

to react with the air.

Noise also can be taking into consideration for cutting operation. Others cutting such

as plasma cutting can reduce noise production and when we measured at conditions that

create a similar gouge size, plasma cutting is 5 to 10 decibels quieter than air arc gouging.

Depending on the current level, the noise level of plasma cutting still may be high enough to

require hearing protection for the operator, but it can eliminate the need for such protection

for nearby workers.

The initial cost of air arc gouging is lower than that of plasma gouging. With air arc

gouging, existing welding power supplies and air supply can be used only a gouging torch

must be added. Air arc gouging with compressed air also costs less than others cutting with

an inert gas supply. Maintenance costs for air arc gouging also can be less than in cutting, in

which the electrode and nozzle must be replaced periodically. Furthermore the air arc

gouging electrode also costs less than a other cutting electrode.

Mostly the air arc cutting process is used for metal cutting; gouge out defective metal,

remove old or inferior welds, for root gouging of full penetration welds, and to prepare

grooves for welding. Air arc cutting is used when slightly ragged edges are not objectionable.

The area of the cut is small since the metal is melted and removed quickly. The surrounding

area does not reach high temperatures and reduces the tendency towards distortion as well as

cracking.

The air arc cutting and gouging process is normally manually operated and the

apparatus can be mounted on a travel carriage. Special applications have been made where

cylindrical work has been placed on a lathe-like device and rotated under the air carbon arc

torch. This is machine or automatic cutting, depending on operator involvement.

The air arc process can be used for cutting or gouging most of the common metals.

Metals include which is aluminums, copper, iron, magnesium, carbon and stainless steels.

The air Arc gouging process is not recommended for weld preparation for stainless steel,

titanium, zirconium, and other similar metals without subsequent cleaning. This cleaning,

usually by grinding, must remove all of the surface carbonized material adjacent to the cut.

The process can be used to cut these materials for scrap for remelting.

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3.8.0 CONCLUSION

Air Arc Gouging is a process where an arc is created between a carbon rod and the

metal to be gouged and the metal is melted. A steady flow of air is blown into that molten

pool removing the molten metal. Increasing the amperage or slowing down the travel speed

allows a deeper gouge. Most of common metals can be gouged by using air arc process. This

cutting process can be automated (automatically functioned) by the aid of carrier or any other

automatic devices. Gouging by using Air Arc Gouging process is much more cheaper

compared to other cutting methods. Air Arc Gouging process has numerous scope of

applications such as cutting and prepare grooves for welding.

3.9.0 RECOMMENDATION

We can see that the gouging process have a defect at the workpiece of material. This

is occur when the handling and technique of the process cannot handle properly. So, it has a

tips to get the gouging process look better at the workpiece. The travel speed is important to

get the better result to determine the depth of the gouge. The faster the travel speed, the

shallower the gouge and a slow travel speed will produce a deeper gouge. A short arc must be

maintained by progressing in the direction of the cut fast enough to keep up with the metal

removal and carbon electrode consumption. The steadiness of the progression controls the

smoothness of the resulting surface.

Second, to get the better result, always use a push technique. Travel speed is continuously

forward with the air blowing behind the arc. Never back up. This will avoid carbon deposits

in the base material that cannot be welded without first re-gouging or grinding to completely

clean the base material. Then, to get the better result have made the proper torch angle to

work about 35 to 45 degrees. The depth and contour of the groove produced are controlled by

the electrode diameter and travel speed. The width of the groove is determined by the

electrode diameter used and is usually wider than the diameter. A wider groove may be made

with a small electrode by oscillating the electrode in a weaving motion.

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3.10.0 REFERENCE

1. Welding Essentials Questions and Answer, 2nd Edition, William Galvery and Frank

Marlow, Industrial Press Inc.

2. Plasma Cutting Handbook HP1569, Penguin Group US, 2011.

3. Welding, Flame Cutting and Allied Processes, HSE Books, 2002.

4. Air Carbon Arc Gouging & Cutting, AWS, 2000.

5. Heavy Piercing With Plasma, Tim Heston, 2012.

6. New Plasma Cutting Technology Takes Care of the "Hole" Issue, Jim Colt, 2010.

7. Recommended Practices for Air Carbon-Arc Gouging and Cutting, American

Welding Society. Arc Welding and Arc Cutting Committee

8. Air Carbon-Arc Guide, Thermadyne Industries, Inc. , 2008

9. http://www.thefabricator.com/article/plasmacutting/gouging-the-other-plasma-process

10. http://www.weldinginfocenter.org/health/hs_02.html

11. http://www.twi.co.uk/technical-knowledge/job-knowledge/job-knowledge-12-air-

carbon arc-gouging/

12. http://www.zena.net/htdocs/welders/Rods/Cut.shtml

13. http://www.weldguru.com/plasma-welding.html

14. http://www.lindeus.com/en/processes/cutting_welding_and_joining/index.html

15. http://www.productionnavigator.nl/ventura/engine.php?

Cmd=see&P_site=790&P_self=339&PMax=1&PSkip=0

16. http://www.welding-robots.com/articles.php?tag=633

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