Gas Cutting by Gnanasekaran

8/20/2019 Gas Cutting by Gnanasekaran

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THEORITICAL TRAINING MANUAL

WELDER

OXY-ACETYLENE GAS CUTTING


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Table of contents

CONTENTS PAGES

1. Introduction to gas cutting…. 3

2. Oxy-Acetylene Gas cutting Equipments & Accessories… 4

3. Safety in Gas cutting Process…. 10

4. Flashback and Backfire….. 13

5. Acetylene Gas…. 15

6. Oxygen Gas… 16

7. Functions of pressure Regulator … 17

8. Single stage regulator 18

9. Two stage regulator 19

10.Cutting Torch… 23

11. Cutting tip or Nozzle 24

12. Assembly of Gas cutting set… 25

13. Cutting operation… 27

14. Job sequence of oxy-acetylene gas cutting 29

15. Operating data for cutting mild steel 33

16. Startup and shutdown procedures 34

17. Difference between cutting blowpipe and welding blowpipe… 34

18. Care and maintenance 35

19. Trouble shooting 36

20. Oxy-Acetylene Machine cutting… 37

21. Manifold system 41

22. Different types of faults in gas cutting… 42

PREPARED BY M. GNANASEKARAN / Senior Training Officer

DATE 12.12.2012

APPROVED BY


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Introduction to gas cutting:

The most common method of cutting mild steel is by an oxy-acetylene cuttingprocess. With an oxyacetylene cutting torch, the cutting (oxidation) can be confinedto a narrow strip and with little effect on the adjoining metal. The cut appears like asaw cut on a wooden plane. The method can be successfully used to cut ferrousmetals i.e. mild steel.

Non-ferrous metal and their alloys cannot be cut by this process.

Principle of Gas cutting:

When a ferrous metal is heated to red hot condition and then exposed to pure

oxygen, a chemical reaction takes place between the heated metal and oxygen. Due

to this oxidation reaction, a large amount of heat is produced and cutting action takes

place.

By this continuous process of oxidation the metal can be cut through very rapidly.

There are two operations in oxy-acetylene gas cutting. A heating flame isdirected on the metal to be cut and raises it to bright red hot or ignition point (900˚C

app.) preheating. Then a stream of high pressure pure oxygen is directed on to the

hot metal which oxidizes and cuts the metal. The two operations are done

simultaneously with a single torch.

The torch is moved at a proper travel speed to produce a smooth cut. The

removal of oxide particles from the line of cut is automatic by means of the force of

oxygen jet during the progress of cut.


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[300 liters of oxygen are required to oxidize one kilogram of iron

completely. The ignition temperature of steel for gas cutting is 875˚C to 900˚C.]

Oxy-Acetylene Gas Cutting Process

Oxy-acetylene welding is a method of cutting and joining metals by

heating them to the melting point using a mixture of oxygen and acetylene

gases.

Equipment and Accessories

1. Oxygen cylinder

2. Acetylene cylinder

3. Oxy-acetylene pressure regulators

4. Rubber hose pipes and connections

5. Cutting/welding torch and nozzle

6. Nozzle cleaner

7. Cylinder Key

8. Spark lighter

9. Trolley

1. Oxygen Gas cylinder: The Oxygen gas required for gas cutting and welding is

stored in bottle shaped cylinders. These cylinders are painted in black color. Oxygen

cylinders can store gas to a capacity of 7m³ with the pressure ranging between 120

to 150 kg/cm². Oxygen gas cylinders valves are right hand threaded.

2. Acetylene Gas cylinder: The acetylene gas used in gas cutting and welding is

stored in steel bottles (cylinders) painted in maroon color. The normal storing

capacity of storing acetylene in dissolved state is 6m³ with the pressure rangingbetween 15-16 kg/cm²

3. Oxy-Acetylene Regulators: This is used to reduce the Oxygen, acetylene

cylinder’s gas pressure to the required working pressure and to control the flow of

oxygen and acetylene at a constant rate to the cutting torch or blowpipe. Oxygen

threaded connections are right hand threaded. The acetylene threaded connections

are left handed.


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4. Rubber hose-pipes and connections: These are used to carry gas from the

regulator to the cutting torch or blowpipe. These are made of strong canvas rubberhaving good flexibility. Hose pipes which carry oxygen are black in color and the

acetylene hoses are maroon color.


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Rubber hoses are connected to regulators with the help of unions. These

unions are right hand threaded for oxygen and left hand threaded for acetylene.

Acetylene hose unions have a groove cut on the corners.

At the blowpipe end of the rubber hoses hose protectors are fitted. The hose

protectors are in the shape of connecting union and have a non-return disc fitted to

protect from flashback and backfire during cutting.

5. Cutting torch: Hand cutting torch is must common use is light duty and

heavy duty. These cutters can be used for cutting mild steel, clad steel and cast iron.

For automatic cutting a motorized special oxy-acetylene cutting machine is used in

place of hand cutting.

The common cutting torch parts are:- acetylene control, hand threads, heating

oxygen control, separate oxygen control, cutting torch head, cutting torch lever,cutting nozzles and guide.

6. Goggles: Fitted with colored glasses should be used to protect the eyes from

harmful heat and ultraviolet rays.


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7. Gloves: Gloves made of leather, canvas and asbestos should be light so that

manipulation of torch may be done easily.

8. Apron: Leather aprons are used to protect the body and clothes from spoiling

and burning due to heat.

9. Wire brush: These are used for removing rust or dirt from metal prior to

welding, removing slag after welding.

10. Cylinder Key: It is used for opening and closing the gas cylinders.


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11. Spanner set: Spanner set is needed for tightening the nuts of blowpipe

connections and regulators.

12. Cutting Tip or Nozzle: To vary the size of the flame and thickness of the

metal, detachable cutting/welding tips varying in size, shape and construction are

used.

13. Tip Cleaner: For cleaning tips or nozzle.

14. Spark lighter: For igniting the flame.

15. Trolley: For holding Gas cylinders


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16. Tong: Is used to hold hot work pieces and to hold the job in position.

17. Chipping Hammer: The chipping hammer is used to remove the slag which coversthe deposited weld bead. It is made of medium carbon steel with a mild steel handle. It isprovided with a chisel edge on one end and a point on the other end for chipping off slag in anyposition.

Other common hand tools

1. Table

2. Tong 350 mm

3. Scriber 200 mm

4. Cold flat chisels 200x20 mm

5. Ball-peen hammer 1kg

6. Hacksaw frame adjustable

7. Flat file bastard 300 mm

8. Bench vice 100 mm9. Screw driver 20 mm

10. Double ended spanner set 4.7 mm to 32 mm

11. Steel rule 300 mm


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Safety in Gas cutting Process

Safety for the operator

Always use safety apparel for the:-

- Protection for your eyes

- Protection from burns

- Protection of clothing

- Prevention of inhaling burnt gases.

Goggles, gloves and other protective clothing must be worn.


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Safety during operation:

Keep the work area free from flammable materials.

Ensure that the consumable material is at least 3 meters away from the cutt ing

op eration area.

In case the flammable material is difficult to remove, suitable fire resisting

guards/partitions must be provided. [Protect your self and others from the flying sparks.

Ensure that the metal being cut is properly supported and balanced so that it will not fall on

the feet of the operator or on the hoses.]

Keep the space clear underneath the cutting job so as to allow the slag to run

freely, and the cutting parts to fall safely.

Be careful about flying hot metal and sparks while starting a cut. Containers

which hold combustible substances should not be taken directly for cutting or

welding.

Wash the containers with carbon tetrachloride and caustic soda before

welding or cutting and fill them argon gas or water before repairing.

Keep fire-fighting equipment handy and ready


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PROCEDURE AND PRECAUTIONS TO BE FOLLOWED IN

OPERATING GAS PLANT

1. Arrange two cylinders in proper position (Vertical)

2. Crack the both cylinder valves before fitting the regulator, so that all the dirtand unwanted material if any may be cleaned out.

3. See that joining surface of the cylinder valve and regulators are free fromgrease

4. Fit acetylene gas regulator and pressure gauge to the acetylene cylinder5. Fit oxygen gas regulator and pressure gauge to the oxygen cylinder

6. Regulators should not be inter-changed.

7. Ensure a gas tight fitting of regulators by soap solution

8. Content the oxygen and acetylene hoses to the respective cylinders and theblowpipe to these hoses

9. Insert the proper size of the tip, after selecting it as per the material to bewelded.

10. Release the pressure on the regulator diaphragm spring by opening the outlet

valves on regulators.

11. Open the cylinder valves by using key, gradually to avoid an abrupt strain onthe pressure gauge.

12. Check that there is no leakage of gases from the regulators.

13. To start the work, turn on the acetylene cylinder first and allow the mixture ofthe gases to pass through the nozzle, so that the hoses and blow pipe arecleared of the air.


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14. Open the acetylene torch valve slightly and light it which spark lighter.Increase the flow of acetylene till a turbulence is created and the sooty smokeis eliminated.

15. Adjust the flame by regulating the supply of oxygen in the correct required

proportion. The flame thus obtained will be in the following order. Carburizing,neutral, oxidizing.

16. To light the gas for flame always open the acetylene valve in torch first and to

put off the flame after completion of work, dip the blow pipe (torch) in to the

water, close the cylinder valves by means of regulators.

17. No oil, grease should be used on any part of cylinders, regulators and torch.

18. Welding goggles pair should be used during welding and cutting.

19. Gloves and apron should be used during cutting and welding.

FLASHBACK AND BACKFIRE

Backfire: At certain times during flame ignition in gas welding or cutting, a small

explosion of the flame occurs at the torch tip.

The flame may or may not go off. This is back-fire.

Causes:

o the gas pressure setting is low

o the nozzle is over heated

o The nozzle orifice is blocked by carbon or spark deposits.

o The nozzle touches the molten pool

o There is leakage in the near the nozzle.

o Eliminate the causes before proceeding further to avoid backfire.

Remedial action

Set slightly higher pressure for both the oxygen and acetylene flames.

Cool the tip. Do not keep the tip very close to the hot molten metal.

Clean the tip by using a tip-orifice cleaner.


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Flash back: Some times during backfire, the flame goes off and the burning

acetylene gas travels backward in the blowpipe, towards the regulator or cylinders.

This known as flash-back.

Indications of flashback

Failure to purge before lighting the torch.

A sharp squealing sound inside the blowpipe may be heard.

Heavy black smoke and sparks come out the nozzle.

The blowpipes handle starts heating.

Overheated tip.

Remedial action

Purge the system properly

Cool the tip

Use a flash back arrester in the line.

HYDRAULIC BACK PRESSURE SAFETY VALVE

It is a cylindrical device which is used to protect the acetylene generator from

dangerous flashback and backfire, during welding and cutting.

It is fitted between the blowpipe and the acetylene generator.

Necessity of hydraulic safety valve: In the low pressure system the oxygen

pressure is always grater than the generated acetylene gas pressure.


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During welding or cutting, due to backfire or nozzle hole blockage, the high

pressure oxygen may enter in to the acetylene passage and either the acetylene

generator, which will lead to an EXPLOSION.

To prevent the entry of high pressure oxygen or backfire to the generator, a

hydraulic safety valve must be fitted in the acetylene pipe line between the blowpipe

and the generator purifier.

Common gases used in gas cutting & welding: Oxygen, Acetylene, Hydrogen.

ACETYLENE GAS:

Acetylene is a fuel gas, which produces a very high temperature flame withthe help of oxygen, because it ha more amount of carbon (92.3 %) than any otherfuel gas. The temperature of oxy-acetylene flame is 3100ºC-3300ºC.

Composition of acetylene gas:

Acetylene is composed of: - Carbon 92.3 % (24 parts)- Hydrogen 7.7 % (2 pars)

Its chemical symbol is C2 H2 which shows that two atoms of carbon arecombined with two atoms of hydrogen.


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Properties of Acetylene gas: it is a colorless gas, lighter than air. It has specific

gravity of 0.9056 as compared with air. It is lightly inflammable and burns with a

brilliant flame. It is slightly soluble in water and alcohol. Impure acetylene has

pungent (garlic like) odour. It can be easily detected by is peculiar smell.

OXYGEN GAS: Oxygen is a supporter of combustion. Its chemical symbol is O2

Characteristics of oxygen gas

Oxygen is a colorless, odorless and tasteless gas

It has an atomic weight of 16.

Its specific gravity at 32º F and at normal (atm) pressure is 1.1053, as

compared with air.

It is slightly soluble in water.

It does not burn itself, but readily supports combustion of fuels.

When compressed oxygen comes in contact with finely divided particles of

combustible material (i.e., coal dust, mineral oil, grease) it will self-ignite them,

leading to fire or explosion. Self-ignition in such cases may be initiated by the heat

given up suddenly by compressed oxygen.

Oxygen becomes liquefied at a temperature of -182.962 C at normal

atmospheric pressure. Liquid oxygen has a faint blue color.


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DIFFERENCE BETWEEN OXYGEN AND D.A. CYLINDER

S.NO OXYGEN CYLINDER D.A. CYLINDER

1 Painted black Painted maroon

2 Longer in length Shorter in length

3 Diameter is less Diameter is more

4 Lighter in weight Heavier in weight

5 Odorless Pungent smell

6 Filling pressure 1850-2000 psi Filling pressure 200-250psi

7 No fuse plug Fuse plug is provided

8 Tilting sound on strinking Dull sound on strinking

9 No safety plug Safety plug is provided

10 Gland nut has no cut mark Gland nut has cut mark

11 All fittings have right hand threads All fittings have left hand threads

12 Seamless cylinder Joints are visible

13 Kept in horizontal/vertical Kept in vertical position only

Functions of pressure Regulators:

In oxy-acetylene welding the oxygen and the acetylene gases are supplied in

cylinders. The oxygen cylinder is filled under a pressure of 2000 lbs/sq.inch pressure

and acetylene cylinder is filled under 0 lbs/ sq.inch to 250 lbs/sq.inch.

For welding and cutting the pressure of a gas can not exceed 0 lbs/sq.inch to

30 lbs/sq.inch in welding torch and therefore it is necessary that some mechanism

should be used to reduce the cylinder, high pressure to working pressure required at

blowpipe.

Types of Regulators:

There are two types of regulators:-

Oxygen regulator (right hand thread) black

Acetylene regulator (left hand thread) Red


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These are furth er classif ied as:-

1. Single stage regulator

2. Two stage regulator

According to the construction single stage regulators are two types

- The nozzle type regulators

- the stem type regulators

The single stage regulator:

The single stage regulator consists of following three main elements. A

pressure spring, diaphragm and a valve.

These are so arranged that the pressure of the spring opens the valve and thegas passing through the part acts on diaphragm, and the later closes the valve when

pressure rises sufficiently to overcome the pressure of spring.

Working pressure is adjusted by tuning the adjusting screw. A pressure gauge

is connected to the opening of regulator to register the cylinder pressure


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.

Two stage Regulators: Single stage regulator as to superseded by two stage

regulator. This reduces the pressure in two stages and it gives much more stable

and uniform pressure than single stage regulator.


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The regulator is fitted in the cylinder valve tuning the wind nut clock wise.

Oxygen regulator wing nut and gauge connections have right hand threads, where

as acetylene regulator wing nut (hexagonal nut, adapter) and gauge connections

have left hand threads.

Oxygen regulator is painted black and acetylene regulator is painted maroon.

No acetylene regulator is used in low pressure gas welding, because the pressure of

the generated acetylene is always sufficient for working i.e. welding and cutting.


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MULTI STAGE REGULATOR

Gas ApplicationMax. Outlet

Pressure

Max. Flow

(L/Min)Model

AcetyleneAll welding,cutting &

heating

1.5 250 A-1.5

OxygenWelding cutting&heating

2.0 250 O-2.0

OxygenPackage toEnsure

10.0 100 O-0

ArgonInert gas seleldir

& purging- 25 Ar 351

NitrogenPressuring

purging &

Refrigeration

10.0 100 N10

HydrogenLaboratory

application10.0 3800

H10

CO MAG welding 35 CO35L

SINGLE STAGE REGULATOR

Gas ApplicationMax. Outlet

Pressure

Max. Flow

(L/Min)Model

AcetyleneAll welding, cutting

& heating1.0 200

IDA-4B, IDA-

4C

OxygenWelding cutting

&heating10.0 1000

IOX-13B, IOX-

13C

OxygenHeavy cutting &

heating14.0 3500 IOX-14

Propane cutting & heating 1.0 200 LPG-18C

Working: after cracking the cylinder valve oxygen regulator is fitted in to the

oxygen cylinder. Acetylene regulator is fitted with the acetylene cylinder.

1. Oxygen hose black and acetylene hose maroon are connected to the

outlet of respective regulators and are secured by means of hose clips.

2. Cylinder valves are opened slowly using two or three turns


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3. It should be noted that high pressure gauge registers the cylinder

(inside) pressure and low pressure gauge registers low pressure or

working pressure at blowpipe.

4. Turning the pressure adjusting screw clockwise slowly, the working

pressure should be adjusted as required at blowpipe.

5. The controls at blowpipe should be kept opened while adjusting working

pressure as per size.

6. Opening the control valves on blowpipe, gas should be lighted and the

flame should adjust.

Uses and care of Regulators:

1. Oxygen and acetylene regulators are used for welding and cutting

metals. These metals should not be interchanged.

2. Always bear in mind while fixing the regulator in the cylinder valve that

oxygen regulator (inlet and outlet) have right hand threads and that of

acetylene have left hand threads.

3. Do not use oil and grease is any parts of the regulator however

glycerin may be used if it is needed.

4. Threads should not be damaged. For this proper shop out fits (tools)

should be used.

5. Before fixing regulator in the cylinder valve, the cylinder must be cracked

to remove dust etc., or any other foreign material.

6. Pressure adjusting screw should be kept loose before operating cylinder

valve to avoid damage to the diaphragm and needle etc., due to sudden

pressure entrance in the body.

7. Listed match should not be used to check leaks.

8. Cylinders fitted with regulators should be kept in right position.

9. Excessive spring pressure should not be used on diaphragm, because it

can damage the diaphragm.

10. After completion of work the cylinder valves should be closed, pressure

from inside the regulator should be released and then adjusting screw

should be unscrewed.


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CUTTING TORCH: The cutting torch differs from the standard Oxy-acetylene

welding torch in respect of a device (usually a lever) which is for controlling the

oxygen cutting stream.

The torch is fitted with two valves which regulate the supply for oxygen and

acetylene which are mixed together for the preheat flame. A third valve controls the

stream of cutting oxygen. The method which is used for mixing of gases for the

preheating flame is the same as that for the welding torch.

The torch may be guided by hand or by a machine. Manual cutting findsextensive use in wide verity of operations. Machine cutting is also used increasingly

in a number of applications due to following reasons.

1. Greater speed

2. Greater accuracy

3. Greater economy

Names and functions of the parts of a cutting torch

No Name Function

1

2

3

4

5

6

7

8

9

10

11

12

Acetylene gas valve

Oxygen hose joint

Acetylene Hose joint

Oxygen conduit

Acetylene gas conduit

Grip

Preheating oxygen valve

Cutting oxygen valve

Injector

Cutting oxygen conduit

Mixed gas conduit

Torch head

To adjust the flow rate of acetylene gas

To connect with the Oxygen gas hose

To connect with the acetylene gas hose

To lead oxygen

To lead acetylene gas.

To hold the Torch

To adjust the preheating Flame

To adjust the cutting oxygen flow rat

To mix the acetylene gas with oxygen

To lead the cutting oxygen

To lead the mixture of acetylene gas and oxygen.

To attach the nozzle


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Cutting Tip:

A typical cutting tip consists of a central orifice with a number of other orifices

around it. The central orifice provides the cutting oxygen stream while those around

it provide the multi flame (preheating flame).

Cutting tips are usually made of a copper alloy. Characteristics of a good

cutting tip are: -

1. High heat resistance

2. High wear

3. Ability to produce sharp cuts and clear faces with a maximum kern.

4. Ability to supply gases continuously without hindrance.

Special purpose of Nozzle: For profile cutting, different types of nozzles are

used for cutting metals in different shapes.


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Nozzles used for cutting profiles are shown in Fig. below.

Assembly of Gas cutting set:

1. Stand both acetylene and oxygen cylinders in vertical position on atrolley and chain them.

2. Open oxygen cylinder valve momentarily (cracking) in order to removedust, dirt or obstruction in cylinder valve, and then close the valve.

3. Open acetylene valve momentarily and then close.

4. Screw oxygen regulator (Painted black) in to oxygen cylinder valve andtighten it.

5. Connect oxygen hose (black) to the outlet of the oxygen regulatorhaving right hand screw threads and secure the end of hose by meansof clips.

6. Connect the acetylene hose to the outlet of acetylene regulator havingleft hand screw threads and secure the end of the hose by means ofclips.

7. Connect the other end of oxygen hose to the blowpipe or cutting torchconnection having right hand screw threads marked ‘O’ and secure theend of hose by means of clip.

8. Connect the other end of acetylene hose to the blowpipe or cutting

torch connection having left hand screw threads marked ‘A’ and securethe end of hose by means of clip.

9. Fix the appropriate size of nozzle to the blowpipe or cutting torch neck.

10. Open the cylinder valve slowly using cylinder valve key. Open two orthree full turns of the cylinder valve spindle/stem.

11. Open the oxygen and acetylene control valves on blowpipe and adjustthe working pressure by means of pressure adjusting screw ofregulators and then to close the valve on blowpipe.


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Cutting operation:

Oxy-acetylene cutting or gas cutting is a process in which a small area of

metal is first preheated to its ignition temperature and a stream of high purity oxygen

is impinged on the heated area to scoop the metal out.

Once the preheat flame raises the cutting area of the work piece to ignition

temperature, (above 870˚C for ferrous alloys) the high velocity stream of oxygen

flows and cuts the preheated metal. When cutting oxygen flows to the heated work,an exothermic reaction produces carbon dioxide and water and releases intense

heat. The oxygen jet blows the metal oxides out of the material to from the cut.

The operations are done simultaneously with a single torch. The torch is

moved at a proper travel speed to produce a smooth cut. The removal of oxide

particles from the line of cut is automatic by means of the force of the oxygen jet

during the progress of cut.


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Oxy-acetylene flame cutting:

The heating flame is adjusted to be neutral. After adjustment of heating flame,

heat the point at which the cut is to start resting the blowpipe is in guide, so as to

keep the blue cone about 6 mm above the metal surface on reaching the metal

surface to a bright re hot, move the blowpipe flame over the edge of the job and

operate the lever to use cutting oxygen pressure.

On this cutting is started. The cutter should move steadily for smooth cut

drawing the cutter towards operator.


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Application of cutting torch:

Oxy-acetylene cutting torch is used to cut mild steel plates above 4 mm

thickness. The MS plate can be cut to its full length in straight line either parallel to

the edge or at any angle to the edge of the plate. Beveling the edges of the plate to

any required angle can also be done by tilting the torch. Circles and any other curved

profile can also be cut using the cutting torch by using a suitable guide or template.


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Heavy duty cutting blowpipe is used for cutting of heavy sections of M.S & CI

nozzle selected according to the thickness of material and then the pressure of

oxygen is adjusted accordingly.

Job sequence of Oxy-acetylene Gas cutting:

Making straight cuts

Wear all safety clothing

Set the gas cutting plant with a cutting blow pipe and cutting oxygen

regulator

Fit the correct cutting nozzle according to the thickness of the metal to

be cut (for M.S.plate 10mm thickness use 1.2mm dia. Orifice cutting nozzle)

Adjust both oxygen and acetylene gas pressure according to the

cutting nozzle size. (Oxygen 1.6 kg/sq.cm and acetylene 0.15 kg/sq.cm)


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While adjusting pressure, keep the cutting blowpipe valves open

Take 200x150x10 thick plate, clean, mark and punch the straight lines

on the plate 15mm apart from one side and 25mm (for straight and

bevel cutting respectively) on the other side.Set the neutral flame

Wear the gas welding goggles

Hold the below pipe at an angle of 90˚ between the line of cut and

cutting nozzle axis and between the nozzle and the surface of the plate

Heat one end of the punched line up to cherry red hot condition

Keep the distance between the work piece and the tip of the nozzle

about 5mmPlace the preheat cone approximate 1.6mm above the plate

Move the flame in circle a little larger than the tip size. When metal is

heated to cherry red, move the tip to the edge of the plate

Operate the cutting oxygen lever immediately and move the torch

slowly along cutting direction.

Skill sequence:

Setting the gas cutting plant: Set the oxy-acetylene gas cutting plant in

the same way as was done for welding, and connect the cutting

blowpipe in the place of welding blowpipe. Also change the oxygen

welding regulator with oxygen cutting regulator.

Setting the job for straight line cutting: Mark and punch straight lines on

the plate 15mm apart for a straight line cut and 3 lines 25mm apart for

bevel cutting on other edge.

Maintain correct torch speed and distance between the plate surface

and the nozzle up to end of the cut.

If long plates are to be cut, to get a good straight gas cut surface,

clamp a straight gas cut surface, clamp a straight edged flat parallel to

the line of cut and use a spade guide attached to the cutting torch.

Move the torch uniformly along the clamped flat and pressing the

spade against the flat.


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On completion of the cut release the cutting oxygen lever and shut off

the flame.

Clean the cut surface by wire brush after chipping off any slag sticking

to the cut edge.

Making bevel cuts:

To the best method for obtaining a good bevel with a minimum slag is

to cut and bevel at the same time

Mark and punch straight lines 25mm apart

For cutting a bevel keep one or two flats on the plates to be beveled

and angle the cutting nozzle by resting the nozzle over the flats.

Hold the torch in left hand, light it, till it to 30-35˚ of perpendicular.

Preheat and start the cut holding the torch on both hands as done in

straight line-cutting. Avoid kerf filling by increasing travel speed.

On reaching the end, cutting should continue for another 6mm or more

to get a complete cut.

Shout of the torch at the end and dip it in water chip of the slag.

Repeat the exercise till a good and smooth cut is achieved.

To bevel the edge of a long plate with a clean and good gas cut

surface, use a beveling attachment to the torch and tilt the nozzle of

the torch to the required angle of bevel.

Set the job on the cutting table, so that the parting piece is free to fall

Ensure that the underside of the cutting line is clear and no combustible

materials are lying nearby.

Adjusting cutting flame: select the cutting nozzle and set the gas

pressure as per the cutting job thickness.

The bevel thickness will be more for bevel cut. When compared with a

square cut for same thickness.

Acetylene pressure should be 0.15 kgf/cm² for all thickness of plates

Select 1.2 mm Ø cutting oxygen (orifice) nozzle for cutting a 10 mm

thick plate

Set 1.6 kg/sq.cm pressure for the acetylene gas.


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Ensure safety apparel is worn.

Fix the cutting nozzle in to the cutting blowpipe correctly.

Check the leakage in the blowpipe connections of oxygen and

acetylene gas lines.

Adjust the neutral flame for preheating

Ensure the flame adjustment is not disturbed while operating the

cutting oxygen lever.

Straight line cutting: keep the hand cutting blowpipe at 90˚ angle with

the plate surface and start cutting a straight line.

Preheat the starting point to red heat before pressing the cutting

oxygen lever.

Keep the distance between the work piece and the nozzle about 5mm

avoid backfire.

Release the cutting oxygen by pressing the cutting oxygen control lever

and start cutting action and move the blowpipe along the punched line

with uniform speed.

Ensure straight travel without any side to side movement

The nozzle angle is 90˚ with the plate surface till the completion of cut

Open the cutting oxygen valve fully

If possible fix a straight edge of template to the plate and fix a support

to the cutting nozzle so as to ensure constant distance between the tip

of the nozzle and the plate surface and maintain a uniform straight cut.

Inspect the cutting for ….

- Uniform and smooth cut or drag line

- Straightness, sharpness- Width of the cut (kerf)

Bevel cutting:

Set the job and hold the cutting blow pipe (nozzle) t required 60 - 55˚

angle so that the bevel angle on the plate will be 30-35˚.

There should be any obstruction at the underside of the cutline and the

parting piece from the job should be free to fall.


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Preheat the starting point to cherry red colour.

Keep the distance between the work piece and the nozzle about 5mm

to avoid backfire

Release extra oxygen by pressing the cutting oxygen lever, observe

the cutting action and start moving along the punched line with uniform

speed.

Keep less cutting speed than you would use for a straight cut for the

same thickness.

Fix one or more straight bars to the cutting job to ensure the cut is

along the straight line and also able to maintain the correct angle.

Inspection of bevel cut:

Clean the slag if sticking to the cut surface by a chipping hammer and

wire brush and inspect for any gas cutting defects.

Good Quality is shown by excellent top edge and extremely smooth cut

face. The cut part is dimensionally accurate.

POOR Quality results in gouging which is a most common fault. These

are caused either by excess speed or too low preheat flame.

Operating data for cutting mild steel

Cutting nozzle size in

mmThickness of the plate (mm)

Cutting oxygen pressure

Kgf/cm²

0.8

1.2

1.6

2.0

2.4

2.8

3.2

3-6

6-19

19-100

100-150

150-200

200-250

250-300

1.0-1.4

1.4-2.1

2.1-4.2

4.2-4.6

4.6-4.9

4.9-5.5

5.5-5.6


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Start Up Procedures:

1. Open the Acetylene tank until gauges move, then ¼ turn more.

2. Open the Oxygen tank until gauges move, then 3 more turns.

3. Check tank pressure gauges.

4. Open acetylene valve on torch slowly.

5. Light torch, add acetylene just until the black smoke disappears.

6. Open oxygen valve on torch slowly.

7. Increase and decrease oxygen and acetylene levels until the flame are

neutral with a small, blue inner cone.

Shut Down Procedures:

1. Turn off the acetylene torch valve.

2. Turn off the oxygen torch valve.3. Turn off both tanks completely.

4. Open the valves on the torch to “bleed” the lines.

5. Close valves – make sure all gauges read “0”.

Difference between cutting blowpipe and welding blowpipe:

A cutting blowpipe has two control valves (oxygen and acetylene) to

control the preheating flame and one lever type control valve to controlthe high pressure pure oxygen for making the cut.

A welding blowpipe has only two control valves to control the heating

flame.

The nozzle of the cutting blowpipe has one hole in the centre for cutting

oxygen and a number of holes around the circle for the preheating

flame.

The nozzle of the welding blowpipe has only one hole in the centre for

the heating flame.

The angle of the cutting nozzle with the body is 90º.

The angle of the welding nozzle with the neck is 120 º.

The cutting nozzle size is given by the diameter of the cutting oxygen

orifice in mm.

The welding nozzle size is given by volume of oxy-acetylene mixed

gases coming out of the nozzle in cubic meter per hour.


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Care and maintenance:

The higher pressure cutting oxygen lever should be operated only for gas

cutting process.

Care should be taken while fitting the nozzle with the torch to avoid wrongthread. Dip the torch after each cutting operation in water to cool the nozzle.

Use the correct size nozzle cleaner. Use an emery paper if the nozzle tip is

damaged to make it sharp and to be at 90º with the nozzle axis.

The cutting oxygen orifice should be cleaned at regular intervals by usingdifferent size wire of nozzle cleaner.

Method of piercing a hole:

Hold the cutting blowpipe at right angles on the point where the hole is to be

made. The point will be brightened. Release the cutting oxygen slowly. Raise the

torch; tilt the nozzle slightly to the left and right directions so that the sparks may not

foul the nozzle. Thus the hole may be pierced.

For cutting of the profile hold the blowpipe head in such a way that the oxygen

stream is directed by the correct tilting of the blowpipe. It is obvious that the angle

between the nozzle and the plate must remain constant and this poses the greatest

difficulty for the beginners.


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Position of the preheating flame as related to the plate surface is very important.

Characteristic of analysis of cutting: This analysis has been made on referring

to the cutting face and the formation of cut in this surface.

This can be analyzed as shown in the figure below.

Trouble shooting

Object TroublePart to be

CheckedMethod Remedy

Torch

Gas leakage

Suction of

Acetylene gas

Preheating flameshape

Cutting oxygen

flow

Hose joint

Valve&Regulator

Cutting tip

attaching part

Injector

Soap water or

WaterSoap water or

Water

Soap water or

Water

Plug the fuel

gas hose mouthwith your

finger

Neutral flame

by visual

inspection

Visible gas

flow by visual

inspection

Tighten further or

ReplaceReplace the torch

Tighten further orReplace

Replace

Clean or Replace

Clean or Replace


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Heat and temperature:

Heat is form of energy, capable of following between two bodies which are atdifferent temperatures. The addition of heat energy to a body increases the kineticenergy of motion of it molecules. Temperature is the degree of hotness of coldness

of a body measured, usually centigrade or Fahrenheit. Temperature is a measure ofthe intensity of heat.

Example: If we ask, “How hot is a substance”, the answer will be, “it is so manydegrees hot. i.e. 40ºC, 50ºC, 150ºF etc.,

Temperature of measurement:

There are two basic scales for measuring changes in temperature.

- Centigrade scale- Fahrenheit scale

In both systems there are two fixed points which indicate:

- the temperature at which ice melts (water freezes)- The temperature at which pure water boils at standard pressure.- Temperature is measured by a unit called “degree”.

Centigrade scale:

This is a system for measuring changes in temperature in which the interval oftemperature between the freezing and boiling points of pure water at standardpressure is divided into 100 equal parts. The freezing point is made zero of the scale(0ºC) and the boiling point is fixed at 100 degree (100ºC), each division part is calledone centigrade degree (ºC). Degree centigrade is also called as degree Celsius.

Fahrenheit scale:

A system for measuring changes in temperature in which the interval oftemperature between the freezing and boiling points of pure water at standard

pressure is divided into 180 parts. The freezing point is made 32 degree of the scale(32 ºF). Each division part is called one Fahrenheit degree (ºF).

Oxy-Acetylene Machine cutting:

There are two types of cutting machines.

Manually driven cutting Machines

Electrically driven cutting machines


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Manually operated cutting machines:

A manually driven cutting machine normally consists of:

A crank or wheel to drive the cutter via screw thread, and this machine can

be used for straight line cutting and bevel cutting.

A system of links or rods which are used with the machines and by which

simple circles, ellipses, squares, etc. can also be cut.

The speed of the manually operated cutting machines is liable to variation and

the range of speed is also limited.

Electrically driven cutting machines:

There are two types of machines available.

1. Portable machines

2. Static machines

Portable Machines:

An electrically driven portable cutting machine generally consists of:

- cutting instruments

- carriage (consisting of a variable speed motor)

- Guide (to guide the carriage).


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This machine can be used for straight line cutting, bevel cutting, circular

cutting and profile cutting. Provision also made to enable full adjustment of the

cutting head to be carried out over the cutting area.

The speed of an electrically driven machine, when set, is constant, and

normally it is able to produce better cuts than a manually driven machine.

The speed range of an electrically driven machine is greater than that of the

manual type and the adjustment of speed helps to control more accurately.

Multiple cutting heads can be mounted to increase the volume of cutting thesecutting heads may be mounted on a adjustable bar extending to either side of the

track at 90º to the direction of travel.

Static machine:

These machines are generally used to produce more accurate work than what

is possible with manually operated or portable cutters.

These machines can be used with radial arm and cross carriage

arrangements. In general the work is required to be brought to the machine. With thismachine straight line cutting, circle and profile cutting can be done.

Profile cutting by templates:

Profile cutting can be done by static cutting machines as per shape of the

templates. The templates are mainly used for reproducing the same shapes in to a

no. of pieces. The templates are made from wood, hardboard, aluminium or steel.


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Two types of templates are in use depending on their size.

outside template

Inside template

Outside template:

The outline of the template will be the shape to be cut, reduced in size by the

radius of the follower wheel or roller which is (knurled) attached with the motor of the

machine.

The size of the template is excluding the radius of the tracing wheel (knurled

wheel)-half of the kerf width.

Example: to cut a circle using an external template

Radius of circle 100 mm

Radius of tracing wheel 6.5 mm (a)

Half the kerf width 0.8 mm (b)

Difference [(a) – (b)] 5.7 mm

So pre radius of external template = 100 – 5.7 mm = 94.3 mm

Note: Kerf width is variable according to the:

nozzle size, type and condition

plate thickness

cutting speed

pressure of cutting oxygen

preheat flame size’

Internal template or inside template

The shape of the template will be the shape to be cut increased by the radius

of the following roller (knurled wheel) + half the kerf width.


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Example: to cut a circle using an internal template,

Radius of circle -100 mm

Radius of tracing wheel (knurled) - 6.5 mm (a)

Half the kerf width - 0.8 mm (b)

Sum of (a) + (b) = 6.5 + 0.8 mm = 7.3 mm

The radius of external template = 100 + 7.3 mm = 107.3 mm

Manifold System:

When large volumes of oxygen and acetylene gas are required on a

temporary or permanent basis for many welding and cutting operations in a

workshop, a manifold system is a most suitable one.

Types: - portable manifold system

- Stationary manifold system

Portable manifold system means two or three cylinders are coupled with a

suitable apparatus – namely “PIG TAIL” and connected to a main distribution pipe.

(Fig. below) Separate arrangements are made for oxygen and acetylene gases.


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DIFFERENT TYPES OF FAULTS IN GAS CUTTING

Common faults in cutting:

The tip is too high of the steel. The top edge is heated or rounded, the cut

face is not smooth, and often the face is slightly beveled where preheat effectiveness

is partially lost due to the tip being held so high. The cutting speed must be reduced

because of the danger of losing the cut.

Extremely slow cutting speed: -

Pressure marks on the cut face indicate too much oxygen for the cutting

condition. Either the tip is too big, the cutting oxygen pressure is too high, or the

speed too slow as shown by the rounded or beaded top edge.


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On reducing the cutting oxygen volume to the correct proportions for

thickness of the cut, the pressure marks will recede toward the bottom edge until

they finally disappear.

Tip too close to the steel, the cut shows grooves and deep drag lines,

caused by an unstable cutting action. Part of the preheat cones burned inside the

kerfs, where normal gas expansion affected the oxygen cutting stream.

Too much cutting oxygen: -

The cut shows pressure marks caused by too much cutting oxygen. When

more oxygen is supplied than can be consumed in oxidation, the remainder flows

around the stags, creating gouges of pressure marks.

Too much preheating: -

The cut shows a rounded top edge caused by too much preheat. Excess

preheating does not increase the cutting speed, it only wastes gases.


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Poor quality bevel cut: -

The most common fault is gouging, caused by either excessive speed or

inadequate preheat flames. Another fault is a rounded top edge caused by too much

preheats, indicating excessive gas consumption.

Slightly too fast a cutting speed: - The drag lines on this cut incline backwards, but a “drop cut” is still attained.

The top edge is good; the cut face is smooth and slag-free. This quality is

satisfactory for most production work.

Slightly too slow a cutting speed: -

The cut is of high quality although there is some surface roughness caused

by the vertical drag line. The top edge is usually slightly beaded. This quality is

generally acceptable, but faster speeds are more desirable because the labour cost

for this cut is too high.

Gas Cutting by Gnanasekaran

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Transcript of Gas Cutting by Gnanasekaran