The common methods used in cutting metal are
oxy-gas flame cutting, air carbon-arc cutting, and
plasma-arc cutting. The method used depends on the
type of metal to be cut and the availability of equipment.
In large high production shops, more advanced cutting methods
such as laser cutting or water jet cutting may also be
used. Most people in the steel trades use the oxy-gas
cutting technique for cutting ferrous metals. Air carbon-arc gouging is also used
to a lesser extent. Plasma-arc is used mainly for cutting
non ferrous metals such as stainless and aluminum,
although it can also be used on ferrous metals also.
The oxygas cutting torch has many uses in steel work
and is an excellent tool for cutting ferrous metals.
This versatile tool is used for operations, such as beveling
plate, cutting and beveling pipe, piercing holes in
steel plate, and cutting wire rope.
When using the oxygas cutting process, you heat a
spot on the metal to the kindling or ignition temperature
(between 1400°F and 1600°F for steels). The term for
this oxygas flame is the preheating flame. Next,
you direct a jet of pure oxygen at the heated metal by
pressing a lever on the cutting torch. The oxygen causes
a chemical reaction known as oxidation to take place
rapidly. When oxidation occurs rapidly, it is called
combustion or burning. When it occurs slowly,
it is known as rusting.
When you use the oxygas torch method to cut metal,
the oxidation of the metal is extremely rapid and part of
the metal actually burns. The heat, liberated by the
burning of the iron or steel, melts the iron oxide formed
by the chemical reaction and accelerates the preheating
of the object you are cutting. The molten material runs
off as slag, exposing more iron or steel to the oxygen jet.
In oxygas cutting, only that portion of the metal that
is in the direct path of the oxygen jet is oxidized. The
narrow slit, formed in the metal as the cutting progresses,
is called the kerf. Most of the material removed
from the kerf is in the form of oxides (products of the
oxidation reaction). The remainder of the material is
molten metal that is blown or washed out of the kerf by
the force of the oxygen jet.
The walls of the kerf formed by oxy-gas cutting of
ferrous metals should be fairly smooth and parallel to
each other. After developing your skills in handling the
torch, you can keep the cut within close tolerances;
guide the cut along straight, curved, or irregular lines;
and cut bevels or other shapes that require holding the
torch at an angle.
Partial oxidation of the metal is a vital part of the
oxygas cutting process. Because of this, metals that do
not oxidize readily are not suitable for oxy-gas cutting.
Carbon steels are easily cut by the oxy-gas process, but
special techniques (described later in this article) are
required for the cutting of many other metals.
Oxy-Gas Cutting Equipment
An oxy-gas cutting outfit usually consists of a cylinder
of acetylene, propane or MAPP gas, a cylinder of oxygen, two
regulators, two lengths of hose with fittings, and a
cutting torch with tips (fig. 4-1). An oxygas cutting
outfit also is referred to as a cutting rig.
Figure 4-1.—Oxy-gas cutting outfit.
In addition to the basic equipment mentioned above,
numerous types of auxiliary equipment are used in
oxygas cutting. An important item is the spark igniter
that is used to light the torch (fig. 4-2, view A). Another
item you use is an apparatus wrench. It is similar in
design to the one shown in figure 4-2, view B. The
apparatus wrench is sometimes called a gang wrench
because it fits all the connections on the cutting rig. Note
that the wrench shown has a raised opening in the handle
that serves as an acetylene tank key.
Figure 4-2.—(A)Spark igniter; (B) apparatus wrench.
Oxygas cutting equipment can be stationary or portable.
A portable oxygas outfit, such as the one shown in
figure 4-3, is an advantage when it is necessary to move
the equipment from one job to another.
Other common accessories include tip cleaners, cylinder
trucks, clamps, and holding jigs. Personal safety
apparel, such as goggles, hand shields, gloves, leather
aprons, sleeves, and leggings, are essential and should
be worn as required for the job at hand.
Figure 4-3.—A portable oxy-gas cutting and welding outfit.
Oxy-Gas Regulators
You must be able to reduce the high-pressure gas in
a cylinder to a working pressure before you can use it.
This pressure reduction is done by a regulator or reducing
valve. The one basic job of all regulators is to take
the high-pressure gas from the cylinder and reduce it to
a level that can be safely used. Not only do they control
the pressure but they also control the flow (volume of
gas per hour).
Most regulators have two gauges: one indicates the
cylinder pressure when the valve is opened and the other
indicates the pressure of the gas coming out of the
regulator. You must open the regulator before you get a
reading on the second gauge. This is the delivery pressure
of the gas, and you must set the pressure that you
need for your particular job.
The pressures that you read on regulator gauges is
called gauge pressure. If you are using pounds per
square inch, it should be written as psig (this acronym
means pounds per square inch gauge). When the gauge
on a cylinder reads zero, this does not mean that the
cylinder is empty. In actuality, the cylinder is still full of
gas, but the pressure is equal to the surrounding atmospheric
pressure. Remember: no gas cylinder is empty
unless it has been pumped out by a vacuum pump.
There are two types of regulators that control the
flow of gas from a cylinder. These are either single-stage
or double-stage regulators. The major disadvantage of single-stage regulators
is that the working gas pressure you set will decrease as
the cylinder pressure decreases; therefore, you must
constantly monitor and reset the regulator if you require
a fixed pressure and flow rate. Keeping the gas pressure
and flow rate constant is too much to expect from a
regulator that has to reduce the pressure of a full cylinder
from 2,200 psig to 5 psig. This is where double-stage
regulators solve the problem.
The double-stage regulator is similar in principle to
the one-stage regulator. The main difference being that
the total pressure drop takes place in two stages instead
of one. In the high-pressure stage, the cylinder pressure
is reduced to an intermediate pressure that was predetermined
by the manufacturer. In the low-pressure stage,
the pressure is again reduced from the intermediate
pressure to the working pressure you have chosen. A
typical double-stage regulator is shown in figure 4-9.
Figure 4-8.—Single-stage regulators.
Figure 4-9.—Double-stage regulator.
Regulators are built with a minimum of two relief
devices that protect you and the equipment in the case
of regulator creep or high-pressure gas being released
into the regulator all at once. All regulator gauges have
blowout backs that release the pressure from the back
of the gauge before the gauge glass explodes. Nowadays,
most manufacturers use shatterproof plastic instead
of glass.
Oxy-Gas Cutting Torches
The equipment and accessories for oxy-gas cutting
are the same as for oxy-gas welding except that you use
a cutting torch or a cutting attachment instead of a
welding torch. The main difference between the cutting
torch and the welding torch is that the cutting torch has
an additional tube for high-pressure cutting oxygen.The
flow of high-pressure oxygen is controlled from a valve
on the handle of the cutting torch. In the standard cutting
torch, the valve may be in the form of a trigger assembly
like the one in figure 4-11. On most torches, the cutting
oxygen mechanism is designed so the cutting oxygen
can be turned on gradually. The gradual opening of the
cutting oxygen valve is particularly helpful in operations,
such as hole piercing and rivet cutting.
Figure 4-11.—One piece oxy-gas cutting torch.
Figure 4-12.—Cutting attachment for combination torch.
Most welding torches are designed so the body of
the torch can accept either welding tips or a cutting
attachment. This type of torch is called a combination
torch. The advantage of this type of torch is the ease in
changing from the welding mode to the cutting mode.
There is no need to disconnect the hoses; you just
unscrew the welding tip and then screw on the cutting
attachment. The high-pressure cutting oxygen is controlled
by a lever on the torch handle, as shown in figure
4-12.
As in welding, you must use the proper size cutting
tip if quality work is to be done. The preheat flames must
furnish just the right amount of heat, and the oxygen jet
orifice must deliver the correct amount of oxygen at just
the right pressure and velocity to produce a clean cut.
All of this must be done with a minimum consumption
of oxygen and fuel gases. Careless workers and workers
not acquainted with the correct procedures waste both
oxygen and fuel gas. This does not seem important when
you are working in a shop, but if you are performing
field work, running out could temporarily halt work
until the cylinders are replaced.
Figure 4-13.—Common cutting torch tips and their uses.
Mapp gas and propane can be changed without having
to change cutting tips. Acetylene uses a slightly different tip.
To make clean and economical cuts, you must
keep the tip orifices and passages clean and free of
burrs and slag. If the tips become dirty or
misshapened, they should be put aside for restoration.
Figure 4-14 shows four tips: one that is repairable,
two that need replacing, and one in good condition.
Figure 4-14.—Four cutting-tip conditions.
In cutting operations,
the stream of cutting oxygen sometimes blows slag
and molten metal into the tip orifices which partially
clogs them. When this happens, you should clean the
orifices thoroughly before you use the tip again. A
small amount of slag or metal in an orifice will
seriously interfere with the cutting operation.
Clean the orifices of the cutting torch tip in the
same manner as the single orifice of the welding torch
tip. Remember: the proper technique for cleaning the
tips is to push the cleaner straight in and out of the
orifice. Be careful not to turn or twist the cleaning
wire. Figure 4-15 shows a typical set of tip cleaners.
Figure 4-15.—Tip cleaners.
In general, the procedure used for lighting a torch is
to first open the torch oxygen needle valve a small
amount and the torch fuel-gas needle valve slightly
more, depending upon the type of torch. The mixture of
oxygen and fuel gas coming from the torch tip is then
lighted by means of a spark igniter or stationary pilot
flame. After checking the fuel-gas adjustment, you can
adjust the oxygas flame to obtain the desired characteristics
for the work at hand, by further manipulating
the oxygen and fuel-gas needle valves according to the
torch manufacturer’s direction.
There are three types of gas flames commonly used
for all oxygas processes. They are carburizing, neutral,
and oxidizing. To ensure proper flame adjustment, you
should know the characteristics of each of these three
types of flame. Figure 4-17 shows how the three different
flames look when using MAPP gas as the fuel.
Figure 4-17.—MAPP-gas flames.
CARBURIZING FLAME.— The carburizing
flame always shows distinct colors; the inner cone is
bluish white, the intermediate cone is white, the outer
envelope flame is light blue, and the feather at the tip of
the inner cone is greenish. The length of the feather can
be used as a basis for judging the degree of carburization.
The highly carburizing flame is longer with yellow
or white feathers on the inner cone, while the slightly
carburizing flame has a shorter feather on the inner cone
and becomes more white. The temperature of carburizing
flames is about 5400°F.
Strongly carburizing flames are not used in cutting
low-carbon steels because the additional carbon they
add causes embrittlement and hardness. These flames
are ideal for cutting cast iron because the additional
carbon poses no problems and the flame adds more heat
to the metal because of its size.
Slightly carburizing flames are ideal for cutting
steels and other ferrous metals that produce a large
amount of slag. Although a neutral flame is best for most
cutting, a slightly carburizing flame is ideal for producing
a lot of heat down inside the kerf. It makes fairly
smooth cuts and reduces the amount of slag clinging to
the bottom of the cut.
NEUTRAL FLAME.— The most common preheat
flame for oxy-gas cutting is the neutral flame. When you
increase the oxygen, the carburizing flame becomes
neutral. The feather will disappear from the inner flame
cone and all that will be left is the dark blue inner flame
and the lighter blue outer cone. The temperature is about
5600°F.
The neutral flame will not oxidize or add carbon to
the metal you are cutting. In actuality, a neutral flame
acts like the inert gases that are used in TIG and MIG
welding to protect the weld from the atmosphere. When
you hold a neutral preheat flame on one spot on the metal
until it melts, the molten puddle that
OXIDIZING FLAME.— When you add a little
more oxygen to the preheat flame, it will quickly become
shorter. The flame will start to neck down at the
base, next to the flame ports. The inner flame cone
changes from dark blue to light blue. Oxidizing flames
are much easier to look at because they are less radiant
than neutral flames. The temperature is about 6000°F.
The oxidizing flame is rarely used for conventional
cutting because it produces excessive slag and does not
leave square-cut edges. Oxidizing flames are used in
conjunction with cutting machines that have a high-low
oxygen valve. The machine starts the cut with a oxidizing
flame then automatically reverts to a neutral flame.
The oxidizing flame gives you fast starts when using
high-speed cutting machines and is ideal for piercing
holes in plate. Highly oxidizing flames are only used in
cutting metal underwater where the only source of oxygen
for the torch is supplied from the surface.
Oxy-Gas Cutting Mild Carbon Steel
To cut mild-carbon steel with the oxy-gas cutting
torch, you should adjust the preheating flames to neutral.
Hold the torch perpendicular to the work with the inner
cone of the preheating flame about 1/16 inch above
the end of the line to be cut (fig. 4-18). Hold the torch
in this position until the spot you are heating is a bright
red. Open the cutting oxygen valve slowly but steadily
by pressing down on the cutting valve lever.
Figure 4-18.—Position of torch tip for starting a cut.
When the cut is started correctly, a shower of sparks
will fall from the opposite side of the work, indicating
that the flame has pierced the metal. Move the cutting
torch forward along the line just fast enough for the
flame to continue to penetrate the work completely. If
you have made the cut properly, you will get a clean,
narrow cut that looks almost like it was made by a saw.
When cutting round bars or heavy sections, you can save
preheating time by raising a small burr with a chisel
where the cut is to begin. This small raised portion will
heat quickly, allowing you to start cutting immediately.
Once you start the cut, you should move the torch
slowly along the cutting mark or guide. Punch marks or
a soapstone drawn line make an excellent mark to guide the cut. As you move
the torch along, watch the cut so you can tell how it is
progressing. Adjust the torch as necessary. You must
move the torch at the correct speed, not too fast and not
too slow. If you go too slowly, the preheating flame
melts the top edges along the cut and could weld them
back together again. If you go too rapidly, the flame will
not penetrate completely, as shown in figure 4-19. When
this happens, sparks and slag will blow back towards
you. If you have to restart the cut, make sure there is no
slag on the opposite side.
Figure 4-19.—The effect of moving a cutting torch too rapidly across the work.
Cutting Thin Steel
When cutting steel 1/8 inch or less in thickness, use
the smallest cutting tip available. In addition, point the
tip in the direction the torch is traveling. By tilting the
tip, you give the preheating flames a chance to heat the
metal ahead of the oxygen jet, as shown in figure 4-20.
If you hold the tip perpendicular to the surface, you
decrease the amount of preheated metal and the adjacent
metal could cool the cut enough to prevent smooth
cutting action. Many steel trade workers actually rest the edge
of the tip on the metal during this process. If you use this
method, be careful to keep the end of the preheating
flame inner cone just above the metal.
Figure 4-20.—Recommended procedure for cutting thin steel.
Cutting Thick Steel
Steel, that is greater than 1/8 inch thick, can be cut
by holding the torch so the tip is almost vertical to the
surface of the metal. If you are right-handed, one method
to cut steel is to start at the edge of the plate and move
from right to left. Left-handed people tend to cut left to
right. Either direction is correct and you may cut in the
direction that is most comfortable for you. Figure 4-21
shows the progress of a cut in thick steel.
Figure 4-21.—Progress of a cut in thick steel. A. Preheat flames are 1/16 to 1/8 inch from the metal surface. Hold the torch in this spot
until the metal becomes cherry red. B. Move the torch slowly to maintain the rapid oxidation, even though the cut is only partially
through the metal. C. The cut is made through the entire thickness; the bottom of the kerf lags behind the top edge slightly.
After heating the edge of the steel to a dull cherry
red, open the oxygen jet all the way by pressing on the
cutting lever. As soon as the cutting action starts, move
the torch tip at a even rate. Avoid unsteady movement
of the torch to prevent irregular cuts and premature
stopping of the cutting action.
To start a cut quicker in thick plate, you should start
at the edge of the metal with the torch angled in the
opposite direction of travel. When the edge starts to cut,
bring the torch to a vertical position to complete the cut
through the total thickness of the metal. As soon as the
cut is through the metal, start moving the torch in the
direction of travel.
Two other methods for starting cuts are used. In the
first method, you nick the edge of the metal with a cold
chisel at the point where the cut is to start. The sharp
edges of the metal upset by the chisel will preheat and
oxidize rapidly under the cutting torch, allowing you to
start the cut without preheating the entire edge of the
plate. In the second method, you place an iron filler rod
at the edge of a thick plate. As you apply the preheat
flames to the edge of the plate, the filler rod rapidly
reaches the cherry red temperature. At this point, turn
the cutting oxygen on and the rod will oxidize and cause
the thicker plate to start oxidizing.
Cutting Cast Iron
It is more difficult to cut cast iron than steel because
the iron oxides in cast iron melt at a higher temperature
than the cast iron itself. Before you cut cast iron, it is
best to preheat the whole casting to prevent stress fractures.
Do not heat the casting to a temperature that is too
high, as this will oxidize the surface and make cutting
more difficult. A preheat temperature of about 500°F is
normally satisfactory.
When cutting cast iron, adjust the preheating flame
of the torch to a carburizing flame. This prevents the
formation of oxides on the surface and provides better
preheat. The cast-iron kerf is always wider than a steel
kerf due to the presence of oxides and the torch movement.
The torch movement is similar to scribing semicircles
along the cutting line (fig. 4-22). As the metal
becomes molten, trigger the cutting oxygen and use its
force to jet the molten metal out of the kerf. Repeat this
action until the cut is complete.
Figure 4-22.—Torch movements for cutting cast iron.
Because of the difficulty in cutting cast iron with
the usual oxy-gas cutting torch, other methods of cutting
were developed. These include the oxygen lance,
carbon-arc powder, inert-gas cutting, and plasma-arc
methods.
Torch Gouging Mild Steel
Cutting curved grooves on the edge or surface of a
plate and removing faulty welds for rewelding are additional
uses for the cutting torch. The gist of groove
cutting or gouging is based on the use of a large orifice,
low-velocity jet of oxygen instead of a high-velocity jet.
The low-velocity jet oxidizes the surface metal only and
gives better control for more accurate gouging. By
varying the travel speed, oxygen pressure, and the angle
between the tip and plate, you can make a variety of
gouge contours.
A gouging tip usually has five or six preheat orifices
that provide a more even preheat distribution. Automatic
machines can cut grooves to exact depths, remove bad
spots, and rapidly prepare metal edges for welding.
Figure 4-23 shows a typical gouging operation.
If the gouging cut is not started properly, it is possible
to cut accidently through the entire thickness of the
plate. If you cut too shallow, you can cause the operation
to stop. The travel speed of the torch along the gouge
line is important. Moving too fast creates a narrow,
shallow gouge and moving too slow creates the opposite;
a deep, wide gouge.
Figure 4-23.—Typical gouging operation using a low-velocity cutting jet for better control of depth and width.
Beveling Mild Steel
Frequently, you must cut bevels on plate or pipe to
form joints for welding. The flame must actually cut
through 2.8 inches of metal to make a bevel cut of 45
degrees on a 2-inch steel plate. You must take this into
consideration when selecting the tip and adjusting the
pressures. You use more pressure and less speed for a
bevel cut than for a straight cut.
When bevel cutting, you adjust the tip so the preheating
orifices straddle the cut. A piece of l-inch angle
iron, with the angle up, makes an excellent guide for
beveling straight edges. To keep the angle iron in place
while cutting, you should use a heavy piece of scrap, clamps, or
tack-weld the angle to the plate being cut. Move the
torch along this guide, as shown in figure 4-24.
Figure 4-24.—Using angle iron to cut bevels on steel plate.
An improvement over mechanical guides is an electric
motor-driven cutting torch carriage. The speed of
the motor can be varied allowing the welder to cut to
dimensions and to cut at a specific speed. A typical
motor driven carriage has four wheels: one driven by a
reduction gear, two on swivels (castor style), and one
freewheeling. The torch is mounted on the side of the
carriage and is adjusted up and down by a gear and rack
The rack is a part of the special torch carrier. The torch also can
be tilted for bevel cuts. This machine comes with a
straight two-groove track and has a radial bar for use in
cutting circles and arcs. A motor-driven cutting torch
cutting a circle is shown in figure 4-25. The carriage is
equipped with an off-and-on switch, a reversing
switch, a clutch, and a speed-adjusting dial that is
calibrated in feet per minute.
Figure 4-25.—Electric motor-driven carriage being used to cut a circle in steel plate.
Figure 4-26 shows an electric drive carriage on a
straight track being used for plate beveling. The
operator must ensure that the electric cord and gas
hoses do not become entangled on anything during the
cutting operation. The best way to check for hose,
electric cord, and torch clearance is to freewheel the
carriage the full length of the track by hand.
Figure 4-26.—Electric motor-driven carriage being used on straight track to cut a beveled edge on steel plate.
When using the torch
carriage, you should lay the track in a straight line
along a line parallel to the edge of the plate you are
going to cut. Next, you light the torch and adjust the
flame for the metal you are cutting. Move the carriage
so the torch flame preheats the edge of the plate and
then open the cutting oxygen valve and turn on the
carriage motor. The machine begins moving along the
track and continues to cut automatically until the end
of the cut is reached. When the cut is complete,
you should do the following: promptly turn off the
cutting oxygen, turn off the current, and extinguish the
flame-in that order. The cutting travel speed depends upon
the thickness of the steel being cut.
Cutting And Beveling Pipe
Pipe cutting with a cutting torch requires a steady
hand to obtain a good bevel cut that is smooth and true.
Experienced pipe fitters use a pipe tape to wrap around
the pipe and a sharpened piece of soapstone to insure
a true straight line around the entire circumference
before attempting the cut. The next step is to cut
the pipe off square, and ensure all the slag is
removed from the inside of the pipe. Next, you
should bevel the pipe. When cutting a piece of pipe, you should keep the
torch pointed toward the center line of the pipe. Start the
cut at the top and cut down one side. Then begin at the
top again and cut down the other side, finishing at the
bottom of the pipe. This procedure is shown in figure
4-27
Figure 4-27.—Cutting pipe with an oxy-gas cutting torch.
When you make T and Y fittings from pipe, the
cutting torch is a valuable tool. The usual procedure for
fabricating pipe fittings is to develop a pattern like the
one shown in figure 4-28, view A-1.
Figure 4-28.—Fabricating a T.
After you develop the pattern, wrap it around the
pipe, as shown in figure 4-28, view A-2. Be sure to leave
enough material so the ends overlap. Trace around the
pattern with soapstone or a scribe. It is a good idea to
mark the outline with a prick punch at 1/4-inch intervals.
During the cutting procedure, as the metal is heated, the
punch marks stand out and make it easier to follow the
line of cut. Place the punch marks so the cutting action
will remove them. If punch marks are left on the pipe,
they could provide notches from which cracking may
start.
An experienced steel trade worker can cut and bevel pipe
at a 45-degree angle in a single operation. A person with
little cutting experience should do the job in two steps.
In that case, the first step involves cutting the pipe at a
90-degree angle. In the second step, you bevel the edge
of the cut to a 45-degree angle. With the two-step
procedure, you must mark an additional line on the pipe.
This second line follows the contour of the line traced
around the pattern, but it is drawn away from the original
pattern line at a distance equal to the thickness of the
pipe wall. The first (90-degree) cut in the two-step
procedure is made along the second line. The second
(45-degree) cut is made along the original pattern line.
The primary disadvantage of the two-step procedure is that
it is time consuming and uneconomical in oxygen and
gas consumption.
The one-step method of cutting and beveling pipe
is not difficult, but it does require a steady hand and a
great deal of experience to turn out a first-class job. An
example of this method for fabricating a T is shown in
figure 4-28. View A of figure 4-28 outlines the step-bystep
procedures for fabricating the branch; view B
shows the steps for preparing the main section of the T;
and view C shows the assembled T, tack-welded and
ready for final welding.
Step 3 of view A shows the procedure for cutting the
miter on the branch. You should begin the cut at the end
of the pipe and work around until one half of one side is
cut. The torch is at a 45-degree angle to the surface of
the pipe along the line of cut. While the tip is at a
45-degree angle, you should move the torch steadily
forward, and at the same time, swing the butt of the torch
upward through an arc. This torch manipulation is necessary
to keep the cut progressing in the proper direction
with a bevel of 45 degrees at all points on the miter. Cut
the second portion of the miter in the same manner as
the first.
The torch manipulation necessary for cutting the
run of the T is shown in Steps 3 and 4 of view B in figure
4-28. Step 3 shows the torch angle for the starting cut
and Step 4 shows the cut at the lowest point on the pipe.
Here you change the angle to get around the sharp curve
and start the cut in an upward direction. The completed
cut for the run is shown in Step 5 (fig. 4-28, view B).
Before final assembly and tack welding of any of
the parts of a fabricated fitting, you must clean the slag
from the inner pipe wall and check the fit of the joint.
The bevels must be smooth and have complete fusion
when you weld the joint.
Oxy-Gas Hole Piercing
The cutting torch is a valuable tool for piercing
holes in steel plate. Figure 4-29 shows the steps you
should use to pierce holes in steel plate. First, lay the
plate out on firebricks or other suitable material so the
flame does not damage anything when it burns through
the plate. Next, hold the torch over the hole location with
the tips of the inner cone of the preheating flames about
1/4 inch above the surface of the plate. Continue to hold
the torch in this position until a small spot has been
heated to a bright red.
Figure 4-29.—Piercing a hole with an oxygas cutting torch.
Then open the cutting oxygen
valve gradually, and at the same time, raise the nozzle
slightly away from the plate. As you start raising the
torch and opening the oxygen valve, rotate the torch
with a slow spiral motion. This causes the molten slag
to be blown out of the hole. The hot slag may fly around,
so BE SURE that your goggles are tightly fitted to your
face, and avoid placing your face directly above the cut.
If you need a larger hole, outline the edge of the hole
with a piece of soapstone, and follow the procedure
indicated above. Begin the cut from the hole you pierced
by moving the preheating flames to the normal distance
from the plate and follow the line drawn on the plate.
Round holes are made easily by using a cutting torch
with a radius bar attachment.
Cutting Rivets
The cutting torch is an excellent tool for removing
rivets from structures to be disassembled or demolished. Rivet cutting
procedures are shown in figure 4-30. The basic method
is to heat the head of the rivet to cutting temperature by
using the preheating flames of the cutting torch. When
the rivet head is at the proper temperature, turn on the
oxygen and wash it off. The remaining portion of the
rivet can then be punched out with light hammer blows.
The step-by-step procedure is as follows:
Figure 4-30.—Using a cutting torch to remove a rivet head.
1. Use the size of tip and the oxygen pressure
required for the size and type of rivet you are going to
cut.
2. Heat a spot on the rivet head until it is bright red.
3. Move the tip to a position parallel with the
surface of the plate and turn on the cutting oxygen
slowly.
4. Cut a slot in the rivet head like the screwdriver
slot in a roundhead screw. When the cut nears the plate,
draw the nozzle back at least 1 1/2 inches from the rivet
so you do not cut through the plate.
5. When cutting the slot through to the plate, you
should swing the tip through a small arc. This slices half
of the rivet head off.
6. Swing the tip in an arc in the other direction to
slice the other half of the rivet head off.
By the time the slot has been cut, the rest of the rivet
head is at cutting temperature. Just before you get
through the slot, draw the torch tip back 1 1/2 inches to
allow the cutting oxygen to scatter slightly. This keeps
the torch from breaking through the layer of scale that
is always present between the rivet head and the plate.
It allows you to cut the head of the rivet off without
damaging the surface of the plate. If you do not draw
the tip away, you could cut through the scale and into
the plate.
A low-velocity cutting tip is best for cutting buttonhead
rivets and for removing countersunk rivets. A
low-velocity cutting tip has a cutting oxygen orifice with
a large diameter. Above this orifice are three preheating
orifices. Always place a low-velocity cutting tip in the
torch so the heating orifices are above the cutting orifice
when the torch is held in the rivet cutting position.
Cutting Cable or Wire Rope
You can use a cutting torch to cut wire rope. Wire
rope consists of many strands, and since these strands
do not form one solid piece of metal, you could experience
difficulty in making the cut. To prevent the wire
rope strands from unlaying during cutting, seize the wire
rope on each side of the place where you intend to cut.
Adjust the torch to a neutral flame and make the cut
between the seizings. If the wire rope is going to go
through sheaves, then you should fuse the strand wires
together and point the end. This makes reeving the block
much easier, particularly when you are working with a
large-diameter wire rope and when reeving blocks that
are close together. To fuse and point wire rope, adjust
the torch to a neutral flame; then close the oxygen valve
until you get a carburizing flame. With proper torch
manipulation, fuse the wires together and point the wire
rope at the same time.
Wire rope is lubricated during fabrication and is
lubricated routinely during its service life. Ensure that
all excess lubricant is wiped off the wire rope before you
begin to cut it with the oxy-gas torch.
Cutting Containers
Never perform cutting or welding on containers that
have held a flammable substance until they have been
cleaned thoroughly and safeguarded. Cutting, welding,
or other work involving heat or sparks on used barrels,
drums, tanks, or other containers is extremely dangerous
and could lead to property damage, loss of life, or both.
Whenever available, use steam to remove materials
that are easily volatile. Washing the containers with a
strong solution of caustic soda or a similar chemical will
remove heavier oils.
Even after thorough cleansing, the container should
be further safeguarded by filling it with water before any
cutting, welding, or other hot work is done. In almost
every situation, it is possible to position the container so
it can be kept filled with water while cutting or other hot
work is being done. Always ensure there is a vent or
opening in the container for the release of the heated
vapor inside the container. This can be done by opening
the bung, handhole, or other fitting that is above the water
level.
When it is practical to fill the container with water,
you also should use carbon dioxide or nitrogen in the
vessel for added protection. From time to time, examine
the gas content of the container to ensure the concentration
of carbon dioxide or nitrogen is high enough to
prevent a flammable or explosive mixture. The air-gas
mixture inside any container can be tested with a suitable
gas detector.
A metal part that is suspiciously light may be hollow
inside; therefore, you should vent the part by drilling a
hole in it before heating. Remember: air or any other gas
that is confined inside a hollow part will expand when
heated. The internal pressure created may be enough to
cause the part to burst. Before you do any hot work, take
every possible precaution to vent the air confined in
jacketed vessels, tanks, or containers.
Judging Oxy-Gas Cutting Quality
To know how good of a cutting job you are doing,
you must understand know what constitutes a good
oxy-gas cut. In general, the quality of an oxy-gas cut is
judged by four characteristics:
1. The shape and length of the draglines
2. The smoothness of the sides
3. The sharpness of the top edges
4. The amount of slag adhering to the metal
Figure 4-31.—Effects of correct and incorrect cutting procedures.
Drag lines are line markings that show on the surface
of the cut. Good drag lines are almost straight up
and down, as shown in figure 4-31, view A. Poor drag
lines, as shown in figure 4-31, view B, are long and
irregular or curved excessively. Drag lines of this type
indicate a poor cutting procedure that could result in the
loss of the cut (fig. 4-31, views B and C). Draglines are
the best single indication of the quality of the cut made
with an oxy-gas torch. When the draglines are short and
almost vertical, the sides smooth, and the top edges
sharp, you can be assured that the slag conditions are
satisfactory and not excessive. A satisfactory oxy-gas
cut shows smooth sides. A grooved, fluted, or ragged
cut surface is a sign of poor quaility.
The top edges resulting from an oxy-gas cut should
be sharp and square (fig. 4-31, view D). Rounded top
edges, such as those shown in view E of figure 4-31, are
not satisfactory. The melting of the top edges may result
from incorrect preheating procedures or from moving
the torch too slowly. An oxy-gas cut is not satisfactory when slag adheres
so tightly to the metal that it is difficult to remove.
Getting good consistent oxy-gas cuts is not rocket science
but, does require some practical knowledge and the ability
to observe the cutting process. The ability to know when to
stop and make corrective adjustments is what skilled oxy-gas
cutters have developed.