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Introduction
Oxygen-fuel cutting is an important industrial process. Much more acetylene is used for cutting metals than for welding them. For many cutting applications, there is no more effective and efficient process. Used in construction, manufacturing, and repair operations, cutting equipment is inexpensive, portable, and easy to use. In some applications, propane or natural gas may be more cost-effective and replaces acetylene as the fuel gas. We will explain how the oxygen-fuel cutting process works, its capabilities and limitations. We will also cover cutting torches, troubleshooting, operating tips, safety, and then present some helpful accessories. Finally we will discuss other thermal cutting processes important in today’s industry.
Process Name
What term does the AWS use for any cutting process using oxygen and any fuel gas?
Oxy-fuel cutting.
What is the AWS abbreviation for oxy-fuel cutting?
The abbreviation for all oxy-fuel cutting is OFC.
What is the AWS abbreviation for the oxyacetylene cutting process?
The abbreviation for oxygen-acetylene cutting is OAC. OAC is just one member of the OFC family.
Equipment
How do you convert oxyacetylene welding equipment into an oxygen-acetylene cutting equipment?
Conventional OAW equipment (outfit) is readily converted to perform light to heavy OAC by exchanging the welding nozzle on the torch handle to a cutting accessory head fitting into the handle, Figure 2–1.
Figure 2–1Oxygen-acetylene cutting equipment
How does an OAC cutting torch or accessory cutting assembly differ from an OAW torch?
The OAC cutting head still contains a means of mixing oxygen and acetylene to produce an approximate temperature of 6300°F (3100°C). But it has added means to deliver a stream of pure oxygen to the cutting point. An oxygen lever opens this pure oxygen stream when the welder, fitter or cutting-operator depresses it. See Figure 2–2.
Figure 2–2Oxy-fuel cutting torches. An injector cutting torch (left) and a mixing chamber positive pressure cutting torch (right)
What are the advantages of a cutting accessory head over a regular cutting torch?
A cutting accessory head is less expensive than a one-piece cutting torch; it is quicker and easier to change back and forth between the cutting and welding functions than between a welding torch and cutting torch. With its greater length, a one-piece cutting head puts more distance between the cutting action and the welder and usually can handle greater oxygen flows for large jobs. Some cutting torches have cutting heads at a particular angle for a given task to relieve operator fatigue. The position of the cutting handle is a matter of preference and varies by manufacturer.
The two cutting torches in Figure 2–2 have different designs. How do they differ and what are their advantages?
The torch on the left side of Figure 2–2 uses an injection chamber or venturi to draw the fuel gas into the oxygen stream and operates with fuel pressures 6 oz/in2, such as those supplied by an acetylene generator or a regulated natural gas system delivering in water column inches or about pound. The torch depicted on the right uses a mixing chamber to bring the gases together and is also known as balanced-pressure, positive-pressure, or medium-pressure torch. The advantage of the mixing chamber design torch is that it operates at higher fuel gas pressures and can supply more heat than the venturi design: the venturi design when adjusted properly creates a near perfect cutting flame which uses the fuel gas more efficiently. See detail in Figure 2–2.
What changes are needed to cut steel thicker than one inch?
Because cutting thick steel requires more oxygen than thin steel, a special oxygen regulator with the capacity of delivering more oxygen volume at higher than welding pressures may be needed. Larger diameter hoses may also be required. The welding acetylene regulator is fine for cutting. Also since there is much higher oxygen consumption and more rapid cylinder depletion than in welding operations, the typical cutting regulator is a two-stage regulator to maintain a constant working pressure as the cylinder gas dwindles. Oxygen regulators specifically for cutting usually have low-pressure gauges (on the output or torch side of the regulators) with higher pressure calibrations than welding regulators. OFC operations on extremely thick metals can require 100 to 150 psi (6.8 to 10 bar) oxygen pressures.
Table 2–1 Optimum pressure and gas flow settings for cutting various metal thicknesses
What important facts should be remembered regarding cutting tips?
•They made are of copper and can easily be damaged if dropped. Tips from one torch maker cannot, in general, be used in another manufacturer’s torch.
•If you have removed the tip nut that retains the torch tip, and the torch tip is stuck in the torch body, a gentle tap on the back of the torch head with a plastic hammer will release the tip.
•Care should be taken when cleaning the tip to avoid breaking off the tip cleaner inside the torch tip.
How do high-speed cutting tips differ from standard ones?
Regular cutting tips have a straight-bore oxygen channel and operate from 30 to 60 psi (2 to 4 bar). High-speed tips have a diverging taper and permit operation at oxygen pressures from 60 to 100 psi (4 to 7 bar). This permits a 20% increase in cutting speed. They are used only on cutting machines. See Figure 2–3.
Figure 2–3Standard cutting tip (left) and high-speed cutting tip (right)
What fuels other than acetylene are used for preheating in the OFC process? These are often called alternative fuels.
•Propane
•Natural gas
•Propylene
•Methyl acetylene-propadiene stabilized (known as MPS or MAPP® )
What changes must be made to OAC equipment to properly utilize alternative fuels?
Torch tip designs are frequently different because alternative fuels may be supplied at lower pressures, have different ratios of fuel to oxygen, and different flame and burn rate characteristics. Several manufacturers offer alternative fuel torches. Figures 2-4 and 2-5 shows alternative fuels cutting tips.
Figure 2–4Alternative fuel gas cutting tip
Figure 2–5Alternative fuel gas tips are two piece tips consisting of [1] an outer shell, [2] an inner member, [3] grooves for preheating flames, [4] extremities of grooves and [5] a cutting oxygen bore
Why are these alternative fuels used when acetylene always produces a higher pre-heat temperature?
Alternative fuels are far more stable than acetylene therefore much safer to handle. An alternative fuel may also offer significant cost savings. Fuel selection is a complex matter involving material thickness, cutting speeds, preheat time, fuel performance on straight lines, curves and bevels, and their impact on the total cost. The availability of fuel, labor, the cost of preheat oxygen, and the suitability of the alternative fuel to perform related processes like welding, heating, and brazing also influence total production cost. While fuels other than acetylene do not produce the high flame temperature of acetylene, some can produce a greater volume of heat output throughout the outer flame envelope. This gives an advantage to some alternative fuels in cutting thick steels.
What other differences are there between cutting with OAC tips and OFC tips?
The main difference is the distance the cutting tip is held above the metal. The acetylene cutting tip is held so the pre-heat inner- cones are just above the metal; the alternative fuel cutting tip should be held outside the skirt which further away from the metal. See Figure 2–5A.
Why is the alternative fuel cutting tip held further away from the metal?
The heat characteristics of acetylene are different from other fuels; in Chapter 1, Figure 1–3 shows the hottest part of an acetylene flame is at the inner-cone. Alternative fuel gases characteristics are different and the hottest part of the flame with these gases is outside the skirt. See Figure 2–5A.
Is there a difference in the way alternative fuel gas tip is lit and adjusted?
The acetylene tips are adjusted by opening the fuel valve until no soot, at the end of the carburizing flame, is visible at the end of the acetylene flame then adjust the flame to neutral by adding oxygen. Once the heat cone flames are at neutral depress the oxygen lever and look at the flame if it appears to have a feather or carburizing flame adjust the oxygen with the oxygen stream lever depressed until the flame again looks neutral; now cutting may proceed.
Alternative fuel tips are adjusted by opening the fuel valve enough so the gas can be ignited followed by adding a small amount of oxygen reducing the flame enough to see the inner-cones; then alternate between opening the fuel valve a small portion at a time followed by opening the oxygen until you see the heat cones sticking out of the end of the tip approximately " then increase the oxygen flow until you hear the tip whistle and see the skirt.
Figure 2–5AAppearance of the OFC flame inner-cones and skirt
What other OAC tip designs are available?
A wide variety of tips are available. See Figure 2–6.
Figure 2–6Special purpose oxy-fuel cutting tips
Process
How does oxyacetylene cutting equipment perform the cutting process?
The oxyacetylene flame brings the steel at the beginning of the cut up to kindling temperature of 1600°F (871°C). At the kindling temperature, steel will readily burn in the presence of oxygen. When the oxygen lever is turned on, the pure oxygen stream with the steel at kindling temperature burns, this combination causes a chemical reaction called oxidation. The mixture of oxides of iron is called slag. This slag has a melting point much lower than the melting point of steel itself that is 2600°F (1427°C) and readily runs out of the cut or kerf. The force of the oxygen stream provides additional help to clear the kerf of molten oxides. In addition to the oxyacetylene preheat flame, the burning of the iron in the oxygen stream releases large amounts of heat. This aids cutting action particularly when cutting thick steel. Moving the torch across the work produces continuous cutting action; straight, curved or beveled cuts are readily made.
What is the kerf of a cut?
When cutting is performed material is removed, the width of the cut is the kerf; when flame cutting the oxidation of the metal along the line of the cut removes a thin strip of metal or kerf which is the thickness of the cut which is also the bore size of the cutting tip. In steel under two inches in thickness, it is possible to hold the kerf to about inch (0.4 mm). In making patterns to fabricate parts by flame cutting them out of flat stock, allowance must be made for the kerf. See Figure 2–7 and Table 1–1.
Figure 2–7Kerf and drag in an oxy-fuel cut
What factors determine kerf size?
•Kerf size depends on the following:
•Torch oxygen bore (orifice) size
•Torch tip design
•Oxygen pressure and flow rate
•Preheat flame size
•Cutting speed
What is the proper size cutting tip to use for various material thicknesses?
Cutting tip bore drill sizing, like welding tip orifice drill sizing, numbering system is not standardized in the welding industry. The drill sizing is standard but the manufacturer’s numbers placed on the tips are not standard. One company may identify a number one tip for cutting one inch steel while the same bore drill size from another company may call for a number two tip. The American Welding Society (AWS) has been urging manufacturers to stamp cutting tips with material thickness size to eliminate confusion with the publication AWS C4.5 Uniform Designation System for Oxy-Fuel Nozzles. Compliance is not mandatory therefore manufacturers have not followed through with using this standard.
What does the bore drill size indicate?
Cutting drill bore size indicates cutting orifice size and material thickness which can be cut. See Table 2–2.
Bore Size for Oxy-Fuel Cutting
| Plate Thickness inches (mm) | Bore Drill Size inches (mm) |
| 1/4-1/2 (6.35-12.7) | 68-53 DR 0.031-0.059 (0.794-1.51) |
| 3/4(19.05) | 62-53 DR 0.038-0.059 (0.965-1.51) |
| 1(25.1) | 56-53 DR 0.046-0.059 (1.18-1.51) |
| l-2 (38.1-50.8 | 51-46 DR 0.067-0.081 (1.70-2.06) |
| 3-5 (76.2-127.0) | 46-44 DR 0.081-0.086 (2.06-2.18) |
| 6-8(152.4-203.2) | 40-39 DR 0.098-0.010 (2.49-2.53) |
| 10 (254) | 39-35 DR 0.010-0.011 (2.53-2.94) |
Table 2–2 Material thickness to bore size for cutting tips
Why does kerf width grow larger with increasing steel thickness?
Cutting thicker steel requires more oxygen, which requires a larger oxygen orifice size, greater oxygen flow rates and a larger oxygen stream. These lead to a wider kerf.
How can the bore size be determined?
Using a tip cleaner find the round file which will fit snuggly into the bore then determine the bore size by the chart list on the tip cleaner’s container.
What is drag?
The distance between the cutting action at the top and bottom of the kerf is called drag. When the oxygen stream enters the top of the kerf and exits the bottom of the kerf directly below, the drag is said to be zero. If the cutting speed is increased (or the oxygen flow decreased), oxygen in the lower portion of the kerf decreases and the kinetic energy of the oxygen stream drops, slowing cutting action in the bottom of the cut. This causes the cutting action at the bottom of the kerf to lag behind the cutting action at the top. Drag may also be expressed as a percentage of the thickness of the cut. See Figure 2–6.
What are the effects of excessive drag?
Excessive drag can cause loss of cutting action in thick cuts and restarting the cutting action may cause the loss of a part being flame cut.
When can reverse drag occur?
Excessive oxygen flow, too slow a cut, or damaged orifices may cause reverse drag leading to rough cut edges and excessive slag adhesion.
What is the chemistry of the OAC process?
There are three principal reactions producing three different iron oxides. Notice that the second reaction releases the most heat that helps sustain the cutting action. The equations show the ratios of oxygen to fuel (iron) needed. To a chemist these equations indicate that about 104 ft3 of oxygen will oxidize 2.2 lb. of steel to Fe3O4.
Fe + O2 → FeO + heat of 267 Kj (Kilojoules)
3Fe + 2O2 → Fe3O4 + heat of 1120 Kj
2Fe + 1.5O2 → Fe2O3 + heat of 825 Kj
What are advantages of the OAC process?
•Low cost compared with machine tool cutting equipment.
•No external power required.
•Readily portable.
•Steels usually cut faster than by conventional machining process.
•Cutting direction may be changed easily.
•OAC is an economical method of plate edge preparation for groove and bevel weld joints.
•Large plates may be cut in place.
•Parts with unusual shapes and thickness variations hard to produce with conventional machinery are easily produced with OAC.
•Can be automated using tracks, patterns, or computers to guide the torch.
What are disadvantages of the OAC process?
•Dimensional tolerance of OAC is dramatically poorer than machine tool based cutting.
•OAC process is commercially limited to steel and cast steel.
•Both the preheat flame and the stream of molten slag present fire and burn hazards to plant and personnel.
•Proper fume control is required.
•Hardenable steels may need pre-heat, post-heat, or both to control the metallurgy and properties of the steel adjacent to the cut.
•High-alloy steels and cast iron need additional process modifications.
What is the maximum steel thickness that may be cut with OAC?
OAC has no practical limit. Steel seven feet thick is routinely cut in heavy industry, and fourteen-foot cuts are not uncommon.
What is the minimum mild steel thickness that may be cut with OAC?
OAC’s lower limit is 20 gauge (0.035 inch or 0.88 mm) steel. Below this thickness the cut becomes irregular with uncontrollable melting, but it can be cut with a large tip-to-plate angle and fast travel speed. Thinner steel sheets are best cut with laser or plasma cutters.
Setup (and Related Safety)
How can the welder determine what cutting tip size and what oxygen and acetylene pressures to use on a given thickness of material?
Given a material and thickness, use a torch manufacturer’s table to convert metal thickness to tip size, starting oxygen pressure and acetylene pressure. Remember these are suggested starting pressure ranges. Fine-tuning of the pressures may be needed to get the best combination of speed and quality.
What steps are required to set up a cutting torch to cut inch carbon steel? Be sure to include all safety precautions.
•Inspect and clean the torch using the cleaning kit.
•Put on your welding safety equipment: goggles with filter lens (or tinted face shield), cap, high-top shoes, fire retardant coat, cape sleeves and bib or cotton or wool long-sleeved shirt, and pants and welding gloves.
•Avoid wearing trousers with cuffs when cutting as they tend to catch hot sparks and can easily catch your pants on fire. Wear no synthetics. If you will be doing overhead cutting, leather skins, fire retardant coats, cape sleeves and bid or aprons are necessary to protect your clothing from falling sparks. Goggles and face shields should be of number 5 shade.
•Firmly secure the oxygen and acetylene cylinders to a welding cart, building column, or other solid anchor to prevent tipping during storage or use. Non-flammable material must be used to secure the cylinders. Remove the safety caps.
•Verify the cutting torch has flashback arrestors installed.
•Check to make sure there are no nearby sources of ignition and then momentarily open each cylinder’s valve to the atmosphere and re-close the valve quickly to purge the valve; this is known as cracking a valve. Cracking serves to blow out dust and grit from the valve port and to prevent debris from entering the regulators and torch. Stand on the opposite side of the cylinder from the valve port when cracking.
•With a clean, oil-free cloth, wipe the valve-to-regulator fittings on both cylinders to remove dirt and grit from the fittings’ connection faces and threads. Cleanse to both regulators’ threads and faces. Remember, to never use any oil on high-pressure gas fittings. Oxygen at high pressures can accelerate combustion of oil into an explosion.
•Check to see that both the oxygen and acetylene regulator pressure adjustment screws are loosened (but not falling out of their threads), then screw each regulator to its respective cylinders. Snug up the connections with a wrench. Caution: Oxygen cylinder-to-regulator threads are right-handed; so are oxygen hose-to-torch screw fittings. Acetylene cylinder-to-regulator fittings and acetylene hose-to-torch fittings are left-handed threads. This arrangement prevents putting the wrong gas into a regulator or torch connection.
•Stand so the cylinders are between you and the regulators. S-L-O-W-L-Y open the oxygen cylinders valves. Be sure to open the oxygen cylinder valve until it hits the upper valve stop and will turn no further.
•With the cylinders between you and the regulators, open the acetylene cylinder valve gradually and not more than one and a half turns. If there is an old style removable wrench on the cylinders, make sure to keep it handy in case you must close the cylinder valve immediately in an emergency.
•Look for the high-pressure—or cylinders side—gauges to indicate about 225 psi (15.5 bar) in the acetylene cylinders and 2250 psi (155 bar) on the oxygen cylinders. These pressures at 70°F (21°C) will indicate the cylinders are fully charged. Note that these pressures will vary with ambient temperature of the cylinders. The pressures given above are for full cylinders at 70°F (21°C), but the actual pressure will vary with cylinder temperature.
•Install the cutting torch on the hoses, or if using a combination welding and cutting handle, install the cutting accessory on the torch handle.
•First, check the area for ignition sources, other than your torch igniter. Then purge each torch hose of air separately: Open the oxygen valve on the torch about three-quarters of a turn, then screw in the pressure control screw on the oxygen regulator to your initial pressure setting. After several seconds, close the torch valve. Do the same for the acetylene hose. Comment: We do this for two reasons, (1) to make sure we are lighting the torch on just oxygen and acetylene, not air, and (2) to get the regulators set for the correct pressure while the gas is flowing through them. If the gas hoses are more than 50 feet (15 m) long, a higher regulator setting will be needed to compensate for the pressure drop in the hoses.
•Test the system for leaks at the cylinder-to-regulator fittings and all hose fittings with soapy water. Bubbles indicate leaks.
•Proceed to light and adjust the cutting torch as detailed below.
What are the steps for lighting and adjusting the cutting torch to cut inch thick mild steel? Include all safety precautions.
•Follow the steps of securing the cylinders, installing the regulators, hoses, and torch, purging the hoses of air, and setting the regulator pressures from cutting reference tables for inch steel: acetylene at 6 psi (0.4 bar) and oxygen at 30psi (2 bar).
•Never adjust the acetylene regulator pressure above 15 psi (1 bar) as an explosive disassociation of the acetylene could occur.
•Open the oxygen valve on the back end of the torch all the way.
•Recheck the low-pressure gauge pressures to make sure the working pressures are not rising. If the working pressure should rise, it means that the regulator is leaking. The cylinders must be immediately shut down at the cylinder valves as continued leaking could lead to regulator diaphragm rupture and a serious accident.
•Light the torch by opening the acetylene valve on the torch handle about turn and light the acetylene using your flint igniter. A large, smoky, orange flame will result. Also, you must have your tinted welding facemask over safety glasses (or your welding goggles with a number 5 lens shade) on prior to lighting the flame.
•Increase the flame size by slowly opening the acetylene valve until most of the smoke disappears.
•Open the oxygen preheat valve on middle of the torch and adjust for a neutral flame.
•Actuate the cutting oxygen lever and examine the preheat flame. Further adjustment of the preheat oxygen valve may be needed to keep the preheat flame large enough when the cutting oxygen is used. This is because cutting oxygen use may cause the hose pressure to drop so much the oxygen to the preheat flame must be increased to keep a proper preheat flame.
•You are ready to begin cutting.
What is the best way to flame cut inch (6 mm) through inch (12.7mm) thick metal?
See Figure 2–8.
•The cutting torch tip is held perpendicular to the metal.
•Start the cut at the edge of the stock by preheating the edge of the stock. In thicker material, the torch may be angled away from the direction of travel so the preheat flame strikes down the edge of the material. When the stock becomes a dull cherry red, begin cutting by squeezing the oxygen cutting lever. Remember to hold the torch tip perpendicular to the surface of the stock when cutting action has begun.
•Move the torch along the cut line in a steady motion. For right-handed welders, cutting from right to left allows the welder to see the marks of the cutting line more easily. Left-handers will usually prefer cutting left to right.
Figure 2–8Position of cutting torch tip on 1/4 inch and thicker plate, starting (left) and cutting (right)
What is the best way to cut thin metal (10 guage- inch or thinner)?
•Utilize the smallest cutting tip available with two preheat flames.
•Hold the torch at a 20 degree to 40 degree angle to the metal surface to increase the kerf thickness.
•Adjust the flame to the smallest preheat flame that will permit cutting.
•Set oxygen pressure at 15psi (1 bar). See Figure 2–9.
Figure 2–9Torch position for cutting thin sheet metal, starting (left) and cutting (right)
When cutting thin-gauge sheet metal, what step can be taken if slag accumulates on the underside of the good part?
Tipping the torch away from the side you will use allows the slag to form on the scrap side of the kerf will keep slag off the good part.
What are the proper steps to shut down an oxyacetylene torch and its cylinders?
•Turn off the oxygen and then the acetylene with the torch handle valves.
•Turn off the oxygen and acetylene cylinders valves on the cylinders.
•One at a time, open and reclose the oxygen and acetylene valves on the torch handle to bleed the remaining gas in the lines and regulator to atmosphere. Verify that both the high-pressure and low-pressure gauges on both gases indicate zero pressure. Bleed off the oxygen first to eliminate the possibility of providing oxygen to the remaining acetylene.
•Unscrew the regulator pressure adjustment screws on both regulators in preparation for the next use of the equipment.
Applications
What metals can readily be cut using the OFC process?
•Cutting of new steel plate, beams, and pipes to size both in mills, in fabrication plants, and on construction sites.
•Cutting risers, gates, and defects from cast iron and steel castings in steel mills.
•Cutting up old steel and cast iron equipment for removal and salvage.
•Removing rivets from old equipment without damaging the surrounding steel.
•Removing damaged parts prior to welding new ones in place.
•Gouging the surface of steel plate edges in preparation for welding.
•Manufacturing steel parts by cutting them out of flat stock instead of machining them.
•Removing backing bars from a weld.
What is stack cutting?
In order to make multiple parts in a single cutting pass, multiple sheets or plates are stacked and clamped or welded together. Then the cutting proceeds manually, or more likely by machine. When the cutting is complete, the stack comes apart, leaving multiples of flame cut parts. Stack cutting is also useful in cutting stock so thin it could not be cut in a single layer. It can be used in place of shearing or die stamping when the production run does not justify making dies. There may also be substantial savings of fuel and oxygen in stack cutting as gas consumption is not directly proportional to total cut thickness. Generally the maximum thickness of plates in stack cutting must not exceed 0.5 inch (12.7 mm). Note that high-quality stack-cut parts require clean, flat plates (or sheets) securely clamped in position with no air gaps between the plate layers. If there is air between the plates, the cutting action will be extinguished and the parts will be ruined.
What metal would OFC definitely not a good choice?
Here are some examples:
•Aluminum
•Brass
•Copper
•Lead
•Magnesium
•Stainless steel
•Zinc
The OAC process can readily cut what metals?
•Mild steel (steel with a carbon content of less than 0.3% carbon)
•Low-alloy steels
•Cast iron (though not readily)
•Titanium
What materials can be cut by the OAC process if additional steps are taken?
•Stainless steel
•High-alloy steel (must be preheated)
Why do high-alloys of steel resist OFC?
As the number and percentage of alloying elements increase, OFC becomes less effective. Oxides of the alloying elements have a higher melting point than the alloying elements themselves and are refractory in nature. (Remember that oxides of iron have melting points lower than the melting point of iron so they become fluid and they readily leave the kerf as molten slag.) Unlike iron oxides, an alloy’s oxides do not readily run out of the kerf to expose new iron to oxygen to keep the burning process going, and cutting becomes more difficult.
By what means can OFC be extended to metals and alloys not readily cut?
•Torch Oscillation
•Waster Plate
•Wire Feed
•Metal Powder Cutting
•Flux Cutting
How does torch oscillation work?
By torch manipulation the entire starting edge of the cut is brought to a bright red color before beginning the cut. This technique is usually used in conjunction with one of the other four cutting enhancement methods on low-alloy stainless steel up to 4 inches thick and on resistant cast iron.
How does a waster plate work?
A low-carbon steel waster plate is secured to the top of the stainless steel to be cut, and the cutting action begun on the waster plate. The heat released from the waster plate’s burning provides additional heat to the cutting action in the stainless below. Hot slag from the waster plate tends to flush the kerf of the stainless steel’s refractory oxides. Waster plate disadvantages are the extra cost of the waster plate, additional set-up time, slow cutting speeds, and rough cut.
How does wire feed cutting work?
A small diameter carbon steel wire is fed into the torch preheat flame just ahead of the cut and melts onto the surface of the alloy steel. The additional carbon steel works just like a waster plate to enhance cutting action. A motor feed and wire guides are needed as accessories.
How does metal powder cutting work?
Powder metal cutting (AWS abbreviation is POC) uses iron-rich powder that is dropped into the kerf or injected into the cutting oxygen stream to add heat. Some powders also chemically combine with the alloying oxides to increase their fluidity and increase the ability of the oxygen jet to wash them out of the kerf. Frequently, cutting speed of POC in high-alloy steels can match OFC in mild steel of the same thickness.
How does flux cutting work?
Flux cutting uses a granular flux introduced into the oxygen cutting stream. The flux combines with the alloying metals’ oxides to lower their melting temperatures to near those of iron oxides and get them to flow out of the kerf. Flux cutting can eliminate torch oscillation and can increase cutting speeds in stainless to that of carbon steel of the same thickness.
Tips, Techniques, and Helpful Accessories
Why is it important to keep the torch tip face clean and flat and to clean out the orifices with tip cleaners regularly?
A damaged tip face or plugged tip orifice can cause an unsymmetrical flame. Such a flame will produce irregular rough edges, and slow cutting action. Dirt inside the torch tip may cause flashbacks.
What are the two best ways of marking the line of cut?
•With welder’s soapstone.
•By a series of center punch marks along the line of cut.
What readily available material may be used to improve the quality of some cutting tasks?
Angle-iron may be used as a straight edge guide, or as a bevel guide. See Figure 2–10.
Figure 2–10Angle iron used as a straight edge (left) and bevel guide (right)
What are two mechanical aids that can be attached to a cutting torch?
•Wheels to keep the torch tip at a constant height above the work and reduce operator fatigue. See Figure 2–11.
Figure 2–11Torch wheels
•Compass attachment to make nearly perfect circles easily. See Figure 2–12.
Figure 2–12Compass attachment
Name the four main types of electrical or electronic aids to guide a cutting torch to increase the accuracy and quality of cutting.
•The motorized cutting head is the most primitive improvement over a handheld torch. Its wheel is motor-driven to maintain optimum cutting speed; the wheel also keeps the torch-to-work distance constant. However, the welder must still guide the motorized head manually. The small wheel in the rear of the unit is useful for cutting accurate bevels. See Figure 2–13.
Figure 2–13A motorized cutting head
•A portable track cutting machine travels along a pair of steel rails driven by a 120 VAC motor. It can make 90° bevel angle and chamfer cuts. It also cuts circles from 4 to 96 inches (0.1 to 2.4 m) diameter. With a second torch, it makes two parallel cuts simultaneously to produce a strip of metal with two parallel edges. It is especially useful in cutting accurate bevels and chamfers for proper fit up. See Figure 2–14 and 2–15.
Figure 2–14Portable track cutting machine
Figure 2–15Two-torch strip cutting attachment for portable cutting machine
•A pattern tracer is the next level of improvement. A stylus follows the edge of a metal pattern, or a photoelectric eye follows the lines on paper and a torch (or could be multiple torches) is directly connected to the pattern tracing mechanism, permitting the torch to reproduce the pattern shape in steel. These systems require heavy cutting operator involvement and supervision.
•Computer driven cutting machines produce the most accurate and best quality cuts. These machines store the path of the cutting torch in their memories. Advanced machines control torch-to-work distance, adjust the torch speed on curves and around corners, adjust the pre-heat and cutting flame to starting and ending cuts. The most sophisticated machines require only loading raw stock and removing scrap and finished parts and can pierce holes to make inside cuts without operator assistance.
What degree of dimensional accuracy can be maintained in cutting machines?
About inch (0.8 mm) can be achieved.
How should you adjust the torch tip preheat orifices in a normal cut?
If there are two preheat orifices, the tip should be rotated in the torch so that a line drawn between orifices will be perpendicular to the cut line. If more orifices, two should fall on the cut line and the rest divided equally on each side of the cut line. This symmetrical preheating improves the quality of the cut. See Figure 2–16.
Figure 2–16Location of preheat orifices in relation to kerf for a normal cut
For making a bevel cut how should you adjust the preheat orifices?
See Figure 2–17.
Figure 2–17Location of preheat orifices in relation to kerf for a bevel cut
What is the best torch technique to start a heavy cut?
See Figure 2–18.
Figure 2–18How to start a cut on heavy steel
What are the proper terminating conditions when making a heavy cut so as not to let the effect of drag permit the cutting action to skip a small triangular area at the bottom of the end of the cut?
As the end of the cut nears, tilt the torch away from the direction of travel. This permits the bottom of the cutting action to proceed ahead of the top cutting action and eliminates premature breakout of the flame which leaves a triangle at the end of the cut. See Figure 2–19.
Figure 2–19How to complete a cut on heavy steel
What is the easiest way to remove a rivet with a cutting torch?
Follow the steps in the Figure 2–20.
Figure 2–20Steps to remove rivet
How can a countersunk rivet be removed?
See Figure 2–21.
Figure 2–21Steps to remove a countersunk rivet
What are two ways to pierce steel with OFC?
•Begin by preheating the material in the pierce location to a dull red color, kindling temperature, with the torch perpendicular to the metal. When metal becomes dull red, slightly raise the torch from the surface and angle the tip away from perpendicular. This prevents the slag blown back from the surface from landing on or in the torch tip. Then squeeze the oxygen lever to start cutting action. As soon as the material is completely pierced, restore the tip to perpendicular and the preheat flame to just above the surface. Complete cutting the opening wanted.
•If a small hole is wanted and the surrounding material is to be protected from cutting action, drill a inch (6 mm) hole at the starting point. Begin the cutting action through the hole. See Figure 2–22.
Figure 2–22Piercing steel
What is the best way to cut out a circle?
Pierce the material inside the circle and away from the finished edge. When cutting action is established, extend the cut into a spiral and begin cutting the circle itself, Figure 2–23. With small circles to avoid damaging the finished edge, drill a inch (6 mm) hole in the center of the circle and begin the cut through the inside of the hole, then spiral out to the edge.
Figure 2–23Cutting a circle
What is the easiest way to sever an I-beam?
Follow the numbered steps in Figure 2–24.
Figure 2–24Severing an “I” beam
What is the AWS designation for oxygen lance cutting?
The abbreviation for oxygen lance cutting is LOC.
What is an oxygen cutting lance and how is it used?
An oxygen lance is a length of steel pipe connected to a source of oxygen. An oxyacetylene welding or cutting torch is used to bring a spot on the work up to ignition temperature. The torch is then removed and the lance pipe end placed over the heated spot and the oxygen supply opened. The lance cuts through the work like a large cutting torch. The steel lance pipe is usually to inch (3 to 6 mm) diameter and is consumed by the cutting action. It has the advantage of being able to poke holes into the work several feet deep. Lances are used to cut large steel or cast iron sections and to cut through reinforced concrete as well. See Figure 2–25.
Figure 2–25Oxygen lance process
Trouble Shooting
What causes a bell-mouthed kerf?
Excessive oxygen pressure, see Figure 2–26.
Figure 2–26Bell-mouthed cut
When the cut is not smooth, how can you determine what corrective action to take?
Compare the defective edge with the drawings in Figure 2–27 to diagnose the problem.
Figure 2–27Cutting Problems and their causes
Air Carbon Arc Cutting
What is the AWS abbreviation for Air Carbon Arc cutting?
The American Welding Society acronym is CAC-A
How does the CAC-A cutting process work?
This process uses an electric arc to melt the metal which is blown away by a high-velocity jet of compress air. See a CAC-A torch in Figure 2–28.
Figure 2–28Air Carbon Arc Torch
Why was this process developed?
Myron D. Stepath originated the process during WW II, while working as a welding engineer with the U. S. Navy, where he conceived the idea to solve the problem of removing defective stainless steel welds in armor plate on warships; the conventional methods, at the time, were chipping and grinding, which had proved infeasible due to time and cost factors.
How is the CAC-A process used?
Today this process is used to rapidly remove defects in welds and base metal.
What are the electrodes made of?
The electrodes are rods made from a mixture of graphite and carbon and most are coated with copper to increase their current-carrying capacity. Manufacturers make both DC and AC rods for this process. See Figure 2–29.
Figure 2–29Carbon electrode air stream and travel direction
What type of power source is required for CAC-A?
Standard constant current welding power sources are used to provide current.
What is the required source of air for this process?
A jet of ordinary compressed shop air is all that is required.
What are the amperage requirements for this cutting or gouging process?
Depending on the electrode diameter and job requirements the amperage ranges can be as low as 60 amperes or as high as 2200 amperes. See Table 2–3.
Table 2–3 Matching electrode diameter to current
Can we calculate electrode consumption?
When used correctly every inch of carbon consumed by the user will get approximately eight inches of groove when making a gouge that is equal in depth to the diameter of the carbon electrode; the gouge should be " wider than the diameter of the electrode. Never burn the electrode closer than three inches from the electrode holder because the heat from the electrode will damage the torch.
What are the advantages of the CAC-A process?
•The primary advantage of this process is the rapid removal of defects so repairs may be made in a timely manner.
•The CAC-A torches are relatively inexpensive.
Are there disadvantages to the use of this process?
•Operator can leave carbon deposits in the area that will be re-welded.
•Carbon deposits must be ground or brushed away before re-welding.
•This process requires compressed air.
Safety for CAC-A
What considerations should be made when using this equipment?
All of the electrical safety considerations covered in Chapter 13 should be followed. The minimum shade lens requirements are the same as those found in Chapter 5, Table 5–7. All of the clothing requirements covered in Chapter 4, Figure 4–26.
Safety
What precautions in handling oxygen and fuel gas cylinders and related equipment apply to OFC?
In addition to the safety precaution covered in this chapter beginning on the next page, all precautions listed in the Safety section of Oxyacetylene Welding, Chapter 1 must be followed.
What are the main hazards of OFC and what safety equipment can prevent these injuries?
•External eye injuries from cutting sparks prevented by safety glasses, or safety shields.
•Internal eye (retinal) damage from viewing hot metal and the radiation coming off it prevented by using a number 5 tinted lens while cutting or oxyfuel welding.
•Burns from weld sparks and hot metal prevented by leather gloves, nonflammable clothing, leather skins when working overhead, cuffless pants, pocketless shirts, a welding cap, and high-top shoes.
•Fires from the welding process are prevented by moving flammables away from the weld zone and having water or fire extinguishers close at hand.
•Fumes from paint or plating vaporized by the cutting process prevented by good ventilation and keeping out of the cutting plume.
What fire safety considerations are important in OFC?
•When cutting near materials that will burn, make sure that flame, sparks, hot slag, and hot metal do not reach them. Cutting creates more sparks than OAW.
•If the work to be cut can be moved, bring it to a safe location before cutting it.
•When flammable materials cannot be moved, use sheet metal shields or guards to keep the sparks away from burnables.
•Prevent sparks from falling into holes or cracks in wooden buildings.
•Do not use tarps or fabric covers to protect other materials from sparks as they will catch fire.
•If cutting on a wooden floor, first sweep it clean, then wet it down before starting cutting.
•Avoid using excessive oxygen pressure while cutting as this will propel sparks farther and make more of them.
•Plan ahead where hot metal will fall when cut; be especially careful to avoid your legs, feet, gas hoses, cylinders, and regulators.
•Have fire extinguishers, buckets of sand, or water on hand should a fire start.
•Make sure jacketed or hollow parts are vented before beginning cutting operations.
What is best way to cut into a sealed tank or container?
Never cut into a sealed container regardless of its size. Even if the container is clean and empty, penetration of the shell could release hot gases or send the cutting flame back toward the welder. If the container is empty and contains no residual vapors, vent it to atmosphere by opening a valve, hatch, bung or drilling a hole, then proceed to cut or weld. See Figure 2–30.
Figure 2–30How to cut into a clean container
How can cutting or welding be done on a tank or container which has contained flammable materials?
An even more dangerous situation results when the vessel contains residual flammable vapors, whether it is vented to atmosphere or not. This will almost certainly result in an explosion. Flood the vessel with water to just below the cutting or welding point. Get the container cleaned usually by boiling with a caustic if the container is small or purged with a non-flammable gas like nitrogen, carbon dioxide or steam. Have the vessel checked for lack of explosive vapors by a qualified person. Then begin cutting or welding. See Figure 2–31.
Figure 2–31Using nitrogen or carbon dioxide to purge oxygen from a container which has held flammable materials
Plasma Arc Cutting
How does plasma arc cutting work and what are its applications?
Plasma arc cutting, AWS designation PAC, is an arc cutting process that uses a constricted arc and removes molten metal with a high-velocity jet of ionized gas issuing from a constricting orifice. There are two variations:
•The first variations are in low-current plasma systems which use the nitrogen in compressed air for the plasma and are usually manual.
•The second variations are in high-current plasma systems which use pure nitrogen for the plasma and are usually automatic.
The PAC torch works very much like the plasma arc welding torch performing keyhole welding, except that the keyhole is not allowed to close. Plasma heat input is very high and melts the work metal. Then the plasma jet blows away the molten material completing the cut. Some PAC systems inject water into the plasma to reduce fumes and smoke; others perform the cutting under water to reduce noise and airborne metal vapor.
What are the capabilities of PAC?
High-current PAC systems cut inch (3 mm) thick metal with a 100 inch/minute (2.5 m/minute) travel speed, 0.050 inch (1.25 mm) thick metal with a 200 inch/minute (5 m/minute) travel speed. The smaller, hand-held torches are used in sheet metal and auto body work. Attachments are available to convert PAW torches for PAC.
Figure 2–32PAC schematic.
What are PAC’s advantages?
•PAC cuts all metals.
•Cutting action is so rapid that despite the high heat input, there is a smaller heat affected zone than in most other processes.
•PAC can pierce metals cleanly without the starting hole needed by OAC.
•PAC is ideal for cutting parts under computer-driven control.
•With its 30,000°F (16,600°C) plasma, it cuts materials with melting points too high for OAC.
•All positions can be used.
•Surface smoothness of the cut edges is equal to or better than OAC.
What are the drawbacks to PAC?
•Equipment may be expensive. Small units today are more cost effective than in the past.
•Metal vapor produced from the cutting must be captured.
•Thick cuts are normally done under water so the metal vapor can be captured; the water container must then be periodically cleaned usually requiring a HAZ MAT crew.
What safety rules should be followed?
All of the safety rules suggested in OAC cutting should be applied to PAC including:
•This process used electricity with voltage ranges from 150 to 400 volts of direct current; this equipment must be properly grounded to avoid electric shock.
•Keep electrical circuits dry.
•Keep all mechanical electrical connection tight; this includes the work lead. Poor electrical connection can cause over heating and fires.
•Proper ventilation is required to prevent inhalation of hazardous metal vapors and gases.
•When securing the equipment always be sure the power supply has been properly shut down and the torch placed in back on it’s proper insulated storage position.
Laser Beam Cutting
How does laser beam cutting work and what are its applications?
Laser beam cutting, AWS designation LBC, is a thermal process using laser beam energy to cut materials by melting or vaporizing. A gas is sometimes used to assist in the removal of melted or vaporized material. Cutting and drilling power densities in the range of 6.5 × 106 to 6.5 × 108 W/in2 (104 to 106 W/mm2) are achieved. Lasers can also drill holes using higher power densities and shorter dwell times than when cutting. Hole dimensions range from 0.0001 to 0.060 inches (0.0025 to 1.5 mm). Although a high-power CO2 laser cuts carbon steel up to 1 inch thick (25 mm), good quality cuts are made on material 0.375 inch (9.5 mm) and less in thickness. The depth of focus limits the quality of thick cuts.
What are the advantages of LBC?
•Narrow kerf widths
•High cutting speeds
•High quality edge surfaces
•Low heat input/minimum distortion
•Cuts most materials
•Easily automated
•Repeatable, precision dimensions
•Multiple layers of material may be cut at the same time
What are the drawbacks to LBC?
•Equipment is expensive.
•Replacement lens’ and consumables are expensive.
See Figure 2–33.
Figure 2–33Schematic of a LBC
