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
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