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
•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
